Wellington Central Library Seismic Strengthening

Wellington Central Library Seismic Strengthening Structural Design Features Report Wellington City Council. Reference: 253267

Revision: E

7 October 2020

Project 253267 File 253267-0013-DFR-ST-0001[E].docx 7 October 2020 Revision E

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Report title Structural Design Features Report

Document ID Project number 253267

File path \\aurecon.info\shares\NZWLG\Projects\253267 WCC Workplace\03 Project Delivery\02 Structural\External Correspondence\WCL\OUT GOING\20200921_WCL_253267_Aurecon_Structural

Client Wellington City Council.

Client contact Peter Brennan Client reference

Rev Date Revision details/status Author Reviewer Verifier (if required)

Approver

A 11 September 2020 Internal Draft I.Black C.Bailey T.Holden J.Finnegan

B 14 September 2020 Internal Draft I.Black C.Bailey T.Holden J.Finnegan

C 22 September 2020 Issue for Pricing I.Black C.Bailey T.Holden J.Finnegan

D 25 September 2020 Issue for Pricing - Updated I.Black C.Bailey T.Holden J.Finnegan

E 7 October 2020 Section 7.1 Updated I.Black C.Bailey T.Holden J.Finnegan

Current revision E

Approval

Author signature Approver signature

Name Isaac Black Name John Finnegan

Title Senior Structural Engineer Title Technical Director

Contents 1 GENERAL ..................................................................................................................................................... 1

1.1 Objective ............................................................................................................................................. 1 1.2 Scope .................................................................................................................................................. 1

2 EXISTING STRUCTURE .............................................................................................................................. 3 2.1 General ................................................................................................................................................ 3 2.2 Gravity Resisting Structure ................................................................................................................. 4 2.3 Seismic Resisting System ................................................................................................................... 4 2.4 Foundations ........................................................................................................................................ 5 2.5 Existing Structural and Geotechnical Assessment Overview ............................................................. 5

2.5.1 Structural Assessments ............................................................................................................ 5 2.5.2 Geotechnical Assessments ...................................................................................................... 7

3 BUILDING STRUCTURE RESILIENCE ....................................................................................................... 8 3.1 Importance Level Commentary ........................................................................................................... 8 3.2 Early Contractor Involvement (ECI) .................................................................................................... 8 3.3 Scheme A – Low Level Remediation .................................................................................................. 8

3.3.1 Hollowcore Units ....................................................................................................................... 8 3.3.2 Diaphragm Strengthening ......................................................................................................... 9 3.3.3 Column Ties ............................................................................................................................ 10 3.3.4 Stairs ....................................................................................................................................... 10 3.3.5 Precast Façade Panels ........................................................................................................... 10

3.4 Scheme B – Mid-Level Remediation ................................................................................................. 11 3.4.1 BRB Frames ........................................................................................................................... 12 3.4.2 Hollowcore Units and Precast Panel Connections ................................................................. 13 3.4.3 Diaphragm Strengthening ....................................................................................................... 13 3.4.4 Foundations ............................................................................................................................ 14

3.5 Scheme C – High-Level Remediation ............................................................................................... 14 3.5.1 Resiliency Rating .................................................................................................................... 14 3.5.2 Isolation System ...................................................................................................................... 15 3.5.3 Hollowcore Units and Precast Panel Connections ................................................................. 16 3.5.4 Diaphragm Strengthening ....................................................................................................... 16 3.5.5 New Frames and Foundations ................................................................................................ 17

3.6 Scheme B Solution – Viscous Damper Discussion .......................................................................... 17 3.6.1 Proposed BRB Solution .......................................................................................................... 18 3.6.2 Viscous Damping Option ........................................................................................................ 18

4 SOIL CONDITIONS..................................................................................................................................... 20 4.1 Description of Site Soil Conditions .................................................................................................... 20 4.2 Soil Design Values ............................................................................................................................ 20

4.2.1 Ultimate Soil Strengths ........................................................................................................... 20 4.2.2 Pile Group Effects ................................................................................................................... 21 4.2.3 Liquefaction Potential.............................................................................................................. 21 4.2.4 Basement Wall Design............................................................................................................ 22

4.3 Groundwater ..................................................................................................................................... 22

5 DESIGN LOADS ......................................................................................................................................... 23 5.1 General .............................................................................................................................................. 23 5.2 Imposed Loads .................................................................................................................................. 23

5.2.1 Vertical loads .......................................................................................................................... 23 5.2.2 Collections Floors ................................................................................................................... 23 5.2.3 Barriers and Handrails ............................................................................................................ 23 5.2.4 Retaining Wall Loads .............................................................................................................. 23

5.3 Wind Loads ....................................................................................................................................... 23 5.3.1 Site Wind Speed Profile .......................................................................................................... 24 5.3.2 Parts of Structure .................................................................................................................... 24 5.3.3 Glazing .................................................................................................................................... 24

5.4 Snow and Ice Loads .......................................................................................................................... 24 5.5 Fibre Reinforced Polymer (FRP) ....................................................................................................... 24 5.6 Means of Compliance ....................................................................................................................... 24 5.7 Alternative Solutions ......................................................................................................................... 24

6 ANALYSIS METHODOLOGY ..................................................................................................................... 25 6.1 Seismic Load Coefficient ................................................................................................................... 25

6.1.1 Seismic Loads ......................................................................................................................... 25 6.1.2 Site Parameters ...................................................................................................................... 25 6.1.3 Superstructure Design ............................................................................................................ 26 6.1.4 Parts and Components ........................................................................................................... 26

6.2 Probabilistic Seismic Hazard Study (PSHA) ..................................................................................... 26 6.3 Basin effects ...................................................................................................................................... 26 6.4 Diaphragms ....................................................................................................................................... 27 6.5 Special Load Cases .......................................................................................................................... 28

7 SERVICEABILITY CRITERIA ..................................................................................................................... 29 7.1 Seismic Deflections ........................................................................................................................... 29 7.2 Wind Deflections ............................................................................................................................... 30 7.3 Gravity Deflections ............................................................................................................................ 30 7.4 Shrinkage and Creep Constants ....................................................................................................... 30 7.5 Design Life for Durability ................................................................................................................... 30

7.5.1 Design Life .............................................................................................................................. 30 7.5.2 Durability Provisions ............................................................................................................... 30 7.5.3 Summary of Surface Treatments ............................................................................................ 31 7.5.4 Maintenance Requirements of Surface Treatments ............................................................... 31

7.6 Floor Vibration ................................................................................................................................... 31

8 SOFTWARE ................................................................................................................................................ 32

9 DRAWING AND SPECIFICATION NOTES ................................................................................................ 33 9.1 Floors ................................................................................................................................................ 33

9.1.1 Design Loads .......................................................................................................................... 33 9.1.2 Fire rating Requirements ........................................................................................................ 33 9.1.3 Propping Requirements .......................................................................................................... 33 9.1.4 Dewatering and Waterproofing ............................................................................................... 33

9.2 Foundations ...................................................................................................................................... 33 9.3 Material Properties (Typical Existing Structure) ................................................................................ 33

9.3.1 Concrete ................................................................................................................................. 33 9.3.2 Reinforcement Steel ............................................................................................................... 33 9.3.3 Steel ........................................................................................................................................ 34

9.4 Material Properties (Typical New Design) ......................................................................................... 34 9.4.1 Concrete ................................................................................................................................. 34 9.4.2 Concrete Masonry ................................................................................................................... 34 9.4.3 Reinforcing Steel ..................................................................................................................... 34

9.4.4 Structural Steel ....................................................................................................................... 34 9.4.5 Fibre Reinforced Polymer (FRP) ............................................................................................ 34

10 Proprietary Systems ................................................................................................................................. 35 10.1 Manufacturer Design Requirements ................................................................................................. 35 10.2 Manufacturer Construction Requirements ........................................................................................ 35

11 Construction Monitoring .......................................................................................................................... 36 11.1 Soil Testing and verification .............................................................................................................. 36 11.2 Materials Testing ............................................................................................................................... 36 11.3 Temporary support and shoring ........................................................................................................ 36 11.4 Inspection Requirements .................................................................................................................. 36

A1 - Significant Design Features, Base Isolation System ................................................................... 38 A2 - Base Isolated Building Business Continuity Design Strategy ...................................................... 39 A3 - Base Isolation Compliance Documents ....................................................................................... 39

Appendix A – Base Isolation Design Commentary

Appendix B – Concept Design – Geotechnical Report

Appendix C – Concept Design – Preliminary Geophysical Result

Project number 253267 File 253267-0013-DFR-ST-0001[E].docx, 7 October 2020 Revision E 1

1 GENERAL

1.1 Objective The Design Features Report (DFR) is a detailed document defining the structure’s design criteria and recording key decisions or outcomes. It outlines design loading, structural modelling assumptions, material properties, foundation requirements and design standards. The DFR also defines the calculation procedure and checking principles to be followed, providing a clear explanation of the full design.

This Design Features Report (DFR) has been written to describe the proposed seismic strengthening design work conducted by Aurecon for the Wellington Central Library (WCL) building at Civic Square in the Wellington Central Business District (CBD).

Specifically, this report is to provide an overview of the structural design for three potential schemes for strengthening, as shown in Table 1- Seismic Strengthening Schemes below:

Table 1- Seismic Strengthening Schemes

Scheme A Low-Level Remediation

~40% NBS (IL3)

Seating angles, column ties, precast panel connections, and diaphragm strengthening provided.

Scheme B Mid-Level Remediation

~80% NBS (IL3)

Concentric Buckling Restrained Brace (BRB) frame strengthening scheme, with seating angles, column ties, precast panel connections, and diaphragm strengthening provided.

Scheme C High-Level Remediation

~100% NBS (IL3)

Base Isolation of the building incorporating concentric brace frames strengthening, with seating angles, column ties, diaphragm strengthening provided.

The report is intended to act as a reflective brief, and it outlines our proposed structure so that the client can ensure the design meets their expectations in terms of function, performance and load capabilities.

1.2 Scope The scope of work associated with this report is in accordance with the Design Brief and Conditions of Engagement.

Aurecon is working on three seismic strengthening options for the Wellington Central Library for our Client Wellington City Council (WCC). The structural scope was to further develop the concept designs prepared previously for the three strengthening schemes noted in Section 1.1. The development of these designs will primarily be to add additional certainty and detail to the design and to add sufficient information to allow the project QS to further refine their cost plan.

The structural design development will clarify the works required allowing the services and architectural teams to better understand the implications of the proposed design options.

The following is a summary of the concept structural works required to seismically strengthen the Wellington Central Library over a range of performance levels. Suitable contingencies should be allowed for when costing the designed described in this report, given that Aurecon has not completed a full detailed seismic design, and that much of the detailing and co-ordination associated with a project of this nature has yet to be addressed at this stage.

In order to achieve adequate life safety performance, “Scheme A” primarily addresses the risks associated with the hollowcore floors, the main interior stairs and the exterior façade precast panel connections.


To achieve improved performance in terms of life safety and damage mitigation, “Scheme B” introduces braced frames, specifically a new Buckling Restrained Brace (BRB) structural system. The introduction of the braced frames, and the higher target performance level, results in significantly greater design actions on the diaphragms which are proposed to be strengthened with the use of an FRP overlay.

To achieve a greater level of damage mitigation, “Scheme C” is a Base Isolation solution which introduces a seismic joint at the Ground Level of the building. In this solution, a combination of Lead Rubber Bearings (LRB) and Slider Plates are placed at the top of the existing Basement columns and walls. The ground floor is partially replaced and built over with a new reinforced concrete overlay in order to provide a sufficient platform to support the isolator movement and lateral force distribution. A new structural steel braced frame with conventional bracing members is introduced to stiffen the superstructure. FRP is used to strengthen areas of the existing floor diaphragms, though to a significantly lesser extent than required for Scheme B.

The major structural works are summarised Table 2 below.

Table 2 Proposed Solutions based on the WCC brief

Scheme A – Low-Level Remediation

Scheme B – Mid-Level Remediation

Scheme C – High-Level Remediation

◼ Installation of structural steel members for hollowcore unit support. (Seating members are modified to better support the units under the larger inter-story drifts)

◼ FRP floor diaphragm strengthening at Ground Level only.

◼ Precast façade panel connection strengthening

◼ Stair strengthening ◼ No new Lateral Brace Frames

are introduced ◼ No new piles introduced

◼ Introduction of new BRB Lateral Brace Frames + New Piles/Foundations (Outside of using BRB’s the new structural steel beams and columns are similar to the Base Isolation solution)

◼ Installation of structural steel members for hollowcore unit support

◼ FRP floor diaphragm strengthening in select areas. (More extensive than for Base Isolation solution)


◼ Stair strengthening ◼ Additional roof structure for

new services plant

◼ Introduction of a seismic joint at Ground Level. LRB and Slider Plates installed at the top of Basement columns and walls. Temporary propping and support to superstructure required during installation works.

◼ Installation of new conventional Lateral Brace Frames + New Piles/Foundations

◼ New Ground Level floor slab and beam supports

◼ Installation of structural steel members for hollowcore unit support

◼ FRP floor diaphragm strengthening in select areas

◼ Basement Retaining Wall modifications


◼ Stair strengthening ◼ Additional roof structure for

new services plant


2 EXISTING STRUCTURE

2.1 General The Wellington Central Library (WCL) building was designed by Athfield Architects, with the structural design by Holmes Consulting Group, and constructed by Fletcher Construction in 1990. The building is located on the corner of Victoria Street and Harris Street in the Wellington CBD. The neighbouring Central Administration Building (CAB) and City Gallery Buildings are situated to the south and east of the WCL respectively.

The reinforced concrete building is irregular in plan and comprises of five levels above the ground level, in addition to a carpark basement. The ground floor, level 1 and level 2 cover the full floor plate, the mezzanine and upper two levels have a reduced floor area. The main entrance to be building is located on the Victoria Street frontage, with large steel nikau palms spanning the height of three floors framing the entrance. Further nikau palms wrap around the Harris Street frontage of the building supporting the level 3 and 4 office space that is suspended out above a pedestrian accessway on both Victoria Street and Harris Street.

The ground floor is typically a double height space except for the south and west sides of the building where a mezzanine is located. A concrete bridge, supported by steel framing, connects the two sides of the mezzanine. The lower three levels of the building are designated Library areas, with a cafe located on the mezzanine level at the south end of the building. Access into the carpark is via a reinforced concrete slab on grade ramp off Harris Street. A full building height glass curtain wall faces Te Ngākau Civic Square. Between the City Gallery and the Library building is a pathway off Harris Street which includes shallow pool water-features and an external concrete stair providing access to the mezzanine level. The other three elevations consist of predominately precast concrete panels that span between the floor levels. The servicing plant room is located at the roof level.

Internal access between the levels is provided by a combination of elevators, steel stairs and escalators. The elevators are located either side of the entrance and provide access from the basement up to level 4. Central to the building are the main steel stairs and escalators that provide access from the ground floor to the mezzanine, level 1 and level 2. A series of precast concrete stairs also provide for emergency egress and internal staff movement.

Figure 1 View looking South along Victoria St


2.2 Gravity Resisting Structure The building is a reinforced concrete moment frame structure with frames at a typical spacing of 8m grid centres. The frames consist of main precast concrete haunched beams supporting a suspended precast flooring system and spanning between 800mm diameter circular concrete columns. At the column face, the depth of the main concrete haunched beams, measured from the top of the slab to the underside of the beam, are a maximum of 1300mm. This reduces down to 865mm at the centre. The concrete columns are typically supported off a single belled concrete bored pile jointed via a pile cap to a grillage of concrete ground beams.

The precast hollow-core floor units span in a north-south direction on each floor excluding the mezzanine. These units typically consist of 200 deep hollowcore flooring with a 65mm concrete topping and single layer of 665 non-ductile mesh. The drawings indicate seating to the hollowcore units of typically 50mm to 60mm (Figure 2), there is no bearing strip noted on the drawings or evident in the building. The mezzanine is comprised of a mixture of precast concrete Double-T beams and rib and infill units, that span on to a combination of concrete and steel framing.

Figure 2 Typical existing hollowcore seating details

At the basement level, gravity and lateral restraint are provided by 250mm thick reinforced perimeter concrete shear walls. The basement carpark extends out beyond the ground floor footprint towards Te Ngākau Civic Square, with an approximate floor area of 1,155m². In localised areas masonry block walls are provided to support the suspended ground floor.

The building is clad with a mixture of glazing and precast concrete panels. The external precast concrete panels are typically connected to the concrete beam by steel angles and TCM anchor connections.

2.3 Seismic Resisting System The lateral load resisting structure comprises concrete moment resisting frames in both the transverse and longitudinal directions. At the basement level, perimeter reinforced concrete shear/retaining walls transfer seismic actions to the ground.

The following is taken from the original building documentation as referenced in Holmes Consulting Group Detailed Seismic Assessment Report, dated February 2013:

◼ The Library Building was designed in accordance with NZS4203:1984 and NZS3101:1982, the relevant loadings and concrete structures standards, respectively, current at the time. Design in accordance with


these standards implies an overall building displacement ductility of approximately μ = 6.0 for a ductile moment resisting frame.

◼ Ductile reinforced concrete frames are designed with the aim of absorbing a proportion of the energy associated with large seismic events through the yielding and hinging of the beams. As the beams yield and hinge, energy is dissipated, reducing the overall demands on the remaining building components. In order for this to be an effective means of lowering the imposed forces on a structure, the columns in the frame need to be protected with a higher strength than the beams that frame into them.

This strength hierarchy requires that a capacity design approach is followed during the original design. Based on the material design and loadings standards current at the time of the original design, a capacity design approach should have been followed and structural members detailed accordingly to support a ductile response. A review of the original detailing supports this aspect and while it may not comply fully with current design standards, it should still serve to ensure an adequate level of ductile response.

◼ Due to the tapered floor plate and differing number of bays in the transverse frames, it appears that the original designers aimed to provide frames of equal stiffness by introducing pin-ended beams in the outer bays of the transverse frames in the lower levels of the building. Biaxial actions on “corner” columns has been avoided by creating pin-end beam connections in one of the frames joining into these columns.

2.4 Foundations The foundation comprises of a mixture of concrete ground beams and concrete bored piles. The concrete slab spans onto the two-way ground beams are typically 250mm and 600-700mm deep respectively. Concrete in-situ precast foundation beam-column joints are provided for the precast elements and connection to the concrete bell piles. The columns are founded on top of 900mm diameter concrete bore piles with a depth less than 6m and the bell between 1200mm to 1800mm. Refer to Section 0 of this report for details of the soil profile of the site.

During the Aurecon geotechnical site investigation, there appeared to be approximately 1.5m head of water pressure from the bottom of the basement slab. This corresponds with the data collected for the local area. Soil classification “C” and “Shallow Soil” accordance with NZS 1170.5:2004, this aligns with Aurecon Desktop Geotechnical Study in Appendix B.

During construction, archive documentation shows that there was sheet piling into the existing CAB basement and around the lift pit with ties back to anchors at a max of 4m spacing and piles around the perimeter on the Harris and Victoria Street sides.

2.5 Existing Structural and Geotechnical Assessment Overview

2.5.1 Structural Assessments

A previous detailed seismic assessment (DSA) of the Central Library was completed by Holmes Consulting Group in February 2013.

A score of 60%-70% (IL2) was attributed to the frames based on the assessed flexural and curvature capacity of the beams and overall inter-storey drift limits. A comparison between Aurecon’s internal verifications and the conclusions reported in the Holmes 2013 detailed assessment report are in general agreement.

Following the Canterbury seismic sequence and the 2016 Kaikōura earthquake, there have been a number of changes to the seismic assessment guidelines and codes. Consideration of key changes for seismic assessments of existing buildings the 2017 Ministry of Business, Innovation and Employment (MBIE) Guidelines and the updated 2018 section C5. Some of the updates include:

◼ Specific guidelines on material properties

◼ Provisions for Beam-column-joint and wall assessment with refined parameters for assessing member rotation capacity and moment curvature

◼ Guidance and improvement of assessing precast flooring systems


◼ Detailing and guidance for stair designs

◼ Improved detailing for cast-in anchor connections

◼ P-delta consideration, ductility and drift limitations

◼ Criteria and assessment provisions for loss of gravity support for columns, walls and flooring systems

In February 2019, Aurecon were engaged by WCC to investigate the potential seismic risk to the Central Library building associated with the existing precast floor system, in particular the hollowcore floor units. This engagement followed previous assessment works completed by Aurecon as part of WCC’s larger “Workplace” project involving multiple municipal buildings in Civic Square.

These previous works had identified that, as well as possible issues with the hollowcore floor units, there were a number of additional potential vulnerabilities which could significantly influence the performance of the building and may have their capacity exceeded at load levels at or below the earthquake prone building threshold 33% NBS(IL3) including:

◼ Insufficient seating and detailing of the central stairs and escalators to allow safe egress of occupants following a major seismic event.

◼ Insufficient movement allowance for the precast cladding panel support fixings which could be compromised under excessive imposed deformations.

◼ Potential floor diaphragm capacity issues including deformation incompatibility and the likely brittle performance of the reinforcing mesh.

The 2019 investigation found that while the lateral frames are generally well detailed for the high levels of ductility targeted in the original design, consideration of the overall building performance including P-delta effects, stability coefficients and the curvature demands associated with the high levels of inter-story drift, an approximate upper bound ductility capacity of μ = 4 can be determined for the building.

A comparison of the seismic design spectra from the 1984 loadings code to the current NZS1170.5 seismic design spectra (Figure 3) shows that the original and current base shear design requirements would be broadly comparable over the period range applicable for this building.

Figure 3 Response Spectrum Comparison (IL2) (Extract from Holmes 2013 Detailed Seismic Assessment Report)


The performance score of the hollowcore floor system has been determined using the latest revisions to Section C5 of the MBIE assessment guidelines. Aurecon considers that the revisions, which are based on the latest academic research, learnings and observed failures following recent major seismic events, and through extensive consultation and technical input from many of the foremost engineering consultancies in New Zealand, represent best practice and provides WCC the most accurate assessment of the hazard which the hollowcore units as installed within the Central Library present.

During inspections for the assessment, some existing damage to the hollowcore has been observed in the basement where cracks the full width of several units have led to the installation of a number of structural steel catch frames. We understand that these were installed following observations made after the 2013 Seddon Earthquake.

The Wellington Central Library Building has a reported performance rating of approximately 20%NBS (IL2) based on the assessed capacity of the critical hollowcore floor units. A separate report is available for this. A review of Aurecon’s findings was conducted by Opus (now WSP). Following their advice that they were in general agreement in March 2019 WCC decided to close the Library building.

It is noted that the above assessment was all completed based on an Importance Classification of IL2. In large part the following %NBS values may be scaled by a factor of 1/1.3 (0.77) in converting to an approximate IL3 score. Further discussion regarding the importance level the building is presented in Section 3.1 of this report.

2.5.2 Geotechnical Assessments

From the current preliminary geotechnical testing and desktop study, it has been established that the depth to bedrock drops away on the southern end of the building towards the CAB and old town hall buildings. Likewise, increased depth to bedrock is expected towards the direction to the sea. This is shown in Appendix C, where both historical and current borehole testing shows that depth to rock is at -5.6m to -8.8m for Harris Street and CAB Building, respectively, and is likewise shown in the Figure 4 below.

Figure 4 - Cross Section - Soil Profile, Line 5 (refer to Appendix C)

Depth to rock from Victoria St towards Te Ngākau Civic Square is -2.9m to -7.9m RL. The depth to bedrock is not a linear transition, and the Remi testing shows this. Additional results will be provided in the October 2020 Aurecon Geotechnical Report. It is likely, the performance of buildings constructed of a similar period that are positioned south-east of the Library would have different seismic performance and affected by lateral spread and possible amplification from the Te Aro Basin Effect.


3 BUILDING STRUCTURE RESILIENCE

3.1 Importance Level Commentary The original building design and subsequent assessment reviews have previously been based on the consideration of an Importance Level 2 (IL2) classification. This classification being based on the library being considered to have no specific post disaster function, and there not being a normal occupancy of more than 300 people congregated in one area. WCC have subsequently reviewed the appropriateness of this classification based on the facilities usage, occupancy, and high importance and value to the public and have subsequently instructed that a revised importance of IL3 be adopted for all strengthening schemes.

3.2 Early Contractor Involvement (ECI) Throughout this design stage, having Early Contractor Involvement (ECI) input provided by LT McGuinness was beneficial. As such, there is a high level of confidence that the three schemes presented consider the critical factors affecting both constructability and buildability allowances.

An example of this is the consideration of pile installation for both Scheme B and C. A key design consideration was managing the hydraulic pressure risk within the basement. The ECI process has determined a methodology of pile installation from the ground floor platform, utilising the double-height library space for plant and employing permanent formwork if the form of a flanged steel tube that will extend up beyond the hydro-pressure level is one way of managing the risk. Other considerations such as avoiding the bell of the existing piles when installing the new piles and understanding any possible impact the new piling work may have and that the existing piles are not undermined during this construction process were discussed and incorporated into the scheme.

3.3 Scheme A – Low Level Remediation Scheme A is intended to achieve a minimum 40%NBS(IL3). As such the structure will have low seismic resilience. This may be expected to equate the following damage outcomes:

◼ 1/25 year event – Moderate level of damage, repair and strengthening potentially required

◼ 1/500 year event – Significant damage, possibly requiring demolition

◼ 1/1000 year event – Significant damage, demolition expected

The primary objective of this scheme is based on improving life safety, and the strengthening design proposed is to allow occupants to evacuate immediately following a significant event.

This scheme principally involves the introduction of seating angles, column ties, precast panel connections, and diaphragm strengthening at the ground floor.

3.3.1 Hollowcore Units Currently, there are five main recognised types of failure mechanisms in hollowcore flooring systems. These include insufficient seating, positive bending moment failure, negative bending movement failure, web cracking and torsion.

The maximum calculated drift for the existing structure is in excess of 3.6% at 100%NBS(IL3) loads. The building drifts associated with 40%NBS(IL3) will exceed the provided seating, positive moment, and web cracking capacity of the hollowcore floor units, with the available capacities being assessed at approximately 0.65% x 2, 0.92% x 2, and 0.90% x 2 respectively when considering the performance uncertainty factor. The saddle bars provided across the top of the concrete beams provide sufficient capacity to accommodate the negative bending moment at the beam.

To address the failure modes described, steel catch frames, angle supports, and stiffened PFC angles with a gap to the underside of the flooring system are proposed (Figure 5). A standard 20mm construction tolerance is applied in the seating requirement calculations. To reduce this tolerance comprehensive on-site


measurements would be required to confirm the range seating lengths. While this review may result in a smaller critical seating length, provision of additional seating will be required in the locations noted.

Figure 5 Typical Flooring System Unit Support

It is important to note that the angle details proposed are part of ongoing industry review into the performance of hollowcore floor systems and may need to be revisited as part of any future design as this industry review progresses.

At the ground level, evidence of some transverse cracking has been observed away from the beam face as shown in Figure 6 below. It is unexpected to observed this type of damage at ground given the limited displacement possible, and it highlights the fragility of the units. For this age of the building, some debonding of the prestressed strand ends was common due to the casting and manufacturing process. Research into the reliability and capacity of the bond is ongoing. Potentially strain tests may be undertaken on select units. In addition, video footage of the interior of select cores parallel to existing beam lines is recommended to establish if any significant web-cracking is observable.

Figure 6 Existing damage to hollowcore units at ground

3.3.2 Diaphragm Strengthening A review of the ability of the building diaphragms (floor slabs) to distribute load to the buildings lateral load resisting system has identified areas requiring upgrade. For Scheme A this is limited to the ground floor only, in areas where load is transferred from the existing flexible moment frames to the stiff basement retaining walls. Where diaphragm strengthening is required this is to be achieved through the provision of layers of fibre reinforced polymer (FRP). Sikawrap Hex-103C has currently been specified. Strain matching has been allowed for between the existing mesh and the proposed FRP to ensure the system will behave as intended.


3.3.3 Column Ties In select locations on the Northern and Southern ends of the building where columns positioned at the exterior of the floor diaphragms are not laterally supported by perpendicular beams or specific reinforcing, unbonded reinforcing ties are provided underneath the floor slab and anchored through to the adjacent bay.

Figure 7 Remedial Column Tie Detail

3.3.4 Stairs The stairs in the building were not designed to accommodate movement between floor levels. Therefore, steel angles are proposed to be provided under the stair landings and the fixed connections released to allow inter-story displacements as shown in Figure 8 below.

Figure 8 Wellington Central Library - Main Stairs

3.3.5 Precast Façade Panels A safety issue to be addressed with all three schemes is the precast concrete facade panel connections. These connections do not appear to have any specific allowance for inter-story drift, having nominally enlarged bolt holes for constructability/ building tolerance. While no damage has currently been observed, without a slot to allow for movement, the precast building connections may have or in the future experience damage similar to that noted on the nearby CAB building. Figure 10 shows that lateral movement can only be provided


through plate bending. Two new slotted connections per panel are currently proposed. An alternative consideration is to remove the panels which would have the additional benefit of reducing seismic demands. The main disadvantage of this option would be that a new external cladding system would be required. The design, aesthetic considerations, and higher costings would need to be fully considered.

Figure 9 Existing Precast Exterior Panels

Figure 10 Remedial Precast Exterior Panel Detail

Scheme A is the lowest initial cost option, but with the lowest level of resiliency and performance. An additional disadvantage of selecting Scheme A is that it is more susceptible to potential changes in the loadings code, material and assessment standards. For example, there are currently a number of technical working groups that are looking at the overall New Zealand seismic hazard as well as potential soil re-classifications, and a requirement to account for potential basin edge effects in the Wellington CBD.

3.4 Scheme B – Mid-Level Remediation Scheme B is intended to achieve a minimum 80%NBS(IL3). As such the structure will have moderate level of seismic resilience. This may be expected to equate the following damage outcomes:

◼ 1/25 year event – Light damage, some repair required

◼ 1/500 year event – Light damage, some repair required

◼ 1/1000 year event – Moderate damage, significant repairs, low probability of demolition


Scheme B incorporates Buckling Restrained Braced (BRB) frames to stiffen the building, as well as, a number of the remedial works identified as part of Scheme A including seating angles, column ties, precast panel connections, and diaphragm strengthening provided. Scheme B is expected to represent a lower cost when compared to Scheme C while providing a significantly higher level of resilience performance than Scheme A.

3.4.1 BRB Frames Buckling Restrained Braced (BRB) frames are proposed to stiffen the building, reducing seismic drifts and the associated impact on the hollowcore flooring and precast façade panels, as well as to provide a controllable means of damping to dissipate seismic energy. A key benefit of the BRB system is that it can withstand symmetrical repetitious cyclic seismic loading for both tension and compression demands.

A concrete wall option was considered and discounted early on because of the additional seismic mass it would introduce into the structure. Even though this would potentially provide a cost effective way of stiffening the building, disadvantages include that it would require significantly more foundation works, significantly limit the functionality of the interior.

It is important that the BRBs are installed, manufactured and sourced from a reputable supplier. For our calculations we have used parameters from Corebrace. A rigorous manufacturing, quality assurance, and testing methodology is required of potential suppliers, as well as, close attention to the design of the connection plates. The possibility of minor residual drift also needs to be considered as part of the overall strengthening design. Another design consideration is the ability to inspect the BRB frames following a seismic event. We have selected pinned BRB’s in the event that these could require replacement in the future.

Figure 11 - Typical BRB Joint

To accommodate the BRB braces into frames, a combination of steel sheet encasement of the existing concrete moment frames at selected locations and new steel frames where there are no existing concrete frames is utilised. The Concentric Braced Frame (CBF) arrangement aids in maintaining the library’s form, space and functionality. To provide an aesthetically acceptable solution which provides sufficient load paths for the beam-column-joint demands, a cradle beam (Figure 14) is has been designed to drag and transfer the loads via an annulus connection (above Figure 11 and location shown in Figure 12 below).


Figure 12 Plan Location and Elevation for a Scheme B and C Typical Frame

3.4.2 Hollowcore Units and Precast Panel Connections While the displacement demands are greatly reduced with the introduction of the BRB frames, seating and catch frames are still recommended for the hollowcore units, in addition to remediation of the precast panel connections. The brittle nature and low redundancy of these elements requires that these issues still need to be addressed, albeit to a lesser extent than for Scheme A.

In regards to the hollowcore units a simple seating angle replaces a more complicated SHS frame and PFC bracketry required for Scheme A in order to prevent positive moment failure.

3.4.3 Diaphragm Strengthening

Stiffening the structure in conjunction with targeting a greater %NBS score requires that the floor diaphragms be further strengthened in order to adequately transfer the increased lateral seismic demands. The extent of FRP works is significant and extends over multiple floors levels, whereas for Scheme A this is limited principally to the ground floor only. As for Scheme A, Sikawrap Hex-103C has been specified, although in localised areas of greater demand a higher capacity product such as Sika Carbodur S is recommended.

It is noted that regardless of the whether the building is stiffened or not, the targeted %NBS rating and IL3 classification of the building means that the floor diaphragms in the building would still require a level of strengthening based on the limited capacity of the existing brittle mesh topping.


3.4.4 Foundations

Beneath the new BRB frames seismic compression and tension loads, as well as, base shear takeout are resisted by newly introduced 900mm diameter bored piles. The depth of these piles is nominally 15m but are dependent on the underlying ground conditions which vary.

To order reduce construction costs the effective pile cap and ground beams for the new frames are proposed to be built atop of the exiting basement slab. This design philosophy has been developed in consultation with WCC and contractor feedback. While the quantity of carparking is limited by placing the foundations within the basement, significant time and cost savings are gained by greatly limiting the amount of excavation and tanking.

3.5 Scheme C – High-Level Remediation Scheme C involves Base Isolation of the building superstructure as well as the majority of retrofit works noted previously for Scheme B, with the BRB’s described for Scheme B replaced with elastic steel tube bracing. Base isolation is considered to be the most resilient solution for the building where both protection of the structure, services, and non-structural fit-out could be controlled up to and beyond the ultimate limit state event.

A base isolation scheme will allow for continuity of use at a 1/500-year event, with limited damage at the Ultimate Limit State (ULS) 1/1000-year event, and a Collapse Avoidance Limit State (CALS) at a return period of ~1800 years. In New Zealand, structures with seismic isolation are designed as an “Alternative Solution”, as there are no Acceptable Solutions (i.e. Standards) or Verification Methods currently available to satisfy the Building Code B1 provisions. To demonstrate compliance with the Building Code to the satisfaction of the Building Consent Authority (BCA), the design will follow industry guidance, international Standards, and will need to be independently peer reviewed. The state of practice in New Zealand is represented by the draft Guideline for Design of Seismic Isolation Systems for Buildings (June 2019). This document (the NZ Isolation Guidelines) was initiated by NZSEE and prepared by a committee of engineers who have experience designing isolated buildings in New Zealand. Nevertheless, the NZ Isolation Guidelines are only a draft version which are written in the form of recommendations (i.e. “should: and “may”) instead of mandatory language (“must” or “shall”). It is still undergoing international review and has been officially released for industry feedback. Our analysis and design approach will be to follow the intent of the NZ Isolation Guidelines with reference to international best-practice to improve upon the Guidelines. Reference documents include the following American Society of Civil Engineers (ASCE) Standards and Federal Emergency Management Agency (FEMA) Guides:

▪ ASCE 7-2016 (new buildings) Chapter 17

▪ ASCE 41-2017 (retrofit of existing buildings) Chapter 14

▪ ASCE 4-2016 (nuclear structures) Chapter 12

▪ FEMA P-1051, 2015 NEHRP Provisions: Design Example, Chapter 15: Seismically Isolated Structures.

3.5.1 Resiliency Rating

The proposed strengthening, as a minimum, is intended to meet the requirements of the United States Resiliency Council (USRC) Four Star rating regarding structural engineering. The design provided, through the utilisation of the Base Isolation methodology, provides a minimum of a Four-Star rating, but with Five Star performance on most of the elements considered (Table 3).

The measured elements comprise Safety, Damage and Recovery. Similar to the developing New Zealand “QuakeStar” rating. Our target at the Concept Design stage is a Five Star rating and will be reviewed and ratified as the design proceeds. A Peer Review from a Structural Engineering Consultancy experienced in USRC Star rating will be undertaken at milestones through the design process.


Table 3 USRC Star Rating

Safety Injuries and blocking of exit paths unlikely 5 Star

Damage Minimal damage – repair cost is less than 5% of the building cost 5 Star

Moderate damage – repair cost is less than 10% of the building cost 4 Star

Recovery Immediately to days 5 Star

Within days to weeks 4 Star

We propose to incorporate measurement instruments into the building to directly measure seismic accelerations (accelerometers) and deflections (scratch plates) so that the building performance can be accurately calculated after a significant event.

3.5.2 Isolation System

The isolation system is a hybrid system consisting of lead-rubber bearings (LRB), and flat slider bearings (FSB). This system we believe gives the most cost-effective isolation solution. It is expected that the maximum displacement at the isolation plane under the CALS design event will be in the order of 800mm.

The number and type of isolation bearings is subject to change at detailed design. The indicative isolation system, based on the design and analysis to date is presented in Table 4:

Table 4: Elastomeric Isolation System – Lead Rubber Bearing and Flat Slider

Bearing Type Number

Lead-rubber bearing (LRB) 24

Flat slider bearing (FSB) 55

TOTAL 79 support locations

The option of using concave sliding bearings (including Triple Pendulum Bearings) was also investigated at concept. This system has the potential to further reduce forces and displacements in the superstructure however it results in much larger displacements at the isolation plane. The current isolation plane has a large number of services crossing it as well as having significant architectural constraints on layout. Added to this, the indicative cost of the concave sliding system would likely be significantly more expensive than the current hybrid system based on the total number of support locations and the current ratio of LRB’s to slider plates.

Figure 13 Lateral Coefficient – Displacement


The current design is limited to an Acceleration Displacement Response Spectra (ADRS) and effective Response Spectrum analysis, with effective stiffness and damping properties modelled within a full 3D numerical model. A full non-linear time history analysis will be required as the design is progressed to validate the initial results. A brief illustration of the ADRS analysis accelerations and isolation plane displacements is provided in the figure above.

Table 5 ADRS Analysis Results

Isolation Type Lead Rubber Bearings

Return Interval Scaling Coefficient Displacement Secant

Stiffness Effective Period

EV Damping

Damping Reduction

Slider Parameters Years Factor (mm) (kN/mm) (sec) %

Weight Supported (kN) 123440 25 0.25 0.099 25.3 734 1.0 0.32 0.67 Coefficient of Friction 0.08 50 0.35 0.098 22.7 819 1.0 0.32 0.67

Max Displacement (mm) 800 100 0.5 0.105 49.7 399 1.4 0.37 0.65

Type Number Lead Force

Rubber Stiffness 250 0.75 0.121 114 201 1.9 0.43 0.63

Qd Kd 500 1.0 0.146 213 130 2.4 0.39 0.64 (kN) (kN/mm) 1000 1.3 0.185 363 96.1 2.8 0.32 0.67

B1 24 375 2.0 1800 1.625 0.231 547 79.9 3.1 0.26 0.71

The following displacements are based on the design parameters noted above. The isolation system has been sized to reduce the accelerations imposed on the superstructure as much as practically possible without the displacements becoming excessive.

Table 6 Target isolator displacements at various Design Level Events

Return Period of Earthquake

(Years) Earthquake Level Approx. Building Movement

(mm)

25 SLS 20

500 - 275

1000 ULS 475

~1800 CALS 800

3.5.3 Hollowcore Units and Precast Panel Connections For the base isolation option, inter-story drifts are limited to approximately 0.8% at the Collapse Avoidance Limit State (CALS). Based on the latest C5 assessment methodology the existing hollowcore seating capacity limit has been assessed at 1.3% drift. Additional seating angles are included for additional resilience however may be removed or the extent limited based on further detailed design and onsite investigations.

In addition, the reliable drift limit on the existing precast panel connections is principally limited by prying forces on the TCM inserts and bending of the angle supports. Insitu testing on these connections may provide sufficient evidence of reliable performance to this limited drift demand which may remove or limit the extent of these retrofit works.

3.5.4 Diaphragm Strengthening

The introduction of base isolation greatly reduces the lateral demands imposed on the superstructure. In regards to the diaphragms the effective peak ground acceleration, which governs the pESA demands for the majority of floor levels in Scheme B, is significantly reduced. As such the amount of FRP required is limited principally to small collector areas in proximity to the new lateral frames.


3.5.5 New Frames and Foundations

In order for the base isolation system to function effectively the existing building is required to be significantly stiffened. The very low yield and high flexibility of the existing frames requires additional stiffness to be introduced similar to that provided in Scheme B. Whereas BRB braces were previously specified to provide a level of energy dissipation, given the hysteretic damping provided through the isolation system, conventional CHS braces acting elastically can be utilised.

Figure 14 Typical Cradle Beam Elevation

The foundation design is similar to that developed for Scheme B, where 900mm diameter bored piles resist seismic shear and axial loads, and the ground beams and pile caps constructed atop the basement ground slab.

At the ground floor level, new Comflor80 is proposed for the sloping and lowered sections on the Victoria and Harris Street levels. However, for constructability and programming, the existing hollowcore units is to remain and a new concrete 600mm waffle slab will be constructed on top at a height above the projected 25-year climate sea level rise. Due to the mass of the additional floor, during construction, a new beam is proposed to reduce the span in addition to providing temporary propping in the basement and temporary works is proposed to support the retaining wall during the physical works.

3.6 Scheme B Solution – Viscous Damper Discussion During the development of a resilient strengthening solution, WCC requested that Aurecon consider the use of Viscous Dampers as part of the intermediate, yet “resilient” design requested (Scheme B).

As described previously, Scheme B provides performance that sits between addressing the immediate life safety risks in the building (Scheme A) and the full high resiliency option (Scheme C).

The performance targets for Scheme B were to achieve a target capacity of approximately 80%NBS(IL3), providing no/low-damage performance for smaller more frequent events while providing allowing a high chance that at the ultimate design event any structural damage would be moderate and could be feasibly repaired. Such an option would present a low risk that the building would require demolition or be uneconomic to repair following a design level earthquake.

As part of a resilient design strategy some key performance criteria were targeted. These included:

◼ Limiting building drift – To both protect the hollowcore floor units, but also to reduce inelastic demands in the precast panels and their connections, as well as, damage internal fitout and non-structural elements.

◼ Reduce the amount of superstructure inelastic demand – The existing superstructure frames are flexible and were designed to be highly ductile. While the frames have been designed to relatively robust capacity design principles, under low levels of seismic demand significant damage could be expected.


3.6.1 Proposed BRB Solution

Scheme B, as discussed in this report, achieves both these key points. The BRB concentrically braced frames (or similar damping frames) provide strength and stiffness to the structure dramatically reducing the deformation demands and inter-story drifts. Due to the stiffness disparity between the new braced frames and existing moment frames the BRB system significantly augments the lateral resistance of the building. As such, the role of the existing moment frames becomes less imperative with the majority of its function being limited to gravity support.

Following a significant earthquake, the reduction in lateral drift will significantly reduce the seismic demands and damage that the existing beams will sustain. As the role of the existing frames is limited to gravity support and minor damage can more readily be assessed and repairs made without as stringent requirements concerning low-cycle fatigue, plastic hinging, and bar buckling which could result in the reinforced concrete frames being un-feasible to repair.

3.6.2 Viscous Damping Option

A considered alternative to the current Scheme B option is the introduction of a series of Viscous Damping devices.

Viscous damping is often used as a way of adding energy dissipation to the lateral motion of a structural system without involving major building modifications. These reduce the response of the building by absorbing seismic energy while not providing any significant additional strength or stiffness. Typically, a single viscous damper would cost more than a BRB or similar damping brace.

For an ideal viscous damping solution, it would be anticipated that significant retrofit to the beams, columns and foundations local to the brace would not be required. Although we note that research has shown a substantial increase in the maximum base shear and column axial forces can occur, which, in practice, would potentially require strengthening of columns and foundations (Hazaveh et al. 2017, Filiatrault et al. 2001, Uriz and Whittaker 2001, Miyamoto and Singh 2002, Martinez-Rodrigo and Romero 2003).

The BRB system described in Scheme B provides a means of ductility without requiring inelastic deformations to the superstructure, but has the downside to stiffening the building, resulting in increased the lateral seismic demands that need to be transferred through the floor diaphragms and foundation system. This requires new piles resist the increased loads transferred to the foundations, and the increased loads in the floor diaphragms require strengthening through the provision of through layers of FRP. We note that regardless of the whether the building is stiffened or not, the targeted %NBS rating and IL3 classification of the building means that the floor diaphragms in the building would still need strengthening, and that peak ground accelerations (PGAs) govern the pESA demands for the majority of floor levels.

There would therefore be a substantial advantage should a viscous damping solution be possible without significant strengthening required for this building. However, following our review, and in regards to the seismic performance under the key criteria noted above, we do not believe that a purely viscously damped solution (without building strengthening) can provide equivalent performance to the current 80% BRB solution.

Figure 15 presents an ADRS plot showing the effectiveness of viscous damping and illustrates the expected viscously damped performance for this building. The red curve represents a design demand scaled to 80% of the current NZS1170.5 code demand (Wellington, IL3 Soil Case C) without accounting for ductility.

Numerous international standards (ASCE7-16, EC8, and the NZ base isolation guidelines) effectively limit the maximum reduction in response due to the introduction of viscous damping in the order of 50%. This is based on the efficiency of damping cycles when subject to irregular and incomplete cycles, which is likely to be exacerbated with near-fault/forward directivity excitation. The red dashed curve represents the 80% NZS1170.5 code demand (Wellington, IL3 Soil Case C) allowing for 50% damping.

Based on Aurecon’s previous seismic assessment works, as well as the findings from the Holmes DSA 2013, the existing frames have a force-based and drift limit of approximately 60%NBS(IL2) at a ductility of 4.0. This is represented by the green horizontal line and is the level at which the moment frames are expected to yield and undergo plastic hinging – i.e. damage.

When the demands are scaled to this level of capacity (the dark blue curve) over the expected period range of the structure (represented by the grey dashed lines), there is a significant shortfall between the yield capacity and the 50% damped demand curve. Because the fundamental capacity of the existing frames is low, even


when additional damping is introduced, the existing frames would need to yield, and inter-story drifts would remain significant. Damage due to ductility demand required in the existing structure would likely be very expensive to repair and may not be feasible after a large event.

Figure 15 ADRS plot showing the effectiveness of viscous damping

A purely viscous damped solution would therefore provide a performance level between the current Schemes A and B but would not be able to achieve 80%NBS. Based on cost, timeframes, and a revised understanding on the resiliency and repairability of the building, WCC could consider this as an alternative option.

During future design stages a dual system incorporating BRB’s, or CBF frames combined with viscous dampers could be investigated further to determine if the potential benefits exceed the additional cost and complexity of incorporating viscous dampers. Noting of course that the current BRB system achieves the required performance criteria. A supplemental viscous damper, as shown in the Figure 16, could be incorporated as a standing single stroke or a knee-supplemental damper within the frame structures.

Figure 16 Potential orientations of viscous dampers


4 SOIL CONDITIONS

4.1 Description of Site Soil Conditions A Desktop Geotechnical Report has been completed. This report completed by Aurecon is titled Central Wellington Library Seismic Strengthening Desktop Geotechnical Review Report” dated 11 September 2020. This report is included in Appendix B

The historic borehole logs indicate a site stratigraphy comprising a surficial layer of reclamation fill underlain by beach deposit and alluvium, which overlie basement greywacke rock from 4.5m to 10.5m depth below historic ground level (BHGL) or a Reduced Level (RL) of -2.9m to -8.8m (refer to site investigation location plan in Appendix B for details). The geology around the margins of Wellington harbour is complex and has been affected by factors such as changes in sea level over geological times and increases in ground level due to faulting. The so called “marginal marine” beach deposits on the geological map therefore represent a wide range of interbedded silts, sands and gravels deposited in both fresh and saltwater environments. Some soils would have been deposited in a higher energy environment say due to flood conditions or wave action, and will be relatively dense, while other deposits will have been laid down in relatively still water and hence will be softer and looser.

Outcomes from the historic geotechnical investigations are generally consistent with published geological information, including the regional maps. However, the regional map (Begg and Mazengarb, 2000 and Begg and Mazengarb 1996) does not indicate the presence of the beach deposits and alluvium shown by the borehole logs. Further description of the site lithology is given below (refer Geotechnical Desktop Review Report in Appendix B for details).

4.2 Soil Design Values

4.2.1 Ultimate Soil Strengths

Based on the available geotechnical information, the depth to the competent bedrock is variable and can be up to 12m below the basement level. The piles are to be socketed at least four times the diameter of the pile into bedrock. However, the depth to bedrock is variable, and hence the pile lengths are also likely to be variable. Our recommendations for the ultimate end bearing capacity for the piles embedded four times the diameter into the competent bedrock is 5.5MPa. Our recommended capacity reduction factors for the bored piles design are as shown in the table 7 below:

Table 7 Capacity reduction factor summary

Case Load Case Reduction Factor

End Bearing

Earthquake over strength load combination

Factored Ultimate Limit State:

- Static (e.g. 1.2G+1.5Q)

- Seismic (e.g. G+Q+E)

Unfactored Serviceability case (e.g. G + Q)

0.7

0.5 (for static)

0.55 (for seismic)

0.25

Skin Friction

Load combinations involving earthquake

Static factored ULS (e.g. 1.2G+1.5Q):

Static unfactored SLS (e.g. G + Q)

0.5

0.75

0.5


The geotechnical report recommended pile skin friction capacity below the basement level is as follows in table 8 and general subsurface profile shown in table 9 below:

Table 8 Skin Resistance for Bored Piles

Depth below basement level (RL-1.3m)

Depth below basement RL level

Shaft Resistance

0 - 6m -1.3m - -7.3m Ignore (fill and less competent alluviums)

6m - 12m -7.3m - -13.3m 150kPa (weak rock)

Below 12m Below -13.3m 250kPa (Competent rock)

Table 9 Generalised subsurface profile - Aurecon Desktop Geotech Report September 2020

Top of Layer (RL)

Geological Description SPT ‘N’ Blow Count

SPT ‘S’ Blow Count

1.6m to 2.1m

Generally, comprises loose to moderately dense, weathered, brown, yellow brown silty gravels and sandy gravels with isolated layers of firm, low to moderate plasticity, dark grey, brown gravelly silt [Reclamation Fill]

4 to 26 8 (equivalent ‘N’

value of 5)

-0.9m to -2.35m

Fine to coarse sand /gravelly sand with numerous shells and shell fragments. [Beach Deposit].

15 to 32 38 (equivalent ‘N’ value of 25)

-2.1m to -3.9m

Intermixed silt, sand silt, silty sand sandy gravel and silty gravel. Silts are typically “firm to stiff”, low to moderate plasticity. Sands/gravels are typically moderately dense to dense. [Alluvium]

11 to 27 21 to 50+ (equivalent ‘N’

value of 14 to 33)

-2.9m to -8.8m

Highly to moderately weathered, yellow brown / brown sandstone/siltstone with argillite seams. [GREYWACKE BEDROCK]

50+ 50+ (equivalent ‘N’

value of 40 to 50+)

4.2.2 Pile Group Effects

Where applicable pile group effects have been included in the vertical load assessment of the bored piles. The pile group effects used are as follows in table 10:

Table 10: Pile Group Capacity Reduction Factors

4.2.3 Liquefaction Potential

As part of the geotechnical investigation the liquefaction potential was assessed. Refer to the geotechnical report in Appendix B. The borehole logs indicate silty sand beach deposits and alluvial silts, sands and gravels.


Some layers of potentially liquefiable soil are present in the logs (such as beach deposits 0.7m to 2.2m thick), however in general the soils are moderately dense to dense and predominantly comprise gravel or silt. Based on the above and considering the proposed building is likely to be founded on deep piles, the impact of liquefaction (even if it occurred) is likely to be insignificant.

4.2.4 Basement Wall Design The following is recommended for assessing basement wall’s capacity:

▪ The angle of internal friction and unit weight for backfill soil placed behind the basement walls should be assumed as 27° and 18kN/m3 respectively. A coefficient of “at rest” earth pressure of 0.55 should be used to estimate earth pressures acting on the wall.

▪ The basement walls need to be assessed for seismic loads. They can be designed as “rigid” or “stiff” with the seismic soil pressures determined from the attached Figure 17 below. These pressures are based on recommendations of Woods and Elm (1990). For the earthquake design, both “at rest” and seismic pressures should be applied to the walls.

Figure 17 Earth Pressure on Stiff (Left) and Rigid (Right) Wall during Earthquake

▪ The effects of groundwater/hydrostatic pressures should be considered in assessing the performance of the existing basement walls. Also surcharge loads from parked vehicles or structures nearby the basement walls should be considered as appropriate.

4.3 Groundwater Refer to the Geotechnical Desktop Report September 2020 and Table 11 below.

Table 11 Summary of groundwater levels

Borehole ID

Date Elevation (RL) Measured depth to water level (BEGL)

Indicative RL depth of water level

Bore 2 17/08/1988 16/10/1988

1.7m 1m 1.1m

0.7m 0.6m

Bore 3 06/09/1988 1.6m 1.3 0.3m Bore 5 09/08/1988

16/10/1988 2.1m 1.5m

1.9m 0.6m 0.2m

Bore 6 10/08/1988 1.8m 1.2m 0.6m BH3 23/03/2001 1.65m 1.1m 0.55m

A recent concrete coring was carried out in the middle of the basement floor to enable a geotechnical borehole to be drilled in August 2020 (the borehole was aborted due to high water inflow rates driven by a groundwater pressure head observed at approximately 1.5m above the floor level in the basement (RL-1.3m)).

It should be noted that the ground water levels are subject to tidal and seasonal fluctuations.


5 DESIGN LOADS

5.1 General For the purposes of consideration of loading, the structure has been designed to an Importance Level 3 in accordance with AS/NZS 1170.0:2002.

5.2 Imposed Loads

5.2.1 Vertical loads

The table 12 below summarizes all vertical loads including both superimposed dead and live loads. In all cases, a minimum superimposed dead load of 1.4 kPa is applied.

Table 12 Imposed Gravity loads

Level/Area Use Live Load Superimposed Dead Load

Library Floors Library, book storage

4.0 kPa 0.5 kPa

Office Floors Office 3.0 kPa 0.5 kPa

Mezzanine Café 3.0 kPa 0.5 kPa

Stairways and Corridors

Access 4.0kPa 1.0 kPa

Roof Non Accessible 0.0 kPa (seismic application)

Timber - 1.0 kPa L03 - 3.3 kPa L05 – 1.8kPa

Plant Room Floors Plant 5.0 kPa – general 5.5 kPa – under tank mezzanine

1.5 kPa (allowance for roof structure)

These values have been retained from previous the original Holmes design and DSA. The book storage loading does appear to be light, but may be appropriate for seismic loading.

5.2.2 Collections Floors

To be confirmed.

5.2.3 Barriers and Handrails

Not applicable for this stage.

5.2.4 Retaining Wall Loads

Design parameters to be provided upon completion of the current geotechnical investigation. It is anticipated that this will be completed prior to the start of the developed design phase.

5.3 Wind Loads Not considered for this stage. However, to be completed in accordance with AS/NZS 1170.2:2002.


5.3.1 Site Wind Speed Profile

Not considered for this stage.

5.3.2 Parts of Structure

Not considered for this stage. Pressure coefficients are used in accordance with NZS1170.4 to give design wind pressures.

5.3.3 Glazing

Not considered for this stage. Wind loads for glazing to be in accordance with the NZ Building Code and NZS 4223:1985, Code of practice for glazing in buildings.

5.4 Snow and Ice Loads The structure is in Region N1, and the elevation is 1m above sea level. Snow and ice are/are not significant loads for this structure.

5.5 Fibre Reinforced Polymer (FRP) This specification is intended to define the minimum requirements of structural strengthening using Surface Mounted bonded fibre reinforced polymer (FRP) composite system.

The work includes the furnishing of all materials, labour, equipment and services for the supply, installation and finish of all structural strengthening using bonded FRP systems.

The general contractor or subcontractor shall furnish all materials, tools, equipment, transportation, necessary storage, access, labour and supervision required for the proper installation of the bonded FRP systems.

5.6 Means of Compliance The design of the structure is generally in compliance with the New Zealand Building Code (NZBC), section B1. The Base Isolation methodology proposed is an alternate solution for compliance with the Building code.

The following standards have been used:

◼ AS/NZS1170:2001 Structural design actions

◼ NZS3101:2006 (amend 1,2,3) Concrete structures standard

◼ NZS3404:1997 (amend 1,2) Steel Structures Standard

◼ SNZ TS 3404:2018 Durability requirements for steel structures and components

◼ NZS2327:2017 Composite Structures

5.7 Alternative Solutions The following alternative solutions have been adopted in the design of the structure:

◼ The draft “Guideline for the Design of Seismic Isolation Systems for Buildings”, dated June 2019 is being used in the design and analyse of the isolation system. The production of this guideline was initiated by the New Zealand Society for Earthquake Engineering (NZSEE).

◼ ACI440.2R-17 Guide to the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures


6 ANALYSIS METHODOLOGY The conceptual seismic analysis has been completed in accordance with AS/NZS 1170.5:2002, using a modal response spectrum analysis. In regards to Scheme C effective secant stiffness values for the isolation system and near fault scaling reduction factors have been used to account for the isolator stiffness and equivalent viscous damping performance. It is anticipated that further analysis including non-linear time history analysis will be used in developing the full design.

Figure 18 ETABS 3D Analysis Model of The Wellington Central Library

6.1 Seismic Load Coefficient Subsoil class C based on the Aurecon Geotechnical Desktop Report dated 11 September 2020, for the Wellington Central Library. Further geotechnical investigations are currently being carried out to confirm site conditions.

6.1.1 Seismic Loads

In accordance with AS/NZS 1170.5:2002 and the current draft of the NZSEE Base Isolation guidelines in the table below.

6.1.2 Site Parameters

In accordance with AS/NZS 1170.5:2002 and the current draft of the NZSEE Base Isolation guidelines in the table 13 below.


Table 13 - Parameters current draft of the NZSEE Base Isolation guidelines

Parameter Scheme A (~40%NBS(IL3))

Scheme B (~80%NBS(IL3))

Scheme C (100%+NBS(IL3))

Soil Class C Importance Level 3 Hazard Factors 0.4 (Wellington CBD) Distance to Fault 0-2km Return Period Factor ◼ SLS ◼ ULS ◼ CALS 1

0.25 1.3 -

0.25 1.3 -

0.25 1.3

1.625 Period (s) ◼ N-S ◼ E-W

1.91 1.49

0.64 0.60

2.9 - ULS

3.2 - CALS Ductility, μ 4.0 3.0 1.0-1.25 (super-structure) Structural Performance Factor, Sp

0.7 0.7 1.0 (Displacements) (0.7+1)/2 = 0.85 (Member

Actions

With the use of base isolation it is anticipated that the 1st Mode periods in each direction will be largely the same.

6.1.3 Superstructure Design

The strength of the superstructure MRF has been checked for ULS demands using the reduced and 5th percentile material properties while the CALS demands have been compared with probable material properties.

6.1.4 Parts and Components

In accordance with AS/NZS1170.5:2002 section 8. Calculations to be developed as part of future design phases.

6.2 Probabilistic Seismic Hazard Study (PSHA) As part of further developed design, a Probabilistic Seismic Hazard Study (PSHA) will be required to accurately account for the seismic hazard (as it is currently understood), as well as, to allow for the appropriate selection of ground motion Time Histories in the use of Non-Linear Time History Analysis.

In the event that the a PSHA results in greater demands than the current NZS1170.5 code requirements an account of these will be included in discussion with WCC. In the case where an increase in seismic demand from a completed PSHA exceeds the current allowances the design could be adjusted to either retain the current drift allowances through stiffening of the isolation system, else additional displacement at the isolation plane could be allowed.

A preliminary PSHA will be conducted for the site to understand the likely loading standard that may be required for the structural strengthening design. This preliminary assessment will follow the issue of this report and will indicate if a more comprehensive PSHA assessment is warranted.

6.3 Basin effects Observations of ground motions in Wellington City, New Zealand’s capital, during the 2016 Kaikōura M=7.8 Earthquake highlighted the significance of 3D basin structure in increasing the severity and duration of ground shaking, and the potential earthquake damage. Observations from the Michoacán earthquake highlighted the significance of 3D basin structure in increasing the level and duration of ground shaking, influencing potential


earthquake damage (Sánchez-Sesma et al., 1988, 1993; Kawase and Aki, 1989; Sánchez-Sesma and Luzón, 1995). Damaging Basin effects are multi-faceted and include

◼ Focusing effects ◼ Scattering ◼ Trapped waves ◼ Vertical and horizontal resonance ◼ Conversion and generation of surface waves at the basin walls ◼ Basin-edge effects, including

− basin-edge-generated surface waves − decrease of sediment thickness (e.g. beach surf)

◼ Amplification behind convex-shaped parts of the basin edges or bottom ◼ Increased shaking duration, weakening building structural elements ◼ Basin resonance ◼ Basin impedance: the increase in amplitude of seismic waves resulting from the transition from

a hard to a soft soil layer

Due to the limited ground motion recordings within the Wellington area to characterise basin effects, 3D ground-motion simulations are a critical towards understanding and evaluating complex basin effects and their impact on ground shaking. Towards characterize the damaging effects of earthquakes in sedimentary basins, there have been many advances in the computation of 3D ground-motion simulations. Such capabilities are vital toward quantifying earthquake ground motion and seismic hazard potential in densely populated areas.

However, until the Wellington area is characterised with adequate detail to be significant in the pursuit of accurate simulations, modelling of basin effects for Wellington is still a work in progress, if understood at the next stage, we recommend basin effects be considered for the next stage of the design.

6.4 Diaphragms Diaphragm loadings are based on pseudo Equivalent Static Analysis loads (pESA) or CALS loading as appropriate.

For the preliminary design phases the following methods have been used to derive design loadings.

◼ For Scheme A and Scheme B floor loadings will consider the follow parameters;

Base shear at over-strength ≈%NBS x 1.8(φos) x ESA(μ=3.0)

Peak ground acceleration =%NBS x C(0)

Two loadings cases considered:

1. For a single floor in isolation – PGA loading in isolation

2. For the building as a whole – over-strength ESA loading concurrently on all the floors.

◼ For Scheme C floor loadings will consider the follow parameters;

Base shear at over-strength = CALS base shear at isolation plane (Upper Bound Properties)

Min individual storey shear (similar to PGA) will be based on guidelines Eq 5-18 & 19 which yields Fi=VCALS x Wi / Ws

Two loadings cases considered;

1. Individual floor loads = Fi on any given floor

2. Transfer floor loads = ESA (based on CALS base shear) applied to all floors


6.5 Special Load Cases Any special collection floor area will likely require to be designed to provide adequate support to mechanical shelving units.

WCC will have a number of fitout items/equipment which will require specific design to ensure the required support is provided. The design of these items will be developed as part of future design phases once the items and equipment have been specified.


7 SERVICEABILITY CRITERIA

7.1 Seismic Deflections Type of Analysis: Response Spectrum/ADRS

The tables below provide the storey drift for the two orthogonal directions between the floor levels, the results in the tables consider the P-Δ deflection of the centre of mass of the storey to calculate the P-Δ Inter-Storey drift. Direction X- represents a North-South orientation, which is the principle spanning direction of the hollowcore units.

Table 14 - Scheme A - 40%NBS IL3 Inter-Storey Drift

SCHEME A – 40% NBS IL3 / (100% NBS Loads IL3) X DIRECTION Y DIRECTION

Storey Drift x µ *kdm (%)

mm Level Height (mm)


mm

1.6 / (1.7) 24 / (64) L05 3,850 0.7 / (1.7) 26 / (66)

0.7 / (1.8) 26 / (68) L04 3,850 0.7 / (1.7) 26 / (67)

0.9 / (2.3) 41 / (103) L03 4,550 0.7 / (1.7) 31 / (79)

1.4 / (3.6) 65 / (162) L02 4,550 1.0 / (2.5) 46 / (115)

1.5 / (3.8) 54 / (135) L01 3,500 1.6 / (4.1) 57 / (143)

1.3 / (3.3) 44 / (109) MEZ 3,325 1.4 / (3.5) 46 / (116)

Note µ=3, and kdm=1.38

Table 15 - Scheme B - 80%NBS IL3 Inter-Storey Drift

SCHEME B – 80% NBS IL3 / (100% NBS Loads IL3) X DIRECTION Y DIRECTION




mm

0.87 / (1.08) 33 / (42) L05 3,850 0.85 / (1.06) 33 / (41)

0.67 / (0.83) 26 / (32) L04 3,850 0.92 / (1.15) 36 / (44)

0.43 / (0.54) 20 / (25) L03 4,550 0.56 / (0.70) 25 / (32)

0.56 / (0.70) 25 / (32) L02 4,550 0.77 / (0.96) 35 / (44)

0.73 / (0.91) 26 / (32) L01 3,500 1.06 / (1.33) 37 / (47)

0.49 / (0.61) 16 / (20) MEZ 3,325 0.77 / (0.96) 25 / (32)


Table 16 - Scheme C - 100%NBS IL3 Inter-Storey Drift

SCHEME C – 100% NBS IL3 X DIRECTION Y DIRECTION




mm

0.36 14 L05 3,850 0.38 15

0.28 11 L04 3,850 0.35 13

0.18 8 L03 4,550 0.23 11

0.23 11 L02 4,550 0.32 15

0.30 11 L01 3,500 0.44 16

0.20 7 MEZ 3,325 0.32 11



Maximum Allowable:

ULS For Scheme B <~1.1% inter-story drift (limit to adequate hollowcore performance)

For Scheme C <~0.5% (limit to achieve low damage design)

(significantly less than code limit of 2.5%)

SLS in accordance with Table C1 NZS 1170.0:2002

(limit inter-story displacements to maximum of h/200)

For isolator bearing displacements refer to Section 3.5.2.

7.2 Wind Deflections Particular elements are designed to the recommended serviceability deflection limits of AS/NZS 1170.0:2002, Table C1.

7.3 Gravity Deflections Particular elements are designed to the recommended serviceability deflection limits of AS/NZS 1170.0:2002, Table C1.

7.4 Shrinkage and Creep Constants The effects of shrinkage and creep in beams and slabs have been accounted for by multiplying the dead and sustained live load deflections by the factor Kp.

7.5 Design Life for Durability

7.5.1 Design Life ◼ Foundations: 50 years

◼ Superstructure: 50 years

◼ Non-Structural / Cladding: 15 years

Note: non-structural elements and cladding specification refer to the Architect’s specification and are not covered by this design features report.

7.5.2 Durability Provisions Durability provisions are achieved by:

Acceptable Solutions B2/AS1

◼ Reinforced Concrete: NZS 3101: 2006 Part 1 Section 5 is an acceptable solution for durability with durability requirements met through covers equal to or in excess of the requirements of the standard.

Verification Method B2/VM1

This is for special cases not covered by the standards requiring a special study or “Durability Evaluation” – SESOC believes this would not be expected in the normal scope of a structural designer, but if required as part of an extended scope, should be covered here.


Alternative Solutions

◼ Structural Steel: There is no acceptable solution available for structural steel and protection is provided through surface treatment in accordance with NZS/AS 2312:2002.

7.5.3 Summary of Surface Treatments The Table 17 below summarises the surface treatments for the structural steel elements covered by this design features report.

Table 17 Schedule of Surface Treatments for Structural Steel elements

Element Design Life

Exposure Category

Surface Treatment in accordance with NZS/AS 2312

Time to first major maintenance

Exposed exterior Structural Steelwork to structure - Portal Rafters

50 Medium Metal Spray Zinc 150 microns NZS 2312 - TZ150

25+

Canopy structures unpainted and painted

50 Medium Hot Dipped Galvanised (after fabrication) 85 microns NZS 2312 - HDG600 Alternative + decorative paint* system HDG + 80 microns NZS2312 - HDG600P2 (*paint system by architect)

25+

Interior Structural Steelwork

50 Low Painted – Zinc Primer with 2 coats Acrylic Latex.

15-25

7.5.4 Maintenance Requirements of Surface Treatments

The maintenance requirements for the above protective coating systems are as per NZS/AS 2312.

7.6 Floor Vibration The new floors will be checked using the vibration criteria proposed in BRANZ report SR 14 (1988) "Serviceability Criteria for Buildings".

Transient vibration limits for the floor units to be not less than 5 Hz.


8 SOFTWARE The following computer applications have been used:

Table 18 Software Used

Analysis type Software used Archive files Seismic response spectra Analyses

ETABS (2018)

General spreadsheet design EXCEL

- General Design Spreadsheets

Member capacity MathCad Gencol

Comflor Flooring design Comflor S&T Structural Steel Connection Design

Limcon V3.63


9 DRAWING AND SPECIFICATION NOTES The purpose of this section is to ensure that the design requirements are included in the drawings or the Specification.

9.1 Floors

9.1.1 Design Loads Refer to the design loads section, gravity deflection and floor vibration section of this report.

9.1.2 Fire rating Requirements To be developed.

9.1.3 Propping Requirements Generally, the propping of composite metal deck slab shall be in strict accordance with the manufacturer’s recommendations. The Comflor will require propping during construction. The temporary propping and handling of all other precast and insitu concrete and structural steel capacity including roof trusses shall be in accordance with the Specification. Generally, this will be the Contractor’s responsibility to ensure compliance with the Building Code and all health and safety regulations.

9.1.4 Dewatering and Waterproofing Generally, this will be the Contractor’s responsibility to ensure compliance with the Building Code and all health and safety regulations.

9.2 Foundations Refer to the Excavation and Concrete - General Sections of the Specification which discuss in detail all requirements for the foundations.

9.3 Material Properties (Typical Existing Structure) Material properties to part C5 technical proposal 2018, the following values are used:

9.3.1 Concrete Based on Holmes DSA report the following values were specified for the original construction:

◼ In-situ f’c=25 MPa ◼ Precast & Beams f’c=30 MPa

To Part C5 (Technical Proposal 2018) the following values are used

◼ In-situ f’p=37.5 MPa, Ec=28,780 MPa ◼ Precast & Beams f’p=45.0 MPa, Ec=31,530 MPa

9.3.2 Reinforcement Steel Steel reinforcement properties as below:

◼ All bars Ers=200,000 MPa ◼ Historic Bars (“yd”) fp=464 MPa ◼ Historic Mesh fy=600 MPa


◼ New HD bars fy=500 MPa

9.3.3 Steel Steel section properties as below:

◼ All steel Es=200,00 MPa ◼ Hot rolled sections (HR) fy=300 MPa ◼ Welded plate sections (HW) fy=300 MPa

Note that built up sections are expected to be butt welded thick plate sections and so HW section slenderness’s have been considered.

9.4 Material Properties (Typical New Design)

9.4.1 Concrete ◼ Foundations: 35 MPa ◼ Elevator pit walls, base slab: 30 MPa ◼ Columns - Insitu: 35 MPa ◼ Beams - Insitu: 35 MPa ◼ Slabs - Insitu: 30 MPa ◼ Floor toppings: 30 MPa

9.4.2 Concrete Masonry ◼ Blockwalls: Grade B

9.4.3 Reinforcing Steel ◼ All reinforcing: 500E MPa (unless noted otherwise)

9.4.4 Structural Steel ◼ Rolled Steel Sections: 300MPa – Grade 300 L0/S0 to AS 3679.1 ◼ Steel Plate General 250 MPa – Grade HA250 to AS1594 ◼ Steel Plate (special) 300 MPa – Grade 300 ◼ CHS Hollow Sections 350MPa – Grade 350 to AS 1163. A large variety and

specification exists for tube from 195 MPa to 450MPa ◼ RHS Hollow Sections: 350MPa – Grade 350 to AS 1163 ◼ Bolt Grades: Grade 4.6 mild steel and Grade 8.8 high strength.

Splices for all lateral load resisting members shall be installed with tensioned bolts.

9.4.5 Fibre Reinforced Polymer (FRP) Sikawrap Hex-103C has been used, properties as below:

◼ Laminate thickness t=1.0 mm ◼ Design tensile strength by laminar thickness

f*fu=1,110 MPa ◼ Design strain ε*fu=1.45% ◼ Interior environment CE=0.95


10 Proprietary Systems There is a diverse range of proprietary systems, manufacturers and products which may be specified to form part of a structural system. The specification and use of proprietary elements may take differing forms according to the level of design input offered by the manufacturer and the degree to which a designer accepts responsibility for the elements performance. In general, the greater the reliance on performance specification, the more responsibility is pushed to the manufacturers and/or their designers. However, the designer responsible for the overall structure still has a duty of care to determine that the proprietary elements meet their performance specification. The designer needs to specify the design expectations (PS1 - PS4) from the manufacturer and provide a summary of the design for the DFR.

The following proprietary elements are included in the project:

Typical systems included in many structures are below – select those that apply, or add more as needed

◼ Cold formed “DHS” purlins / rafters - Dimond

◼ Cold formed Studs

◼ Steel Tray Composite flooring – Comflor

10.1 Manufacturer Design Requirements Isolator Manufacturer requirements to be developed.

10.2 Manufacturer Construction Requirements Inspection QA statements, testing results and PS3 Statements of conformance to be provided by the Contractor as noted in the Specification.


11 Construction Monitoring The design is based on the verification of specific design aspects of the construction by a suitably qualified Chartered Professional Engineer in accordance with ACENZ/IPENZ level CM 3. A more stringent level on monitoring can be provided based on Client requirements.

11.1 Soil Testing and verification Visual inspection of excavation as work proceeds. This may include the removal and filling of soft spots with compacted hardfill as required by the Specification.

11.2 Materials Testing The seismic isolators will be tested as part of the design process to be developed. Reference to the proposed testing procedure is provided in the Structural Specification.

11.3 Temporary support and shoring Temporary propping and shoring is the responsibility of the Contractor. For further information reference should be made to the Specification.

11.4 Inspection Requirements A schedule of inspections based on the required level of Construction Monitoring will be developed as part of the Detailed Design phase of the project.


Appendix A Base Isolation Design Commentary


A1 - Significant Design Features, Base Isolation System

The seismic design philosophy for the majority of modern buildings in New Zealand is based on the allowance of structural damage in the event of a significant earthquake. Seismic energy is dissipated through inelastic deformations to the structure which may not be practicable or economic to repair. Deformation incompatibility and high imposed accelerations can also result in significant damage to non-structural elements and services which may be significant in regards to capital loss and business continuity.

The design developed for Wellington Central Library incorporates a base isolation system, where seismic energy is dissipated through movement in the bearings rather than the structure. This reduction in imposed demand allows the superstructure to be designed to remain essentially undamaged following a significant design level event. The deformations and accelerations imposed on the building are significantly reduced which greatly increases the resilience of the facility and post-disaster functionality. The isolation system developed for this development would allow the building to be re-occupied following a major event with little or no downtime.

Table 3.2 of the New Zealand Standard AS/NZS 1170.0:2002 describes the Importance Levels (IL) of buildings in New Zealand with typical office buildings being IL2 and buildings with crowds or of a higher importance being classified as IL3. These Importance Levels dictate the size of the earthquake event (or Return Period) that the buildings must be designed for. Essentially, per NZS1170, an IL3 building is designed for 30% greater earthquake loadings than that of an IL2 building.

A summary of the IL limit state to design event return periods interpreted from the latest draft New Zealand base isolation guidelines are provided in the table below and is valid for buildings with a design life of 50 years.

Table 19 Design Return Period event in relation to Importance Level

Limit State IL3

Return Period Factor, R Return Period

SLS1 0.25 1/25 year

SLS2 -

ULS 1.3 1/1,000 year

CALS 1 1.63 1/1,800 year

- CALS 1 has been assumed in regards to the moat conditions for the Central Library design

Table 20 Return Periods NZS 1170.5 C3.3

Return Periods on interpolation of Figure C3.3 of NZS1170.5

SLS1 The structure and non-structural components do not require repair.

SLS2 The structure and non-structural components do not require repair.

ULS Probability of collapse (and therefore the risk to human life) is at an acceptable level.

CALS 1 Isolation system constrained by rattle-space on all sides RCALS=1.5 x R/α where α=1.2.

Following the recent series of devastating earthquake events to affect New Zealand, structural engineers are increasingly placing greater emphasis on how structures will perform following a larger than design ULS (Ultimate Limit State) earthquake. Consideration of this CALS (Collapse Avoidance Limit State) is particularly important for base isolated structures where a failure in the bearings due to excessive displacement could have catastrophic consequences. In Wellington, the expected CALS movements for an IL3 building under the draft base isolation guidelines proposed would currently be in the order of 800mm +/-200mm, compared to 600mm +-200mm for an equivalent IL2 structure.

In discussing resilience as it relates to the use of base isolation it is important to define the performance required at both the SLS and beyond ULS events.


A2 - Base Isolated Building Business Continuity Design Strategy

The business continuity design strategy for a base isolated building can be adjusted to suit the proposed tenant but in general the table above shows which building elements and services can be readily designed to continue operating after a seismic event of a certain level.

Other building operations, in addition to services, may be affected by the seismic movement and need to be reviewed by the appropriate disciplines. These include but may not be limited to, elevators, elevators and computer servers.

Figure 19 Scheme C – Basement Cross Section - Isolation system for Harris St

A3 - Base Isolation Compliance Documents

In New Zealand, structures with seismic isolation are designed as an “Alternative Solution”, as there are no Acceptable Solutions (i.e. Standards) or Verification Methods currently available to satisfy the Building Code B1 provisions. To demonstrate compliance with the Building Code to the satisfaction of the Building Consent Authority (BCA), the design will follow industry guidance, international Standards, and will be independently peer reviewed.

The state of practice in New Zealand is represented by the draft Guideline for Design of Seismic Isolation Systems for Buildings (August 2017). This document (the NZ Isolation Guidelines) was initiated by NZSEE and prepared by a committee of engineers who have experience designing isolated buildings in New Zealand. Nevertheless, the NZ Isolation Guidelines are only a draft version which are written in the form of recommendations (i.e. “should: and “may”) instead of mandatory language (“must” or “shall”). It is still undergoing international review and has not been officially released for industry feedback.

Hence our analysis and design approach is to follow the intent of the NZ Isolation Guidelines with reference to international best-practice to improve upon the Guidelines. Reference documents include the following American Society of Civil Engineers (ASCE) Standards and Federal Emergency Management Agency (FEMA) Guides:

◼ ASCE 7-2016 (new buildings) Chapter 17

◼ ASCE 41-2017 (retrofit of existing buildings) Chapter 14

◼ ASCE 4-2016 (nuclear structures) Chapter 12

◼ FEMA P-1051, 2015 NEHRP Provisions: Design Example, Chapter 15: Seismically Isolated Structures.

Project number 253267 File 253267-0013-DFR-ST-0001[E].docx, 2020-10-07 Revision E 40

Appendix B Concept Design – Geotechnical Report

Project number 253267 File 253267-0013-DFR-ST-0001[E].docx, 2020-10-07 Revision E 41

Appendix C Concept Design – Preliminary Geophysical Result

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NOTES:Aerial image and property boundaries sourced from the LINZ Data Service and licensed for re-use under the Creative Commons Attribution 4.0 New Zealand LicencePropery boundaries and contours sourced from WCC's Open Data site(https://data-wcc.opendata.arcgis.com)

DISCLAIMER: This product is for informational purposes and may not have been prepared for, or be suitable for legal, engineering, or surveying purposes. Users of this information should review or consult the primary data and information sources to ascertain the usability of the information. Aurecon cannot accept any responsibility for any errors, omissions, or positional accuracy of the data.

ISSUEDIssuer

_PROJECT No. WBS TYPE DISC NUMBER

_ __REVISION

_

LEGENDBore_investigations!( Aurecon core hole 2020

!A Aurecon borehole 1988

!A WSP borehole 2001

WCL Basement

WCL_extent

Vict

oria

St

Harris St

Rock = RL-2.9mRock = RL-5.6m

Rock = RL-7.9m

Rock = RL-8.8m

Line

4

Line

5

Line 1

Line 2

Line 3

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Document prepared by Aurecon New Zealand Limited Spark Central Level 8, 42-52 Willis Street Wellington 6011 PO Box 1591 Wellington 6140 New Zealand T F E W

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Wellington Central Library Seismic Strengthening

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