Report - Waitaki District...ultimate limit state as an indicator of the vulnerability to collapse....

$: Report - Waitaki District...ultimate limit state as an indicator of the vulnerability to collapse. That state occurs at a certain fraction of the earthquake intensity assumed for the$
Director L M Robinson BE (Hons), NZCE (Dist), FNZSEE, FIPENZ (Geotechnical, Structural, Fire), CPEng, IntPE 469 George Street, PO Box 6068, Dunedin, Phone (03) 477 8923, Fax (03) 477 0608

Email [email protected] Website: www.hadleyrobinson.co.nz

Report

Forrester Gallery Thames Street, Oamaru Detailed Seismic Assessment

Report prepared for Waitaki District Council

Version Date Description

V1 February 2017 Completed; first publication

mailto:[email protected]

Hadley & Robinson Limited—CONSULTING ENGINEERS

Forrester Gallery, Thames Street, Oamaru Version 1: February 2017

Detailed Seismic Assessment Page 2 of 17

Forrester Gallery Thames Street, Oamaru Detailed Seismic Assessment

Summary Analyses of the building for earthquake effects shows that it is not earthquake prone as

defined in the Building Act 2004. A building is defined as earthquake prone if it would collapse in a moderate earthquake, defined as one that produces effects at the site one-third those assumed for the design of a new building.

The NZSEE (New Zealand Society for Earthquake Engineering) Guidelines (see later in this report) are used for assessing this building. Those guidelines use the attainment of the ultimate limit state as an indicator of the vulnerability to collapse. That state occurs at a certain fraction of the earthquake intensity assumed for the design of new buildings, with that fraction commonly expressed as X%NBS (X% of New Building Standard), which is imprecise in terms of earthquake intensity to cause collapse, but useful in recording a rating of the earthquake resistance of the building relative to the resistance of a new building.

The concept of reaching an ultimate limit state is taken up in the revised legislation on earthquake prone buildings. That legislation becomes operative in 2017.

This building is unlikely to reach an ultimate limit state in an earthquake that produces shaking at the site less than 100% of the intensity of shaking that is assumed for the design of new Importance Level 3 buildings. We therefore report that it rates at 100%NBS (IL3).

Legal Provisions: Earthquake Proneness The provisions of the current Building Act with respect to earthquake prone buildings are noted in Appendix A. In brief, a building is defined as being earthquake prone if it would collapse in a moderate earthquake. A moderate earthquake is defined as one that would produce shaking at the site one-third as great as would be assumed for the design of a new building at the site. If a building is assessed as earthquake prone, the Territorial Authority may require the danger presented by that condition to be removed within a specified time, by demolition or by improvement of the building. In discharging these duties the Territorial Authority is required to produce policies with respect to these matters. The Waitaki District Council has such a policy, but that is likely to change as a result of recent changes to the legislation, so is not quoted here. The earthquake prone legislation has recently been amended. It takes effect during 2017. It introduces more prescriptive measures for assessing buildings that are potentially earthquake prone and for setting deadlines for improvement. The present threshold changes from a concept of collapse to one of reaching an ultimate limit state. As explained above, this is not what the present legislation states (which is more about the intensity of earthquake to cause collapse, rather than the attainment of an ultimate limit state), so represents an increase in performance expectations. However, the notion of collapse is retained by stating a rider to the attainment of an ultimate limit state (ULS), whereby a building would only be judged earthquake prone if as a consequence of any collapse injury or death might ensue. The required rating to be achieved in any upgrade that is necessary for an earthquake prone building remains just sufficient for the building to pass the test for earthquake proneness if that test was reapplied after an upgrade. This is indeed the present position, also, notwithstanding the policies of some Territorial Authorities—a position clarified in a recent judicial review and in the new legislation.




Relative Risks New buildings of that house collections of high value to the community are categorised as IL3 (importance level 3). An IL3 building with a design life of 50 years, is designed for an earthquake with an average annual expectation of occurrence of 1/1000, or 0.1% average annual expectation. The return period (or recurrence interval) is 1000 years. The chance that the design earthquake (or a larger one) occurs over the 50 years design life is about 5%. A moderate earthquake against which earthquake proneness is assessed, has a return period of about 100 years for an IL3 building. The average annual risk of occurrence is about 10 times that of the design earthquake for a new building. The return period of an earthquake of intensity 67% of the design earthquake for a new IL3 building is about 300 years. A building that can just sustain such an event would therefore have an associated annual risk about 3 times that of a new building, still high relative to a new building but a considerable improvement over the risk posed by an earthquake prone building. Processes to be employed for this Report The Waitaki District Council is the owner of the Building. The Gallery wishes to upgrade and extend, and intend applying for grants for that purpose. This will require a report on the current earthquake status of the building, and, in due course the status after any extension and upgrade This report was therefore commissioned for that purpose and, more importantly, to determine if occupants of the gallery would be safe in an earthquake. For the purpose of this report, the threshold of unacceptable danger has been taken as that associated with earthquake proneness as contemplated in the most recent revisions to the Building Act 2004. Assessments therefore start at the present configuration—with any deferred maintenance that might affect performance in earthquake first completed. If the building is assessed as not earthquake prone, then no improvement is required by legislation (unless there is a change of use) but improvement might be considered desirable, anyway. If the building is assessed as earthquake prone then it will need to be improved within a timeframe set by Council (or perhaps by further changes to legislation). The standard of improvement would need to be such that the building would not be considered earthquake prone if the test for that condition were reapplied after the improvements were completed. That is, the building needs to be brought up to a level of performance such that the ultimate limit state would not be reached in a moderate earthquake. The time for completion of improvement would be set by Council or by legislation. It would be at least 35 years. Higher standards of improvement than just those sufficient to meet earthquake prone tests might well be set by the owner, who might also apply greater urgency to complete the improvements than legislation or Council policies might otherwise impose. For completeness, then, further analyses may need to be undertaken. At the conclusion of the assessment of the building as-is, it will be established what intensity of earthquake the building is capable of sustaining. If that intensity is less than the intensity that would be assumed for the design of a new building, assessment of the results of further improvement could be made. In this report it is assumed that any necessary improvements would be incremental, first by improving the strength of building elements and their interconnection and then by insertion of additional structure. The results for any such step would be reported and discussed—but there is none required.




Construction and Condition of the Building The building is constructed with unreinforced Oamaru stonework with limited use of unreinforced concrete. Timber is used in floors and roof framing. No drawings describing the original construction of the building were available. Measurements and a visual survey by Hadley & Robinson were undertaken following commissioning, to check the general form of the original construction and for the creation of an analytical model. The building is in fair condition, standards of maintenance having been reasonable. There is some cracking of the masonry and some concrete, though insufficient to seriously affect the integrity of the construction.

Methods used in this Assessment The assessment methods used in the preparation of this present report are those outlined in the document “The Seismic Assessment of Existing Buildings”, a revision of the original “Assessment and Improvement of the Structural Performance of Buildings in Earthquakes”, which the New Zealand Society for Earthquake Engineering prepared for the Department of Building and Housing (now part of the Ministry of Business Innovation and Employment).

The NZSEE document is intended primarily for the assessment of earthquake prone buildings and the improvement of performance of a building to the extent that it would not then be earthquake prone. However, the methods are also useful in assessments for other purposes, such as improving performance to higher standards. The document is presently being revised. The unreinforced masonry sections have been refined, leading to higher performance levels, and the release of the whole revised document is imminent. It is to be noted that the NZSEE methods and procedures relate to the building reaching the limit of its strength or displacement capacity, called its Ultimate Limit State, or ULS. In this respect it does not strictly adhere to the definitions in the current Building Act, which are about reaching a state of collapse. The difference between the two states can be quite wide, with collapse occurring at an intensity of earthquake shaking that is commonly 150% or more of the level of shaking associated with reaching the ULS condition. In this respect the NZSEE procedures provide a conservative assessment of earthquake proneness, but one that is reflected in the recently revised legislation, as explained above. The analyses reported here include numerical integration time history methods. These are methods where ground accelerations that have been recorded during an earthquake (using and accelerometer) are applied to an analytical model of a building and the response of the building is calculated by numerical integration. However, most analyses used were based on the equivalent static force method, principally using static pushover. The pushover is a method that applies loads in a particular pattern to the building, with these loads gradually increased until failure of the building is signalled as imminent. The pattern of forces is a key to the analysis, and modal response spectrum analysis was undertaken to provide pointers to a more appropriate pattern than the default pattern commonly used in the equivalent static procedures.

Design Parameters As a precursor to the analysis for earthquake effects, certain basic parameters are assessed as in Appendix B. In this report it has been assumed that the building is Importance Level 3, appropriate to a building with contents of a high value to the community, as instructed by Waitaki District Council. The seismic coefficient for the equivalent static procedure depends on the mode of failure




and the energy dissipation. Where response is likely to be limited by the strength and energy dissipation in the masonry, it is about 0.26 g at this site. If a building is earthquake prone, it is sufficient compliance with the legislation to upgrade to meet the test for earthquake proneness if it were reapplied. Where a higher level of upgrading is be considered, such as for a change of use, or just from considerations of additional safety or property protection, there is a question of what constitutes a “reasonably practicable” level of shaking to assume, taking into account heritage values of the building and how these might be jeopardised by inappropriate intervention into the existing fabric. It is often assumed that 67%NBS is a reasonable target level to which to upgrade.

Analysis Methods The building is constructed with unreinforced masonry walls with limited use of unreinforced concrete. The walls dominate the mass and stiffness of the building. The roof and walls are not well connected to the walls. While configuration and connection are insufficient to allow the roof to act as a bracing diaphragm, demands on diaphragms can be high anyway due to the large mass of the masonry. The rather massive walls suggest that they might be capable of resisting significant earthquake motions without the assistance of roof or floor diaphragms. That possibility is tested in the analyses and found to be valid. A three-dimensional model of the buildings was prepared. It is shown in Figure 1. The roof and floors or other parts can be incorporated into the model or excluded from it. Anyway, individual elements can be excluded from the display to keep the presentation clutter-free.

Figure 1a

Isometric of the three-dimensional model used in this study, with roof, floors and cornices included, from NE




Figure 1b

Isometric of the three-dimensional model used in this study, without roof, floors and cornice, viewed from NE.

Figure 1c Isometric of the three-dimensional model used in this study, with roof, floors and cornice, viewed from a

diagonally opposite point from SW.




The purpose of the model is to explore the potential for earthquake proneness, and to analyse incremental improvement measures, though, as subsequently explained, it is scarcely necessary for that latter purpose. In all analyses, the strength of the masonry was set using a Mohr-Coulomb approach as in the NZSEE Guidelines. There is a known deficiency of that modelling as it was originally proposed. That is excess dilatancy. Dilatancy is the tendency of a material to expand in volume as it is sheared. The original Mohr-Coulomb criterion exaggerates that expansion. To overcome that, actual implementation was carried out using modified approaches. Past studies of this nature have commonly used a Drucker-Prager approach. In this case, a modified Mohr-Coulomb approach, which is due to Menétrey and Willam (1995), was used. For this building mid-range values for material strengths were input, consistent with the state of the masonry and mortar. The friction angle was input at 31 degrees (friction coefficient about 0.60), dilation angle at 20 degrees—so, a non-associative flow criterion was employed—and with cohesion of 500 kPa1. Progressive reduction in the cohesive strength of the mortar under reversed cycling in the numerical time history analyses was modelled using a damage evolution rule. For the concrete elements, the material model adopted was a smeared concrete cracking representation. In that representation discrete cracks are assumed adequately modelled by smearing them as a continuum. Other features then assumed are:

Concrete crushing strength is 30 MPa

Concrete direct tensile strength is 2.7 MPa

Ratio of biaxial compressive stress to uniaxial ultimate stress is 1.16

1 It should be noted that a non-associated flow rule will lead to numerical difficulties unless a non-symmetric stiffness

matrix formulation and solution strategy are implemented. The friction angle and cohesion are consistent with the

values suggested in the revised NZSEE Guidelines.

Figure 1c Isometric of the three-dimensional model used in this study, without roof, floors or cornice, viewed from a

diagonally opposite point from SW.




Concrete tensile stiffening exists up to a strain of 0.001 (about 10 x the tensile cracking strain). There is a linear falloff in tensile strength between the cracking strain and the strain of 0.001, at which the tensile strength is assumed exhausted.

Ratio of the principal component of plastic strain at ultimate stress in biaxial compression to that in uniaxial compression is 1.28.

Ratio of tensile principal stress at cracking in plane stress, when the other nonzero principal stress component is at the ultimate compressive stress value, to the tensile cracking stress under uniaxial tension is 0.33.

Other considerations for the model include:

The walls are not connected to the roof of floors in a sufficiently robust way to ensure that walls do not move relative to the roof or floors in a direction normal to the walls. However, the displacements are unlikely to be sufficient to cause disengagement—which is subsequently demonstrated. The security of the roof and floors was checked by similar back analysis. In any case, the contribution of the roof and floors to the stability of the building was ignored in an extension of this study, though the mass of the roof and floors was included in the analyses for earthquake effects. This is explained in part at the close of this report.

Similarly, the mass of the cornices and other ornamentation (e.g. the ornamental column capitals) is included in the analyses, but the contribution to strength and stability is otherwise ignored. This is a pragmatic step to enable analysis runs to be undertaken in reasonable time without modelling the intricacies of the cornices and other ornamentation.

There is considerable restraint afforded by intersecting masonry properly bonded at the intersections. In that respect two-way action is reliable. However, one-way action of the walls was also considered by the displacement based procedures of the NZSEE Guidelines for walls loaded out of plane (face loading).

Analysis Results Numerical Integration Time History Analysis The first analyses used numerical integration time history analysis for the reduced north-south component of the 1940 El Centro accelerogram (a de facto standard). This record has a peak ground acceleration of 0.34 g, which is scaled by 0.65 to match the predicted peak ground acceleration for this site. The first 13.0 seconds of the record was used in the analysis. Results are not reported here for reasons of brevity. However the results were used to verify the results of subsequent analyses. Modal Response Spectrum Analyses In the modal response spectrum method, the building is first analysed to extract independent ways in which it can vibrate. Then each mode is analysed for the selected earthquake in a particular direction and the maximum value of each response quantity of interest is found. These quantities can be displacement, stress or any other quantity of interest. The results for each mode are then combined in a way that is statistically derived to achieve an overall result for all significant modes. In the implementation for this study, the earthquake spectrum is that defined for new buildings in NZS 1170.5. Modal results are combined using the complete quadratic combinations method. Sufficient modes are combined so that 90% of the mass of the building is mobilised.




Incremental Static Pushover Analyses As reported, static incremental pushover analyses were also employed. They gave similar results to the numerical integration time history analysis, and to the modal response spectrum analysis. This latter result is to be expected, as the modal response spectrum results were employed to provide a better approximation to the pattern of equivalent static forces, and there is low inelastic strain in the static procedure. Pattern of displacement is similar. Figure 2a and 2b show resulting displacements when the earthquake direction is in the X-direction. Figures 2c and 2d show the damage (regions of inelastic strains, or cracking) for the same direction viewed from opposite points. Figures 3a, 3b, 3c and 3d show results when the earthquake direction is in the Z-direction.

Figure 2a

This shows the displacements in the X-direction when the earthquake is in that direction (highly exaggerated).

The original position of the structure is shown transparent.




Figure 2c

This shows the regions subject to inelastic straining (cracking) at the end of the pushover in the X-direction.

Figure 2b

This shows the displacements in the X-direction when the earthquake is in that direction (highly exaggerated)

from the diagonally opposite view. The original position of the structure is shown transparent.




Figure 3a

This shows the displacements in the Z-direction when the earthquake is in that direction (highly exaggerated). The

original position of the structure is shown transparent.

Figure 2d

This shows the regions subject to inelastic straining (cracking) at the end of the pushover in the X-direction viewed

from the diagonally opposite point.




Figure 3c

This shows the regions subject to inelastic straining (cracking) at the end of the pushover in the Z-direction

Figure 3b

This shows the displacements in the Z-direction when the earthquake is in that direction (highly exaggerated) from

the diagonally opposite view. The original position of the structure is shown transparent.




The analyses show that the rating is at least 100%NBS for earthquake in the direction parallel to and at right angles the street. As has been well established in recent years, a displacement-based approach will produce more reliable results. Analysis based on that approach and developed from the NZSEE Guidelines confirm that the building is stable at greater than 100%NBS. Further to that, it was discussed above about the flexibility of roof and floor connections to the masonry walls. The displacement based procedures were applied for reassurance that even should the roof or floors connections fail the building as a whole would remain secure.

Conclusions The foregoing analyses show that the building is not earthquake prone. Indeed, it can sustain 100% of the earthquake intensity assumed for the design of new buildings of Importance Level 3. In reaching that conclusion, certain assumptions have been made. These include the assumptions that walls are well bonded together at intersections and corners. On the basis of evidence on site, this is a reasonable assumption. Other assumptions, not vital to the survival of the buildings in an overall sense but important to ensure damage initiation is not too early or too severe have been validated by further analysis. They include investigation of incompatible displacements between component parts of the building, and the security of the roof and floors in extreme events. Similarly, while the strength of the cornices and other ornamentation is not included in the strength assessments for the building in an overall sense, the mass of these features is included. The

Figure 3d

This shows the regions subject to inelastic straining (cracking) at the end of the pushover in the X-direction (at

77%NBS), viewed from the diagonally opposite point.




deformations of the underlying substrate are then imposed on the omitted features, which are then checked to ensure that they do not fail and fall from the building. By using a parametric approach, whereby variations in the assumed design inputs are varied in a statistical way, it can be reported with high confidence that the building rates 100%NBS (for IL3).

DIRECTOR Hadley & Robinson Limited Consulting Engineers




Appendix A: Earthquake Proneness

NOTE: This is an extract from the present building act. The provisions have been modified with the recent enactment of the Building (earthquake prone) amendment act 2016, which has not yet come into force.

Legal Requirements—earthquake proneness The test for earthquake prone buildings is defined in section 122 of the Building Act 2004, and in associated regulations.

122 Meaning of an earthquake-prone building (1) A building is earthquake prone for the purposes of the Act if, having regard to its condition

and the ground on which it is built, and because of its construction, the building— (a) will have its ultimate capacity exceeded in a moderate earthquake (as defined in the

regulations); and (b) would be likely to collapse causing—

(i) injury or death to persons in the building or to persons on any other property; or

(ii) damage to any other property.

(2) Subsection (1) does not apply to a building that is used wholly or mainly for residential purposes unless the building— (a) comprises 2 or more storeys; and (b) contains 3 or more household units.

The regulations referred to in s122 were promulgated in 2005/32 on 21 February 2005. Regulation 7 defines a moderate earthquake.

7. Earthquake-prone buildings: moderate earthquake defined For the purposes of section 122 (meaning of earthquake-prone building) of the Act, moderate earthquake means, in relation to a building, an earthquake that would generate shaking at the site of the building that is the same duration as, but is one-third as strong as, the earthquake shaking (determined by normal measures of acceleration, velocity, and displacement) that would be used for the design of a new building at that site.




Appendix B: Earthquake Design Parameters

In the design of new buildings using NZS 1170.5, the seismic coefficient is derived as follows:

uu

p

d RRZk

STCTC 03.0)02.020/(

)()(

Where

),()()( 1 DTZRNTCTC h

In these expressions, T is the period of vibration in any mode. For the equivalent static procedure, only the first mode is considered, and T is then replaced with T1. For a short-period building, assumed with 5% of critical damping and on firm ground, the hazard spectrum has the following values:

Period, T, seconds Hazard spectral value, Ch(T), g

0.0 1.33

0.1 2.93

0.2 2.93

0.3 2.93

0.4 2.36

0.5 2.00

For the equivalent static procedure, a short-period building is taken as having a period of 0.4 seconds. Assume the building is one housing contents of a high value to the community. With the site remote from an active fault and composed of shallow soils, the following parameters apply at Oamaru.

00.1),(3.113.036.2)( 11 DTNRZTCh

Hence,

399.0)( 1 TC

In this building, there will be energy dissipation by one principal mechanism: dissipation by equivalent viscous damping due to frictional sliding and rocking in the masonry. NZSEE Guidelines, Section 8, suggest that 15% equivalent viscous damping may be assumed. This is taken into account by the additional factor

642.02

75.0

k

So, for the equivalent static method

255.0)( 1 TCd

For the static procedures implemented in this report, this coefficient is applied.




In accordance with the test specified for earthquake prone buildings, only one-third this level of shaking needs be assumed in testing for that condition. For the modal response spectrum method, use is made of the hazard spectrum tabled above to extract earthquake effects in each mode. The other parameters (Z, N, R, Sp, kμ and kξ) apply as for the equivalent static procedure. However, for the MRS analysis implemented in this study, the modal damping values can be stated directly for each mode. Therefore, the NZS 1170.5 spectrum at the 5% level is used. The design spectrum is therefore given for the assessment by:

TCTC hd 17.0

In considering peak ground acceleration, as for example in correlating with numeric integration time history analysis, the period is taken as zero, and otherwise the basic relation is applied without the building dissipation parameters. Accordingly

gZRNCCPGA h 224.013.113.033.1)0()0(

One method of scaling earthquake records, especially for short period buildings, is on the basis of peak ground acceleration. For the El Centro record, which has a PGA of 0.34g, scaling by 0.66 is therefore appropriate.

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