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Canadian Wood Council

The Mid-Rise Wood-Frame Construction Handbook: Overview and Structural Design Aspects

Marjan Popovski, Ph.D., P. Eng. Principal Scientist, FPInnovations

Adjunct Professor, University of BC

February 04, 2016

Continuing Education Course and Credits

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Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.

This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. _______________________________________ Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

Course Description

To facilitate the design and construction of mid-rise wood-frame buildings in Canada, FPInnovations, in collaboration with CWC, NRC, and WoodWorks has developed the Mid-Rise Wood-Frame Construction Handbook.

The Handbook has been prepared to assist architects, engineers, code consultants, developers, building owners, and Authorities Having Jurisdiction (AHJ) in understanding the design and construction of mid-rise wood-frame buildings in Canada.

The presentation will provide overview of all chapters of the handbook with emphasis on structural analysis and design aspects.

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Learning Objectives

At the end of the this course, participants will be able to: ▫ Understand the current code status of the mid-rise wood-

frame construction in Canada ▫ Get an overview of the content of the mid-rise handbook

and all chapters ▫ Get familiar with the structural analysis and design

aspects of mid-rise buildings ▫ Get familiar with the structural analysis and design

aspects of podium buildings

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Start of Mid-Rise Wood-Frame

Construction: Code Change in BC

Limit raised to 6 storeys in BC in April 2009 Intensive input from leading experts in the field (including FPI

staff) along with stakeholders from the residential building industry

APEGBC developed Technical & Practice Bulletin for mid-rise wood-frame buildings

72 buildings constructed or underway and 129 in design phase

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Photo Courtesy of WoodWorks!

Midrise Wood-Frame Construction in

Rest of Canada

April 2013: Régie du Bâtiment du Québec (RBQ) allowed wood-frame construction up to 6 storeys

January 2015: Ontario Building Code revised

March 2015: Alberta Building Code Revised

Canadian Commission on Building and Fire Codes (CCBFC) approved 5- and 6-storey wood-frame construction in 2015 NBCC

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Midrise Construction in the US

Already Code Approved in California, Washington and Oregon for about a decade

Allowed in 2012 IBC

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Photo: BC WoodWorks!

The Handbook

With funding from NRCan, the Provinces of BC and Québec, and in partnership with CWC, WoodWorks and NRC, FPInnovations compiled the state-of-the-art technical information on Midrise Wood-Frame Construction

10 Chapters on multi-disciplinary topics involving 42 industry, research and design experts

In accordance with 2015 NBCC provisions and CSA O86-14

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The Handbook (cont.)

Complementary to existing manuals ▫ CWC Wood Design Manual (2010) ▫ APEGBC Bulletin for 5-and 6-storey

wood-frame structures in BC ▫ Quebec RBQ Guidelines

The Handbook will help facilitate adoption of midrise wood-frame construction in Canada

Ensure that buildings meet the applicable codes and exhibit good performance in every aspect

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Chapter 2: Structural Products,

Components and Assemblies

Products and Components ▫ Dimensional lumber, FJ lumber, panels, I-joists

Trusses, Glulam, SCL, CLT Structural Assemblies

▫ Conventional floor/roof/wall, mid-ply shearwalls

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Chapter 4: Floor Vibration Control

Fundamentals of floor vibration Review of existing design methods/gaps A new design method for determining

vibration controlled floor span Design examples using the method Field control and remedies

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Fundamentals of Floor Vibration

Causes of floor vibration Critical design parameters for vibration control Construction details affecting floor performance such as:

glue between floor joists and the subfloor, lateral reinforcements, concrete topping etc. are discussed

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S. Ohlsson, 1984, "Springness and human induced floor vibration – A design guide”

Existing and New Design Methods

The current method in NBCC only works for floors with joists but without concrete topping

A new design equation was proposed for determining vibration-controlled floor span

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15.014.0

284.0)(

22.8

1

Lscl

eff

mF

EIl

l = vibration-controlled span (m) EIeff = effective bending stiffness of the T-beam (N*m2) mL = linear density of the T-beam (kg/m) Fscl = none-zero and ≤1 factor related to stiffness contribution of subfloor and topping to reduce the 1kN static deflection

Equation Assumptions and Field Control

The new design equation assumes that the floor joists sit on a rigid foundation

To ensure satisfactory floor performance, construction details should have adequate floor support and proper floor stiffness

Methods for enhancing floor stiffness are provided in situation where floor stiffness is not adequate

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Chapter 5: Design for Vertical Differential

Movement

Vertical Differential Movement (VDM) was identified as one of the key design issues for mid-rise wood frame construction

Content: ▫ Causes of VDM ▫ Predicting VDM ▫ Methods to reduce and accommodate VDM ▫ Recommendations for on-site moisture

management and construction sequencing

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Causes of Vertical Differential Movement

Wood shrinkage (major cause) ▫ Primarily contributed by horizontal wood members ▫ Amount depends on MC change and shrinkage coefficient

Loading (relatively small cause) ▫ Closing of gaps between members

(settlement, bedding-in) ▫ Elastic compression ▫ Time-dependent deformation

(creep) ▫ Influenced by loads and wood MC

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Design for Vertical Differential Movement

Always design to allow certain differential movement ▫ Detailing for major interfaces provided in the chapter, such as

masonry cladding , balconies, elevator shafts and stairwells, etc.

Measures to reduce/accommodate wood shrinkage and differential movement ▫ Use and maintain drier wood in

construction ▫ Use engineered wood for floor joists ▫ Use good construction sequencing to

reduce wood wetting, encourage drying, and allow settling before enclosure

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Chapter 6: Fire Safety Design

Fundamentals of fire safety in buildings Fire separations and service penetrations Fire-resistance of elements Firewalls Concealed spaces and fire blocks Flame spread of interior finishes Automatic sprinkler protection Exterior cladding Guidance on podium structures Wood-based vertical shafts Preventing fires during construction

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Fire Separations and Penetrations

Fire separations required for walls, floors and roofs Properly detailed and built so that continuity is maintained Service penetrations passing through a fire separation need to be

sealed with a fire stop system Info on fire stops: NRCC publication “Best Practice Guide on Fire

Stops and Fire Blocks and Their Impact on Sound Transmission”

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Podium Buildings from Fire Prospective

Widely used in Western Canada and the West coast of the US Codified in the US Not explicitly addressed in NBCC, use alternative solutions A guideline has recently been prepared by LMDG provides an

overview of the NBCC implications on podium building design

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Vertical Shafts

Various systems, such as wood-framed, nailed-laminated timber and CLT, can be designed to achieve the required fire performance for vertical shafts

These systems have been widely used in BC, QC and the US More info on elevator shafts in Chapter 9

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Fire Safety During Construction

Great risk during construction as the structure is most vulnerable Documents related to construction site fire safety are referenced

with safety objectives: ▫ Reduce the risk of starting fires ▫ Increase the likelihood of early detection if fires do start ▫ Provide fire protection measures to mitigate damage

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No more!

Chapter 7: Noise Control

Fundamentals of building acoustics Review of 2015 NBCC requirements Strategy for controlling noise transmission Noise control through design & installation Acceptable wall and roof/floor assemblies

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C. Benedetti 2010, “Timber buildings”

Sound Transmission Paths

Direct path and flanking part 2015 NBCC takes into consideration flanking paths through Apparent

Sound Transmission Class rating for the control for airborne noise In the past, NBCC sound transmission ratings requirements did not

consider flanking paths

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Three Lines of Defense Approach

An effective strategy for controlling noise transmission in buildings: ▫ Reduce noise transmission through walls or floors ▫ Reduce noise level by reducing the vibration of walls

or floors caused by the noise source ▫ Prevent the vibration of walls or floors to be

transmitted to adjacent units

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Noise Control by Design and Installation

Based on the Three Line Defence Approach, the noise control through design & installation can be achieved by: ▫ Using sound-absorbing materials with low porosity surface

to reduce airborne noise ▫ Decoupling and discontinuing of building components, if

possible ▫ Reducing impact sound transmission through wood floor

by using: • Floating topping with weight ≥ 30kg/m2 • Resilient underlayment to reduce impact noise

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Chapter 8: Durable & Efficient Building

Enclosure (Building Envelope)

Increased environmental loads on the envelope Design for higher wind and stack effect Construction moisture management Exterior moisture management Thermal design Durability and maintenance

Increased Environmental Loads

No specific envelope provisions for mid-rise buildings, however, increased wind loads require stronger materials and assemblies

Higher wind-driven rain requires more attention to water management and drainage systems than in lower buildings

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More robust air barriers and detailing for higher wind / stack effects More attention to preventing on-site wetting Promoting drying - typically prolongs construction More robust and durable building envelope design and detailing

(e.g. drained and ventilated rain screen walls)

Solution Examples

Chapter 9: Elevator Shafts and Stairwells

Relevant code requirements for elevator shafts Various design issues and considerations related to

elevator shafts that influence the choice of materials ▫ Non-combustible shafts ▫ Wood-based shafts ▫ Hybrid shafts

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Code Requirements in Canada

Although mid-rise buildings are permitted in BC, Ontario, Alberta and Quebec, the requirements for elevator shafts and stairwells are currently different

In BC building code, combustible shafts/stairwells with a minimum of 1-hour fire-resistance rating are allowed, (consistent with 2015 NBCC)

In Quebec only non-combustible elevator shafts and stairwells are allowed with 1-hour rating

In Ontario non-combustible stairwells are required with 1.5-hour fire-resistance rating

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Design Considerations

Fire control and separations Noise control Vertical differential movement

between elevator shaft and the building

Interaction of loads and deflection between shaft and the building under wind and seismic loads

Requirements for connecting the elevator to the shaft Design team needs to reach a collective design that accounts for

all these design considerations Innovative solutions presented

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Chapter 10: Prefabricated Systems

Overview of various prefabricated systems and advantages Preconstruction process Manufacturing Transportation Installation and site procedures Certification standards

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Prefabricated Element Categories

Components ▫ Beams, Columns, Trusses, mass timber frame elements

Panelized building elements ▫ Walls, floors, ceilings, mass timber plates

Volumetric systems ▫ 3-D modules that include floor, walls and ceiling

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Standardization in Canada

CSA A277 "Procedure for certification of prefabricated buildings, modules and panels" (Available Fall 2015)

Procedures for certification of prefabricated buildings, modules and panels completely revised

Applies to all forms of prefabricated systems and buildings of all occupancies

Focus on compliance markings, such as labels, stamps and specification sheets

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Chapter 3: Structural Design

Code requirements General analysis and design Fundamental building period Deflection of multi-storey shear walls Linear dynamic analysis Diaphragm flexibility Capacity-based design High-capacity shear walls and

diaphragms Force transfer around openings Design of podium structures

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2015 NBCC Requirements

For continuous wood construction of more than 4 storeys in moderate and high seismic zones (Ie Fv Sa(0.2) ≥ 0.35) shall not have irregularities of type 4 and 5 (in-plane and out-off-plane)

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2015 NBCC and 2014 CSAO86

Requirements (cont.)

When building period Ta is determined in ways other than the NBCC formula, the earthquake shear force V determined according to the Equivalent Static Force Procedure (ESFP) shall be multiplied by 1.2 (but not exceed the cut-offs)

When Ta is determined using dynamic analysis, the design base shear Vd shall be taken as the larger of: ▫ 100% of the base shear V obtained using the ESFP

▫ Force from dynamic analysis obtained as: 𝑉𝑑 =𝑉𝑒𝑑

𝑅𝑑𝑅𝑜 𝐼

CSAO86 2014: For buildings higher than 4 storeys, contribution of the gypsum wallboard shall not be accounted for in the seismic resistance

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Building Period

Significant role in calculation of the design base shear Preliminary design to be done using the NBCC formula

Once shearwall detailing is completed (preliminary design), the period can be recalculated using methods of mechanics such as Rayleigh's method

Make sure period is not exceeding the upper limit of 2Ta

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Gypsum Wallboard and Stucco

Significant influence on the building period Although gypsum wallboard shall not be taken in the resistance,

its stiffness and that of the stucco shall be included when determining the building period

The initial stiffness can be calculated using the slope between the points of 0% and 40% of capacity (ASTM E2126)

Gypsum wallboard and stucco shall not be accounted for in lateral drift calculations (NBCC, as not part of SFRS)

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Deflection Single Shear Wall

Deflection of a single-storey shear wall can be determined per CSA O86 accounting for bending and shear deformation, nail slip and anchorage elongation:

This assumes shear and moment distribution as given below

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Deflection of Stacked Multi-storey SW

Moment at the top of the storey is not zero (except top one) Effect of the top moment and the cumulative effect of rotation at

the bottom of the SW has to be considered (Newfield et al. 2013)

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Suggested Formula for Stacked Multi-

storey Walls

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Linear Dynamic Analysis (LDA)

Use of LDA should be encouraged in analysis and design Benefits of LDA are:

▫ Considers the effect of higher mode participation ▫ Better determines building deflections and storey drifts ▫ Allows for three-dimensional modelling ▫ Reduces the minimum torsional effect required under the ESFP ▫ Better considers the effect of vertical changes in RdRo (podiums)

Challenge: the stiffness properties and other input parameters are not easily determined

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Proposed Steps of LDA

Step one (preliminary analysis): Perform an initial analysis and design to determine the properties of each wall forming part of the LLRS ▫ Allows designers to get the information required to determine stiffness

and deflection characteristics of the shearwalls Step two: Use the preliminary analysis info to generate input data

for LDA for a multi-level structure The design base shear must be the larger of the dynamic design

force Vd and the 100% of static design force V.

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Mechanical Properties of Shear Walls for

LDA

SW can be modeled as beam elements in commercial software Guidelines for calculating equivalent beam element properties

(such as flexural and shear stiffness) are given based on the basic wall parameters

Example: the shear modulus used for LDA

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Diaphragm Flexibility (In-Plane)

In-plane diaphragm stiffness affects the overall response of the building lateral forces

Whether a diaphragm is treated as flexible, rigid, or semi-rigid, depends on the in-plane stiffness of the diaphragm relative to the stiffness of the vertical LLRS underneath

Suggested to use ASCE 41-13 (flexible if: MDD > 2 ADVE)

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Capacity Based Design

Widely used for seismic design of concrete and steel structures, but only recently made inroads into wood design standards

By choosing desirable deformation modes of the SFRS, certain parts of it are designed for yielding and energy dissipation ("plastic hinges" or "dissipative zones")

All other structural elements are designed not to yield (capacity protected and designed based on over-strength)

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CSAO86 Provisions on Capacity Design

Increased design loads on critical system components and force transfer elements

Anchor bolts, inter-storey connections, and hold-downs to be designed for seismic loads that are at least 20% greater than the force that is being transferred

Intent: To ensure that the desired ductile nail yielding is achieved throughout the structure without any failure in the hold-downs and shear transfer connections (Popovski et al., 2009).

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CSAO86 Provisions (cont.)

To avoid a soft-storey mechanism at the bottom two storeys, check for over-capacity ratio of the vertical SFRS (C2/C1), where:

𝐶𝑖 =𝑉𝑟𝑖

𝑉𝑓𝑖 ; Vri = Factored resistance of SW at storey "i"

Vfi = Factored seismic shear at storey "i"

It is recommended that the C3/C2, C4/C3 and C5/C4 ratios be checked for 5- and 6-storey buildings

Diaphragm coefficients CDi are also introduced, being the lesser of Ci or 1.2

Handbook contains main steps of the design process for shearwalls and diaphragms

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High-Capacity Shear Walls and

Diaphragms

May be needed in mid-rise buildings in high seismic zones and in commercial buildings with large openings

The Handbook introduces: ▫ Midply shearwalls ▫ Diaphragms with multiple rows of fasteners

Both to be designed using the mechanics-based approach for shear walls and diaphragms in 2014 CSA O86

Design and detailing requirements, and factored resistances of some configurations of Midply walls are provided

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Regular vs Midply Shearwall

38 89 mm lumber stud spaced at 406 mm o.c.

Wood-based panel fastened to the narrow face of framing

members

Developed by FPI and UBC

Studs rotated 90 degrees (on flat) 610mm o.c.

Wood-based panel at the center of the wall fastened to the

wide face of framing members

Standard shear wall 2x4 studs

16” 16” 16”

Sheathing

Drywall/Sheathing

24” 24”

Midply shear wall

Drywall/Sheathing

Cladding/Sheathing Sheathing

Nails work in double shear

thus increasing the lateral load capacity

Greater edge distance - panel chip out failure is reduced

Nail head away from panel surface - nail pull through failure is prevented

Capable of accommodating additional sheathing (Double Midply)

Reasons for Improved Performance of

Midply Walls

Nail in single shear

Nail in double shear

Sheathing Stud or

Plate

Grain direction

89 mm

Stud or Plate

38 mm 38 mm

Application of Midply Walls

Elderly care facility in Tokyo, the largest contemporary wood building in Japan

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Force Transfers Around Openings

Most diaphragms have openings for elevator shafts, stairwells, skylights, pipes, ducts, etc.

This induces more shear demand on the diaphragm (higher design forces)

This "weakening effect" depends on the ratio of the opening size vs. the area of the entire diaphragm

Solution: Design for the increased shear around the opening Three methods available:

▫ Drug strut analogy: Consistently unconservative ▫ Cantilever beam analogy: Most conservative ▫ Vierendeel Truss analogy: Reasonable agreement with measured

forces, but cumbersome. Design example provided.

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Analysis NOT Needed if ALL Conditions

Below are Met

Opening depth ≤ 15% diaphragm depth LD; Opening length ≤ 15% diaphragm length L

Distance from any diaphragm edge to the nearest opening edge is ≥ 3a where a is the larger opening dimension

Diaphragm portion between opening and the edge meets the maximum aspect ratio requirement

Opening corners are reinforced for a load 50% of the maximum diaphragm chord force

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a

Podium Buildings

Several storeys of wood-frame construction built over one or more storeys of elevated concrete podium

Especially prevalent in the Western North America during the last two decades

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Current Code Status and Approaches

Not explicitly included in 2015 NBCC and 2014 CSA O86 Designers can choose between two methods that implicitly cover

podium buildings in NBCC First: Linear Dynamic Analysis (LDA) as default NBCC approach

▫ Analytical model should include both concrete and wood portions with their own strength and stiffness properties

▫ Distribution of linear shear forces along the height is obtained ▫ Corresponding RdRo factors for each storey are used to determine the

design shear forces

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NBCC Equivalent Static Procedure

Seismic interaction of concrete and wood-frame portion is ignored Wood portion is treated as a separate building supported on the

ground designed with its own Rd Ro Shear forces and overturning moments from the wood portion are

applied to the concrete slab below Concrete podium designed as separate building with its own Rd

and Ro factors No criteria in main body of NBCC when to use this approach

▫ Commentary J note 151 states that such procedure can be used when the stiffness Kpodium > 3 Kwood

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ASCE-7 Two-stage Analysis Procedure

Two-stage procedure can be used if the structure complies with both requirements: ▫ Stiffness of the podium Klower ≥ 10 times that of the wood Kwood ▫ Period of the entire structure Ta ≤ 1.1 Twood (as a separate structure)

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Twood Kwood

Klower Tlower

Ta

ASCE-7 Two-stage Procedure

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Upper portion designed as a separate structure using R (RdRo = 5.1) and ρ (Redundancy factor = 1.0);

Lower portion as a separate structure using appropriate R and ρ The reactions from the upper portion must be amplified by the ratio of

(R/ρ) upper / (R/ρ) lower. Ratio > 1.0

Rupper = (5.1); upper = 1.0

Rlower ; lower

𝑉𝑙𝑜𝑤𝑒𝑟 =

𝑅

𝑢𝑝𝑝𝑒𝑟

𝑅

𝑙𝑜𝑤𝑒𝑟Vupper

Conclusion

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The handbook provides guidelines for early adopters and mainstream practitioners to design and construct mid-rise wood frame construction in compliance with the 2015 NBCC, Provincial Codes, and 2014 CSA O86

A total of 42 industry, research and design experts have been involved in the development of the mid-rise handbook

The information shall be used in addition to the info already available in CWC’s Wood Design Manual (2010), the APEGBC Bulletin for design and construction of 5-and 6-storey wood-frame construction, and the 2013 Quebec guidelines from Régie du bâtiment du Québec

Thank You

This concludes The American Institute of Architects Continuing Education Systems Course

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Canadian Wood Council www.cwc.ca

Wood WORKS! BC www.wood-works.org