Canadian Wood Council G063 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 October 27, 2015 1
Continuing Education Course and Credits 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. 2
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. 3
Learning Objectives At the end of the this course, participants will be able to: Understand the current code status of the mid-rise woodframe 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 4
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 Photo Courtesy of WoodWorks! 5
Midrise Wood-Frame Construction in Rest of Canada April 2013: Régie du Bâtiment du Québec (RBQ) allowed woodframe 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 woodframe construction in 2015 NBCC 6
Midrise Construction in the US Already Code Approved in California, Washington and Oregon for about a decade Allowed in 2012 IBC Photo: BC WoodWorks! 7
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 8
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 9
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 10
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 11
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 12 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 l 1 8.22 ( EI F eff 0.14 scl 0.284 ) m 0.15 L l = vibration-controlled span (m) EI eff = effective bending stiffness of the T-beam (N*m 2 ) m L = linear density of the T-beam (kg/m) F scl = none-zero and 1 factor related to stiffness contribution of subfloor and topping to reduce the 1kN static deflection 13
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 14
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 15
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 16
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 17
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 18
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 19
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 20 Photos: G. Triggs
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 21
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 22 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 C. Benedetti 2010, Timber buildings 23
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 24
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 25
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/m 2 Resilient underlayment to reduce impact noise 26
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 27
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 28
Solution Examples 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) 29
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 30
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 31
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 32
Chapter 10: Prefabricated Systems Overview of various prefabricated systems and advantages Preconstruction process Manufacturing Transportation Installation and site procedures Certification standards 33
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 34
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 35
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 36
2015 NBCC Requirements For continuous wood construction of more than 4 storeys in moderate and high seismic zones (I e F v S a (0.2) 0.35) shall not have irregularities of type 4 and 5 (in-plane and out-off-plane) 37
2015 NBCC and 2014 CSAO86 Requirements (cont.) When building period T a 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 T a is determined using dynamic analysis, the design base shear V d shall be taken as the larger of: 100% of the base shear V obtained using the ESFP Force from dynamic analysis obtained as: V d = V ed R d R o CSAO86 2014: For buildings higher than 4 storeys, contribution of the gypsum wallboard shall not be accounted for in the seismic resistance I 38
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 2T a 39
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) 40
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 41
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) 42
43 Suggested Formula for Stacked Multistorey Walls
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 R d R o (podiums) Challenge: the stiffness properties and other input parameters are not easily determined 44
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 V d and the 100% of static design force V. 45
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 46
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) 47
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) 48
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). 49
CSAO86 Provisions (cont.) To avoid a soft-storey mechanism at the bottom two storeys, check for over-capacity ratio of the vertical SFRS (C 2 /C 1 ), where: C i = V ri V fi ; V ri = Factored resistance of SW at storey "i" V fi = Factored seismic shear at storey "i" It is recommended that the C 3 /C 2, C 4 /C 3 and C 5 /C 4 ratios be checked for 5- and 6-storey buildings Diaphragm coefficients C Di are also introduced, being the lesser of C i or 1.2 Handbook contains main steps of the design process for shearwalls and diaphragms 50
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 51
Regular vs Midply Shearwall Standard shear wall 2x4 studs Sheathing 16 16 16 Drywall/Sheathing 38 89 mm lumber stud spaced at 406 mm o.c. Wood-based panel fastened to the narrow face of framing members Midply shear wall Sheathing Cladding/Sheathing 24 24 Developed by FPI and UBC Studs rotated 90 degrees (on flat) 610mm o.c. Drywall/Sheathing Wood-based panel at the center of the wall fastened to the wide face of framing members
Reasons for Improved Performance of Midply Walls 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) Stud or Plate Grain direction 89 mm Nail in single shear Sheathing 38 mm 38 mm Stud or Plate Nail in double shear
Application of Midply Walls Elderly care facility in Tokyo, the largest contemporary wood building in Japan 54
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. 55
Analysis NOT Needed if ALL Conditions Below are Met Opening depth 15% diaphragm depth L D ; 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 a 56
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 57
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 R d R o factors for each storey are used to determine the design shear forces 58
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 K podium > 3 K wood 59
ASCE-7 Two-stage Analysis Procedure Two-stage procedure can be used if the structure complies with both requirements: Stiffness of the podium K lower 10 times that of the wood K wood Period of the entire structure T a 1.1 T wood (as a separate structure) T a K wood T wood K lower T lower 60
ASCE-7 Two-stage Procedure Upper portion designed as a separate structure using R (R d R o = 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 R upper = (5.1); upper = 1.0 R lower ; lower 61 V lower = R upper R lower V upper
Conclusion 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 62
Thank You This concludes The American Institute of Architects Continuing Education Systems Course Wood WORKS! BC www.wood-works.org Canadian Wood Council www.cwc.ca 63