Canadian Wood Council
Conseil canadien du bois
Engineering Guide for Wood Frame Construction 2009 Edition
Engineering Guide for Wood Frame Construction 2009 Edition
Canadian Wood Council Conseil canadien du bois
© 2009 Copyright Canadian Wood Council Conseil canadien du bois Ottawa, Ontario, Canada ISBN 978-0-9783213-4-5 Printed in Canada
Design and production: Accurate Design & Communication Inc., Ottawa, ON
Engineering Guide for Wood Frame Construction
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Preface The Canadian Wood Council (CWC) is the national federation of forest products associations responsible for development and communication of technical information about the use of wood products in construction. Ensuring that this information is in tune with technical changes and users’ needs is accomplished through the publication and updating of CWC manuals, software, technical bulletins and case studies. The Engineering Guide for Wood Frame Construction has been produced by CWC to provide guidance to engineers, building designers, building officials, builders, and students of these disciplines, on the structural design of wood elements and connections for wood frame buildings that: (a) fall within the scope of Part 9, “Housing and Small Buildings,” of the National Building Code of Canada, and (b) due to the size, location or configuration of the building, engineering design is required in order to augment the prescriptive requirements in Part 9 (See the Foreword for additional explanation). This is the third edition of the Guide and it supersedes the previous editions published in 2001 and 2004. The Guide was revised, in this 2009 Edition, in order to conform to changes in the 2010 edition of the National Building Code of Canada. Article 9.4.1.1 of the 2005 National Building Code of Canada references the “Engineering Guide for Wood Frame Construction,” published by the Canadian Wood Council as an example of good engineering practice for the design of Part 9 structural members and their connections. Design Requirements are provided in Part B of the Guide and Supplementary Design Tables are provided in Part D of the Guide. Guidance on the National Building Code of Canada Part 9 prescriptive requirements can be found in Part C “Supplementary Guidelines on Applicability and Scope” of the Guide. It is intended that the Guide be used in conjunction with competent engineering design, accurate fabrication and adequate supervision of construction. Every effort has been made to ensure that the data and information in the Guide are accurate and complete. The CWC does not, however, assume any responsibility for errors or omissions in the Guide nor for designs or plans prepared from it. This edition of the Guide was developed with technical input from a committee of engineers, builders, building regulators, researchers and industry representatives. The Canadian Wood Council gratefully acknowledges the contribution of the following people: Thomas Abbuhl Michael Bartlett David Bowick Steve Boyd Brent Bunting Y.H. Chui Steve Copp Bruno DiLenardo Paul Jaehrlich Keith Jansen Dominique Janssens
BC Institute of Technology University of Western Ontario Blackwell Engineering Quaile Engineering Simpson Strong-Tie Canada Ltd. University of New Brunswick Steve Copp Construction Ltd. National Research Council of Canada, Canadian Construction Materials Centre CertiWood™ Technical Centre RCG Developments Consultant
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Preface
Robert Kok Kenneth Koo Frank Lam Chun Ni Thor Tandy Cathleen Taraschuk
Brockport Home Systems Ltd FPInnovations – Forintek Division University of British Columbia FPInnovations – Forintek Division UNISOL Engineering Ltd. National Research Council of Canada, Canadian Codes Centre
With the permission of Canadian Standards Association, material is reproduced from CSA Standard CAN/CSA-O86-09, Engineering Design in Wood, which is copyrighted by Canadian Standards Association, 178 Rexdale Blvd., Toronto, Ontario, M9W 1R3. While use of this material has been authorized, CSA shall not be responsible for the manner in which the information is presented, nor for any interpretations thereof.
For additional technical information or to receive more information on CWC design tools, call this toll-free number 1-800-463-5091 or visit the CWC web site at www.cwc.ca.
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Table of Contents A
Foreword
B
Design Requirements Scope.................................................................................................................... B-3 Definitions and Symbols....................................................................................... B-7 Objectives and Design Requirements................................................................ B-10 Loads.................................................................................................................. B-12 Roof Design........................................................................................................ B-17 Floor Design........................................................................................................ B-23 Wall Design......................................................................................................... B-30 Column Design................................................................................................... B-35 Diaphragms......................................................................................................... B-36 Shearwalls.......................................................................................................... B-47 General Construction Details.............................................................................. B-62
Supplementary Guidelines and Tables Supplementary Guidelines on Applicability and Scope C D
Supplementary Tables Load Tables Gravity Load Tables..............................................................................................D-3 Wind Load Tables.................................................................................................D-9 Seismic Load Tables..........................................................................................D-35 Design Tables Roof Design Tables............................................................................................D-61 Floor Design Tables............................................................................................D-73 Wall Design Tables.............................................................................................D-83 Beam and Column Design Tables....................................................................D-121 Diaphragm Design Tables................................................................................D-135 Shearwall Design Tables..................................................................................D-149
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Wood Frame Construction Guide
Engineering Guide for Wood Frame Construction
Foreword
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Foreword The objective of the Engineering Guide for Wood Frame Construction is to provide acceptable structural design solutions for wood elements and connections in wood frame buildings that are 3 stories or less in building height, 600 m2 or less in building area, and roofs, walls and floors are generally constructed using systems of repetitive wood members spaced no more than 600 mm on centre. These solutions apply to conditions where the element or building being designed is within the scope of this Guide, and additional information is provided to supplement the prescriptive wood frame construction requirements in Part 9, Housing and Small Buildings, of the 2010 edition of the National Building Code of Canada (NBCC). The provisions and solutions provided in the Guide address structural design considerations. For design requirements related to other issues such as fire, durability, sound transmission and building envelope refer to the NBCC. The Design Requirements in Part B of the Guide are based on design calculations that reflect the prescriptive provisions of Part 9 where appropriate, and provide structural solutions for designs that are beyond the prescriptive solutions in Part 9. Buildings or building elements beyond the scope of Part 9 and the Guide are designed in accordance with Part 4 of the NBCC. It is recommended that all post-disaster buildings essential to services in the event of a disaster also be designed in accordance with Part 4 of the NBCC. The Guide also contains supplementary information to assist in identifying situations, where engineering analysis is required and structural solutions in the Guide are appropriate. Structural design provisions are provided in the Design Requirements, Part B, of the Guide. Supplementary information is included in the sections: Part C – Supplementary Guidelines on Applicability and Scope, and Part D – Supplementary Tables Builders, regulatory authorities and building designers may wish to consult the section on “Supplementary Guidelines on Applicability and Scope” as a guide to determining whether a building or element falls outside the assumptions on which the Part 9 prescriptive requirements are based. The Design Requirements and Tables in Parts B and D of the Guide have been written on the assumption that structural design will be carried out by a Professional Engineer who is qualified for such design. PART 9 OF THE NATIONAL BUILDING CODE OF CANADA, “HOUSING AND SMALL BUILDINGS” General Part 9 of the NBCC is a set of primarily prescriptive requirements covering the design of residential, business, personal service, mercantile and some industrial buildings, 3 stories or less in building height and 600 m2 or less in building area. Part 9 and Wood Frame Construction Part 9 includes prescriptive solutions for wood systems where; • wall, roof and floor planes are generally comprised of repetitive wood structural members spaced no more than 600 mm o.c., • walls, roofs and floors are clad, sheathed or braced on at least one side, • clear spans of wood members are limited to 12.2 m, and • the floor live load does not exceed 2.4 kPa.
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The solutions were developed as a simple set of minimum requirements that could be used without the assistance of an architect or engineer. The Part 9 requirements for wood frame construction are based on a combination of calculated designs and solutions based on performance history. There is some overlap between these two categories; i.e., calculations include performance considerations. In some circumstances new products are introduced into Part 9 solutions using engineering design. Examples of elements in Part 9 that are explicitly based on calculations include: • dimension lumber floor joists • dimension lumber and glulam floor beams • dimension lumber roof rafters • dimension lumber roof joists • dimension lumber ceiling joists • trusses designed in accordance with TPIC procedures • dimension lumber lintels, and • anchor bolt connections. Examples of elements in Part 9 that are based mainly on performance history include: • notching and drilling limitations • bearing requirements for rafters, joists and beams • header joist, and trimmer joist sizes • hip and valley rafter size and connection • wall plates • stud size and spacing, and • column sizes. The span tables in Part 9 are based on gravity loads - dead loads, occupancy loads and balanced snow loads. In addition, elements such as roof rafters and roof trusses that are designed for gravity loads also withstand lateral loads without being explicitly designed for that purpose. The gravity loads used in Part 4 are in some cases higher than the gravity loads used in Part 9 calculations. Some, though not all, of the elements sized by calculation in Part 9 would be larger if they were sized in accordance with Part 4 due to both load differences and differences in design assumptions. Section 9.4 of the NBCC allows Part 9 structural members and connections to be designed according to Part 4 using the loads and deflection and vibration limits specified in Part 9 or Part 4. Gravity loads in the Guide are consistent with loads in Part 9 of the NBCC and may be used to design repetitive framing members and their supporting members. The NBCC 2010 introduced prescriptive requirements for lateral resistance of Part 9 buildings. The Engineering Guide for Wood Frame Construction specifies lateral loads and lateral design solutions, and the section on Applicability and Scope includes a framework for considering where the lateral load provisions in the Guide may be applicable. Consistent with Part 9, the Engineering Guide for Wood Frame Construction applies to the design of residential, business, personal service, mercantile and medium or low hazard industrial buildings, 3 stories or less in building height and 600 m2 or less in building area. The Design Requirements and Tables in Parts B and D of the Guide use an engineering mechanics approach to assess the lateral resistance of a wood frame building, with some simplifying assumptions for practical purposes. In this approach, the roof systems and floor systems are assumed to act as diaphragms to transfer the lateral forces to wall systems acting as shearwalls. Non-structural elements that contribute to the overall building performance are
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disregarded. Only adequately anchored floors, roofs and walls sheathed with structural wood panels (alone or combined with gypsum board in the same wall or combined with gypsum board within a storey) are considered in the engineering calculations. Using this approach, many wood frame buildings based only on the Part 9 prescriptive requirements would appear to be inadequate for resisting lateral loads; however, performance history indicates that this is not the case. The performance history of small wood frame structures cannot be explained completely by Part 4 structural calculations using simple 2-dimensional load path assumptions. Traditional wood-frame construction is difficult to model mathematically due to the many load paths in the indeterminate structural system and the contributions of “non-structural” elements. Various aspects of building performance have been investigated experimentally during the past fifty years (for example Dorey and Schriever 1957, Boughton and Reardon, 1982, Boughton, 1988, Ceccotti, 1990, Fischer et al. 2001, Paevere, 2002, UWO 2002, Doudak, 2005). Most of these studies have focused on one- and two-story structures on rigid foundations. These reports gave more insight into the mechanisms of structural deformation including the importance of load sharing among the structural and “non-structural” elements within the structure: 1. Interior finishes and many types of exterior cladding contribute to the lateral resistance of the structure. Both the ultimate load capacity and the lateral stiffness are improved by the addition of architectural components. 2. Non-loadbearing partitions stiffen and strengthen the structure so that the building acts as a rigid box rather than a series of diaphragms and shearwalls. 3. Other non-structural elements such as stairs, closets and cabinetry also contribute to the lateral resistance of the building. 4. The performance of wood light frame systems is enhanced by the load sharing and composite actions. The overturning resistance of a wall is enhanced through “corner effects” that engage adjacent walls. Roof and floor diaphragms, if adequately connected, will transfer lateral wind and earthquake loads to all supporting walls, including walls parallel and perpendicular to the direction of loading that normally may not be considered in design. Although to a large extent the structural stability of Part 9 buildings relies on these non-structural elements, to date, this action has not been quantified in a systematic manner suitable for use in structural design. The numerous wood frame buildings throughout Canada and elsewhere represent countless “prototypes” subject to field-testing over many decades. However, some wood frame buildings covered by Part 9 differ sufficiently from the norm that they cannot be counted on to demonstrate similar performance if their design is based only on the prescriptive provisions of Part 9. WOOD FRAME CONSTRUCTION – PART 9 AND THE GUIDE The Part 9 structural requirements for wood frame construction are derived from a combination of calculated designs and solutions based on performance history. In the Guide, calculations are used to develop structural requirements. The Guide calculations use: • Gravity loads based on Section 9.4 of the NBCC • Wind and earthquake loads from Part 4 of the NBCC, and • Resistances for lumber members, sheathing, I-joists, structural composite lumber, shearwalls, diaphragms and connections based on CSA Standard O86, Engineering Design in Wood with some modifications. Lateral Design
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Performance of wood frame construction under high wind loads The wind design provisions in the National Building Code of Canada and the Engineering Guide for Wood Frame Construction are intended to simulate peak gusts in storms having a 1 in 50 probability of occurring every year. Studying the damage from hurricane force winds in other parts of the world provides insight into how wood frame construction behaves under high wind loads. Similar forms of damage have been reported in wood frame houses exposed to the direct paths of tornadoes. (Allen, 1984; Allen 1986; Dagliesh and Allen, 1994) The damage to housing in hurricanes and tornadoes shows that: 1. Sheathing attachment, gable end details and attachment of roof framing to walls are critical. (Douglas, 1992; Keith and Rose, 1992; Sheffield, 1993; Wolfe, Riba and Triche, 1993) 2. When it occurs, structural damage is usually related to the roof system. Damage to walls and foundations are rarer. (Crandell, Gibson, Laatsch, Nowak and vanOvereem, 1993) 3. Major damage occurs at gradient wind speeds of 70 m/s or greater and minor damage occurs at gradient wind speeds below 50 m/s (Sparks and Bhinderwala, 1993). Neither the National Building Code nor this guide is intended to provide design solutions against the direct force of tornadoes. Based on observations made in Canadian tornadoes, the Structural Commentaries to the National Building Code state, “It is generally not economical to design buildings for tornadoes beyond what is currently required by NBCC Subsection 4.1.7 because of the low risk of loss to individual owners. It is, however, important to provide key construction details for the safety of building occupants….anchorage of home floors, is essentially covered by NBCC Article 9.23.6.1 for normal housing with permanent foundations.” (Canadian Commission on Building and Fire Codes, 2010) Performance of wood frame construction under earthquake loads Canada has not experienced an earthquake that has caused widespread damage to wood frame buildings. The damage from the 1989 earthquake in the San Francisco area was studied by Canadian researchers (Rainer, Jablonski, Law and Allen, 1990), leading to the following observations concerning the performance of wood frame construction: 1. Foundation walls weak in racking resistance—such as cripple-stud walls—led to failure of buildings. 2. Openings for doors in the ground floor of multiple storey buildings created “weak storeys” which led to damage of the buildings. 3. Most of the serious structural failures that occurred to residential construction were due to deficiencies prohibited by California building codes and reflected in the 1985 Uniform Building Code (UBC). The authors concluded: “Nominal lateral resistance in the UBC is achieved by specifying minimum percentages of shear panels in the walls.” Similar observations were made following the 1994 Northridge earthquake in California (NAHB Research Center, 1994; American Forest and Paper Association, 1994). Structural damage to wood frame construction in this earthquake was attributed to site conditions or non-conformance with building codes.
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In a survey of wood frame building construction from around the world (Rainer and Karacabeyli, 1999) the authors concluded that, “…despite some specific shortcomings and resultant failures, wood frame construction has performed exceedingly well, both from the perspective of life safety and incidence of damage.” Lateral Resistance in the Guide The section on “Applicability and Scope,” provides guidelines that are additional to the Part 9 prescriptive requirements of the NBCC. The detailed design information in the Guide can be used where a full lateral design to resist wind and earthquake loads is considered necessary. In the Design Requirements, lateral design for wind and earthquake loads includes the design of fully detailed roof and floor diaphragms supported on shearwalls. Gravity Loads The prescriptive requirements in Part 9 are limited to buildings with occupancy loads of 2.4 kPa and the floor joist, lintel and floor beam span tables in Part 9 apply only to residential loads of 1.9 kPa even though Part 9 may apply to occupancies which have specified live floor loads up to 2.4 kPa. Sometimes only a small portion of a building will have higher loads and engineering may be required to address an element supporting a combination of loads. The Guide provides design solutions for small wood frame buildings with occupancy loads up to 2.4 kPa. Snow loads in the Guide are calculated in accordance with Part 9 or, where required, Part 4 of the NBCC. The dead loads in the Guide reflect actual construction and are provided for floors with normal weight finishes and concrete toppings. New Materials Increasingly, wood frame buildings incorporate engineered, proprietary wood products such as wood I-joists, floor trusses, laminated veneer lumber, parallel strand lumber, laminated strand lumber and proprietary wood framed roof and wall systems and their connections. Builders, designers and building officials have sought guidance in using these products with Part 9 requirements. One of the objectives of the Engineering Guide for Wood Frame Construction is to clarify load requirements for engineered wood products used in small wood buildings. The Guide may be used with proprietary wood products that are manufactured in accordance with a quality assurance program supervised by an independent third-party certification organization with design values developed in accordance with Clauses 13 and 14 of CSA Standard O86 and are designed and installed in accordance with the manufacturer’s material evaluation report. Design of Individual Elements Studs NBCC Part 9 stud provisions are limited to stud lengths of 4.2 m for interior studs and 3.6 m for exterior studs. In some situations, such as entrances and gable end walls, longer studs are used. The stud design procedures and stud tables in the Engineering Guide for Wood Frame Construction have been developed using wind load data and considering the composite action of wall elements.
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Beams, headers and lintels supporting point loads Part 9 provides beam, header and lintel tables for built-up wood members supporting uniform loads. The Guide provides design solutions for built-up wood members supporting point loads. Column design Part 9 prescriptive requirements for columns are limited to columns supporting 2 floors, a maximum tributary width of 5 m and a maximum of 2.4 kPa occupancy load. The Guide section on “Applicability and Scope,” provides guidance on limits to the Part 9 prescriptive requirements and the “Design Requirements” provide column design solutions for a broad range of loads and sizes. Roof rafters Part 9 rafter spans are limited to rafters tied at the eave. The Guide provides rafter design for rafters with raised ties. In addition, the Guide provides structural design solutions for hip and valley rafters. Floor members The Guide may be used to design floor support members supporting non-residential floor loads. In addition, the Guide may be used for designing header and trimmer joists for openings larger than allowed with the prescriptive requirements of Part 9.
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REFERENCES Allen, D.E. 1984. Tornado Damage at Blue Sea Lake and Nicabong, Quebec, July 1984. Building Research Note 222. Division of Building Research. National Research Council of Canada. Ottawa, Canada. Allen, D.E. 1986. Tornado Damage in the Barrie/Orangeville Area, Ontario, May 1985. Building Research Note 240. Institute for Research in Construction. National Research Council of Canada. Ottawa, Canada. American Forest and Paper Association, 1994. The Northridge Earthquake – A Preliminary Report. American Wood Council. Washington DC Boughton, G. N. and Reardon, G. F. (1982) “Simulated wind test on a house: Part 1 -Description” Tech. Rep. No. 12, James Cook Cyclone Testing station, Townsville, Queensland, Australia. Boughton, G. N. (1988) “Full scale structural testing of houses under cyclonic wind loads” Proceedings of the 1988 International Conference on Timber Engineering, Seattle, WA, Vol. 1, 82-88. Canadian Commission on Building and Fire Codes, 2005. National Building Code of Canada. National Research Council of Canada. Ottawa, Canada Canadian Commission on Building and Fire Codes, 2005. User’s Guide - NBCC 2005 Structural Commentaries. National Research Council of Canada. Ottawa, Canada Ceccotti, A. (Ed.) 1990. Structural Behavior of Timber Constructions in Seismic Zones, Commission of the European Communities and Florence University, Dipartimento di Ingegneria Civille, Florence University, Italy, 427 pp. Crandell, J.H., M.T. Gibson, E.M. Laatsch, M.S. Nowak and J.A. vanOvereem. 1993. Statistically-Based Evaluation of Homes Damaged by Hurricanes Andrew and Iniki. Hurricanes of 1992: Lessons Learned and Implications for the Future. American Society of Civil Engineers. New York, New York. Dalgliesh, W.A. and D.E. Allen, 1994. Tornado Damage in Aylmer, Quebec on August 4, 1994. Internal Report No. 669. National Research Council of Canada. Ottawa, Canada. Dorey, D. B. and Schriever, W. R. (1957) “Structural test on a house under simulated wind and snow loads.” Special tech. Pub. No. 210. American Society for Testing and Materials, Philadelphia, PA. Doudak, G., 2005. Field Determination and Modeling of Load Paths in Wood Light-Frame Structures, PhD thesis, McGill University, Montreal, QC. Douglas, B.K., 1992, Hurricane Andrew, Part 2 Wood Building Analysis and Recommendations. A Special Report of the National Forest Products Association Fischer, D., Filiatrault, A., Folz, B., Uang, C-M., and Seible, F. (2001) “Shake Table Tests of a Two-Story Wood-fame House. CUREE Publication No. W-06. CUREE, Richmond, CA. International Conference of Building Officials, 1994. Uniform Building Code. International Conference of Building Officials. Whittier, California. Keith, E.L. and J.D. Rose, 1992. Hurricane Andrew – Structural Performance of Buildings in Southern Florida (August 24, 1992). APA Report T92-21. American Plywood Association. Tacoma, Washington. Paevere, P. (2002) “Full-scale Testing, Modeling and Analysis of Light-Frame Structures Under Lateral Loading”, Ph.D. Thesis, Department of Civil and Environmental Engineering, University of Melbourne, Australia. Rainer, J.H., A.M. Jablonski, K.T. Law and D.E. Allen, 1990. Earthquake Damage in the San Francisco Area and Projection to Greater Vancouver. Canada Mortgage and Housing Corporation. Ottawa, Canada.
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NAHB Research Center, 1994. Assessment of Damage to Residential Buildings Caused by the Northridge Earthquake. U.S. Department of Housing and Urban Development, Office of Policy Development and Research. Rainer, J.H. and E. Karacabeyli, 1999. Performance of Wood frame Building Construction in Earthquakes. Forintek Canada Corp, Special Publication SP 40. Vancouver, Canada Sheffield, J.W. 1993. A Survey of Building Performance in Hurricane Iniki and Typhoon Omar. Hurricanes of 1992: Lessons Learned and Implications for the Future. American Society of Civil Engineers. New York, New York. Sparks, P.R. and S.A. Bhinderwala, 1993. Relationship Between Residential Insurance Losses and Wind Conditions in Hurricane Andrew. Hurricanes of 1992: Lessons Learned and Implications for the Future. American Society of Civil Engineers. New York, New York. UWO. 2002. “Mitigating housing losses in extreme natural events”, Proceedings of Workshop, 2 & 3 December 2002, Toronto, University of Western Ontario, ON, Canada (on CD). Wolfe, R.W., R.M. Riba and M. Triche. 1993. Wind Resistance of Conventional Light-Frame Buildings. Hurricanes of 1992: Lessons Learned and Implications for the Future. American Society of Civil Engineers. New York, New York.
