Light-frame timber construction -- Comparison of four national design documents

ISO/TR 12910:2010 provides an introduction and synopsis of comparisons among the following four national design documents on light-frame timber (wood) construction: - AS 1684-1, AS 1684-2 and AS 1684-3; - the Engineering guide for wood-frame construction; - NZS 3604; - the Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings (Chapter 1: General information, Chapter 2: Engineered design and Chapter 3: Prescriptive design). Each of the four light-frame texts compared in ISO/TR 12910:2010 is based on a national timber design standard that includes provisions for assemblies and systems, which go beyond single-member design methodology.

Construction à ossature légère de bois -- Comparaison de quatre documents nationaux pour la conception

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TECHNICAL ISO/TR
REPORT 12910
First edition
2010-04-01
Light-frame timber construction —
Comparison of four national design
documents
Construction à ossature légère de bois — Comparaison de quatre
documents nationaux pour la conception
Reference number
ISO/TR 12910:2010(E)
ISO 2010
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ISO/TR 12910:2010(E)
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ii © ISO 2010 – All rights reserved
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ISO/TR 12910:2010(E)
Contents Page

Foreword .............................................................................................................................................................v

Introduction........................................................................................................................................................vi

1 Scope......................................................................................................................................................1

2 Document design principles ................................................................................................................1

2.1 Basic principles .....................................................................................................................................1

2.2 Compliance through pre-engineered solutions .................................................................................2

2.3 Design requirements matched to risk level........................................................................................2

2.4 Relationship to engineering design ....................................................................................................3

2.5 Evolving understanding of load-resistance interaction....................................................................3

3 Coverage and limitations......................................................................................................................3

3.1 General ...................................................................................................................................................3

3.2 Definition of light-frame construction .................................................................................................4

3.3 Prescriptive and performance-based approaches.............................................................................4

3.4 Integration of national code provisions..............................................................................................4

3.5 Scope of structural design ...................................................................................................................5

3.6 Beyond structural requirements, such as durability .........................................................................5

4 Loads and load factors .........................................................................................................................6

4.1 General ...................................................................................................................................................6

4.2 Basic design framework and philosophy ...........................................................................................6

4.3 Load comparisons.................................................................................................................................6

5 Material specifications..........................................................................................................................7

5.1 General ...................................................................................................................................................7

5.2 Product standards.................................................................................................................................7

5.3 Proprietary wood products...................................................................................................................7

6 Member design ......................................................................................................................................7

6.1 General ...................................................................................................................................................7

6.2 Member vs. system design...................................................................................................................8

6.3 Roofs.......................................................................................................................................................8

6.4 Walls .......................................................................................................................................................9

6.5 Floors......................................................................................................................................................9

7 Lateral load systems design ................................................................................................................9

7.1 General ...................................................................................................................................................9

7.2 Linking lateral design to analysis models ..........................................................................................9

7.3 Load action assumptions ...................................................................................................................10

7.4 Uplift design.........................................................................................................................................10

7.5 Racking and overturning design .......................................................................................................11

7.6 Building plan irregularities.................................................................................................................11

7.7 Wall openings ......................................................................................................................................11

7.8 Elevation irregularities........................................................................................................................12

8 Connection design ..............................................................................................................................12

8.1 General .................................................................................................................................................12

8.2 Fastener schedules .............................................................................................................................12

8.3 Proprietary connections .....................................................................................................................13

9 Other .....................................................................................................................................................13

9.1 Miscellaneous ......................................................................................................................................13

9.2 Definitions of terminology..................................................................................................................13

© ISO 2010 – All rights reserved iii
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ISO/TR 12910:2010(E)

Annex A (informative) Comparison chart .......................................................................................................15

Bibliography ......................................................................................................................................................50

iv © ISO 2010 – All rights reserved
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ISO/TR 12910:2010(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies

(ISO member bodies). The work of preparing International Standards is normally carried out through ISO

technical committees. Each member body interested in a subject for which a technical committee has been

established has the right to be represented on that committee. International organizations, governmental and

non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the

International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards

adopted by the technical committees are circulated to the member bodies for voting. Publication as an

International Standard requires approval by at least 75 % of the member bodies casting a vote.

In exceptional circumstances, when a technical committee has collected data of a different kind from that

which is normally published as an International Standard (“state of the art”, for example), it may decide by a

simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely

informative in nature and does not have to be reviewed until the data it provides are considered to be no

longer valid or useful.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent

rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO/TR 12910 was prepared by Technical Committee ISO/TC 165, Timber structures.
© ISO 2010 – All rights reserved v
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ISO/TR 12910:2010(E)
Introduction

Light-frame timber construction is the dominant construction practice for housing and other types of buildings

in some countries. In these countries, it has gained widespread acceptance due to its many benefits, including

ease of construction, cost-effectiveness, adaptation to energy efficient buildings and proven performance. This

Technical Report is intended to provide an overview of the common elements in existing national structural

design documents on light-frame timber construction.

The comparison chart (see Annex A) is intended to assist in that process. This Technical Report draws

attention to several common themes and identifies some differences in the documents reviewed.

vi © ISO 2010 – All rights reserved
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TECHNICAL REPORT ISO/TR 12910:2010(E)
Light-frame timber construction — Comparison of four national
design documents
1 Scope

This Technical Report provides an introduction and synopsis of comparisons among the following four national

design documents on light-frame timber (wood) construction:
a) AS 1684-1 (including AS 1684-1:1999/Amd.1:2002), AS 1684-2 and AS 1684-3;
b) the Engineering guide for wood-frame construction;
c) NZS 3604;

d) the Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings (Chapter 1: General

information, Chapter 2: Engineered design and Chapter 3: Prescriptive design).

Each of the four light-frame texts compared in this Technical Report is based on a national timber design

standard that includes provisions for assemblies and systems, which go beyond single-member design

methodology. Other jurisdictions also have similar design documents on light-frame timber construction.

Although not all jurisdictions have design documents on light-frame timber construction, timber design

standards typically address assemblies and systems (See 6.2).
2 Document design principles
2.1 Basic principles

AS 1684-1, AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction, NZS 3604 and the

Wood Frame Construction Manual each comply with a national code, which defines the higher level building

design principles and conditions that need to be met for light-frame timber buildings, including strength and

serviceability criteria, specified loads and material design performance.

In general, AS 1684-1, AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction, NZS 3604

and the Wood Frame Construction Manual (design documents) share basic principles related to

demonstrating how light-frame timber construction can comply with structural requirements, particularly how it

can resist high-wind and/or seismic loading conditions and provide additional guidance concerning system

design and construction methods.

At the same time, AS 1684-1, AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction,

NZS 3604 and the Wood Frame Construction Manual do not all seek to have the same coverage or topics

(see Clause 3). Some deal with housing only, others with housing and small buildings; a few with wood

structural design only and others with other structural components or design aspects of the building. These

differences complicate direct comparisons of the documents on a detailed level.
The following are some observations on general principles found.

1) The terms “wood” and “timber” are used interchangeably in this Technical Report.

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ISO/TR 12910:2010(E)
2.2 Compliance through pre-engineered solutions

To demonstrate how to comply with national code requirements for light-frame wood construction,

pre-engineered solutions and tables to accompany engineering design methods are included in AS 1684-1,

AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction, NZS 3604 and the Wood Frame

Construction Manual. This approach can be more conservative in some cases, but can also facilitate design

and regulatory checking, particularly in areas less familiar with the construction system.

The following are examples.

⎯ AU: AS 1684-1, AS 1684-2 and AS 1684-3 cover design of timber-framed construction conforming to

loading and performance requirements for Class 1 (housing or hostels) and Class 10 (non-habitable

structures) buildings as defined by the national code; less conservative designs can be permitted by other

building regulations or standards.

⎯ CA: The Engineering guide for wood-frame construction covers a subset of the national code — Part 9

(Housing and small buildings). Part 9 typically permits less conservative measures for housing and small

buildings, particularly for buildings subjected to significant lateral loading.

⎯ NZ: NZS 3604 provides acceptable “non-specific” design solutions to the performance-based national

code, including many detailed construction provisions.

⎯ US: The Wood Frame Construction Manual is referenced in US national codes for design of wood-frame

construction, particularly in higher wind and seismic conditions; other provisions permit less conservative

conventional construction or engineered designs.
2.3 Design requirements matched to risk level
2.3.1 General

AS 1684-1, AS 1684-2 and AS 1684-3, the Engineering guide for wood-frame construction, NZS 3604 and the

Wood Frame Construction Manual all, in one way or another, match design provisions to the type and

intensity of loading as well as vulnerability of the building structure. This can in some cases lead to design

discontinuities where special solutions are specified only for high- or low-risk areas, but also helps to provide a

better fit to the scale of the problem.
2.3.2 Examples

⎯ AU: Separate cyclonic and non-cyclonic documents deal with high- or low-risk cyclonic events. AS 1684-1,

AS 1684-2 and AS 1684-3 increase requirements for higher risk areas, with up to half of racking force

permitted to be resisted by nominal wall bracing; AS 1684-2 is published for simplified design in non-

cyclonic areas.

⎯ CA: Part C of the Engineering guide for wood-frame construction advises on applicability of solutions

under lateral loading conditions, based on spacing and location of braced walls; national codes permit

prescriptive construction that are more liberal than Part C of the Engineering guide for wood-frame

construction.

⎯ NZ: Prescriptive provisions include restrictions on bracing elements in terms of spacing, minimum

capacity and location; if spacing of bracing lines not evenly distributed, spacing is reduced to coincide

with a line of supporting members.

⎯ US: Separate “prescriptive design” document in tabular format is based on engineering principles;

national codes permit conventional construction in lower risk situations.
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ISO/TR 12910:2010(E)
2.4 Relationship to engineering design

AS 1684-1, AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction, NZS 3604 and the

Wood Frame Construction Manual share the objective of maintaining continuous load paths, but it is

recognized that they cannot fully replace the design process. Common themes include agreement that not all

aspects of the complete structure are fully addressed and conservative assumptions are made to present

efficient design aids. The documents are intended to be used principally by engineers or other competent

designers.
The following are examples.

⎯ AU: Intended to provide building industry with design procedures and details for use in cyclonic and

non-cyclonic areas; another simplified version (AS 1684-4) was published for non-designers.

⎯ CA: Intended to be used in conjunction with competent engineering design, as well as to provide

guidance to a wide range of the building community (e.g. builders, code officials).

⎯ NZ: Intended to be used by a wide range of the building industry, while recognizing that due to national

code requirements, users would be mainly fulfilling the role of a designer.

⎯ US: Intended to be used in conjunction with competent engineering design, providing guidance and

saving time for the design professional.
2.5 Evolving understanding of load-resistance interaction

There is widespread recognition of the complexity of load and resistance distribution and interaction in

light-frame construction systems under gravity and lateral loading patterns. AS 1684-1, AS 1684-2, AS 1684-3,

the Engineering guide for wood-frame construction, NZS 3604 and the Wood Frame Construction Manual

include varying approaches to incorporating system action provisions and calibrating to the long performance

history of these systems. These provisions are likely to evolve to include more advanced design tools as

knowledge grows.
The following are examples.

⎯ AU: Load distribution and strength-sharing effects are included in design methods for framing members,

including rafter, studs and joists.

⎯ CA: Design of rafters, studs, joists, headers and beams includes consideration of system action and

strength sharing; shearwall design is a mechanics-based approach with additional consideration of

shearwalls without hold-downs where sheathing is used to resist overturning and/or uplift.

⎯ NZ: Nominal bracing systems are provided with prescribed bracing capacities; performance-based design

(national code) permits use of alternative design solutions.

⎯ US: Wall stud design includes system factors, load-sharing increase factors provided for joists, rafters

and other members; shearwall design includes a mechanics-based approach for shearwalls with

hold-downs with additional consideration of the empirical based perforated shearwall model (without

hold-downs).
3 Coverage and limitations
3.1 General

The light-frame construction documents are limited in terms of building size, loads and other parameters.

These limits are dictated largely by national codes, yet committees also make many decisions on what to

include within the scope and limitations of light-frame construction documents.
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ISO/TR 12910:2010(E)
3.2 Definition of light-frame construction

AS 1684-1, AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction, NZS 3604 and the

Wood Frame Construction Manual all address small light-frame buildings constructed primarily of closely-

spaced repetitive wood-framing members connected together in roof, wall and floor systems. Construction

details are based on traditional domestic methods, and consequently, they differ to some extent on restrictions

on building height, width, size and occupancy. The differences are less significant than common elements of

structural design interest.
The following are examples.

⎯ AU: Maximum building of two stories above substructure, maximum 8,5 m height; storey height up to

3,0 m, except up to 3,6 m where increased loads considered in design.

⎯ CA: Building height not greater than three stories and maximum storey height not greater than 3,5 m.

⎯ NZ: Timber-framed buildings up to three stories high, total height not more than 10 m.

⎯ US: Building not more than three stories or a mean roof height of 10 m, maximum story height of 3,6 m

for engineered design or 3,0 m for prescriptive design.
3.3 Prescriptive and performance-based approaches

Design documents are formulated on the basis of either prescriptive or performance-based designs, or

combinations of both. Both approaches can take the form of pre-engineered solutions and it is sometimes

difficult to disentangle them in a standard. In general, one would expect prescriptive measures to be more

limited in application or more conservative than a fully engineered approach, but this is not always so.

The following are examples.

⎯ AU: Bracing walls with minimum shear capacity require nominal fixing only and prescriptive fixings are

provided for such cases; design criteria include system-based assumptions.

⎯ CA: Mechanics-based lateral design method supplemented by tables; prescriptive bracing provisions,

including consideration of non-structural elements included in Part C of the Engeneering guide for

wood-frame construction.

⎯ NZ: Prescriptive measures are based on tabulated bracing units equivalent to shear force (kN) per unit

length or unit area; detailed provisions depend on building location and climatic data.

⎯ US: Engineering lateral design method supplemented by prescriptive tables and specific framing details

based on engineering assumptions and calculations.
3.4 Integration of national code provisions

National building codes vary from country to country and can include other sections related to light-frame

construction, such as conventional construction provisions, loading and load factors, design principles and

methods. On the other hand, national codes cannot cover all that is needed to fully address light-frame

construction. Therefore, the documents either duplicate some of the national code provisions or make

reference to them. Duplication is more complete, but can also lead to errors, revisions and national code

interpretation questions.
The following are examples.

⎯ AU: AS 1684-1, AS 1684-2 and AS 1684-3 include many prescriptive and engineering design and loading

provisions, as well as reference to other documents forming part of the criteria for residential timber-

framed construction to show conformance to national code requirements.
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ISO/TR 12910:2010(E)

⎯ CA: National code includes separate provisions for both fully-engineered and prescriptive design of wood

buildings, therefore the Engineering guide for wood-frame construction includes selective provisions on

loads and design assumptions to ensure understanding of which are applicable it.

⎯ NZ: National code is a high-level performance-based document without specific provisions for light-frame

construction, therefore NZ 3604 includes all aspects of design for these types of buildings, but does not

include all load and design provisions.

⎯ US: National codes include specific provisions for prescriptive braced walls which may be used for

conventional construction, but these provisions are not included in the Wood Frame Construction Manual;

the Wood Frame Construction Manual includes design provisions with reference to the national code for

further load information.
3.5 Scope of structural design

Structural design of light-frame construction can include design of foundations, components, ancillary

buildings, hybrid concrete-wood construction and other types of structures. Some of the documents include

provisions for design aspects of these non-wood structures, while others are limited to the basic wood-frame

construction system. AS 1684-1, AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction,

NZS 3604 and the Wood Frame Construction Manual stress the need for load path continuity throughout the

structure.
The following are examples.

⎯ AU: AS 1684-1, AS 1684-2 and AS 1684-3 include detailed provisions for concrete foundations, piers,

stumps, columns and slabs; AS 1684-1, AS 1684-2 and AS 1684-3 also includes building practice and

procedures to assist in correct specification and construction procedures.

⎯ CA: The Engineering guide for wood-frame construction does not cover specific foundation design, but

includes forces and design provisions for attachment of walls to foundation and anchor bolt capacities; it

also contains detailed prescriptive provisions for connecting braced walls to foundation.

⎯ NZ: Includes detailed provisions for site requirements, foundations, expansive soils, slabs, piles, lintels,

claddings, linings and ceilings; concrete masonry walls included as part of framing system; some design

aspects covered by reference to other standards.

⎯ US: Does not cover specific foundation design, but includes forces and connection design provisions for

attachment of walls to foundation or crawl spaces, and anchor bolt capacities.
3.6 Beyond structural requirements, such as durability

Regulation of wood-frame construction does not end with structural requirements. Durability issues are of

equal concern and can also be linked to structural issues resulting from high-wind and seismic events, e.g.

corrosion, decay, mould and ventilation. Some of these documents are limited to structural design, while some

cover other aspects of national code requirements.
The following are examples.

⎯ AU: Includes durability provisions for timber, hardware, service conditions by reference to other standards

and informative appendices on durability classes and timber properties.

⎯ CA: Addresses structural requirements only; durability is outside scope, other than a general requirement

to consider decay resistance in design of the structure.

⎯ NZ: Includes section on durability covering timber, hardware, service conditions, claddings, underlay,

sheathing, concrete, sealants and flashings; it does not include full details related to protection of the

building envelope.

⎯ US: Addresses structural requirements only; it is not applicable to durability.

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ISO/TR 12910:2010(E)
4 Loads and load factors
4.1 General

Because loading is dictated largely by national codes, there are load differences between AS 1684-1,

AS 1684-2, AS 1684-3, the Engineering guide for wood-frame construction, NZS 3604 and the Wood Frame

Construction Manual. However, these differences are not so large as to change the nature of the light-frame

construction system or the demands placed on the system.
4.2 Basic design framework and philosophy

All except the Wood Frame Construction Manual are based on factored load and resistance Limit states

design format. While the Wood Frame Construction Manual is in unfactored or Working stress design format

in accordance with general US practice, the referenced US wood design standard is available in dual (factored

and unfactored) format. Another framework choice relates to decisions about design for load combinations,

including the probability of simultaneous occurrence of maximum design loads.
The following are examples.
⎯ AU: Loads and capacities based on Limit states design; calculations in
...

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