ISO/TR 18267:2013
(Main)Timber structures — Review of design standards
Timber structures — Review of design standards
ISO/TR18267:2013 documents the findings of a review of design standards undertaken by ISO/TC 165 to assess how an ISO International Standard for the design of timber structures might be developed and whether development of such an International Standard should be initiated.
Structures en bois — Revue des normes de calculs
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 18267
First edition
2013-04-01
Timber structures — Review of
design standards
Structures en bois — Revue des normes de calculs
Reference number
ISO/TR 18267:2013(E)
©
ISO 2013
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ISO/TR 18267:2013(E)
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ISO/TR 18267:2013(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Current and draft ISO design standards . 1
2.1 Basis for design . 1
2.2 Scope and limitations . 1
2.3 Standard or guide . 2
2.4 Rules-based or engineered approach . 2
2.5 Material specifications . 2
2.6 Load provisions . 3
2.7 Assemblies or components . 4
2.8 Construction practice . 4
2.9 Commentary . 4
2.10 Relationship of the draft laminated bamboo structural design standard . 4
3 Adoption or use of these design standards . 5
4 Relationship to selected national timber standards . 5
4.1 Basis for design . 5
4.2 Scope and limitations . 6
4.3 Standard or guide . 6
4.4 Rules-based or engineered approach . 6
4.5 Material specifications . 6
4.6 Load provisions . 7
4.7 Assemblies or components . 8
4.8 Construction practice . 8
4.9 Commentary . 8
5 Considerations for an ISO/TC 165 design document. 8
6 Conclusions . 9
7 References .10
Annex A (informative) Factors, issues and options considered by ISO/TC 165 .12
Annex B (informative) Overview of ISO structural design documents .18
Annex C (informative) Outline for a Basic Level Standard .25
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ISO/TR 18267:2013(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2. www.iso.org/directives
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received. www.iso.org/patents
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
The committee responsible for this document is ISO/TC 165, Timber structures.
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ISO/TR 18267:2013(E)
Introduction
In the 1980s ISO Technical Committee 165 prepared and circulated a draft International Standard based
on a Structural Timber Design Code that was put together by a Working Group of CIB (International
Council for Building Research Studies and Documentation). The stated purpose of the ISO/TC 165 draft
and the CIB code was to “provide an agreed background for the national committees and international
bodies responsible for formulating timber design standards, to ensure a reasonable and consistent
quality of timber structures.” Neither document included safety factors, partial coefficients, or loads,
since they were considered to be the responsibility of national authorities.
Engineering material design standards demonstrate compliance with the structural requirements
of building codes; therefore, they have a special relationship to national codes and related legal
considerations and are even called “codes” in many places. Design standards provide regulatory bodies
with information needed to protect public safety.
The standards also provide a mechanism for adopting advances in structural modelling, material behaviour
science, reliability and other subjects. They are vehicles for introducing research into practice. They are
also a means for receiving feedback and modifying design requirements based on practical experiences.
Beyond this, design standards also create a framework for specifications of construction products. This
provides a context for recognition and approval of structural initiatives using referenced products.
In 2006 ISO/TC 165 prepared a business plan that identified “standards to support harmonization of
design and building codes internationally” as a Committee priority. Around the same time, an ISO/TC 165
Framework document included design requirements on the list of work areas, although it did not specify
what those requirements might include.
ISO/TC 165, at its 25th meeting in 2011, reviewed a report on current national design standards for timber
and international design standards for other construction materials. The committee concluded that, at this
time, there was not sufficient interest to undertake development of a structural timber design standard.
However, the information provided was deemed to be a valuable resource that should be formally retained
for use by ISO/TC 165 and member bodies in future development of timber design standards.
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TECHNICAL REPORT ISO/TR 18267:2013(E)
Timber structures — Review of design standards
1 Scope
This Technical Report documents the findings of a review of design standards undertaken by ISO/TC 165
to assess how an ISO International Standard for the design of timber structures might be developed and
whether development of such an International Standard should be initiated.
NOTE Annex A summarizes the factors considered and the activity of the TC leading to the preparation of
this Technical Report.
2 Current and draft ISO design standards
The five structural design standards and technical reports shown in the following table are published
ISO structural design documents. They provide a selection of different approaches for review to help
understand what is involved in preparing an international design standard and what the end product
might look like.
Designation Title
ISO/TR 11069:1995 Aluminium structures — Material and design — Ultimate limit state under static
loading
ISO 10721-1:1997 Steel structures — Part 1: Materials and design
ISO 9652-2:2000 Masonry — Part 2: Unreinforced masonry design by simple rules
ISO 15673:2005 Guidelines for the simplified design of structural reinforced concrete for buildings
(ISO/TC 71)
ISO 22156:2004 Bamboo — Structural Design (ISO/TC 165)
2.1 Basis for design
All structural design standards are based on a design philosophy that defines the relationship between
load requirements and material resistances. International standard committees make basic choices on
ground rules, leading to different design approaches in these standards.
Most of the reviewed ISO documents are established in Limit States Design format. It is instructive to
see how the limit states are defined in each document, influenced to some extent by material-specific
considerations. Limit States Design format also includes partial coefficients in the calculation of loading
and resistance, and these factors are handled differently (or are unspecified) in the documents.
Although quantified reliability is implied by Limit States Design philosophy, none of the documents
quantify reliability targets or indices. The overall view is that the subject is too dependent on regional
conditions and applications to be defined in an international standard and so is left to the discretion of
the adopting country or region.
Serviceability Limit States are even more dependent on regional cases and conditions than Strength or
Ultimate Limit States, and are largely left out of these documents.
Annex B of this report provides a short summary for the reviewed documents.
2.2 Scope and limitations
In each of the reviewed ISO documents, the scope and limitations give an indication of intended purpose
and audience. Since the general aim of material design standards is to facilitate engineering design of
structures, the target audience is taken to be the professional community qualified in this practice.
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In some cases, the scope of a standard is limited to make the task more manageable, or to make room for
design aids such as rules-of-thumb or other tools for the purpose of design simplification. Simplification
can be a particular priority where the primary audience is very diverse or is located in countries that do
not have existing national standards for the purpose at hand.
It should be noted, however, that simplifying assumptions or design aids do not necessarily result in
“simple” standards. There is always a need for professional engineering knowledge and interpretation
of design requirements. In other words, scope limitations do not necessarily diminish the need for
complete structural design.
In some cases, the limitations may be defined in terms of the geometry or layout of structures; in other cases,
limitations are defined by the extent or application of design requirements (see Annex B for examples).
2.3 Standard or guide
Some of the ISO documents under review are written to guide as well as to standardize design
requirements. Some put more emphasis on the guidance, others on the normative function.
Design shortcuts are provided in some cases where a more comprehensive design procedure would
result in smaller sections. This is generally the role of a handbook or guide rather than a Standard;
however, in the case of an international standard, there may be no handbook or guide to consult. Design
shortcuts are included to simplify or facilitate the presentation, or to ensure that a requirement is not
overlooked or misinterpreted.
Terminology used in the documents to express mandatory or non-mandatory actions can provide a
clue as to which provisions are intended to “advise” or “guide” practice rather than “regulate” it. From
an implementation standpoint, standards written in mandatory language with specific quantifiable
requirements are easier to enforce. But this is difficult to achieve in a standard that is intended to be
applied across national boundaries (see Annex B for examples).
2.4 Rules-based or engineered approach
The reviewed ISO documents, like most material design standards, are intended for use by engineers and
are typically formulated on the basis of principles and assumptions that lead to calculated design solutions.
Prescriptive rules-based standards, on the other hand, are sometimes written for a non-engineering
audience and are formulated to be carried out without further structural analysis or calculations. In
the process, rules-based standards can yield more conservative solutions than would result from a fully
engineered approach. Also, rules-based approaches are limited in applicability due to assumptions that
don’t fit all cases.
From time to time, a rules-based engineering standard is developed for one of a number of reasons, such
as: a) to illustrate a particular type of structural design problem and solution, b) to provide a readily-
enforceable document for building regulators, or c) to provide a baseline for acceptable solutions.
Most standards combine elements of prescriptive and performance-based standards writing, and
some include provision for interpolating or extrapolating beyond the scope of rules-based provisions.
Standards can be placed on a scale ranging from fully-engineered to rules-based provisions (see Annex B
for examples).
2.5 Material specifications
Before a product can be utilized in design, it must be clearly defined and the structural performance
characteristics of the product defined. Product definition is generally done through material
specifications and performance characteristics established by testing and evaluation protocols.
Design standards provide a framework for referencing standards for construction products because
it is preferable to make normative references to specification or testing standards for this purpose.
International standards have special challenges in this area.
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Although ISO material specification and testing standards are referenced where available, the general
trend in the reviewed documents seems to be to recognize that national material standards are at least
as likely to be used and are referenced as well. This is done even though the specifications and the related
properties may be based on standard dimensions and grade qualities that may not be recognized in all
countries (see Annex B).
A further complication is the number of proprietary structural products that have emerged in recent
years. Proprietary products can play a significant role in current design practice, which can vary
regionally and over time. Generally such products have not been addressed specifically in the reviewed
ISO standards.
Unless the design standard is to become an omnibus standard addressing many different subjects, it
is preferable to have a separate referenceable standard to deal with basic material properties and the
development of characteristic values. This helps to relieve the design standard from the need to address
many material specification issues.
2.6 Load provisions
Load requirements are the domain of national building codes and standards, and it seems
counterproductive for an international ISO design standard to specify load provisions incompatible with
those that are already in use in interested countries. Complete load requirements include the factors or
coefficients that are used to calculate load actions, as well as parameters or references for specified loads.
Nevertheless, some aspects of load requirements are inseparable from the design provisions in these
standards. Examples include the limitations to application of a standard, load interaction effects. In the
case of timber or timber-based products, there is also the special duration of load effect that ties load
and resistance effects together.
Lateral load design complicates design standards significantly, and some standards avoid the topic
entirely. Others provide partial lateral load design requirements (Annex B).
The ISO TC responsible for Bases of Design, TC 98, has developed a number of load standards, some of
which are referenced in the design standards as listed below.
ISO 15673 (concrete) references:
ISO 4354, Wind actions on structures
ISO 2103, Loads due to use and occupancy in residential and public buildings
ISO 2633, Determination of imposed floor loads in production buildings and warehouses
ISO 3010, Basis for design of structures — Seismic actions on structures
ISO 4355, Bases for design of structures — Determination of snow loads on roofs
ISO/TR 11069 (aluminium) references:
ISO 2394, General principles on reliability for structures
ISO 3898, Bases for design of structures — Notations — General symbols
ISO 8930, General principles on reliability for structures — List of equivalent terms
ISO 10721-1 (steel) references:
ISO 2394, General principles on reliability of structures
ISO 3898, Bases for design of structures — Notations — General symbols
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2.7 Assemblies or components
Most of the reviewed ISO documents address structural members and typical connections, focusing on
the most basic and generally applicable design cases. The material behaviour formulae are based on
generally accepted models, and some of these are specific to material types and connections. In general,
the standards do not address more complex structural assemblies or components.
Connection design can be quite involved and complicates even a limited-scope standard. Today’s practice
also includes proprietary types of connections and designs, which have not been included specifically in
the reviewed documents.
Although a few of the standards address specific types of applications, or assembly design procedures,
none of them go into detail (Annex B).
2.8 Construction practice
The reviewed ISO documents include general comments on construction and workmanship, without
going into much detail. Construction practice requirements are not always included in national design
standards, and it is even more difficult to develop construction requirements that would be applicable
across different countries or regions.
In some cases, the reviewed standards refer to other documents that cover construction and fabrication
details (Annex B).
2.9 Commentary
The reviewed ISO documents include varying amounts of commentary, ranging from very little to
more than half of the document. In general, commentary is an important part of any structural design
standard because the user needs to understand the principles behind the requirements, and standards
are written to be enforceable rather than easy to understand. In the case of an international standard,
however, the need for commentary may be overlooked (Annex B).
2.10 Relationship of the draft laminated bamboo structural design standard
The Draft design code for laminated bamboo submitted by INBAR in 2010 is in most cases identical to
the existing bamboo standard (ISO 22156). A few differences of note in the laminated bamboo draft
are as follows:
• Terms and definitions are added for glubam, plybamboo and lamination joint.
• Design by “concepts based otherwise” are permitted in lieu of calculations, as in the bamboo standard,
although design based on “experience from appropriate timber construction requirements” has
been substituted for design based on “previous generations.”
• The formula for calculating characteristic values from test data are retained, with the suggestion
that a smaller sample size is possible for glubam.
• The factor of safety and duration of load factors are retained, including the same assumption about
standard deviation.
• The design assumptions have been amended with respect to joints, initial curvature in shape, and
shear stresses.
• A new section on “shear walls,” defined as panels formed with framing elements and sheathing
panels, has been added.
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3 Adoption or use of these design standards
As discussed in the Introduction, a draft International Standard (designated N95) was prepared in the
1980s based on the CIB Structural Timber Design Code. The CIB document received input from both
ISO and CIB committees, and a revised version was eventually adopted as the European Structural
Timber Code (Eurocode 5). Later in the 1990s, ISO TC 165 agreed that work on the draft (N95) should be
suspended until Eurocode 5 was finished. No further activity has been recorded.
The most recent “systematic review” of current ISO material design standards resulted in two positive
responses on the question of whether they had been adopted: Austria for ISO/TR 11069 (aluminium)
and Colombia for ISO 15673 (concrete). A few of the other countries indicated that they made use of one
or more of the design standards in some way, without adoption.
In the ballot responses, there were a few comments coming mainly from European countries, indicating
lack of interest at the national level or suggesting the standards be modelled after Eurocodes. A
lengthy comment from Germany noted the intensive effort required by Eurocodes to reconcile national
differences, and indicated that ISO work had been put on “standby” due to priorities. This comment went
on to say: “Now this work is complete and it is time to think about the next step in the internationalization
of masonry construction and about applying the experience gathered by others. It is also an opportunity
to streamline the work on and for masonry and its worldwide application.”
4 Relationship to selected national timber standards
The next step is to compare and contrast the nature of some current national timber design standards in
the same terms that have been discussed for the ISO standards. The timber design documents shown in
the following table are all long-established standards that have been adopted and used widely in design
practice. These standards are referred to here as “national” standards even though the Eurocodes are
applied beyond national boundaries.
Reference Designation Title
NDS ANSI/AF&PA NDS (2005) National Design Specification for Wood Construction ASD/LRFD
CSA O86 CSA O86-09 (2009) Engineering Design in Wood
EC 5 Eurocode 5 Design of Timber Structures, Part 1-1: General, Common rules and
(EN 1995-1-1:2004+A1) rules for buildings
4.1 Basis for design
All of the above timber design standards include Limit States Design (or Load and Resistance Factored
Design) requirements. The NDS also provides design requirements in Working Stress Design (or
Allowable Stress Design) format, which is more commonly used in the US.
Target reliabilities and partial coefficients for loading and resistance in these standards are different,
since they are determined by code bodies in various countries and are influenced by other engineering
materials besides timber. The Foreword to Eurocode 5 notes that the same partial coefficients and
reliability parameters have to be assigned when the standard is used as a base document by other TCs.
CSA O86 states that any load requirements other than those given in the standard would necessitate
review of the appropriateness of applicable factored resistance values. The NDS refers to the applicable
building code or, in the absence of a governing code, the ASCE standard on minimum design loads.
Serviceability limit states are also very important for timber design, perhaps more than for other
structural materials, and are treated in detail in these standards. EC 5 addresses serviceability criteria
for beam deflection, floor vibration and joint slip; CSA O86 addresses beam deflection and floor vibration;
NDS addresses beam deflection. Details of deflection provisions are significantly different in the three
standards, although the overall goal of preventing unacceptable deformation is the same.
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4.2 Scope and limitations
All of the national timber standards are designed to address fully engineered structures, regardless
of occupancy or size. In general, the scope of these standards is not limited for purpose of design
simplification, since there are many design tools like handbooks and software to aid in interpreting and
applying the design requirements of each standard. Where limitations are imposed, they are typically
related to material properties or environmental conditions.
In addition to purely structural concerns, the standards include some provisions for durability and fire
resistance. While not the main subject of the standards, selected requirements have been included in
recognition that structural integrity can be impacted by these issues. EC 5 has a separate section (1995-1-
2) for structural fire design, and the NDS has a section on design procedures for exposed timber members.
From time to time, some timber committees have considered the possibility of creating a “two-level”
standard: one level for a novice or occasional timber designer, and a higher level for an experienced
timber designer. The typical reason for this suggestion is that novice designers find some aspects too
complicated or unfamiliar for occasional use. A two-level approach could facilitate design at the basic or
lower
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