ASTM D6555-00a
(Guide)Guide for Evaluating System Effects in Repetitive-Member Wood Assemblies
Guide for Evaluating System Effects in Repetitive-Member Wood Assemblies
SCOPE
1.1 This guide identifies variables to consider when evaluating repetitive-member assembly performance for parallel framing systems.
1.2 This guide defines terms commonly used to describe interaction mechanisms.
1.3 This guide discusses general approaches to quantifying an assembly adjustment including limitations of methods and materials when evaluating repetitive-member assembly performance.
1.4 This guide does not detail the techniques for modeling or testing repetitive-member assembly performance.
1.5 The analysis and discussion presented in this guideline are based on the assumption that a means exists for distributing applied loads among adjacent, parallel supporting members of the system.
1.6 Evaluation of creep effects is beyond the scope of this guide.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
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Designation: D 6555 – 00a
Guide for
Evaluating System Effects in Repetitive-Member Wood
Assemblies
This standard is issued under the fixed designation D 6555; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The apparent stiffness and strength of repetitive-member wood assemblies is generally greater than
the stiffness and strength of the members in the assembly acting alone. The enhanced performance is
a result of load sharing, partial composite action, and residual capacity obtained through the joining
of members with sheathing or cladding, or by connections directly. The contributions of these effects
are quantified by comparing the response of a particular assembly under an applied load to the
response of the members of the assembly under the same load. This guide defines the individual effects
responsible for enhanced repetitive-member performance and provides general information on the
variables that should be considered in the evaluation of the magnitude of such performance.
The influence of load sharing, composite action and residual capacity on assembly performance
varies with assembly configuration and individual member properties, as well as other variables. The
relationship between such variables and the effects of load sharing and composite action is discussed
in engineering literature. Consensus committees have recognized design stress increases for
assemblies based on the contribution of these effects individually or on their combined effect.
The development of a standardized approach to recognize “system effects” in the design of
repetitive-member assemblies requires standardized analyses of the effects of assembly construction
and performance.
1. Scope 1.7 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This guide identifies variables to consider when evalu-
responsibility of the user of this standard to establish appro-
ating repetitive-member assembly performance for parallel
priate safety and health practices and determine the applica-
framing systems.
bility of regulatory limitations prior to use.
1.2 This guide defines terms commonly used to describe
interaction mechanisms.
2. Referenced Documents
1.3 This guide discusses general approaches to quantifying
2.1 ASTM Standards:
an assembly adjustment including limitations of methods and
D 245 Practice for Establishing Structural Grades and Re-
materials when evaluating repetitive-member assembly perfor-
lated Allowable Properties for Visually Graded Lumber
mance.
D 1990 Practice for Establishing Allowable Properties for
1.4 This guide does not detail the techniques for modeling
Visually-Graded Dimension Lumber from In-Grade Tests
or testing repetitive-member assembly performance.
of Full-Size Specimens
1.5 The analysis and discussion presented in this guideline
D 2915 Practice for Evaluating Allowable Properties for
are based on the assumption that a means exists for distributing
Grades of Structural Lumber
applied loads among adjacent, parallel supporting members of
D 5055 Specification for Establishing and Monitoring
the system.
Structural Capacities of Prefabricated Wood I-Joists
1.6 Evaluation of creep effects is beyond the scope of this
guide.
3. Terminology
3.1 Definitions:
This guide is under the jurisdiction of ASTM Committee D07 on Wood and is
the direct responsibility of Subcommittee D07.05 on Wood Assemblies.
Current edition approved Oct. 10, 2000. Published December 2000. Originally
published as D 6555 – 00. Last previous edition D 6555 – 00. Annual Book of ASTM Standards, Vol 04.10.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 6555 – 00a
3.1.1 composite action, n—interaction of two or more 5.1.1 Load sharing reduces apparent stiffness variability of
connected wood members that increases the effective section members within a given assembly. In general, member stiffness
properties over that determined for the individual members. variability results in a distribution of load that increases load on
3.1.2 element, n—a discrete physical piece of a member stiffer members and reduces load on more flexible members.
such as a truss chord. 5.1.2 A positive strength-stiffness correlation for members
3.1.3 global correlation, n—correlation of member proper- results in load sharing increases, which give the appearance of
ties based on analysis of property data representative of the higher strength for minimum strength members in an assembly
species or species group for a large defined area or region under uniform loads.
rather than mill-by-mill or lot-by-lot data. The area represented
NOTE 2—Positive correlations between modulus of elasticity and
may be defined by political, ecological, or other boundaries.
strength are generally observed in samples of “mill run” dimension
3.1.4 load sharing, n—distribution of load among adjacent,
lumber; however, no process is currently in place to ensure or improve the
parallel members in proportion to relative member stiffness.
correlation of these relationships on a grade-by-grade or lot-by-lot basis.
Where design values for a member grade are based on global values,
3.1.5 member, n—a structural wood element or elements
global correlations may be used with that grade when variability in the
such as studs, joists, rafters, tresses, that carry load directly to
stiffness of production lots is taken into account.
assembly supports. A member may consist of one element or
multiple elements.
5.1.3 Load sharing tends to increase as member stiffness
3.1.6 parallel framing system, n—a system of parallel variability increases and as transverse load-distributing ele-
framing members.
ment stiffness increases. Assembly capacity at first member
3.1.7 repetitive-member wood assembly, n—a system in failure is increased as member strength-stiffness correlation
which three or more members are joined using a transverse
increases.
load-distributing element.
NOTE 3—From a practical standpoint, the system performance due to
3.1.7.1 Discussion—Exception: Two-ply assemblies can be
load sharing is bounded by the minimum performance when the minimum
considered repetitive-member assemblies when the members
member in the assembly acts alone and by the maximum performance
are in direct side-by-side contact and are joined together by
when all members in the assembly achieve average performance.
mechanical connections or adhesives, or both, to distribute
5.2 Variables affecting Load Sharing Effects on Stiffness
load.
include:
3.1.8 residual capacity, n—ratio of the maximum assembly
5.2.1 Loading conditions,
capacity to the assembly capacity at first failure of an indi-
5.2.2 Member span, end conditions and support conditions,
vidual member or connection.
5.2.3 Member spacing,
3.1.9 sheathing gaps, n—interruptions in the continuity of a
5.2.4 Variability of member stiffness,
load-distributing element such as joints in sheathing or deck-
5.2.5 Ratio of average transverse load-distributing element
ing.
stiffness to average member stiffness,
3.1.10 transverse load-distributing elements, n—structural
5.2.6 Sheathing gaps,
components such as sheathing, siding and decking that support
5.2.7 Number of members,
and distribute load to members. Other components such as
5.2.8 Load-distributing element end conditions,
cross bridging, solid blocking, distributed ceiling strapping,
5.2.9 Lateral bracing, and
strongbacks, and connection systems may also distribute load
5.2.10 Attachment between members.
among members.
5.3 Variables affecting Load Sharing Effects on Strength
4. Significance and Use include:
5.3.1 Load sharing for stiffness (5.2),
4.1 This guide covers variables to be considered in the
5.3.2 Level of member strength-stiffness correlation.
evaluation of the performance of repetitive-member wood
assemblies. System performance is attributable to one or more
6. Composite Action
of the following effects:
4.1.1 load sharing,
6.1 Explanation of Composite Action:
4.1.2 composite action, or 6.1.1 For bending members, composite action results in
4.1.3 residual capacity.
increased flexural rigidity by increasing the effective moment
4.2 This guide is intended for use where design stress of inertia of the combined cross-section. The increased flexural
adjustments for repetitive-member assemblies are being devel-
rigidity results in a redistribution of stresses which usually
oped. results in increased strength.
4.3 This guide serves as a basis to evaluate design stress
6.1.2 Partial composite action is the result of a non-rigid
adjustments developed using analytical or empirical proce- connection between elements which allows interlayer slip
dures.
under load.
6.1.3 Composite action decreases as the rigidity of the
NOTE 1—Enhanced assembly performance due to intentional overde-
connection between the transverse load-distributing element
sign or the contribution of elements not considered in the design are
and the member decreases.
beyond the scope of this guide.
6.2 Variables affecting Composite Action Effects on Stiff-
5. Load-Sharing
ness include:
5.1 Explanation of Load-Sharing: 6.2.1 Loading conditions,
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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D 6555 – 00a
6.2.2 Load magnitude, 8.2 Analytical Method:
6.2.3 Member span,
8.2.1 System effects in repetitive-member wood assemblies
6.2.4 Member spacing,
shall be quantified using methods of mechanics and statistics.
6.2.5 Connection type and stiffness,
8.2.2 Each component of the system factor shall be consid-
6.2.6 Sheathing gap stiffness and location in transverse
ered.
load-distributing elements, and
8.2.3 Confirmation tests shall be conducted to verify ad-
6.2.7 Stiffness of members and transverse load-distributing
equacy of the derivation in 8.2.1 to compute force distribu-
elements (see 3.1.5).
tions. Tests shall cover the range of conditions (that is,
6.3 Variables affecting Composite Action Effects on
variables listed in 5.2, 5.3, 6.2, 6.3 and 7.2) anticipated in use.
Strength include:
If it is not possible to test the full range of conditions
6.3.1 Composite action for stiffness (6.2),
anticipated in use, the results of limited confirmation tests shall
6.3.2 Location of sheathing gaps along members.
be so reported and the application of such test results clearly
limited to the range of conditions represented by the tests.
7. Residual Capacity of the Assembly
Confirmation tests shall reflect the statistical assumptions of
7.1 Explanation of Residual Capacity
8.2.1.
7.1.1 Residual capacity is a function of load sharing and
composite action which occur after first member failure. As a
NOTE 6—When analyzing the results of confirmation tests, the user is
result, actual capacity of an assembly can be higher than
cautioned to differentiate between system effects in repetitive-member
wood assemblies that occur prior to first member failure and system
capacity at first member failure.
effects which occur after first member failure as a result of residual
NOTE 4—Residual capacity theoretically reduces the probability that a
capacity in the test assembly (See Section 7).
“weak-link” failure will propagate into progressive collapse of the
assembly. However, an initial failure under a gravity or similar type 8.2.4 If increased performance is to be based on material
loading may precipitate dynamic effects resulting in instantaneous col-
property variability, the effects of the property variability shall
lapse.
be included in the analysis.
7.1.2 Residual capacity does not reduce the probability of
8.2.4.1 For material properties which are assigned using
failure of a single member. In fact, the increased number of
global ingrade test data, the effects of the property variability,
members in an assembly reduces the expected load at which
including lot-by-lot variation, shall be accounted for through
first member failure (FMF) will occur (see Note 5). For some
Monte Carlo simulation using validated property distributions
specific assemblies, residual capacity from load sharing after
based on global ingrade test data (Practice D 1990).
FMF may reduce the probability of progressive collapse or
8.2.4.2 For material properties that are assigned using mill
catastrophic failure of the assembly.
specific data, the effects of the property variability shall be
accounted for using criteria upon which ongoing evaluation of
NOTE 5—Conventional engineering design criteria do not include
factors for residual capacity after FMF in the design of single structural
the material properties under consideration are based.
members. The increased probability of FMF with increased number of
8.2.5 Extrapolation of results beyond the limitations as-
members can be derived using probability theory and is not unique to
signed to the analysis of 8.2.1 is not permitted.
wood. The contribution of residual capacity should not be included in the
8.3 Empirical Method:
development of system factors unless it can be combined with load
sharing beyond FMF and assembly performance criteria which take into
8.3.1 System effects in repetitive-member wood assemblies
account general structural integrity requirements such as avoidance of
quantified using empirical test results shall be subject to the
progressive collapse (that is, increased safety factor, load factor, or
following limitations:
reliability index). Development of acceptable assembly criteria should
8.3.1.1 For qualification, a minimum of 28 assembly speci-
consider the desired reliability of the assembly.
mens shall be tested for a reference condition. Additional
7.2 Variables affecting Residual Capacity Effects on
samples containing 28 assembly specimens shall be tested for
Strength include:
additional loading and test conditions.
7.2.1 Loading conditions,
Exception: When system factors are limited to serviceability,
7.2.2 Load sharing,
the number of assembly tests need not exceed that required to
7.2.3 Composite action,
estimate the mean within 65 % with 75 % confidence.
7.2.4 Number and type of members,
7.2.5 Member ductility (brittle versus ductile),
NOTE 7—The minimum sample size of 28 was selected from Table 2 of
7.2.6 Connectio
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