ASD/LRFD Manual - American Wood Council

ASD/LRFD MANUAL FOR ENGINEERED Wood Construction
131
Metal Plate Connected Wood Trusses
Generally, a fire endurance rating of 1-hour is mandated
by code for many of the applications where trusses
could be used. All testing on these assemblies is performed
in accordance with the ASTM’s Standard Methods for
Fire Tests of Building Construction and Materials (ASTM
E119).
The two primary source documents for fire endurance
assembly results are the Fire Resistance Design Manual,
published by the Gypsum Association (GA) and the Fire
Resistance Directory, published by Underwriters’ Laboratories,
Inc. (UL). Warnock Hersey (WH) assemblies are
now listed in the ITS Directory of Listed Products. These
tested assemblies are available for specification by Architects
or Building Designers, and for use by all Truss
Manufacturers where a rated assembly is required, and
can generally be applied to both floor and roof assembly
applications.
According to the UL Directory’s Design Information
Section: “Ratings shown on individual designs apply to
equal or greater height or thickness of the assembly, and to
larger structural members, when both size and weight are
equal or larger than specified, and when the thickness of
the flanges, web or diameter of chords is equal or greater.”
Thus, larger and deeper trusses can be used under the auspices
of the same design number. This approach has often
been used in roof truss applications since roof trusses are
usually much deeper than the tested assemblies.
Thermal and/or acoustical considerations at times
may require the installation of insulation in a floor-ceiling
or roof-ceiling assembly that has been tested without
insulation. As a general ‘rule,’ experience indicates that
it is allowable to add insulation to an assembly, provided
that the depth of the truss is increased by the depth of the
insulation. And as a general ‘rule,’ assemblies that were
tested with insulation may have the insulation removed.
To make a rational assessment of any modification
to a tested assembly, one must look at the properties of
the insulation and the impact that its placement inside the
assembly will have on the fire endurance performance
of the assembly. Insulation retards the transfer of heat, is
used to retain heat in warm places, and reduces the flow
of heat into colder areas. As a result, its addition to a fire
endurance assembly will affect the flow of heat through
and within an assembly. One potential effect of insulation
placed directly on the gypsum board is to retard the dissipation
of heat through the assembly, concentrating heat
in the protective gypsum board.
In some cases specific branded products are listed in
the test specifications. Modifications or substitutions to
fire endurance assemblies should be reviewed with the
building designer and code official, preferably with the assistance
of a professional engineer. This review is required
because the final performance of the assembly is a result
of the composite of the materials used in the construction
of the assembly.
The following pages, courtesy of WTCA – Representing
the Structural Building Components Industry, include
brief summaries of wood truss fire endurance assemblies
and sound transmission ratings. For more information, visit
www.sbcindustry.com. Also, several truss plate manufacturers
have developed proprietary fire resistant assemblies.
These results apply only to the specific manufacturer’s
truss plates and referenced fire endurance assembly system.
For more detailed information on these assemblies, the
individual truss plate manufacturer should be contacted.
Complete specifications on the UL, GA, and WH assemblies
are available at their respective websites.
M16: FIRE DESIGN
16
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