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[SEAOC] Truss discussion.

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At 09:39 PM 8/3/96 -0700, you wrote:
>
>Question:  Have you ever been provided loads of the lateral bracing so that
>you could provide connections and resisting members by your local truss
>supplier?

Tom and Neil:

This is a good question and it has been fixed at least "on paper."  If
you will refer to WTCA 1-1995 (Standard Responsibilities in the Design
Process Involving Metal Plate Connected Wood Trusses), Section 6.0 Truss
Designer Responsibilities, paragraph 6.2 states "For each Truss Design Drawing,
set forth as a minimum the following:.......................................

6.2.11 Maximum axial compression forces in the Truss members to enable
the Building Designer to design the size, connections and anchorage of
permanent continous lateral bracing.  Forces may be shown on the Truss Design
Drawing or on supplemental Documents; and
6.2.12 Required permenent truss member bracing location.

So to calculate the lateral bracing loads, you need the information from
6.2.11 from the Truss Designer and a factor to convert from truss member
force to a required lateral restraining force. TPI literature has given
a 2% rule for many years; ie, "The force required to restrain the top chord
against buckling in the lateral directiom is assumed to be two percent
of the maximum axial force, per lateral brace, per truss braced."(Paragraph
H-13.1.2.1, HIB-91 Handling, Installing & Bracing Metal Plate Connected
Wood Trusses, Truss Plate Institute)  The TPI telephone number is
608-833-5900 if you would like to purchse HIB-91, a 109 page pocket size
manual.

The word "Maximum" in 6.2.11 is very important because trusses are now
analyzed under 3 or 4 load cases.  So--to design the lateral bracing,
you need the maximum compression in each member for ALL load cases used in
the truss design. Sincerely, Frank Woeste


Frank E. Woeste, Ph. D., P. E.
Biological Systems Engineering
Virginia Tech
Blacksburg, VA 24061-0303
Home:(540)951-0469
Office:(540)231-6093
FAX:(540)231-3199

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