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Re: Wood Moment Frame in historical building - retrofit

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JZ,

Wood bow-string trusses are generally designed to very efficiently support
distributed gravity loads applied at the roof.  For  loads of any other
kind, they become quite fragile.  Any loading that reverses the direction of
load in an individual member from compression to tension, or from
compression to tension, or that puts bending into a member, is likely to
lead to bad news.

I would advise against in any way including a bowstring truss in a seismic
lateral load resisting path.  If there is no alternative [for example if the
choice is between doing nothing or doing something that includes the trusses
in a retrofit seismic load resisting system], I would use every truss in the
system to drag load from the diaphragm to each end of each truss, into
vertical lateral load resisting elements that are fixed at the base.  I
would not try to use a truss as a girder that provides fixity for the top of
a frame column.  If you can accomplish this, lateral load will not be
accumulated into a single truss, and will not be concentrated into one end
of a truss.  The load delivered to a vertical lateral load resisting element
should not be great enough to overcome the tension in the bottom chord or
the compression in the top chord at the heel of the truss -- if this cannot
be accomplished, I think you have to find another way.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net


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