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Seismic Beams

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I am fairly new in the steel building design department and I have a
general question
for you experts on the matter.  What is the purpose of a seismic beam in
a steel building
with a heavy gage roof sheathing?  Wouldn't a stiff angle do the same
job much cheaper?
I have analyzed some existing steel buildings with a deep (3 inch ribs)
26 gage pan deck
and found that the seismic beam used at the top of the CMU wall was much
less stiff
than the steel pan deck sheathing.  With the given purlin spacing and
attachment, it was
obvious that the sheathing would absorb the vast majority of the lateral
load from the wall
because of the relative stiffnesses involved.  Are there no provisions
for steel sheathing
diaphragm shear development?  I have many resources showing accepted
methods for
calculating the allowable steel diaphragm shear (given an immense amount
of data involved).
Couldn't one verify that the sheathing will absorb this load and thereby
release the need
for the seismic beam.  A simple angle could be used for a continuous
positive connection
as required by code.  I could be way off, but it seemed a worth while

Mark Pemberton, P.E.