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Re: Seismic Beams

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Jessica Pemberton wrote:
> 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
> question.

The seismic beam that you refer to, can  be acted as chord beam of the
diaphragm, steel deck or steel angle is too slender for compression. 
Instead of chord beam, some engineers add additional continuous bars in
the CMU wall to act as chord beams also.


Tom Chiu, SE