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Seismic Provisions

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Dear Mr. Turner:

A typical chevron-braced (or stacked chevron-braced) structure will not be able to withstand base shears
that will cause tensile yielding; the beam tends to yield after one of the braces buckles.  Thus the maximum
force that a connection undergoes may be close to the buckling load of the brace.  Note that this is not true
if the beam is very strong, nor if the chevron is paired with an inverted chevron, making a two-story "X."  

You did not state the type of structure (or structures) you are designing, nor the exact location.  I understand
that even in some areas of moderate seismicity (that is, areas exempt from the almost-yearly rattling of the
Chardonnay glasses that we experience in northern California), the intensity of the maximum credible
earthquake is not far below that of ours.  It may therefore be necessary to consider your performance goals.  
If you are designing a hospital or a 911 switching station, I would suggest that neither tension-only bracing 
nor ordinary chevron-braced frames should be used.

As you have realized, there are difficulties taken on when one attempts to design a system that will perform
well in both the elastic and, should it be necessary, in the post-elastic range.  In order to function as a fuse,
the brace must be the weakest link in the force path.  Any increase in its capacity should be followed through 
to connections, collectors, etc., so arbitrary increases should be avoided.  Of course, designing the braces to 
an extremely low capacity may result in a building with the distinction of being the only one in town to have 
buckled braces.

Designing the system as a whole design is harder, specifically harder than your competitors may be working.  
In areas of high seismicity this work is appreciated by some clients, at least to a degree; elsewhere I'm sure 
it's not the case.  And that is how codes can be useful.

Rafael Sabelli