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Seismic Prov. Bracing Connections

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This issue may have come before, but I don't remember if
dealing exactly with the question I want to pose, right now.
The AISC Seismic Provisions state for bracing connections
in SCBF, ( 13.3 in the 97 version ) a required strength being the
lesser of :
a. The nominal axial tensile strength of the bracing member,
    determined as RyFyAg.
b.  The maximum force, indicated by analysis, that can be
     transferred to the brace by the system.
We know that usually b is the governing case, but where does
provision a come from ? Since Ag in most of the cases, results
from reasons other than the section capacity, when the a
provision is to be considered, this results in a large number of
bolts, enormous gousset plates, etc.
What are the real factors determining this provision ?
For small structures, the outcome of a may not be that important,
but when we are dealing with high rise, heavy industrial buildings,
the difference in between a and b may be dramatic.
Appreciate your comments/opinions.
Best Regards,
Raul Labbé S.E.