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Rigid Diaph. Analysis

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I am presently designing a two story Type V, (steel/wood), commercial building which has blocked plywood roof and floor diaphragms and a combination of lateral resistive elements; plywood shear walls, OMRF's, and at the center of the building in one direction, a two story chevron braced frame, (2 bays). I have used an R of 4.5 due to the mixed use of lateral resistive elements, and of course, the requirements of the '97 UBC.
 
My problem is, in using an "envelope" solution for the lateral forces, the braced frame due to it's stiffness, takes almost the entire lateral load in one direction when the diaphragms are assumed rigid. I know, from a practical stand point, that due to the rigidity of the braced frame in comparison to the OMRF's and plywood shear walls it will take more load, but taking almost the entire load causes all sorts of problems such as increasing the rho, overturning, collector forces, etc. In relationship to the stiffness of the braced frame, the diaphragm should be considered flexible, but in relationship to the other elements, rigid. Is there some rationalization, code exception, or modeling, that would allow me to be more practical in how much load this braced frame will take?
 
Thanks in advance,
 
Larry Hauer SE