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Re: Dual System (OBF/OMF)

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In a message dated Thu, 11 Oct 2001  6:52:14 PM Eastern Daylight Time, "D W" <lateral(--nospam--at)> writes:

> Why is the OBF/OMF Dual system penalized with an R(4.2) lower than the OMF R(4.5)? The dual system requires that the moment frames "be designed to independently resist at least 25% of the design base shear." Does this mean that two moment frames on opposite sides of the building are designed to resist a total of 25% of the load regardless of the braced frame stiffness?<


You've got a good point in regrad to the R factor. It makes no sense to assign a lower R to a dual system using the same type of moment frame. A dual system certainly has more redundancy and should get the benefit of that. 

In regard to your second question; the answer is yes, the moment frames have to independently be able to resist 25% of the load. Keep in mind that to verify this , you need to take out the braced frames and analyze the building as if they do not exist. However, you check the remaining moment frames for only 25% of the base shear. the reason for this exercise is that loads get distributed differently with a rigid diaphragm if you leave everythinbg in the model. and that is not consistent with the intent of this provision.

Ben Yousefi, SE
San Jose, CA

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