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Confused about Redundancy Factor for ASCE 7-05

Confused about Redundancy Factor for ASCE 7-05

Confused about Redundancy Factor for ASCE 7-05

(OP)
I'm still getting acquainted with California's adoption of ASCE 7-05, and have a hard time understanding the idea of the new redundancy factor calculations (ASCE 12.3.4.2).  Below is a couple of questions that I have in regards to the section, I'm hoping that someone can help me out.  

Table 12.3-3 states shear wall or wall pier with height to length ratio of less than 1 shall be removed and checked that it would not result in a 33% reduction of story strength (in order to maintain a rho of 1).  My understanding is that the code is asking me to eliminate a weaker pier/wall (in terms of rigidity) and check for the 33% reduction.  Shouldn't I be more concerned about eliminating a stronger pier/wall (i.e. wall to height ratio of greater than 1)?  Based on wall rigidity, a stronger wall/pier will take more shear force, and eliminating one of them will be more critical,... right?  Also, if my shear walls are all less than 1 in height to length ratio, does that automatically mean the rho is 1?  

I am also starting to get the impression that a rho of 1.3 will practically never occur for a structure with shear walls as the lateral resisting element.  For example, if we are assuming a simple box structure with a flexible diaphragm, the perimeter walls on each side will take 50% of the base shear.  Assume that there are only 2 shear walls at each side, this leads to 50% of the base shear * 50% for each shear wall.  If I am to eliminate one of them, that only leads to a 25% reduction of story strength.  So it seems that the only time rho of 1.3 applies is when there is only one shear wall on the perimeter of a building.

Sorry for the long post, but this section has got me scratching my head.  Any help or tips will be much appreciated.  Thank you.   

RE: Confused about Redundancy Factor for ASCE 7-05

I'm pretty sure the provision is written that way because the weaker wall will be the one that fails first.  They want you to check the L/H>1 walls because they'll still be there when the L/H<1 wall has failed.  The provision likely comes from field observations of how structures with multiple shear walls fail.



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