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.
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.
