jdgengineer
Structural
- Dec 1, 2011
- 748
I have a basic question that I don't think is explained well in the code that I wanted to see how you all handle it. In my opinion it appears that the redundancy factor check in ASCE 7 is most applicable to rigid diaphragm type buildings and that some intrepation is needed to apply it to a flexible diaphragm.
I don't have the code in front of me but I believe for a shearwall type building not meeting the prescriptive requirements (certain number of bays of walls at each exterior wall) that the code requires that you remove walls with a high aspect ratio one at a time to see if you either reduce the story shear strength by a certain percentage (I think 33%) or if you create an extreme torsional irregularity. As torsional irregularities don't apply to flexible diaphragms I have seen some engineers neglect this portion of the check which I don't think is the way to approach it.
The way I see it there are a variety of possible ways to implement it and wanted to see what others do.
1) apply redundancy factor on a line by line basis (I don't think that this is the intent of the code)
2) treat the diaphragm as rigid for the purposes of checking redundancy (this doesn't seem appropriate)
3) ignore the torsional irregularity check and instead check to make sure that the diaphragm is stable (ie walls on all 4 sides - it's a flexible diaphragm so 3 sided doesn't work) and that you don't exceed allowable cantilever lengths at the exterior walls (15% of the diaphragm length or whatever it is). This seems to make sense to me. In a complicated roof layout it's possible you may have several roof diaphragms. This redundancy check would then be done independently at each roof diaphragm. I suppose conservatively you could use 1.3 if any of the roof diaphragms fail or possiblly consider a different redundancy factor for each roof plane (I don't think this is the intent with the way the code is written but seems to make sense)
What do you all do? My office typically nearly always consider redundancy of 1.3 based on an approach similar to 3) described above. Essentially, nearly any residential flexible diaphragm building we work on has multiple roof planes and generally at least one of them will have an unstable diaphragm if one of the walls is removed so we conservatively use redundancy if 1.3
I don't have the code in front of me but I believe for a shearwall type building not meeting the prescriptive requirements (certain number of bays of walls at each exterior wall) that the code requires that you remove walls with a high aspect ratio one at a time to see if you either reduce the story shear strength by a certain percentage (I think 33%) or if you create an extreme torsional irregularity. As torsional irregularities don't apply to flexible diaphragms I have seen some engineers neglect this portion of the check which I don't think is the way to approach it.
The way I see it there are a variety of possible ways to implement it and wanted to see what others do.
1) apply redundancy factor on a line by line basis (I don't think that this is the intent of the code)
2) treat the diaphragm as rigid for the purposes of checking redundancy (this doesn't seem appropriate)
3) ignore the torsional irregularity check and instead check to make sure that the diaphragm is stable (ie walls on all 4 sides - it's a flexible diaphragm so 3 sided doesn't work) and that you don't exceed allowable cantilever lengths at the exterior walls (15% of the diaphragm length or whatever it is). This seems to make sense to me. In a complicated roof layout it's possible you may have several roof diaphragms. This redundancy check would then be done independently at each roof diaphragm. I suppose conservatively you could use 1.3 if any of the roof diaphragms fail or possiblly consider a different redundancy factor for each roof plane (I don't think this is the intent with the way the code is written but seems to make sense)
What do you all do? My office typically nearly always consider redundancy of 1.3 based on an approach similar to 3) described above. Essentially, nearly any residential flexible diaphragm building we work on has multiple roof planes and generally at least one of them will have an unstable diaphragm if one of the walls is removed so we conservatively use redundancy if 1.3
