ASCE 7-05 Increasing Factors (overstrength, redundancy)

[nr123]

I have been going through the ASCE 7-05 as I am starting to get into higher seismic areas. I think I have a grasp as to when the redundancy factor would be greater than 1.0 however I am somewhat confused as to when the overstrangth factor comes into play. I know that you would apply the overstrength factor to say a collector or that you would apply a 1.4 increase for elements of the wall anchorage system per 12.11.2.2.2, however Im unsure as to when else I would apply the overstrength factor that is given in table 12.2-1. Can anyone shine some light on the issue or any other increasing factor that I have not mentioned.

 

[Teguci]

Battered piles

 

[JAE]

The overstrength factor is used, as you say, with collectors. Also brace connections and diaphragms. Basically the intent is to ensure that rigid, brittle connections don't control the failure - that brace members go into yield prior to connections failing in rupture.

The code usually states that the overstrength factor would be applied only where it is specifically required.

 

[nr123]

JAE,

I didnt see anywhere that the overstength factor would apply to the diaphragm. When you say diaphragm are you referring to the attachment of the diaphragm to the steel supports?

 

[JStructsteel]

12.11.2.2. requires the load to be increased by 1.4 for Cat. C-F

 

[JAE]

The diaphragm requirement is rather cryptic (and might be argued both ways) but I found the reference through the AISC seismic requirements.

I've attached a memo that we used on another project and it is for a specific situation - but the logic flow might help you see why diaphragms could be considered as part of the seismic force resisting system: