AISC (ASD) Allowable Stress Increase for Wind/Seismic 1

[PVGuy]

It is my understand that the newest version of ASD no longer allows for a 1/3 increase in the allowable stress for the member in question when wind/seismic loading is applied. Is this the case or is there more context to this?

 

[UcfSE]

The 1/3 increase was removed with the 2001 supplement to the 1989 ASD 9th edition spec.

 

[PVGuy]

UcfSE,

Thankyou for the quick answer. I do not delve into the AISC code often so it is good to know this stuff when I need to.

 

[Taro]

It should be noted that they didn't just remove the 1/3 increase by itself. It goes along with the newer ASCE 7 load combinations that include a 0.75 load factor on load combinations that include more than one transient load. An interesting history lesson on the subject is Ellifritt's article "The Mysterious 1/3 Stress Increase" in the AISC Engineering Journal, 4th Quarter 1977.

 

[VoyageofDiscovery]

0.75 on the load side is the same as 1/3 increase on the capacity side. So is this really a change?

VOD

 

[UcfSE]

It is not the same. The 1/3 increase was applied to stresses due to all types of load, including dead load. The 0.75 factor appearing in some of the ASCE equations only applies to two or more transient loads occuring at the same time, such as full live and full wind plus dead. Combinations such as dead and wind now receive no load reduction and no stress increase.

 

[mrMikee]

Here's some more discussion about this.. thread507-142825

-Mike

 

[PVGuy]

mrMikee,

Thanks for the thread.

 

[Bagman2524]

ASCE 7-98 sec 2.4.3 allows the 0.75 reduction with two transient loads, but if the loading with only one of the transient loads without the reduction produces more severe effect, then that load combination governs.