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Re: Alternative Load Combination in UBC97 (1612.3)

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I'm not familiar with UBC, but ASCE7, from which (I think) UBC got its load
combinations, specifically forbids the 1/3 allowable increase.

Phil


"Soojin, Hur" wrote:

> I am designing a pipe rack structure for gas pipe lines to UBC97 and
> AISC ASD.
>
> As per UBC97, there are two methods in load combination for ASD
> especially combined with wind or earthquake load as follows:
>
> Chapter 1612.3.1 : 1) D + (W or E/1.4)
>                    2) 0.9D + E/1.4
>                    3) D + 0.75[L + (Lr or S) + (W or E/1.4)]
> Chapter 1612.3.2 : 1) D + L + (W or E/1.4)
>                    2) 0.9D + E/1.4
>                    3) D + L + S + E/1.4
>
> As a one-third allowable stress increase is permitted only in case of
> chapter 1612.3.2, I think the later case always result in more
> economical solution than 1612.3.1. If I am correct, may I disregard
> 1612.3.1 for my pipe rack design? What is the background reason for the
> provision  of the captioned two alternative methods in UBC97?
>
> Thanks for your help in advance.
>
> Soojin, Hur