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RE: Alternative Load Combination in UBC97 (1612.3)

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Where live load/snow loads are low, I agree that the alternate method
should generally be more economical.  As long as you consider all of the
load cases under that section, then 'yes' you can use 1612.3.2 and
disregard 1612.3.1.  I don't know the reasons for the new basic load
combinations in 1612.3.1, but the combinations in 1612.3.2 were most likely
kept to allow a more traditional approach.  

> -----Original Message-----
> From: Soojin, Hur [mailto:sjhur(--nospam--at)hec.co.kr]
> Sent: Monday, September 13, 1999 12:51 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Alternative Load Combination in UBC97 (1612.3)
> 
> 
> 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
> 
> 
>