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Alternative Load Combination in UBC97 (1612.3)

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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


--- Begin Message ---
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



--- End Message ---