I think this is a case of two codes out of step. The formula with the 1.5
factor on earthquake loads is copied verbatim from ACI 318, which even in
the 1999 edition still footnotes the source as ASCE 7-88. In ASCE 7-88, E is
calculated on an allowable load basis and then factored up for use in
ultimate strength load combinations. In the current version, ASCE 7-98, E is
calculated on an ultimate load basis and the factor in the same formula is
now only 1.0. It looks like an editorial lapse. When the code writers can't
even keep track, it makes me wonder about the rest us.
From: Dan Goodrich [mailto:dang(--nospam--at)karren.com]
Sent: Thursday, February 22, 2001 12:50 PM
Subject: Alternate load combinations
UBC 19184.108.40.206 has the following load combo:
5. 1.2D + 1.5E + (0.5L or 0.2S)
The combo in UBC 1612.2.1 has:
1.2D + 1.0E + (f1L + f2S)
Is the 1.5 factor on the seismic force correct in
chapter 19 combo? Why is it 50% larger?
Dan Goodrich, P.E.