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RE: Question on ASCE 7-02 Load Combinations

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Wow! I'm not sure the text in those sections in Chapter 9 could be any more
confusing of what I think is a fairly simple concept. Self-criticism, of
course, since I probably voted for the language I just criticized when ASCE
7 was balloted.

In both Sections 9.5.2.7 and 9.5.2.7.1, there are two equations for the
amplified seismic load. In each of the first, you have a function of QE plus
0.2SDSD. In each of the second, you have a function of QE minus 0.2SDSD. The
text tells you (in many, many words) to substitute the first for E in load
combinations in Sections 2.3 and 2.4 when the earthquake effect is additive
to the other loads and use the second when the earthquake effect is to be
counteracted by the other loads.

So, I think ASCE 7 has an editorial error in the referencing to the ASD load
combinations as pointed out. Bill Scott already knew this and asked the
question with the answer in it. (-:

I'll pass this along to ASCE so it can be corrected. Thanks.

Charlie

-----Original Message-----
From: Scott, William N. [mailto:William.Scott(--nospam--at)veco.com]
Sent: Monday, December 29, 2003 2:54 PM
To: seaint(--nospam--at)seaint.org
Subject: Question on ASCE 7-02 Load Combinations


The following are exerted from ASCE 7-02. 

2.4.1 Basic Combinations. Loads listed herein shall be
considered to act in the following combinations; whichever
produces the most unfavorable effect in the building,
foundation, or structural member being considered. Effects
of one or more loads not acting shall be considered.
4. D + H + F + 0.75(L + T ) + 0.75(Lr or S or R)
5. D + H + F + (W or 0.7E)
6. D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr
or S or R)
7. 0.6D + W + H
8. 0.6D + 0.7E + H"

"9.5.2.7 Combination of Load Effects. .....
for load combination 5 in Section 2.3.2 or load combination
4 in Section 2.4.1:
E = ?QE + 0.2SDSD (Eq. 9.5.2.7-1)
for load combination 6 in Section 2.3.2 or load combination
3 in 2.4.1:
E = ?QE - 0.2SDSD (Eq. 9.5.2.7-2)
where
E = the effect of horizontal and vertical
earthquake-induced forces

Should the load case and LC 5 for ASD instead of LC4?

"9.5.2.7.1 Special Seismic Load. ...the special seismic load
of Eq. 9.5.2.7.1-2 shall be used to compute E in load
combination 7 in Section 2.3.2 or load combination 5
in Section 2.4.1:
E = =oQE + 0.2SDSD (Eq. 9.5.2.7.1-1)
E = =oQE - 0.2SDSD (Eq. 9.5.2.7.1-2)"

Should the load case be LC 8 for ASD instead of LC 5?


Bill Scott, P.E., S.E.
Principal Engineer
VECO Alaska, Inc.
949 E. 36th Street, Suite 500
Anchorage, AK  99508
Phone 907-762-1655, FAX 907-762-1734



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