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RE: Question on ASCE 7-02 Load Combinations

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In both cases I think the answer is yes.  The idea of the first combination (Eq 9.5.2.7-1) is a condition when seismic loads create additional axial load.  On a column for example.  I would use that with load combination 5 in section 2.4.1.  The opposite happens with the other combination.  You are trying to envelope the uplift demand.  So reduce the dead-load as required by load combination 8 and use the E as defined by equation 9.5.2.7-2. Load combination 6 appears to cover the case of live & snow with seismic.  I don't see where load combination 4 would be affected by the seismic loads.

HTH,

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Scott, William N. [mailto:William.Scott(--nospam--at)veco.com]
Sent: Monday, December 29, 2003 1:54 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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