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Re: ASD Load Combinations

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1997 UBC Equation 12-16-1 is posted by ICBO as an errata.  It reads 0.9D
plus or minus E/1.4.

Rick Drake, SE
Fluor Daniel, Aliso Viejo

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James_F_Fulton(--nospam--at)RohmHaas.Com (James F Fulton) on 11/15/2000 06:13:48 AM

Please respond to seaint(--nospam--at)seaint.org

To:   seaint(--nospam--at)seaint.org
cc:

Subject:  ASD Load Combinations


Based on commentary in ASCE 7-95, the 1/3 allowable stress increase on
combined stress, or its equivalent the 0.75 load factor, is allowed only
when
two or more "transient" loads are contained within the load combination.
[This of course is counter to long past practice, as reflected for one in
the
AISC Spec (see ASD 9th, A5.2) , but it appears to be the re-think today].
Equations (12-7) through (12-11) are consistent with this new thinking,
except it appears for (12-8) since there is a low probabiliity that L and
Lr
or S would occur at the same time (but not as low of a probability of L and
W
or E occuring at the same time -- say OK).

Equations (12-12) through (12-16) are also consistent with the new criteria
if one interprets the INTENT of the equations as written this way: In
(12-13)
for example, D + L + (W + E/1.4), the L cannot be taken as zero such that D
+
(W +E/1.4) results to which a 1/3 stress increase is applied. In other
words,
in all the equations for the 1/3 increase to apply, ALL individual loads
must
be included, and none excluded. This is the interpretation I have to make
for
this to be consistent and make sense. However,  suppose you do not have a
"L", which can occur for non-building structures such as vessels/ tanks, or
"L" happens to be very small for some reason. Looks like there is no load
combination in the (12-12) through (12-16) group that covers this situation
because there is no D + (W or E/1.4) combination similar to (12-9). So, you
are stuck with (12-13) which becomes in effect D + (W or E/1.4) to which
allowable stresses are increased by 1/3. To me this appears to be
inconsistent and an oversight.









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