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Re: ASD Load Combinations

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On 11/15 Jame Fulton wrote:
++++++++++++++++++++++++++++++++++++++

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.
++++++++++++++++++++++++++++++++++++++++


The ASD Load Combinations in ASCE -7 were reformulated a few years ago, to
provide roughly the same or at least not less protection against failure than
would be obtained by the application of the LRFD or Strength load combinations.
In the LRFD combinations, the same approach of permitting reductions in load
factors on transient loads only when multiple transients are present is
included.  In adopting the new ASD combinations, ASCE felt that the new versions
were both more consistent with the LRFD approach ( which was based on somewhat
rigorous statistical evlaluation of failure probabilities) and also would not
end the previous regime in which ASD load combinations would result in a more
economical design than LRFD, thus, discouraging the move to LRFD.


Both the 1997 UBC and the 2000 IBC adopt the ASCE-7 load combinations (both for
strength and ASD) but also include an alternative set of ASD load combinations
that look more like the ones engineers have used in the past, with the 1/3
increase permitted, regardless of the number of transients present.  It is
unclear if NFPA-5000 will also include these additional load combinations.
However, the code and standards community has pretty much decided that the old
style "alternative" ASD load combinations will be phased out in teh future, and
only the ASCE versions will remain.