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ASD Load Combinations
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- Subject: ASD Load Combinations
- From: James_F_Fulton(--nospam--at)RohmHaas.Com (James F Fulton)
- Date: Wed, 15 Nov 2000 09:13:48 -0500
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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