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In ASCE 7-16 there is a load combination as: 1.2D + 1.0E + L + 0.2S but then there is also another combination as: 0.9D + 1.0E In both of them earthquake load is the same so why would we use the second one, while the first one would create the worse combination with 1.2 instead of 0.9 for dead load? What am I missing exactly?

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Dead loads can, in some cases, be beneficial to the structure's safety.

For instance, say you have a simply-supported beam with a cantilever and you want to find its bending moment envelope. Depending on the length of the cantilever, it's possible the simply supported segment will mostly be under negative bending moment. So when calculating the positive bending moment envelope (from dead and live loads), the dead loads will actually "cancel out" some of the live load's results. In such cases, you'd then want to use 0.9 instead of 1.2 as the dead load factor.

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  • $\begingroup$ Good point. Still, we would arrive this conclusion only after comparing both combinations correct? I mean we must still calculate both and then choose the worse one right? $\endgroup$ – user3600630 Jan 27 at 15:21
  • $\begingroup$ And what you described applies to whole structure as well too correct? I mean we compare the two combinations for the whole structure, and still, for some members as you said the 0.9DL+1E may create the worse effect so for those member we apply this combination, but for other members we use the 1.2D + 1.0E + L + 0.2S? so we design all members for whichever combination is worst for them, so there can be more than one combination used for the same structure? $\endgroup$ – user3600630 Jan 27 at 15:25
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    $\begingroup$ @user3600630: Exactly. All the combinations must always be checked (unless you're dealing with really trivial cases where one combination is clearly useless, such as a simply-supported beam, which will only ever have positive bending moment). $\endgroup$ – Wasabi Jan 27 at 15:25
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    $\begingroup$ @user3600630: As for your second comment, yes. Remember, the final objective of structural analysis is to come up with the envelope of internal forces at each point in the structure. We want to find the maximum (positive and negative) force at each and every point of the structure from any and all combinations. $\endgroup$ – Wasabi Jan 27 at 15:29

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