Dead loads can, in some cases, be beneficial to the integrity of the structure.
For example, suppose you have a beam simply supported with a cantilever and you want to find the envelope of the bending moment. Depending on the length of the cantilever, it is possible that the simply supported part is often under a negative bending moment. So when calculating the envelope of positive bending moment (from dead and live loads), the dead loads actually “cancel” some of the live load results. In such cases, you may then want to use 0.9 instead of 1.2 as the dead load factor.
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