Load combinations for DL & LL from ACI 318
Posted 11 October 2011 - 02:50 PM
I am quoting from CERM pg 50-3 (11th ed):
"According to ACI 318 9.2, required strength U must be at least equal to the factored loads [in the following equations]. When dead load and live loads only are considered,
U = 1.2D + 1.6L"
Then further down the same page CERM states
"According to ACI 318 App.C, when only dead and live loads act, the load combination that is used for design is
U = 1.4D + 1.7L"
Why is ACI 318 9.2 different than ACI 318 App C? Which combination do we need to use for the exam?
A little confused here.
To make matters worse, CERM Example 50.1 calculates the factored moment for a beam using each of the load combinations above (once following ACI 318 9.2 and once for ACI 318 App C) and (of course) gets 2 different answers for the ultimate load, but doesn't specify which one is correct.
Do we need to check both and pick the highest for the test? Any help here would be greatly appreciated!
Posted 11 October 2011 - 03:03 PM
Posted 11 October 2011 - 05:05 PM
I believe that the capacity reduction factors are modified in the new code as well to offset the reduction in load factors. correct?
Use the load combinations and factors from Section 9.2. The load combinations in Appendix C can technically still be used for design, but they are a carry-over from earlier editions of ACI 318. I believe it was 318-02 when they changed the load combinations to match IBC, ASCE 7, and AISC LRFD combinations. Any question asked during the exam will need to be answered with the 1.2D + 1.6L.
Posted 11 October 2011 - 05:18 PM
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