Load combinations for DL & LL from ACI 318

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#1 Construction PE

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Posted 11 October 2011 - 02:50 PM

I need clarification on the equations below...

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!

#2 Ble_PE

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Posted 11 October 2011 - 03:03 PM

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.

#3 MA_PE

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Posted 11 October 2011 - 05:05 PM

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.

I believe that the capacity reduction factors are modified in the new code as well to offset the reduction in load factors. correct?

#4 Ble_PE

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Posted 11 October 2011 - 05:18 PM

I think you're right. I believe they went to the variable phi factor for flexure in 318-02,and they might have changed other reduction factors as well, but I have to qualify this by saying that I never did any design with a code earlier than 02.

#5 civilized_naah

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Posted 11 October 2011 - 07:58 PM

Note 3 in the current Codes and Standards list for the Structural Depth exam specifically states:
Appendix C does not apply to the Civil Structural examination.

#6 Construction PE

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Posted 12 October 2011 - 07:04 AM

Thanks to all for the info! Great help as always.

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