Composite Beam Superimposed DL

[mfstructural]

I was going through the problems in the NCEES practice problems for the Structural I exam, and a problem came up which I don't really understand the solution. here is the question:

A simply supported composite girder has a span length of 76ft. The girder is not shored.
Design Data:
Weight of steel girder 822 PLF
Superimposed DL 344 PLF of girder

What is the portion of the DL moment (ft-kips) carried by the composite section?

The solution is the moment wl^2/8 based on a w = 344 plf.

I don't understand, won't the composite section carry the total DL on the beam and not just the superimposed DL. Also, I don't understand what is meant by 344 PLF of girder??

Any insights?
Thanks,

 

[Teguci]

Consider the sequence of construction
1. Steel beam is placed over the span - it bends due to its own weight
2. Concrete topping is placed over steel beam - The steel beams bends due to the concrete weight
3. Concrete cures in deflected shape
4. Superimposed dead load is placed over concrete - the composite steel beam and concrete deck deflect due to this additional load.

This composite section does not get stressed due to step 1 and 2.

 

[Lion06]

In the terms of this question, they are clearly looking for
(344*76*76/8)/((344+822)*76*76/8) = 344/(344+822) = 0.295 = 29.5%. This is because of what Teguci mentions above.

That being said, the new 13th edition AISC manual clearly states that the bare steel section be checked for the loads it supports prior to composite action, BUT that the total moment (pre-composite, AND post-composite) is resisted by the COMPOSITE section as ultimate strength. Since the composite beam analysis is now done using ultimate strength methods (not the old elastic ASD method), the entire DL moment is resisted by the composite section.

This is clearly illustrated on page 3-7 of the 13th edition manual. The section labeled "SHORED AND UNSHORED CONSTRUCTION" states, "The available flexural strength is identical for both shored and unshored construction. In unshored construction, issues such as lteral support during construction and construction load deflection may require consideration".

I'm sure that's not what they're looking for on the test, though.

 

[mfstructural]

Concrete cures in deflected shape, hence suppplying beams with camber...

Thanks,