NCEES sample exam (Comp. Stl unshored)

[spatrum]

Problem 121 from the NCEES Structural I sample book asks for dead load moment carried by the composite section, and problem 119 from the NCEES SE sample book asks for dead load moment carried by the steel section.

Can someone explain to me why the weight of girder is not included in the calculation of dead load moment for Problem 121? This is very confusing to me. Thanks in advance.

 

[patik83]

Problem 121 from the NCEES Structural I sample book asks for dead load moment carried by the composite section, and problem 119 from the NCEES SE sample book asks for dead load moment carried by the steel section.
Can someone explain to me why the weight of girder is not included in the calculation of dead load moment for Problem 121? This is very confusing to me. Thanks in advance.

I got this problem wrong the first time I did it, and I agree that it's a bad question. I think the key may be in the wording, where they say "the portion of the dead load moment carried by the composite section," which may imply that the composite section (steel + concrete connected w/ shear studs) carries the superimposed load), while the beam alone carries that portion of the dead load moment from its own weight.

But despite that, I still agree with you that the composite section supports the load of the beam and the superimposed load. Maybe they mean "carried" in a literal sense, implying that the load would rest on top?

Hopefully someone can respond and justify that this is actually a good question and you and I are missing something. Anyone? Beuller?

 

[lhpriest]

Problem 121 from the NCEES Structural I sample book asks for dead load moment carried by the composite section, and problem 119 from the NCEES SE sample book asks for dead load moment carried by the steel section.
Can someone explain to me why the weight of girder is not included in the calculation of dead load moment for Problem 121? This is very confusing to me. Thanks in advance.

I got this problem wrong the first time I did it, and I agree that it's a bad question. I think the key may be in the wording, where they say "the portion of the dead load moment carried by the composite section," which may imply that the composite section (steel + concrete connected w/ shear studs) carries the superimposed load), while the beam alone carries that portion of the dead load moment from its own weight.

But despite that, I still agree with you that the composite section supports the load of the beam and the superimposed load. Maybe they mean "carried" in a literal sense, implying that the load would rest on top?

Hopefully someone can respond and justify that this is actually a good question and you and I are missing something. Anyone? Beuller?

One way to look at the problem would be to consider the dead load carried by the composite section is everything not carried by the steel section. That is, whatever is carried by the steel section is present before the section behaves compositely and therefore must be resisted by the steel section alone at some point in time.

In problem 119, "the portion of the dead load moment carried by the steel section alone" = wet weight of concrete + self weight of beam

In problem 121 "the portion of the dead load moment carried by the composite section" = dead load not carried by steel section alone = superimposed dead load.

I do agree, this is a confusing question because once the section is composite, the steel section no longer acts alone and therefore its own self weight must be resisted by the composite section.

 

[spatrum]

Thank you both for the replies. I understand it much better now.

My boss said to try to look at composite section as "Phase II" process, and not the final stage.