Prob 521 NCEES Civil: Structural Sample Exam

[petrographer77]

Any idea why they did not incorporate Cb in the problem? When I saw the beam diagram with the off center point load I thought for sure they wanted me to incorporate it. Is it simply because the max moment occurs away from the brace point? Any help would be appreciated

 

[petrographer77]

Maybe describing the problem will help...

The W10x22 steel beam (Fy = 50 ksi) shown in the figure is only braced at the center of the span.

Work either ASD or the LRFD version of the question

ASD

The allowable moment capacity (ft-kips) of the beam is most nearly:

(A) 45

(B) 51

© 56

(D) 65

LRFD

The moment capacity Mn (ft-kips) of the beam is most nearly:

(A) 68

(B) 76

© 84

(D) 97

Figure shows 20 foot long simply supported beam with a point load 8 feet from the right end of the beam.

 

[petrographer77]

The solution basically just uses the beam tables for a 10 foot unbraced length. After typing in the problem I guess they don't say the following...

"Determine the maxiumum allowable moment for the loading in the below figure"

or

"Determine the maximum allowable point load in the below figure"

In these cases it would be more obvious to utilize the Cb factor. Thoughts?

 

[SF_century]

Did you find a reason why they did not use Cb? I also feel the Cb should be used.

 

[phecke]

In order to determine Cb you need to know what the max moment is and what the moment at the quarterpoints in any unbraced segment of a beam are. If you don't know what the point load is, you can't determine that, therefore, you cannot calculate Cb.

You are also correct that they just asked for the max allowable moment for the beam. Now, to do this you could ignore Cb and go about calculating Lp and Lr and solving for equation F2-1, F2-2, or F2-3 in AISC360 for whichever case you have, or you can do it the much simpler way and save tremendous amounts of time.

Keep in mind the AISC360 has TONS of useful tables and design aids. Check table 3-10 out for this case. I found the page where the available moment values were around those given in the problem to get me in the ballpark, then found the unbraced length of 10 feet and then found the W10x22 line. Go over to the left for that point and you'll see the answers are exactly 51 k-ft (ASD) and 76 k-ft (LRFD).

 

[JimmerC]

In order to determine Cb you need to know what the max moment is and what the moment at the quarterpoints in any unbraced segment of a beam are. If you don't know what the point load is, you can't determine that, therefore, you cannot calculate Cb.

You are also correct that they just asked for the max allowable moment for the beam. Now, to do this you could ignore Cb and go about calculating Lp and Lr and solving for equation F2-1, F2-2, or F2-3 in AISC360 for whichever case you have, or you can do it the much simpler way and save tremendous amounts of time.

Keep in mind the AISC360 has TONS of useful tables and design aids. Check table 3-10 out for this case. I found the page where the available moment values were around those given in the problem to get me in the ballpark, then found the unbraced length of 10 feet and then found the W10x22 line. Go over to the left for that point and you'll see the answers are exactly 51 k-ft (ASD) and 76 k-ft (LRFD).

I disagree with your statement. In order to determine Cb you need to know the shape of the moment diagram, you dont need to know the values. You can compute Cb based on moments being in terms of P.

 

[BoilerEng]

In order to determine Cb you need to know what the max moment is and what the moment at the quarterpoints in any unbraced segment of a beam are. If you don't know what the point load is, you can't determine that, therefore, you cannot calculate Cb.

You are also correct that they just asked for the max allowable moment for the beam. Now, to do this you could ignore Cb and go about calculating Lp and Lr and solving for equation F2-1, F2-2, or F2-3 in AISC360 for whichever case you have, or you can do it the much simpler way and save tremendous amounts of time.

Keep in mind the AISC360 has TONS of useful tables and design aids. Check table 3-10 out for this case. I found the page where the available moment values were around those given in the problem to get me in the ballpark, then found the unbraced length of 10 feet and then found the W10x22 line. Go over to the left for that point and you'll see the answers are exactly 51 k-ft (ASD) and 76 k-ft (LRFD).

I disagree with your statement. In order to determine Cb you need to know the shape of the moment diagram, you dont need to know the values. You can compute Cb based on moments being in terms of P.

I agree with jimmer. It seems to me the problem was worded poorly. It should've either left the loading off and said "find max moment" or left the load on and went with "find max moment for the following load configuration".