Beam selection

[Brendan Webb]

If the unbraced length of a beam is 3 m and the maximum bending moment in this unbraced segment is 312 kN-m, and also the maximum moment in the braced segment of the beam is 350 kN-m, select an economical section just based on the moment requirements. Use Table A.2M included in the course materials.

I am having problems with this question I am not sure how to address the maximum moment in the braced section of the beam. I know that the beam's Lp has to be higher than the unbraced length as Lp denotes the maximum un-braced length of the compression flange for which the maximum design stress for a compact symmetrical shape may be used. So Lp > 3m. I also know that the beam's resistance to the maximum bending moment in this unbraced segment must be greater than 312kN - m. So MLp > 312 kN - m.

For the braced length I believe have to find an appropriate section modulus.
So:

0.9 Fy = M/Zx


Zx = (350kN - m * (1000^3))/(0.9 * 345000kpa)

Zx = 1,127000 mm^3

So the Zx of the beam must be greater than this (Zx > 1,127000mm^3).

Based upon this I would select W310 * 97 as all of its properties are larger than what's required. It is also the lightest beam that is able to do this.

Any pointers on if I did this right? Attached is the Table.

Thanks

 

[Greg Bernhardt]

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

 

[Nidum]

Question is difficult to interpret with certainty . Nice clear diagram would help .

 

[Brendan Webb]

Question is difficult to interpret with certainty . Nice clear diagram would help .

Thanks for the reply, I believe I solved the problem (or at least I sent in my assignment with my interpreted answer). Next time I post I will include the diagram and make my thoughts extra clear.

Cheers