Select an Economical Beam Section for Moment Requirements

[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/ZxZx = (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

 

[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