Critical Bending Moment

[Anonymous]

Analysing the calculations of the critical bending moment for the same beam (assuming the same length but obtainingin it in different ways), I've noticed some issues I couldn't explain.

Let's assume a simply supported 16m long beam, with bracings every 4m.

1. One bar with 16m. 

Code parameters:

Lz=16m

Llt=4m (defined with bracings)

2. 4 bars, 4m each

Coed aprameters:

Lz=4m

Llt=Lz (Lcr=L0)

3. 4 bars, 4m each

Code Parameters:

Lz=16m

Llt=4m (Lcr=0.25xL0)

Runnig then calculations, we get to the following Critical moments (which I thought should be the same in every situation sicne de the critical length for bending is the same):

 1.

2.

3.

As we can see, the first 2 situations lead to the same value of the critical moment, but the last one leads to a much smaller one. It seemes RSA is some how afecting the values of kz in the critical moment formulation, since the values of C1 and C2 are the same for all the cases.

Does anyone knows why this happens?

Thanks in advance.

[Anonymous]

(File attached)

[Artur.Kosakowski]

You should use the first definition. I'd suggest to watch recording of the 3rd RSA webinar on steel design (http://forums.autodesk.com/t5/robot-structural-analysis/robot-structural-analysis-webinars/td-p/5523...).

See the slides from this presentation:

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.