Primary beam parallel to secondary beams (steel moment frame)

[LuisUgarte]

Hello Everyone,

Imagine we have a steel primary beam running parallel to secondary beams. The floor system is a concrete-filled metal deck and is attached to the primary beam via shear studs. Under lateral loads, this beam will experience double curvature. If the beam was not attached to the slab we would design it with unbraced length equal to the entire span (see image). But, what happens is we use shear studs over the entire length?

Are you still design it with Lb equal to the clear span?

What is your standard practice for the design of these primary beams?

Do you consider some restraint of the slab, even for a negative moment?

I think that the deck slab provides a restraint that prevents the lateral torsional buckling to be fully developed.

Thanks in advance for your comments and references

Regards

 

[lexeng18]

In your example, the beam does not see any flexural loading so lateral torsional buckling can not occur. If you were to consider flexure alongside your lateral loading, you’d just have to revisit your moment diagram and figure which flanges to be in compression vs tension. Cracked or not, the slab will brace the top flange so your Lb will be the length of the bottom flange in compression. For what it’s worth, I have always been taught to ignore composite action when designing a moment frame beam.

Under lateral load, the beam will fail in constrained axis flexural torisional buckling with the axis of buckling midpoint in the slab. When doing the constrained axis FTB calculation, which can be found in a paper online I believe, the length of beam used is the overall length of the member. In this case, from column to column.

I can try to dig up the paper if you’d like.