Composite Beam Design? 1

[Dlmyln]

Hello everyone.

I am currently analyzing a composite beam consisting in two cold formed steel channels bolted together and filled with 3000PSI concrete (as shown in the attached picture).

I did a model with ETABS of the whole structure using this beams. ETABS allows you to model and design "Tubes Filled with concrete" and I can obtain the stress/ratio capacity and
all internal forces of the member.

The thing is, this is all assuming that both channels behave as one steel tube. For this to happen, I need to provide enough anchors along the span of the beam to resist all internal forces.

How can I analize this? Is the maximum torsion and moments (Mu2, Mu3) in the beam supported by all bolts together?

 

[Shu Jiang PEng SE]

The key to design composite beam is to guarantee the integration of steel component and concrete component, especially at section of the member where it experience tension, like the bottom of beam in sketch. The current layout you have does not work.

 

[Dlmyln]

Thanks for your feedback Shu Jiang. So in order to have a more integrated steel component what could I do? I have seen cases where they join both channels with a through bolt. In this case could I put it joining the top and the bottom of the beam instead of joining it at the sides?

 

[WARose]

To me (for composite action) this is missing additional mechanical anchors at the top & bottom.

I'm also a little wary of calling this fully "composite" when there is nothing really holding this concrete together internally (since it is 4" x 8").

You also have to watch out if this thing is picking up axial loads. That can change your hold down needs considerably.

 

[PaulG93]

Why do you have to use channels?

The thing is, this is all assuming that both channels behave as one steel tube. For this to happen, I need to provide enough anchors along the span of the beam to resist all internal forces. How can I analize this? Is the maximum torsion and moments (Mu2, Mu3) in the beam supported by all bolts together?

AISC 360-10 Section F13.4 describes build up sections in flexure:

Where two or more beams or channels are used side by side to form a flexural member, they shall be connected together in compliance with Section E6.2.

Section E.6 describes built up sections in compression. Get you max compressive force from your moment envelope (Mu/d) and determine a, the distance between connectors in accordance with E6.2. This will only ensure the two channels work together as one built up section, not as a steel/concrete composite section. Other requirements are necessary to develop a fully composite section.