Lateral support of a member by steel trapezoidal sheeting

[mikesg]

Hallo community,

I am working on a single level structure that consists of a system of moment frames (braced in longitudinal direction). The roof is covered with trapezoidal sheeting supporterd directly on the beams of the frame (on the top flange).
The sheeting is placed in such a way that the orientation of the ribs (its stiff direction) is transverse to the beam.

I want to know the extent to which I can rely upon this sheeting for providing lateral support to the top flange of the beam and the design checks needed to choose the fasteners between sheeting and beam that will transfer the forces providing such extent.

Should I design for a percentage (say 2%) of the force in the flange distributed between all fasteners? Now should I go for checking whether the in-plane stiffnes of the sheeting is sufficient for providing a bracing (as described in Yura's "Fundamentals of beam bracing")?

Also, in case I can rely upon this lateral bracing support should I provide additional torsional bracing for the beam?

Many thanks,

Mike

 

[ishvaaag]

I would say some rational approach would be:

1. Assume some out of straight initial shape for your beams in plan, say some parabolic 1/500, 1/1000, 1/1500 sagitta.
2. Model Stucture plus deck in FEM with that explicit initial deformed shape.
3. Apply the lateral loads that would increase the lateral deformation.
3. Analyze it with P-Delta included.
4. If stresses in the elements do not warrant some reduction in stiffness, and the structure remains stable for the factored loads, you have finished the check. If not, you need to reduce the pertinent element's stiffness and revert to point 3.
5.Forces at the connections may be found from the result of the satisfactory analysis. Members and connections are to be designed or redesigned for one of these satisfactory checks (whole set of hypotheses)

Now, in this process the deck acts like some web for in-horizontal plane forces. The top and bottom chords at least of the more or less flexible diaphragms would then be expected to be continuos, and so you would be having lower compressed beam flanges at supports, Your scheme then would be providing overall bracing to the structure and top flange of beams where compressed, but not LTB bracing for the compressed portions of the beams where negative moments act. This however needs not to be a problem other than the moment there be allowable for moments with bracing with lengths between braces equal to that between support point. I think for this instance and double tee shape, the addition of stiffeners wouldn't be a mostly adequate solution, and it would be a far more satisfactory (and costly one) to convert the compressed sections to double tee plus 2 plates parallel to the web for the compressed lengths. You may also, of course to place some second structural element with no other main function than to take any rotational instability the beam may try to develop; mere inclusion in the model will allow Lb then to be reduced when satisfactory to the distance from support to such points or between them.

 

[mikesg]

Thanks a lot!

Although it seems a little complicated for my current skill level, I will try experimenting with this approach.

Mike