Stair Stringer - 20' radiused channel

[courtnvm]

I have a radiused stair stringer that is approximately 20' in arc. Is there any reasons/requirements as to using a steel channel. I analyzed the stair stringer w/out the radius in it. I was just wondering if anyone knew of any thing that I should check b/c of it being radiused. Thank you so much for your time and attention.


Val

 

[UcfSE]

Is it a vertical arc or horizontal? Horizontal would introduce a lot of torsion and probably make the channel shape not work.

 

[courtnvm]

It is Horizontal.

 

[JStephen]

Are you saying that is formed into a helical shape, but you analyzed it as though it was just a straight stringer?

If so, the difference would depend on how large the radius is- the flatter the thing is, the closer to the straight stringer analysis it would be.

Note that generally the AISC allowable stress equations don't address a curved beam- all the buckling critera assume a straight beam to begin with. You're on your own.

 

[courtnvm]

Yeah. I knew that because it was horizontally radiused it would have torsion just rationally speaking. I was just wondering if anyone knew of any literature or procedures on designing horizontal radiused beams/stringers. I don't think it is a very simple task by any means.

Val

 

[haynewp]

Can you use a HSS section instead? That will be better for torsional resistance. I think it would be tough to get a good connection at each end of a channel to achieve a torsionally fixed end.

 

[JStephen]

The analysis of the forces on it would depend on the end conditions. By careful selection of a free body diagrams, and use of symmetry if appropriate, you may be able to find the internal shears and moments without too much trouble. I've done this on curved beams loaded perpendicular to the plane of the curvature, not on helical. It may help to intentionally make one or both ends pinned instead of fixed, just to simplify the analysis.

You could use structural software, and break the beam up into a number of short segments.

It would simplify things if the beam were symmetrical, and would allow much higher torsion (and better analysis of it) if the section were tubular as suggested above. (Or solid rectangular bar would also work handily for analysis.)

 

[shepherd]

You also may want to consider designing treads between stringers to help with the torsional loading.
Try

http://www.naamm.org/literature.php

metal stair manual for more info.