Steel Channel Cantilever Beam

[WpgKarl]

Let's say you had a series of steel channels embedded 16 inches into a large concrete footing and the channels cantilevered vertically from the top of the footing about 5 ft. Clear spacing between channels is 12 inches.

These are being designed as precast panel storage "racking" blocks, with the idea being the panels are placed on their edge between these channels and wood blocking is wedged between the channels, to hold the panels upright. The loads applied near the tips of the channels (at the wedge points) are from wind and from tilt of the panels and or footings. Channels are loaded in the strong direction.

The way I see it, the channels are prevented from twisting and rotating by a moment connection at the footing. However the part of the channel that is above the footing is not braced in any direction, so I'm worried about lateral torsional buckling of the channel under loads. Can I use the general unbraced flexural resistance eqns for steel beams to check these channels, which are cantilevered? What would the unbraced length be, 5 ft or 10 ft, (because its a cantilevered and free to translate at the end - (like k = 2.0)?

How about loading the channels slightly off the shear center? The load will come into the channels on the flanges, close to the webs, and the shear center of a channels is probably about 1/2" to 1" off of the cl of the flanges.

PS - Cdn steel design code, but I believe the US code is very similar.

 

[Lion06]

actually, I think the shear center for a channel is to the opposite side of the web from the flanges. It is likely considerably more than 1/2" to 1"

 

[youngstructural]

StructuralEIT is dead right: If you're loading at the tips of the webs, you're quite far from the shear centre in a chanel. The shear centre is not coincident with the centre of mass because of shear flow. You're going to have to check your channels for torsion, and that torsion is going to be due to a significant (relative to the chanel's flange width) eccentricity.

I'm going to give this a bit of thought, and probably post again. I've done similar calculations for stairs with just one stringer (where a person walking down one side of the stairs puts a torsion load on the stringer).

Cheers,

YS



B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[civilperson]

No worry, the described problem has only flexure from wind/seismic. Buckling of compression flange would require folded light gage channels.

 

[Lion06]

civilperson-
If you are loading the channels through the flanges, there will be torsion on the channels.

 

[hokie66]

YS and SEIT are right about the torsion, but I would think your wood wedges would provide adequate bracing for the flanges.