Odd Stair Tread Design 1

[SeanD]

I am checking the design of a stair tread for a fabricator and am having difficulty in finding any help whatsoever. The tread is an unequal l shape bent in the shop out of A36 sheet. It has 4" downward vertical leg, 12" horizontal and is 42" long and .25" thick. A wood piece for decoration will be attached to the top by the GC. The problem is that it is supported only at the middle by a single TS. My concerns are that traditional methods of calculating the section properties and checking for bending as a cantilever member are insufficient. I asked AISC for help and they referred me to NAAMM. NAAMM responded to check with AISI. I am assuming they would respond by saying check with AISC since it is a thick member. Any thoughts would be greatly appreciated.

 

[haynewp]

Have you figured the section properties for the shape and checked the outstanding leg for buckling?

I would think an 'I' shape that is 1/4" thick with a 4" leg should be pretty strong for a stair tread.

 

[Ron]

SeanD....I think you can still get pretty close with traditional methods considering the following:

1. For cantilever bending, consider only 4 inches of the tread and neglect the remaining 8 inches of plate (for now). Compute bending as you would for an equal leg angle, considering the buckling of the "down" leg as haynewp mentioned.

2. Separately compute the bending resistance in the remaining 8 inches of plate (as if it were just a plate tread, no "down" leg). Convert this to an equivalent load and subtract from your equal leg angle load to get the equivalent effect (after recomputing the angle bending).

3. Check for torsional bending with the tread load placed at the outside edge and inboard from the riser.

Like haynewp, I doubt there's an issue.

 

[SeanD]

I am sorry to have mislead you. The shape is L, not l (lowercase L). Thanks for your reponses though.

 

[blake989]

with most stair loads around 100psf, I would try for a little more section on the back side of the tread.

 

[SeanD]

Thats my concern, the flimsyness of the backside. The fabricator already has this in the shop, (yes he is irresponsible) but the architect specified the design shapes. Calculating the section properties for an L of these dimensions, and using an Fb=.6Fy, the calculations show it to be ok, although I have not checked the end of the 4" leg that is in compression for buckling yet.

 

[Ron]

SeanD...if you have the opportunity at this point to modify the design, why not have a leg down on the front of the tread and leg up on the back. This would give equal bending resistance to the section and cut the flimsy plate problem.

 

[SeanD]

True, however I suggested this at which point they said they would need to weld a 3/4" leg upward. I do not know what the minimum weld requirement would be to develope combined section properties. They have told me that the welding of a leg that small will warp it proportionally to the amount of welding. In other words CP weld is not acceptable. Any ideas would be great. By the way I also suggested they bend the tread twice to achieve the desired tread and they are having difficulties achieving it with the equipment available. Any ideas would be great, or assistance in the welding specifications to achieve combined section capabilities would be much appreciated.

 

[haynewp]

Can you fillet a 2x2 angle below at the backside or will the bottom of the treads be exposed? The weld could be staggered.

Since it is cantilevered, I think bounciness (like a diving board) would be my concern.

 

[Ron]

haynewp's suggestion is good as the angle could be rolled to the radius. Stitch welding the angle would negate the warping problem.

Another solution would be to roll a thin "I" or "S" section and then flame cut the opposing flanges off (or cut only the front edge down to the wood height), being mindful of the potential for warping.

 

[SeanD]

The latest meeting has resulted in the architect has approved the addition of a flat bar at the back side of the tread up in direction. Now my question becomes, what spcific welding guidlines or specifications to follow so as my section properties calculations become valid. A complete pentration weld is not an option. Stich welding at 2" every 6" o.c. has been shown to be able to maintain a straight look with little or no warping. I.e. architect is happy!

 

[Ron]

Check your moment at the cantilever. Replace it with a couple applied to the bar standing up (and thus the stitch welds). As long as the shear capacity in the welds exceeds the couple load, the section properties are valid.

 

[SeanD]

Thanks for the help