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courtnvm (Structural) (OP)
22 Mar 07 15:13
I have a question regarding kinked stringer design. The NAAMM standard for metal stairs design kinked steel stringers based on a uniform load that is projected along the length of the stringer. When I look at a kinked steel stringer it appears that there would be an outward thrust at each end of the steel stringer.

What would be the correct way to design a kinked steel stringer? Is there an industry standard regarding designing for the thrust?

Thank You,
Val
haynewp (Structural)
22 Mar 07 16:47
I think you are talking about what I call crippled stringers, going flat for a landing at the top.

There is a thrust due to live load due to the end restraints, but live load deflection usually controls my stringer designs. And I think it is conservative if you look at the stringer as taking all the vertical load in bending.

To be exact, you would have to analyze it as taking combined bending plus the axial component plus thrust for the sloped part, then bending plus thrust for the horizontal landing part. But I just keep it simple and use what has worked for thousands of stringers. I flatten it out and check stresses and deflection for bending.

jike (Structural)
22 Mar 07 17:59
There can be no thrust developed if the supports and the connections cannot restrain that thrust.

For this reason I would recommend analyzing the stringers as simple beam (as if there is no bend in it).
haynewp (Structural)
22 Mar 07 18:06
There is always some amount of thrust unless the stringer is on an idealized roller at one end. But it doesn't matter anyway.

courtnvm (Structural) (OP)
22 Mar 07 21:28
Well I normally do not design my crippled/kinked stringers for thrust b/c most typical stair connection will not develop the thrust. What I typically do is resist one end for horiozontal movement and allow the other end to expand. Usually my maximum horizontal movement is between 1/16" and 1/8" depending on the stiffness of the stringer, but this particular sitation I have 3/8" horizontal movement which I feel is too much. The stair sringer is attached to a masonry wall using a single clip angle on one end and the other end is attached using an angle seat welded to the top flange of a steel floor beam. I really don't think I can develop the thrust. NAAMM does not show crippled stringers being designed for thrust.   

Val Courtney, PE
Valstone Engineering, Inc.

haynewp (Structural)
23 Mar 07 10:21
Can you slot and bolt the end that attaches to the masonry? I would be more concerned about the connection failing from the thrust.




'Bringin' the mullet back into style'

courtnvm (Structural) (OP)
23 Mar 07 10:28
I built a RISA model of the stringer with the CMU wall stiffness included. I believe the CMU wall will move enough locally (about 1/4") to relieve most of the thrust that was occuring. The thrust in the stringer is approximately 2 kips and I am comfortable with that.
csd72 (Structural)
23 Mar 07 12:33
Be aware that your vertical deflection is proportionate to the actual length of the stringer. Bending is proportionate to the horizontal span.

Also ensure that the stringer is laterally restrained at the point of the kink ohterwise it will just tend to twist.
JLNJ (Structural)
23 Mar 07 12:48
Val

I'd be less worried about the thrust and more worried about the kink. How do you design the kink? I know how I'd do it but most stairs I've seen are not reinforced at the kink (or knee). Maybe I'm missing something.

csd72 (Structural)
23 Mar 07 14:23
Good point JLNJ it is essentially a pre-buckled flange. You really need to put a stiifener plate at the joint to equalise the unbalaced flange force components.
courtnvm (Structural) (OP)
23 Mar 07 14:57
I guess I didn't explain that this stringer is an HSS12x3.

Val Courtney, PE
Valstone Engineering, Inc.

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