Steel stringer support at top

[buleeek2]

Hello,

I am looking for opinions about the proper support condition at the top of the stringer. I ran 3 analyses of the same typical steel stringer: HSS tube with 6" bent at the top and bottom. The bottom has a base plate with 4 anchor bolts and a foundation. The top of the stringer will most likely be attached to the LVL beam. Please take a look at the attached pdf and let me know your opinion on which model you consider the most accurate. I am trying not to go with "A" (pin connection) as it yields uplift and huge thrust. Analysis B is not accurate, as the wood beam provides some lateral stability and horizontal reaction. Is "C" the most accurate?

Thank you,

 

[JAE]

I would presume that the LVL is integrated into the upper floor (diaphragm) such that there is lateral resistance to movement - thus your Option B with a roller isn't accurate and you are rightly doubtful as to its accuracy....but for design of the stringer itself, I would be OK with using this one.

A pure infinitely fixed support at the top (horizontally) isn't possible either so Option A is not very accurate as there will be some "give" in the floor diaphragm.

I don't believe that your connection to the LVL will really produce any meaningful moment rigidity so that tiny bit of negative moment at the top of your stringer probably isn't something to worry about. The LVL probably has relatively tiny stiffness rotationally as the wood is much softer than the steel and the connections at the LVL ends probably don't produce much torsional resistance either.

 

[TRAK.Structural]

I would check how much horizontal translation is happening in model "B". If that amount of movement is very small than it likely won't develop the horizontal reaction as the system will inherently have some flexibility.

As another option you could provide a horizontal slotted connection at the bottom to the slab to relieve any horizontal reaction that wants to develop.

 

[phamENG]

Determine the lateral stiffness of the floor diaphragm that it's tying into and put horizontal springs supporting the top of the stair. See what that does to your reactions. Pin-pin would be the appropriate model, unless you're actively trying to fix it to the concrete with a moment connection. You won't get meaningful fixity at the LVL.