Stair stringer modelling in RISA 3d

[ChangeOrder]

I am designing a steel platform with 2 C12 stringers coming down to a concrete landing. What end boundary and end conditions should be used for stairs. I have a pinned connection at the top of the stringer and at the bottom, i have reactions in the Y and Z (assuming stringer runs along the Risa 3d global X axis). Thoughts?

 

[WinelandV]

I think that's fine. Most stairs are designed with a pin-roller boundary condition assumption. Just make sure your platform lateral system in the global X direction is sufficient for the stair stability.

Please note that is a "v" (as in Violin) not a "y".

 

[KootK]

1) I don't like pinned - pinned for most, new build situations. It tends to depend on axial forces in the stringers and large forces in the stringer connections that can be a hassle to deal with.

2) When I'm desperate to make the upper support work, I'll occasionally do pinned at the bottom and vertical roller at the top. I suspect that this is the most realistic model for a lot of stairs in the wild.

3) For new build situations where I'm not desperate, I like pinned at the top and horizontal roller at the bottom. This has the following benefits:

a) Doesn't rely on axial force in the stringer or heavily taxed connections to make it work.

b) Produces a conservative design for the upper stair support in many situations.

c) In some jurisdictions, inter story drift requires a slip connection in the stair. I find that this is most easily done with a horizontal slip connection at the bottom of the stair.

 

[JLNJ]

Be careful at the “knees” where your stringer is bent. Keep your stresses low or reinforce that joint.

 

[SeizeTheMoment]

It depends on how the rest of the stair is designed so that you don't need to have a moment connection at the end connections. If the members and anchorage works with your pinned connection, then that's fine. But if you're finding that you don't like the deflection or members are failing because they're not stiff enough, then you might have to add braces with your pinned connections or change it to a moment connection.

Don't bend to the stress, seize the moment!

 

[Ron]

Bottom connection is fixed...top connection is pinned.

 

[hokie66]

I just wonder why you need RISA 3d, or any other computer program, to design a stair stringer.

 

[WinelandV]

hokie66 - I'd guess that the steel platform is being designed with RISA, so may as well get the stringer design done at the same time.

Please note that is a "v" (as in Violin) not a "y".

 

[ChangeOrder]

It is more about getting the accurate loading to the platform created by the stringers. If the stringers are fixed at the bottom, it appears to give the platform stability that cannot necessarily be counted on. Thank you all for your comments. Pinned top - roller bottom connection looks like the consensus for most.