Lateral Design of Steel Stairs
Lateral Design of Steel Stairs
(OP)
I am working on the design of a 3-story light gage steel and structural steel building in Northern Calif. The stairs are steel and tied into the lateral system of the building. What do I design the connection of the stairs to the building for?
I do not think it would be an Fp calc since the stairs are part of the building structure.
Thanks for any advice.
I do not think it would be an Fp calc since the stairs are part of the building structure.
Thanks for any advice.






RE: Lateral Design of Steel Stairs
Can you expand upon this? In a high seismic environment, I would expect the bottom of the stairs to be as sliding joint such that lateral drift was not transmitted to the stairs (acting as accidental bracing).
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Lateral Design of Steel Stairs
RE: Lateral Design of Steel Stairs
Design the stairs independent of the structure. They exert load on the structure....the structure should not exert load on the stairs.
If you have a seismic lateral issue to deal with as KootK notes, then try to design such that the stair sections are isolated from the building's lateral response.
RE: Lateral Design of Steel Stairs
For what it is worth, the shearwalls on this building are Sureboard and are very stiff. My amplified seismic shearwall deflection at the 3rd floor (top of the stairs) is only 0.81".
Thanks!
RE: Lateral Design of Steel Stairs
RE: Lateral Design of Steel Stairs
RE: Lateral Design of Steel Stairs
RE: Lateral Design of Steel Stairs
You can use slotted connections parallel to the stringers to allow for displacements without attracting lateral forces from the main structure. You don't need to worry about slotting perpendicular to the stringers, because they will have an "accordion" affect.
RE: Lateral Design of Steel Stairs
Cal91, if I slot the connections of the stringers to the main floor beam, what is the lateral system for the stairs parallel to the stairs? The stringers themselves?
If I tie the stairs to the building, I do not think I can use the Fp from Table 13.5-1 as it says "Egress stairways not part of the building structure".
After looking in ASCE more, I think the 2nd half of 12.14.7.1 Connections may apply: 0.2Sds or 5%.
RE: Lateral Design of Steel Stairs
You've got two options:
1. Slot base of stringers, don't slot connection to structure. This will induce lateral forces on the main structure.
2. Slot connection to structure, but not the base. This will induce lateral forces on the foundation.
I don't think that 12.14.7 applies - only members of the seismic force resisting system need comply with this section. The stairs are not part of the SFRS. I could be wrong, however.
RE: Lateral Design of Steel Stairs
1. It will be tough to detail stairs as an Ordinary Concentric Braced Frame; columns (similar to zipper columns) may be needed at the stringer to beam connections. Even harder (more than likely impossible) to detail them as a Special Concentric Braced Frame. If the building is in Seismic Design Category C or lower, a Steel System Not Detailed for Seismic Resistance may be used.
2. The stairs would need to included in the lateral analysis and there is a good chance the stringers will attract large loads requiring stout connections, robust stringers and a real foundation. Depending on where the stairs are located in plan, they could cause torsional irregularities (playing the devil's advocate, they might also be used to tame torsional irregularity).
3. Unless there is some huge architectural constraints, it probably is a lot more trouble than it is worth. I am guessing the stair cost will significantly increase with negligible savings on the rest of the lateral system.
4. If you need steel braced frame in the lateral system, it is easier to add a real one.
5. I don't think it is not a good idea to use a critical life safety component as part of the seismic force resisting system. The stairs could experience inelastic deformations. I wouldn't feel comfortable using a lateral element for evacuation after an earthquake. If it has to be done, I would want them to be extremely robust and designed to remain elastic.
I am with the others who say design it as a component per chapter 13. If they are egress stairs, don't forget to use Ip = 1.5 since they are needed for life safety.
RE: Lateral Design of Steel Stairs
So here is my current thinking:
1) Parallel to the stringers: Slot the connections from the stringers to the 2nd and 3rd floors. Tie the stringers into the foundation.
2) Perp to the stringers: The intermediate landings will be supported by a steel column. The horizontal reaction from the intermediate landing will hit mid-height of the column and then transfer into the floor diaphragms. The stiffness of the column is low enough that the building would not impart load on the stairs. The top of the stringers would be bolted to the stair beam.
3) Design all of this for Fp force with ap = 1.0 and Rp = 2.5 and Ip = 1.5.
What do you think?
Thanks!
RE: Lateral Design of Steel Stairs
I think if you tied the stairs in on only one side, the stairs would have enough flexibility to not attract forces of any significance from the building.
If it's what I'm picturing, I'd apply standard bolt connections at the following locations.