Lateral Design of Steel Stairs

[davidfi]

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.

 

[KootK]

Are we talking about the connection of the stairs themselves (stringers, treads, etc) or the connection of an entire external shaft to the main building? If it's the stairs only, I'd go with the 5% stuff in ASCE7 12.1.4. If it's the whole shaft, I'd go with 12.1.3.

The stairs are steel and tied into the lateral system of the building.

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.

 

[JLNJ]

If the stair forces are forcibly dragged into the main structure, then I would look at the Omega combinations for the connective tissue.

 

[Ron]

The stairs are not (and should not be) part of the building structure. They are; however, life safety means of egress, so their design should be taken seriously. AISC disavows any relationship with stairs! They are not considered "structural steel" according to AISC.

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.

 

[davidfi]

Why do you recommend not tying the stairs into the structure? Space is very limited, and I do not have room to make the stairs isolated from the structure.

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!

 

[gwynn]

If the deflection at the top of the stair tower is that small, I would determine what forces are induced in the stair tower by that amount of deflection. Design the stair tower for those forces (or 5% out of plane loads, etc., whichever governs). If displacement loads govern, this would be iterative. Then determine the stiffness of the tower and compare it to the rest of the structure. If the stiffness of the stairs is negligible compared to the rest of the structure (which I suspect it may be), I would then call it a day.

 

[Ron]

davidfi....perhaps I wasn't clear. I don't mean to not attach the stairs to the structure, but to do so such that the stair section does not received significant load from the structure. Some "backloading" is inevitable, such as a portion of lateral load from the building, imparted by lateral deflection as you noted from your shearwall. I would not design the stair section to resist that load in total, but to allow that load to move the stair section without a great deal of resistance to that movement by the stairs. While stair sections are designed for certain mandatory live loads (100 psf tread, 200 lbf/50 lb/ft rail loading, they are not robust enough to accept significant load transfer from the structure due to the required loading on the structure itself.

 

[sandman21]

Fp fr egress stairs, ap=1.0 RP=2.5

 

[cal91]

You need to determine Fp - see ASCE 13.3 Seismic Demands on Nonstructural Components, ap=1.0 and Rp=2.5 from Table 13.5-1.

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.

 

[davidfi]

Thanks for all of the advice.

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%.

 

[cal91]

Yes, the stringers will act as braces. I've designed around 8 of these in the last 6 months, and the stringers (usually C12x20.7, or even C10x15.3) have always been overkill acting as braces.

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.

 

[wannabeSE]

I have never seen stairs designed as part of the lateral force resisting system. I am not saying that it can't be done. But here are a few reasons not to use then in high seismic areas:

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.

 

[davidfi]

I guess I may have been confused on how I want to design the stairs laterally. Originally, I was thinking I would just throw the weight of the stairs into the building lateral system; there would be no separate lateral system for the stairs. But I was not accounting for the stringers acting as a stiff lateral system and sucking load from the building.

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!

 

[cal91]

My only concern would be that the stairs are 2 stories tall and their only lateral restraint parallel to the stringers is at the base.
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.