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Stair Serviceability Lateral Stiffness

Stair Serviceability Lateral Stiffness

Stair Serviceability Lateral Stiffness

(OP)
Residential (single family) stair in a loft type condo, 3' wide. The treads are 2.5" thick oak (per architect), open risers. On one side there is an hss stringer (hidden in a wall). The other end of the tread is supported by (2) 3/4" rods that hang from above. The rods go through the tread and connect to a plate on the bottom.

My worry is about lateral stiffness, i.e. the stair 'swinging' forward/back from a lateral component of the stair load. The rods will provide almost no resistance in this direction so I need to count on some type of fixity at the hss stringer side.

I posted a similar question a while back on here about what load should be applied laterally. There didn't seem to be a consensus but most agreed that maybe 10% to 20% of the vertical for service makes sense. At this point for fixity I'm thinking a plate or angle supporting the tread at the hss side with a bunch of screws.

See attached sketch. Think this is going to be a problem?

RE: Stair Serviceability Lateral Stiffness

I would use 20%. Maybe come out 6" with a plate and install (4) 1/4x 1 1/2" Simpson SDS screws.
I honestly would have probably not even thought to check serviceability in that direction - good call.

RE: Stair Serviceability Lateral Stiffness


First I would ask if each tread is carried by 2 rods independent of the next tread up or down?

If each rod were to go thru 2 treads, then much of your concern about lateral movement (perpendicular to the riser) is moot, as the rods will tie each tread to the next tread up or down, this providing a load path for lateral back to the upper and lower landings.

It seems implausible to me that each tread would be independently suspended at rod end and would really be a waste of resources. But then, architects always seem to have their own ideas as to what makes sense.

Ralph
Structures Consulting
Northeast USA

RE: Stair Serviceability Lateral Stiffness

(OP)
Ralph - Each is independent. I only showed one in my sketch but each is the same, no overlap. I voted for an overlap or some type of connection to tie them all together that could fit in the aesthetic but it was shot down.

XR - That's basically what I have shown at this point but I'm second guessing myself. Wondering if it will loosen up over time with back and forth loading.

RE: Stair Serviceability Lateral Stiffness

Don't you need the 1" overlap for tread nosing?

RE: Stair Serviceability Lateral Stiffness

(OP)
1" is not sufficient to fit a 3/4" rod through.

RE: Stair Serviceability Lateral Stiffness

what if instead of the wood treads being structural, a plate is used and clad with the wood? The plate could be welded directly to the stringer, and the tread cladding could take care of any tolerance issues with the plate being skewed and such. Could give the added benefit of hiding the connection hardware.

RE: Stair Serviceability Lateral Stiffness

My recommendation is the same or similar to structSU10's. make it a structural steel plate surrounded by wood.

RE: Stair Serviceability Lateral Stiffness

Trying to clad a potentially skewed steel plate can be difficult as well.

RE: Stair Serviceability Lateral Stiffness

Here are a couple thoughts:
1. I would verify the local code will allow completely open risers before spending much time on the design.
2. Has the force of a person falling been considered? Perhaps I am risk adverse. I would design the lateral connection at the HSS to withstand at least a 200lbs horizontal load at the hangers. Maybe use 250 lbs which is consistent with grab bar loads.
3. You may have already considered this. But I would include some screws through the 1/2" plate into the tread at the hangers. This will prevent the tread from lifting when a persons heel or toe catches the bottom of the tread.

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