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?
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 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
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
RE: Stair Serviceability Lateral Stiffness
RE: Stair Serviceability Lateral Stiffness
RE: Stair Serviceability Lateral Stiffness
RE: Stair Serviceability Lateral Stiffness
RE: Stair Serviceability Lateral Stiffness
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.