Stair Stringer Fully Braced
Stair Stringer Fully Braced
(OP)
I am designing a simple stair stringer that spans from landing to landing. The stringer is 18'-6" long (horizontal). The tread width is 3'-6". I am using LL=100 psf and DL=50 psf. Also I am using "Metal Stairs Manual" NAAMM STANDARD AMP 510-92. The manual states: "Stringers are considered to be laterally braced by attached treads and risers." The question is do you consider the stringers laterally braced by the treads and risers, and if so would you still consider the stringers braced if there were no risers just treads?
Thanks!
J
Thanks!
J






RE: Stair Stringer Fully Braced
RE: Stair Stringer Fully Braced
I'm not aware any of problems with stair stringer failures within our, so I have been comfortable with my conclusion. I do avoid using plain plates for the stringers for which the Metal Stairs Manual provide load tables for; at least even a minimal channel flange improves the buckling resistance. If anyone does know of stringer failures, please share the conditions in which that occurred!
Regarding StructuralEIT's question, if it is assumed that the tread/stringer connection has sufficient capacity, then could the buckling capacity for the entire stair run be based on the entire assembly with the "width" of the member equal to the width of the stair + the width of the flanges? This of course assumes that the single stringers have the buckling capacity for the length (distance) between nosings.
RE: Stair Stringer Fully Braced
With discrete treads it is not as simple, even though the treads do act as a torsional restraint it may be hard to justify a fully restrained condition.
I would try the beam as unrestrained for the full length, I usually find that it makes no difference.
csd
RE: Stair Stringer Fully Braced
- LTB of the individual stringers is local to the stringer itself.
- If one wants to consider buckling of the stringer+thread/riser assembly+stringer then the geometric properties of the entire stair assembly should be taken into consideration.
RE: Stair Stringer Fully Braced
RE: Stair Stringer Fully Braced
RE: Stair Stringer Fully Braced
If these two stringers reach their buckling capacity together, how can one brace the other?
I would agree if you were getting some kind of composite action out of the treads, but I don't think you are.
RE: Stair Stringer Fully Braced
In this case it is not about direct connection to a solid bracing element, it is more like a horizontal vierendeel truss spanning to the ends and bringing the bracing loads out to the supports.
As I said above the treads do provide some bracing, but it is difficult to justify full restraint.
csd
RE: Stair Stringer Fully Braced
RE: Stair Stringer Fully Braced
Thanks to everyone for your valuable comments!!
RE: Stair Stringer Fully Braced
Visualize the stringer if you laid it flat (instead of upward sloping) - the treads now are very similar to vertical rigid diaphragms that you see on bridge girders.
But instead of being vertical, they are at an angle - perhaps as much as 57 degrees from the vertical(7" on 11"). But that is still adequate for bracing and there is one every 11" or so.
RE: Stair Stringer Fully Braced
RE: Stair Stringer Fully Braced
I find that stairs which just touch side walls perform better and do not bounce using customary size of stringers found in NAAMM. this may be due to the fact that when the stair lurches, it leans on the side wall and provides some bracing laterally to the stair system.
RE: Stair Stringer Fully Braced
RE: Stair Stringer Fully Braced
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RE: Stair Stringer Fully Braced
J
RE: Stair Stringer Fully Braced
I don't think the position of the treads relative to the top matters that much. Bracing against lateral torsional buckling can be in the form of either resistance to lateral translation of the compression flange or by resistance to torsional twist of the section.
In the case of the treads, they are really working as elements resisting twist (a rotational restraint) and not a compression flange translational restraint.
Almost any tread connection that has some depth to it would work (there are so many treads).
RE: Stair Stringer Fully Braced
Quoting from the commentary to the Australian code AS4100;
My view is that stairs are restrained by a combination of Vierendeel truss action and torsional restraint (but I've seen nothing to support this view).
Where these assumptions really affect design is if using flat plate as stringers. In my experience some practices are comfortable with using flat as stringers, while others insist on using channels. It would be good to see something authoritive on the subject.
RE: Stair Stringer Fully Braced
I think the something authoritive is the fact that these flat plate stringers are not collapsing. It doesnt take much to buckle an unrestrained vertical plate so if they are working as stringers then the treads must be providing restraint to some degree.
I have specified plate stringers but only with continuous folded plate treads.
csd
RE: Stair Stringer Fully Braced
Just reflecting on what Yura at Univ. of Texas has taught for many years.