wood stair stringer design 3

[Cannon9]

I have a interior stair design in a public building. Client wants all wood construction for ease of fabrication. Residential live load in Canada for stairs is 40 psf, but for a public stairway live load is 100 psf. The stringer is 18 ft long between landings. A 2x12 stringer with a 12" run and 7" rise results in only 5.5" of actual cross sectional depth, so for load carry capacity the stringer is really a 2x6. No resonable number of 2x6 wood members can span 18 ft and support 100 psf. Any ideas as how to support the stair treads for this load besides stringers spanning bewteen landings? (In wood framing)

 

[msquared48]

No way to post the stringers midspan?

Try a microlam or parallam, or 4X12 or 6X12 stringers? I see 6X12's used a lot ads exterior stringers with no cuts for treads in apartment buildings. Treads are bolted to inside of stringers.

Mike McCann
McCann Engineering

 

[JAE]

For that long of a stringer, you'd have to do what msquared48 suggests - an LVL or large timber beam.

One thing you can do is use a full depth stringer (i.e. don't cut it for the treads) and then add triangular steps on top of it to support the treads. That way you have the advantage of a full section to span the distance.

First choice though is to add an intermediate post to cut the span.

 

[jike]

Another idea is use the cut 2x12 to support the treads and then nail it to a full 2 x 12 stringer.

If there are walls on each side of the stairs, you can nail the stringers directly to each stud.

 

[MarcbSE]

I just finished designing a very similar problem. Wood stair framed roughly 16' long for a commercial building in Alaska. I ended up using (3) 2x12 stringers: the two outside stringers were double 2x12 with the center stringer a triple 2x12.

To make the 2x12 members work I specified two intermediate support walls or posts located at the third points of the span. If the contractor can not locate 2x12's that long he will have to use an LVL to get the extra length.

 

[csd72]

With timber you have to be really careful about overcut.

If your 5.5" depth only just works then any overcut of the triangle for the stringers will reduce the capacity significantly.

Have you considered a fletch beam with a 5" deep plate?

Or provide a timber beam with triangular spacers over (less cutting than cutting it out of the stringer).

csd

 

[jaypartain]

Have you checked for any requirements limiting the overall run/rise of stairs between landings?

 

[SperlingPE]

When cutting 2x12 for treads, depending on location of imperfections and grade of wood, you may no longer have the engineering properties that you started with.

 

[Cannon9]

Thanks everybody, I'm sure a combination of these suggestions will solve my problems.

Thanks again