Notched stair stringer

[SperlingPE]

I am checking the stringer capacity for an existing stair. The stringer is a Trus Joist 1.3E LSL 2x12 (1 1/2"x11 1/4"). The stringer has been notched for steps (7 3/4" rise, 10" run). There are 12 steps which makes the total run 120" and the total rise 93". The pair of stringer are 36" apart. I am using 10psf dead load and the required 40psf live load. This stringer does not work. Is there something that I am missing?

 

[boo1]

There is several options to commonly used to reduce the striger stresses. A few are place a load bearing wall under the stringer, nail the stringer to a wall, attach a 2x4 to the stinger under the stairs, reduce stringer spacing, and adding an end cap to the stairs with an uncut stringer.

With wood contruction I use stinger spacing of 18" or 24" OC (reduces the tread deflection and stringer stress). Loading I use for stair systems is 100 psf. Dont forget about the hand rail loads and resisting its rotation. Consider nailing and gluing the treads and risers.

 

[SperlingPE]

These are existing stairs. I am re-checking calcs and then will decide how to fix if needed.

 

[boo1]

At what stage is the home, is the bottom accessable? The 100 psf i refered too is LL.

 

[SperlingPE]

The home is complete and occupied. The basement is finished. I re-checked the calc's using 40 psf live load (residential) and 5 psf (steps only) dead load. I only used the remaining un-notched portion (approximately 1 1/2"x5 1/8") of the stringer for the calc's. Is my approach correct? There is access to the under side of the stairs.

 

[KarlT]

What are you assuming for the span? The diagonal stringer length or the projected horizontal span of the stringers?

Is part of the stringer cut flat on the bottom? Maybe you can pick up a few inches here and there to reduce the actual clear span to make it work for moment (assuming that is what is governing)

 

[SperlingPE]

I have checked it using the diagonal stringer length and transformed loads. I will check using horizontla span length and applied loads also.

 

[bylar]

Am I missing something!
You said a Truss Joist which is an "I" member with two flanges and a web. When you notched the member you destroyed the flange and ruined the member. The standard stringer is a 2x12 at 18" o/c for a 36" stair.

 

[mkberry]

bylar: Truss Joist makes several products. LSL material is a solid member and resembles OSB while Microllams are like plywood.

My quick check indicates that the net effective member (1 1/2" x 5 1/8") spanning 10 feet fails in both bending and deflection using 100 psf. At 40 psf bending works, but it's a little bouncy. I think most codes allowthe use 40 psf for residential stairs that serve one dwelling, but I prefer 100 psf. 100 psf is required for egress stairs on multi-family residences.

Make sure they didn't over-cut the stair notches!

 

[boo1]

Mkberry has a good point, if the beam is saw cut the blades over cut creating a stress concentration.

 

[SperlingPE]

From the responses, I presume that I am following standard practice. I am now going to design a fix. Does anybody see anything wrong with making this a 3 span stringer?

 

[boo1]

I would have required it to be a 3 stringer system. Consider adding a SYP 2x6 to the bottom of each stringer.

 

[SperlingPE]

A 2x6 flat to the bottom edge of the stringer will not be able to be centered.
The inside face of the stringer is accessible as is the bottom edge.

 

[boo1]

SperlingPE, consider some way of adding to the stringers, with the 5 1/4" effective depth they may still be bouncy. We add the 2x6 or 2x4 to the inner side of the stringers, under the stairs in new constuction before its all covered up.

 

[SperlingPE]

My suggestion to the owner.
Add 2x4 (or 2x6) to the inside face of stringer.
Add 4x4 posts ((2)-2x4's) at two locations at the underside of stringer.