Cutting Existing 4 x 2 Wood Floor Trusses and Repair Guidance

[KirkGH]

I'm cutting a couple of existing floor trusses down from 15 ft to 10 ft to create head room for a new stair in a residence (replacing trusses with new LVL or similar is is not possible because there are air ducts in the way). My planned repair is to toenail 3 vertical 2x4 at the end as stiffeners, and add 6'-0" x 1/2" plywood each side nailed with 10d @ 6" o.c. to the chords and struts. I know it's a composite beam and not elastic and there's slip at the connections, but as an approximation I modeled it elastic, homogenous, no slip. I calculated the maximum shear stress V to size the plywood, and shear flow v at the nailed connection to calculate the required nail size and spacing.

My questions:

Is there a more exact way of modeling the repair short of a finite element analysis? I looked around and it looks like the APA may have something that could fit the bill but still looked more complicated than I prefer.

Is there a rational way to determine the required length of the plyood to develop the forces in the truss? I assumed about 2'-0" of damage, and twice that to develop forces.

I know this method of reinforcement works in theory and have seen it in the field but I'm having a difficult time comprehending the analysis requirements. Any guidance or insight is appreciated.


Kirk

 

[dhengr]

KirkGH:
How about providing an existing framing plan, which shows what you want to do; along with a larger scale side view of the trusses involved? The biggest problem in doing this is re-analyzing the trusses for the new length and studying the member forces, and more particularly the joint forces to determine if the connector plates are o.k. at each panel point. What you say you are doing sounds like about the right approach. I take it that you are cutting in a new floor opening for the stair framing.

 

[KirkGH]

16" deep trusses by the way.

 

[KirkGH]

Elevation view

 

[dhengr]

KirkGH:
1.) Aren’t there vert. web members from each bot. chord panel point up to the top chord?
2.) I would stagger the nails through the plywd. and into the edges of the 2x4 truss members to prevent splitting, and driven toward the mid thickness of the 2x4, tipped up or down a bit. Maybe use 8d instead of 10d nails and space a little closer for the same reason, and stagger a bit from side to side also. You won’t be able to nail too near existing panel points because of the existing connector plates. You might use some structural adhesive btwn. the plywd. and the truss members.
3.) The vert. bearing members (new vert. chord members) at the end should be cut to fit well/accurately/tightly. Put the toenails in the wide face not the edges, and face nail these together first. Then nail the plywd. to these also.
4.) You’re building a box-beam or torsion box of sorts, tension and compression chords and plywd. shear panels. If you can stop the plywd. on a vert. web member that’s probably a good detail too.

 

[RFreund]

I think Dhenger covered this with his last comment but I was curious what you would do at the but joints of the plywood. I suppose you would need a vertical member there(as mentioned) or need two layers of plywood. Or else the top and bottom chords would be carrying all she shear that those points (right?).

EIT

http://www.HowToEngineer.com

 

[dhengr]

Yup..., you have to block any panel butt joints, with a 3x4 or 4x4, the added thickness is to get enough nailing width for the two panels, just as you do on shear walls or horiz./roof diaphragms to get the max. shear values out of the shear panels. Look at the APA tables for blocked and non-blocked shear values. As long as you keep the adhesive joints dry, and with the newer adhesives which don’t age and lose their strength, the adhesive are an improvement in these details. They take the shear force in fewer lbs./inch (a shear flow) than the higher shear force at discrete nails. Now, the nails act as much as clamps during the glue curing, and don’t take much of the shear loading once the glue has cured, since they actually have to flex a bit to go into bearing on the wood before they pick up a shear loading.

 

[hokie66]

Why would you have butt joints in a 6' length of plywood?

 

[RFreund]

I was thinking that the entire truss needed to be sheathed. I see now that it is just the ends where the truss is getting shorter. Sometime I just skip to DH's responses...

EIT

http://www.HowToEngineer.com