wood truss modification

[jeng123]

I have some existing pre-engineered wood trusses on a house that I would like to cut back because the bearing location is being moved in toward the ridge.The trusses have a 12:12 pitch and currently spans 25' the trusses would be trimmed back approx. 5' and new bearing support provided. I am planning to sheath both sides of the trusses in a 4 ft width from the new bearing location. The sheathing would be attached to the truss bottom & top chord and 2x4 blocking at the other two panel edges; the sheathing would be attached with a dense nailing pattern. Any thoughts on this approach or alternate solutions? Anyone of aware of any modification design guidance from SBCA or WTCA? thanks

 

[dik]

Can you check with the truss supplier for a fix?

Dik

 

[jeng123]

I would like to but trusses are older, don't know the supplier and it would be a miracle if they still had the drawings

 

[dik]

If you are moving a 25' span support 5' you could be modifying the member forces and bracing requirements significantly even though the span is relatively short. If you know someone that has truss design software or if your local lumberyard has had dealings with you in the past, you may be able to get them to run the truss... else, you can enter it in one of the 2D frame programs and just see what the effects are.

You should have a diagonal web member going from the bottom support to one of the top panel points.

Dik

 

[woodman88]

Calculate the forces in the current design, than calculate the new forces for the design you wish to change it to. Repair any joints with new or higher forces. If a current force is compression and changes to tension, some truss programs would of plated for only half the compression force so it would be best to repair any new tension forces that are greater than half the current compression force.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[TXStructural]

And you have to know the existing plate capacity. You can refer to ICC-ES and prior reports.

Sheathing the sides with plywood and nailing will seldom achieve a desirable end. I have seen this done and seen it result in very poor/disastrous results. You need to re-engineer the entire truss, with no shortcuts. Look at TPI 1, and use the NDS for wood and fastener values. You will probably find that 3/4" plywood and 1/2" bolts will work best, and be sure to maintain truss action. If the plywood is too large, you will end up fixing the members, changing the action of members, and introducing flexure into tension/compression members.

But, you will likely also find that for closely-spaced trusses on 20 foot spans, there isn't that much load anyway.

 

[demayeng]

Yes, like the others said, just install new vertical/diagonals and design it using frame analysis.

The easiest connection is plywood and bolts like TXStructural mentioned.
You can check it, but I don't think fixing the joint would have much effect. You are reducing the span of the trusses so the forces should be fairly low.