Wood Truss Fix... 1

[Thmierzwa]

Hi all,

I have a 24' long Spruce Pine Fir Fink Truss (all 2x4) that have a broken top and bottom chord along the left end of truss before the web splice plates. The peak is 4' high and the web diagonals connect the bottom chord at 8' from the end (spacing is 8-8-8). The top chord connection for the webs is about 6'6" along the top chord from the end (corresponding to about 6 feet in the x direction from the bottom chord end). I want to design a fix since replacement would be way to costly.. My loads are 50# snow and 12# DL.. The spacing is 2 feet on center. Currently I plan on splicing along the top chord, second web leg, and bottom chord on good wood using 3/4" CDX sheathing and a plethora of 10d?? nails/screws. Any idea on a pattern, size, length of the splices/gussets on the members?

 

[jike]

If you know the truss supplier, he can design a fix for you. They do it all the time!

See if you can find a grade mark on the members that need to be spliced. If they are MSR (machine stress rated) then they might be alot stronger than the material that you are going to splice with. Either obtain the same MSR material from a truss supplier or make sure that you use something equivalent in strength.

 

[msquared48]

Any idea why that particular truss failed in the top and bottom chords, and only that truss?

Sounds like it could be bad materials or knot locations. If so, be cautious with the fix.

Mike McCann
MMC Engineering

 

[K2skis]

I do truss repairs day in and day out draw me a picture and I can give you a repair quickly (ken@kweng.net). One truss rarely if hardly ever just fails. It wouldn't surprise me if they installed it originally like that. I have seen it all in regards to truss repairs I think and what people do with them out in the field.

1/2" plywood should be fine but nothing wrong with 3/4"

I would use 8d 2-1/2" max length nails if you have the room you can use 12d or 16d and get double shear, but I have seen this poorly done in the field many times. They drive all the nails from one side pushing the plywood on the opposite face out away from the truss.

 

[KootK]

I like wood screws in combination with glue. Numerically, I wouldn't count on the glue at all but it's nice to have for reserve capacity.

I like both the screws and the glue foor the same reason: they help to hold the repair tightly together.

I used to work for the Wood Truss Council of America. Alhough the industry doesn't advertise it much, both truss plates and nails have a tendancy to work their way out over time. This is particularly the case where there are large fluctuations in the ambient humidity. Scary, I know...

Plate manufucturers usually provide values for their plates assuming that the plates will not be snug tight against the wood. Ostensibly, this is to account for truss fabrication tolerances though.

 

[Thmierzwa]

The reason why they failed is because a large tree pierced the roof and snapped the chords...

 

[archeng59]

There was a great article in the Nov. 2008 issue of Structure magazine about repairing wood trusses.

http://www.structuremag.org/article.aspx?articleID=797

 

[KootK]

That article's a great find. Thanks for sharing ArchEng. Pretty tough to argue with Fox's credentials...

 

[MiketheEngineer]

And I too like plywood, glue and all-purpose screws. Much easier to intall and MUCH less chance of cracking the ceiling drywall. A hammer just vibrates everything....

As to the actual fix - and not knowing all the details - probably sister rafters on each side about 4' long would more then solve the problem. IF the break is at a panel point where the webs come in - then 1/2'' or 3/4'' plywood is the rule.

This is a SMALL truss - so the forces are really quite low.

Try fixing a 70' or 80 truss - now you have some problems...

Good luck

 

[KootK]

Specifically, I've been using the simpson SDS screws. They're self tapping and easy to come by. You know, that, and they make me feel all warm and fuzzy inside.

 

[Thmierzwa]

In calculating the forces, I came up with about 3500# axial compressive forces in the top chord at the end and about 3300# axial tensile forces on the bottom chord (using 50# 12# DL) (spacing is 2' on center)...

A low value for 10D common nails in single shear on OSB is about 95# ea more or less right?