wood truss repairreplacemen

huangyhg

wood truss repair/replacement
i have a new project to repair factory fabricated wood trusses that have a failed bottom chord.  the building was constructed about 30 years ago and two trusses have failed because of knots located in the bottom chord (2x6).  the span is about 40' and spacing is about 2' o/c.  the trusses are parallel chord and the o/o depth is 4'.  both bottom chords have broken through and opened about 1/2".  failure is likely overstress due to large knots taking up approximately 1/3 of the section area.
i'm not sure what precipitated the failure; the trusses have worked fine for 30 years with much heavier snow loadings... possibly one truss failed progressively due to the knot and the overload on the adjacent truss due to failure precipitated the failure of the adjacent truss...
the problem with repair is the large amount of mech and elect equipment in the ceiling space.  it's not an easy repair.  i don't know if the truss plates have been compromised, although they look ok.  
any repair suggestions... plating the chords with steel strapping and glue? (i'm big on glue these days)replacing the trusses? repair using cables?  carbon fibre?
dik
dik:
if, in your opinion, the break is casued by the knots in the orginal material, can you use new 2x6's each side of the broken chord   
the problem is that it is in a very high stress area... it's possible to jack up the offending trusses, cut out the failure area and to strap the removed material... i was a little reluctant to use a wood repair...  but will consider this.
dik
if it is always in tension, can you use nailing plates either side of the bottom chord? simpson has a number of strap nailing plates that would possibly do the job.
check the deflection.  jack back into position if necessary - go a little beyond.
find tension axial forces.  wl^2/8 / depth is close enough at centerline.
make a wood or steel strap repair based on those forces.  do not cut any material.  why bother....
release jacks.
you might want to post in the truss engineering design forum.  there are some sharp guys there that do this every day....
thanks, mte... i was planning to preflex the truss a bit and was thinking of cutting the broken area away to remove any interference with the jacking operation.  it's the first time i've seen a pure tension failure of a bottom chord and here i have two of them... a little more of a curiosity, is that this has not happened in the 30 year history of the truss... and the loading wasn't that excessive (i still have to look at the possibility of some ponding).
i'll post a thread link in the truss forum...
dik
i think anything i add would echo what's been said about plating either side with wood or steel.  i would make note to make sure your replacement also provides the same or better axial and flexural rigidity.  you don't want to add any local flexibility by using a stronger, more flexible fix such as a simple steel strap checked only for strength.
dik:
might consider using a simpson rps strap ties each side of the failed chord to carry the tension load. you can add mutliple straps to each side of the 2x6.
see page 179 of the simpson wood construction connectors c-2007  catalog.
make sure you check wind uplift on the truss. if the bottom chord can go into compression, you might want to consider the wood solution. you may be able to get msr wood from the nearest truss manufacturer to deal with the higher stresses.
dik,
i have designed channels and threaded rods to repair trusses where the plates were not correctly installed on the bottom chords.  i called for a channel on each side of the bottom chord on each side of the break (4 total) through bolted to the wood.  each channel had a plate welded to the end near the break with a threaded rod attaching it to the channel on the opposite side of the break.  the threaded rods were then tightened to take out any slop in the bolt holes.  i did not try to correct the deflection.  hope this helps.
these are all interesting solutions.  i am very curious to know the cause of the failure.  i also have a truss failure i am working on (tension breaks also).  
i would think that as far as the knots go, that would go into the initial visual grading and allowable stress of the lumber.  i see where the knot itself creates a problem by weakening the board, but this should be accounted for in the grading process.