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130 year old girders/trusses bowed

130 year old girders/trusses bowed

(OP)
We are rehabbing a 130 year old courthouse. The roof framing clear spans about 50 feet over a central core using 7' to 9' deep girders built up of 1x12's. The webs are two layers of 1x's (flat) diagonally opposed to each other. The chords are then nail laminated to them and consist of 6 or 8 1x12's with staggered splices and staggered laps at the corners. The ceiling joists frame into the bottom chords. In some areas the rafters frame into the top chords, bracing them. In other areas the roof is higher and the top, compression chord, is not braced. These unbraced top chords have bowed up to 4" in 15' of unbraced length.

I plan to stabilize them by through bolting WT's to each side. My question is should I just stabilize and shim out to the WT's or should I attempt to straighten them by slowly tightening the through bolts? The finishes are being completely removed thus damaging them is not an issue. The duration of the project will be many months allowing some time for the wood to deflect but not 130 years. I have contemplated calculating the WT deflection for 50% of Fb allowable and telling the contractor to tighten the bolts to that deflection and then shim and snug them up. Has anyone had experience with this type of repair?

Thanks for your help.

Ken

RE: 130 year old girders/trusses bowed

I have never done this type of thing before, but with wood this old and the memory it has incurred, I would be very reticent without more information to try to bring the top chord back into plumb. The forces to do that could be tremendous. I don't know what would happen, and any failure could be catstrophic in nature.

I would lean to just stabilizing the top chord and preventing further lateral movement, pending further investigation.

Mike McCann
MMC Engineering

RE: 130 year old girders/trusses bowed

(OP)
Thanks Mike. I tend to agree but am concerned that without some pre-loading there will be additional deflection as the WT's take load. They are relatively flexible but unfortunately there is nothing handy to run a perpendicular brace to.

Ken

RE: 130 year old girders/trusses bowed

I would avoid drilling any holes through the chord!
Can the trusses be jacked up to relieve the load and see it they straighten out? I think I would try that rather than trying to force the chords in line with the side members. There is probably a good bit of load sharing going on with the adjacent trusses and it would be a good idea to temporarily support them all before attempting anything.
Where in the country is this? In snow country?

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