We are jacking up floor trusses in 3

[BCMIENG]

We are jacking up floor trusses in a 3 story wood frame condo 32 years old. Trusses under a bay cantilevered over a concrete bearing wall are sagging up to 2 inches. Also, two internal trusses are sagging up to 1.5 inches between the cross beam and the bearing wall. Structural Engineer has determined the fix which is LVLs nailed to trusses. Contractor wants to jack up the trusses all at once. I want to jack up in 1/8 inch or 1/4 inch increments at intervals of three days or a week to minimize drywall cracking, doors not opening, window popping, etc.. Could anyone recommend jacking-up intervals and procedures? It would be greatly appreciate. Thanks.

 

[Ron]

The deflection didn't happen all at once, so don't try to recover it all at once. If it were steel it would be no big deal, but when wood deflects, the fibers re-orient and should be taken back slowly. The tension side won't be a problem...the compression side will likely crack if you reverse the loading too quickly. About 1/4" with an 8 to 12 hour recovery time at each interval should be enough to allow relaxation. This is only a guess and I have no technical reference for you....just a bit of common sense applied. Would be great if you could wet the wood prior to jacking. Saturated fibers move easier than dry ones and without as much damage.


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[Agent666]

I'd think you'd only be able to take out the instantaneous deflection from the loads currently on the truss, the rest will be locked in deflection due to long term creep (i.e. occurred over the 32 year life of the structure), and not removable by even applying the load slowly.

Once you jack past the point of taking out the existing loading, then you are loading the trusses in the opposite direction which is potentially a bad thing (think elements designed as slender tension elements now in compression).

I don't wholley really agree with Rons approach depending on how much of the excessive sag deflection is due to long term creep and how much is instantaneous deflection. If there is no creep then I agree with what Ron is saying, but after 32 years 1" of that 1.5" deflection might be due to creep, and you're not going to recover this in my view by jacking unless you wanted to jack it over a very long time period (could take 32 years if you apply the same load in the reverse direction!).

The method being proposed is like imposing an elastic deflection of 1.5" on the current truss arrangement in the upwards direction to take out the instantaneous + creep components. Truss may be only able to take 0.5" instantaneous deflection before members fail elastically for example (a guess but demonstrating an effect).

Over the short term of this recovery loading I'd imagine there is minimal creep going on. So I doubt the truss could handle this negative loading condition, but at the very least it's something that needs to be assessed, and presumably the locked in loads in this condition need to be accounted for in the final arrangement after the addition of the strengthening and then release of the jacking and ultimately some sharing of the total locked in loading, normal original loading, etc.

If it were me i'd jack out the current loads or some proportion of them, then strengthen on the basis of the new and old sharing the loads based on the final arrangement. If there were still residual deflections, then I'd look at treating this by some other means like redoing ceiling battens or redoing flooring, etc.

 

[BCMIENG]

Thanks for the great input. We are trying to get the structural PE back in tomorrow to look at our jacking plan and methods and make a recommendation. For the bay area that is cantilevered over the concrete bearing wall, not leveling it is not really an option. I am thinking we are going to have to do the slow and steady method (not 32 years, I am 67) but we will see what the PE says. The negative loading during the jacking is something I am somewhat worried about, perhaps it would be better to jack up the OSB sub-floor on either side of the truss and then attach the LVL. What do you think about that method? Thanks.

 

[BCMIENG]

The more I think about it trying to jack up the OSB would not work at all.

 

[KootK]

Another consideration here is that, in a wood truss, much of your creep will actually take place in the metal plate connector connections. And there's really no way to reverse that without creating slop in the connections that would just get taken up again under reloading.

Will the LVL be designed to take 100% of the load going forward? If that's the case, it may simplify some things.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[Triangled]

i agree with incremental approach, and, in addition to the incremental dimensions, i'd consider some kind of incremental force application too.

that's a lot of deflection

a contractor may sees his job as to just jack up 2" no matter the force involved. if the required force for an incremental dimensional "lift" exceeded the anticipated force by "much", i may want to study the situation a little more.

 

[Tomfh]

The main thing is to lift entire walls together so they keep their shape, as opposed to doing it piecemeal. You can do it all at once with synchronised jack system, or incrementally with separate jacks. I straightened out my house with a couple of bottle jacks. Cracked a bit of drywall here and there, but nothing too catastrophic. I did 5mm or so at a time. If it was all synchronised I’d happily do it in one lift.

 

[BCMIENG]

Thanks for all input, jacking is progressing in increments and almost complete. A couple doors need to be trimmed and minor drywall cracking otherwise all is good so far.

 

[Ron]

As expected!


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