Jacking Pre-engineered wood floor trusses

[jplay2519]

I have some 12" pre-engineered wood trusses that I have to provide shoring and jacking plan for. I don't like trying to jack a bottom chord at a panel point where diagonals come in (i'll run an analysis to see if the diagonals can handle it I suppose) or a between panel points. Is something like a parallel member under each truss approximately 4' long (length can be changed) then having your typical jack and beam perpendicular to push everything up work? The thought is to help spread the load out but I feel like if you hit the point load that parallel member won't spread the load much unless it's exponentially stiffer than the truss itself. Any thoughts? This is just a general question on procedure, i'll have to run loads and everything but some things just aren't good to do no matter what the loads say.

 

[CANPRO]

What is the purpose of the jacking? That in part will dictate your jacking location. You're going to reduce your clear span of the joists by placing the jack points some distance from the end, but you'll also create a cantilever where the joist spans past your jacking point. Do you need to jack just a few joists or a series of joists? If your "parallel" beam is stiff enough, you may effectively reduce your main span and cantilever of the joist.

How are the webs connected to the chords - finger joint, metal plates?

 

[jplay2519]

The purpose of the jacking is to do repairs to supports that have been damaged. It will be 4-5 trusses I'm series. Once the damage is repaired we will release the trusses. They are attached with plates that have been compressed to install at joints.

 

[KootK]

I'd jack at the joints if you can and not bother with the spreader thing. The competing stiffnesses of the systems is a valid point. That said, it's probably 2x4 on the flat chords and it would take much of a spreader beam to compete with the inter panel stiffness of that. I'm just not sure what the extra effort really saves you as you should still wind up having about the same induced shear in all of the joints.

 

[jayrod12]

I'd be putting a shoring beam under the bottom chord at the first interior panel point that supports all affected trusses, and put a couple of shoring posts under that beam. In most cases, it is my experience that the web members are actually better in compression than tension due to the connections. So if you magically were inverting the load on the last diagonal temporarily, it would not be a catastrophic issue.

 

[KootK]

I agree that a distribution member perpendicular to the trusses may be a logical thing to do.

 

[CANPRO]

If your jacking point is centered on a panel point, I don't think you'd ever get the spreader beam to truly distribute the concentrate load from the jacking point to adjacent panel points. If you really need to split the load between multiple panel points for some reason, I'd center the jacking point between to panel points and then use your spreader bar (which would need to be much stiffer than your bottom chord) to distribute your load evenly between adjacent panel points. But with a shortened span your web loads should go down - you're probably going to turn a tension web into a compression web doing this, but for a wood joist I don't think that should matter much.

I'd run a single jacking beam under the panel point to span your 4-5 joist and use (2) jacks per jacking beam. If needed, you could add blocking in between the top and bottom chords at your jacking points.

 

[jplay2519]

Thanks for all your input. I think loading it at the panel points and having some jacks with a perpendicular member is the best option. I looked at the site and there are 3 elevated levels that need to be done so I'm going to have to figure out how to stack these. Panel points loaded from the bottom don't line up with the ones above likely. Sigh this is turning into a cluster extremely quickly. All because the original contractor did not provide flashing at the deck and the wall and header are rotted now down the building.

 

[jayrod12]

I think you'd be pleasantly surprised at how closely the panel points line up provided loading and spans are typical floor to floor. Make sure you solid block the floor cavity if you're providing shoring posts on each floor level (which to me would be recommended to avoid having the lower level need to bring 100% of the wall load over to a single shoring beam.

 

[OldBldgGuy]

Whenever jacking or temporarily shoring OWSJ or parallel chord wood trusses I always locate the support at a diagonal intersecting the top chord and put in vertical supports from the bottom chord to the top chord. That essentially ensures that all you are doing is shortening the span on the original design.

 

[CANPRO]

Beware when stacking your shoring that you could overload your webs from the new point loads. You should specify the joists be blocked at the bearing points.

Or better yet, run your jacking beam perpendicular to the joists, and offset the jacking posts from the joists...line up the jacking posts from floor to floor and you have direct bearing through the sheathing.