Unloading existing joists for reinforcing

[haynewp]

I apologize if this is hard to follow.

For the process of reinforcing existing floor joist diagonals due to occupancy change:

Option 1. Not installing shoring to counterbalance the existing dead load and welding on the reinforcing would leave prestress in the existing joist members and the new reinforcing could only share the subsequent new loads. The prestress in the existing members would have to be accounted for in the design.

Option 2. Jack up the joists from the bottom chord to counteract the existing slab dead loads would put all web members temporarily in compression.

Option 3. Jack up the joists from the top chord to unload the joists. This would eliminate the web members being in compression when the reinforcing is welded on and after unshoring, all old and existing members could share the load without prestress.

#3 appears to be the obvious easiest answer for me. #1 is pretty straightforward to account for the prestress in the existing members.


If #2 was chosen, I would have some concern about the joists bucking to the side during the lifting process underneath the bottom chords. Just for discussion, assume the contractor braces off the bottom chords of the joists so sideway buckling is not possible and this is the desired method of unloading the joists prior to installing the reinforcing. With all existing web members in compression, the new reinforcing is welded on under option #2.

Stage 1:
The bottom chord shoring is assumed to be slowly released. The existing diagonals have built in compression that would try to go back to equilibrium by applying tension into the new web reinforcing. How much tension is applied to the new reinforcing is a function of the amount of compression that was put into the existing diagonal and the ratio of the area of new/old steel at each diagonal.

Stage 2:
1. For the diagonal web members that would naturally be in tension under simple span floor loads, additional tension would be applied to the new reinforcing and tension would be applied to the adjacent existing diagonals that were initially compression prestressed from the shoring. However, the new reinforcing would see tension from the floor loads combined with tension transferred from the existing diagonals that were prestressed so this new diagonal reinforcing should tension yield prior to the existing adjacent diagonal yielding.

2. For the diagonal web members that would naturally be in compression under simple span loads, the existing diagonals would have some built-in compression remaining from the jacking and reinforcing operation. New compression would added from the floor loads that could cause these existing diagonal members to start to buckle prior to the new reinforcing buckling which is attached to them.

Any opinions on what I have stated for the option #2 analysis?

 

[JedClampett]

I would ask, "How badly does using option 1 impact design?" If it's a little more steel or harder computation, I would weigh that vs. the complexity of a shoring system that a contractor could easily screw up. And you're putting joists in a loading condition that they've never been designed for. You might get local deformations (crushing) that could seriously weaken the chords. The shoring might get in the way of the fixes.
Unless there's a doggone good reason, I would follow the KISS principle and reinforce in place, as is. Watch out for welding adjacent to the roof, as the roof membranes are normally flammable. If there's a way to minimize the floor loads (removing furniture, carpeting, interior walls, etc.), do that.

 

[BAretired]

I don't like welding joists in place because it weakens the members where they are heated. I would shore the structure to the underside of deck and perhaps snug up the jacks a bit to relieve the members of some of the dead load stress. Live load should not be permitted during remedial work.

And finally, I would overdesign the new members if residual stress is a concern.

BA

 

[haynewp]

Thanks, I think option #2 would just be too much aggravation and putting compression in members that were not intended for it.

 

[dicksewerrat]

I'd go with BARetired, placing the shoring parallel to existing.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Ron]

Agree...shore the deck, not the joists, but be careful of joist/slab connections if that's an issue.