1940s post tensioned slab...stressing out?

[efsinc]

The existing slab exhibits significant sag, the client's rep tells me. I am thinking the solution may be to bypass any slab strength at all, to jack the slab up to level and to install a completely new structure in steel to support the slab from underneath. At the end, the existing slab becomes just a kind of antique topping on my new steel floor system.

I am not well experienced with either post tensioned slabs nor 1940s construction methods.

Is there a jacking procedure or cautions you can guide me to?

Any pitfalls in this concept you can steer me away from?

Thanks!

 

[ishvaaag]

I would say one should ensure none but those essential to the work be present. Then proceed always slowly, and always all in an almost completely shored status, that must be inmediately restored or automatically maintained by some kind of continuous mechanical cleavage. Also, investigate if there is any likelihood of lateral failure, or buckling at the shoring or jacking parts, local or overall. When jacking, in my view, it is more efficient to follow the deflections. So, map your deflections and where more, jack up. Less force to exert. This should be amenable to some somewhat regular pattern that you may jack up iteratively in a way that following the structural adaptation can be analyzed and followed. It also would guide about wanted or permissible interferences with your new steel structure.

 

[theonlynamenottaken]

If the sag is really that significant" you may have quite a few cracks appear in your slab's top face as you jack. Just something to consider - in case the client would want that addressed for aesthetics. Of course, I've sold exposed minor structural damage to architects and owners before as being "character". They eat that trash up... just call it "distressed".

 

[InDepth]

What's your planned sequencing? Are you shoring, then releasing the PT tension, and then jacking?

Why don'y you call these guys to do a design-build? They would probably suggest an external post-tensioned system (this minimizes weight and you can weave it around any mechanical obstructions).

http://www.structural.net/Structura...ralPreservationSystems/tabid/413/Default.aspx

 

[BAretired]

I don't believe post-tensioning was being done in the 1940's. If there is sag in the slabs, I would be inclined to assume that the post-tensioning was flawed and cannot be trusted. I agree with your suggestion of providing an independent structure underneath. And even at that, I wonder about the capacity of the slab to span between beams.

BA

 

[msquared48]

What indicators do you have that it is a PT slab?

If so, how thick is the slab and what are the spans between column lines? Any drop panels? Is this the podium slab for an existing apartment building above?

If it is a PT slab, I would be worried about any jacking as you are creating support points, even though they are temporary, for which the slab is not designed. There could be a sudden uplift failure.

Mike McCann
MMC Engineering

 

[ishvaaag]

What msquared48 introduces is an important worrying aspect needed to be cared for. If one can ascertain is truly postensioned, you may need to anchor and counterweigh as well against upwards movement prior to attempting to release the actual prestress force.

Foremost, the sequence of the works needs to be analyzed for the standing presstress present. Jacking up and external postensioning both will enter the corrective upwards movement that introduces the risk, and the assessment of it will depend on the standing prestress forces and the applied forces in magnitude and position.

If ungrouted all this can be forfeited by just relieving the strands of jacking force after shoring; making cuts in the strand at the slab points where the strand shows adhesion, if some.

But if grouted this can be a worrying problem since there's no way of getting rid of what is an excessive not counteracted level of prestress force for the introduced shorter spans. For that case you may use your new structure as an additional one to that existent, the sum of the two making for the wanted behaviour in the bigger thickness of the structural floor. Yo do not move the existing slab, just shore it structurally and add a leveling layer atop. The new structure needs then afford for whatever the old does not provide, or even by the two.

 

[rapt]

ishvaaag,

"an excessive not counteracted level of prestress force for the introduced shorter spans"

I think you should have another think about your comments and the problem you are suggesting exists.

Yes, there could be a problem as Mike suggests with support points where there is no top reinforcement. Or positive moment points where there is little or no bottom reinforcement and capacity. That does not create the problem you are suggesting.

My first thought on reading this is I doubt that it would be post-tensioned in the 1940's. The first major briges were post-tensioned successfully in the mid 1930's I think. I did not think post-tensioning was used in buildings until the mid 1950's.

 

[msquared48]

If the slab is not PS, this just could be a long term creep issue.

Mike McCann
MMC Engineering

 

[ishvaaag]

rapt, surely, I was thinking on the scenario suggested by msquared48 when jacking up a point and say halving the effective span ... and of course no counteracting (enough rebar or whatever); sorry because I may have messed a bit the issue with the whole second post yet anyway something to consider...if postensioned. I also think it unlikely for that era anywhere, but that's what is said as a given ... to check.

 

[rapt]

Mike,

It could be worse than that if it is an RC flat slab. If I remember correctly, a lot of flat slabs were designed/built in those times based on designing for half of the load in each direction. If this is one of those, then it is under strength by 50%.

Otherwise, yes it could be a creep/shrinkage deflection problem. Most designers, even now, do not consider this adequately in their design, using simple deflection check deemed to comply rulkes rather than checking it properly. There are lot of flat slabs out there with very high deflections!