Cutting existing precast hollowcore slabs 2

[clarke1973]

Hello,

I need to create an opening in an existing concrete roof structure, which consists of 5m long 250mm thick precast hollow core slabs spanning onto precast concrete beams.
obviously we cannot cut through the pre-stressed slabs, so we are proposing to cut and remove the full slab units (3 no. x 1.2m wide), add some steel trimmer beams to frame out the opening and add some new shorter span HCS spanning onto the steel beams.
can anyone offer some guidance on this? We are proposing to cut along the interface of the HCS slabs with the supporting RC beam (i.e where the top part of the beam was cast insitu with the HCS screed). Assuming the HCS is not bonded with the beam surface (supported on a form of bearing pad), the units should be able to be lifted straight off, once cut...? (in theory anyway)
Does anyone have any experience with this who can offer some advice?
Also, is it possible to fix a lifting eye to the slabs for cranage (assuming the strands can be located, and avoided)?

Thanks in advance

 

[KootK]

I'd think that the planks would come out without too much trouble. You never know for certain though unless you have detailed drawings/shops. There could be plank keyway grouting or potentially rebar in the cores at the end of the planks. And I'm sure that you could through bolt a lifting eye into the planks somehow.

If you're installing new trimmer steel anyhow, are you sure that the existing, cut planks would not work without replacement? It seems as though you'd have a fighting chance there. Perhaps I do not fully understand the situation in plan.

Take care that:

1) the supporting precast beams do not become torsionally unstable in the temporary condition. Highly unlikely but not impossible.

2) the continuity of the roof diaphragm is reconstituted somehow.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[structSU10]

how large of a hole do you need? you could gpr to locate the strands and cut a hole. you can cut a strand if you have to, as long as the plank has reserve capacity. since it is a bonded PT strand, it is no different than when they cut the strands in the shop.

 

[Agent666]

If you had room under the units why not put in supporting steelwork under the slab, resupporting the edge of the existing planks where you were planning on cutting them across the 3.6m width. It sounds like you are doing that anyway? (a sketch or two would help)

The key is to get some load into the steel so it's supporting the hollowcore before you cut it. For this in the past I have used something that you slip into the narrow gap and grout is pumped in under pressure to preload the beam, I think they were called flat jacks but don't quote me on it as it was years ago. Some careful analysis was required to work out the required deflection of the beam to ensure sufficient weight was carried by the beam so that it wouldn't drop when the slab was cut. You then grout all of the gap between the beam and underside of the slab. This way there is no replacing of the units like you are suggesting, it's just a resupporting of the existing hollowcore at the new opening edge.

 

[clarke1973]

Thanks for the responses guys.

I just assumed that you cannot cut these units as if you cut through the strands it would ruin the slab, and render it useless?
The opening is to be 2.8m x 0.5m, so I would need to cut through at least 2 units. Are you saying that I could simply cut the units for the opening (once the supporting steel work is in place of course)? Would the strands not need to the anchored somehow at the edge of the opening?

Thanks

 

[Ingenuity]

Would the strands not need to the anchored somehow at the edge of the opening?

It is unusual for cut strands to pretensioned elements to be anchored, but there are some proprietary products/techniques that are able to be used.

Typically, where the strand is cut and NOT anchored, the engineer will need to consider the transfer and development lengths of the prestressing strand, and it effect on structural capacity of the remain element and it's new support/span configuration.

This old PCI paper [PCI Paper Link] has some good background on transfer and development lengths from an analytical perspective.

 

[KootK]

I just assumed that you cannot cut these units as if you cut through the strands it would ruin the slab, and render it useless?

No, there usually some utility left in the plank.

Are you saying that I could simply cut the units for the opening (once the supporting steel work is in place of course)?

Yes, that is normally the case unless the transfer length business doesn't work out. If your new opening is at the end of the plank span such that you'd be losing 0.5m from a 5m span, I doubt that you'd have a problem.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[OldBldgGuy]

I have no idea how many times I've done this & unless your manufacturers are a lot different than the ones around here, your biggest problem is preloading your new support to remove deflection. The planks only gain more capacity as you shorten them, the strands are fully bonded. Cut a long plank & you improve the performance of what's left. Forget grouting, it only secures what's already there, the best I've come up with is have the new support made with enough camber that it takes the load when you jack it into place. Cut out the slabs & Koot's your uncle.

 

[KootK]

KootK's not your uncle. He's your daddy! As evidence, I submit the slick preloading detail below.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[OldBldgGuy]

He's your daddy