Plumber Going Crazy on Structural Slab!

KarlT · Oct 23, 2003

I am involved in looking at an existing cast in place floor slab that has had numerous holes cut out of it.

The slab is a 7" concrete structural slab spanning between walls and it sits on 6" of void form. Below the void form is sand. We do have drawings of the other areas of the shopping center but not in this particular location.

I was called down the other day to look at an L-shaped opening that the plumber had cut into the floor slab. The contractor realized it was a structural slab and called us in to look at it. (He was assuming it was a slab on grade based on the other visible areas that had obviously been cut out in the past to install other plumbing).. Some years ago another plumber had cut out large diameter circles from the slab, done his piping and then in-filled it with a 4" slab, ignorning the rebar that was cut.

I came up with a detail to restore the capacity at the new opening in which the slab at the opening is chipped back to expose a min. of 8 inches of the existing slab steel. (It appears to be a two way slab with 5/8" bars @ 8" o/c e.w. bottom) Then a full-strength tension splice is made between the existing steel at each end with new bars using a flat plate and double flare groove welds.

Now, this recent opening is a result of a large renovation that is underway to an existing dental office. the shopping center is concerned about the other areas of the floor that had been cut in the past and they want our recommendations on what to do. The last thing we want to do is further disrupt the tenant and tear up the whole floor area, it would effectively shut down their operation.

Any ideas on what to do?

Maybe we can pour some local footings under the slab?

PS-The soil below the slab should be fairly stable since there are basements on either side of this main floor slab.

SlideRuleEra · Oct 23, 2003

Could you fill the entire void between the sand and bottom of the slab with flowable fill (mixture of fly ash and cement)? This may effectively make the existing slab a slab on grade. Flowable fill can be easily dug out in the future with conventional equipment if access in need for maintenance.

rlewistx · Oct 23, 2003

I would answer the question "why was it built that way?" first. There must be a reason whay there is a void space or they wouldn't have gone to the expense of putting in a structural slab. Is it on expansive soil?

The second issue I would address is why are they concerned about past holes cut. Are there signs of structural distress? If so then address those areas. I would then come up with a maximum hole size whereby you could say to the owner "if you have cut holes bigger than XX square feet in the past then we should repair them. Holes less than XX square feet should be okay as is. That will minimize your investigation. Keep in mind that there may be special conditions where the XX square feet may not apply.

HTH

cap4000 · Oct 23, 2003

Short of doing nothing, you are going to have disruptions.
I think you have the best fix, chip back 8 inches all around and reinforce accordingly.The ("Sure Void"?) forms is there because of perhaps of an expansive soil condition and thus the structural slab design. Footings are not recommended. I suggest you get a geo-tech involved as the sand may be just a cushion layer. Good Luck.

Bbird · Oct 23, 2003

Seems to me the shopping centre is under continuous development. Damage may not be serious if there is no noticeable settlement in the slab. You should get advance warning in the cracking of the slab and the floor finishes. Restoring the reinforcement and patch up the slab structurally is the right thing to do. I know it isn't a nice job to tidy up other people's loose ends but the services were obviously needed by the tenants otherwise the plumbers would not have to butcher the slab. They are not structural guys and would not know how to make good or go about the cutting.

It is not bad idea to reach an agreement with the shopping center that all future opening up of the slab over a certain size must be reviewed by you before the execution, otherwise the builder’s work by plumber, electricians and the alike could eventually cause expensive damage to the shopping center. The plumbers must get permission from someone before they can bring in the heavy gears to hack out the 7” thick concrete slab.

To beef up the slab one can always pressure grout its underside.

Focht3 · Oct 24, 2003

If problems haven't manifested themselves by now, then the previous repairs may be okay. This becomes something of an owner's risk/reward decision unless you can identify specific problems that you know must be repaired.

For small areas, you can epoxy a "thin" steel plate to the concrete slab. "Enough" overlap should be included to provide sufficient protection from a catastrophic failure. This type of repair could be done on the weekend or at night to reduce disruption. Of course, the edge details and floor coverings could make this approach problematic. Not to mention the problems that could arise if the floor isn't very flat to begin with...

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KarlT · Oct 24, 2003

Thanks for all the responses everyone!

Had a big meeting today with the client. We may be able to get existing drawings of the slab from the city so that would help a lot.

It is possible that some else examined the existing openings and determined they didn't need further reinforcing. The main reason why they probably went with a structural slab is that there is fill below the slab, so if they did a slab on grade it would have settled after construction. However, the slab was designed assuming it was in a shopping mall and then the Dental Office became a tenant so there is a possibility that the original engineer wasn't aware of these slab holes.

The other nice thing is that there are a couple of 1000# sterilization units that are sitting on an area of floor with a previous trench. (By the way.....Does anybody have a method for analyzing a two way or even a one way slab with a point load on it other than yield line theory?)

What I'll have to end up doing is map out the openings relative to the info on the structural drawings and then see if these opgs are ok.

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Plumber Going Crazy on Structural Slab!

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