Shoring Point Loads on a 5 inch Slab

[chamokinawan]

I have a 5" thick post tension slab in a parking garage. It's a one way slab spanning 18' from beam to beam. The contractor is putting about a 17 kip (D+L) point load (shoring load for floor above) about 3' from the beam on each side. He says he's done it without cribbing before (the footprint is a 7" square plate). That's a huge point load on a 5" slab. I checked punching and it is fine. But my negative bending above the beam is almost 65% overstressed. I can't figure a way around it nor how other engineers were able to justify it. Any ideas? Thanks.

 

[theonlynamenottaken]

That fails the first-pass sanity check on my end too. Any chance you could diplomatically ask who had approved this in the past? I'd be curious as to whether or not another engineer had actually said yes to this without wanting shoring below or cribbing above.

 

[hokie66]

The contractor is a cowboy, and now that you are involved, he wants to put the burden of risk on you. Insist on supporting the load as you would in any multi-storey construction.

 

[Archie264]

You've got your answer but for clarity on my end do you mean two 17k point loads straddling the beam, 3' away from it on each side, for 34k total?

 

[chamokinawan]

@Archie264, Yes.
I figured it was practical, I just thought I was missing something. They'll definitely need cribbing.

 

[Archie264]

Wow. Well, one thing to add then, make sure you protect yourself in being absolutely crystal clear on how long they have to leave it shored. I know of a building that collapsed during construction because the contractor pulled the shoring (or back-props) too soon. The contractor had violated the engineer's instruction on that issue but in the litigation that got glossed over and they still ran the engineer out of business. Something to that effect, at least; I don't know all the details.

 

[MrHershey]

The 'this is how I've always done it' may be true, and other engineers may have even justified a similar scheme in the past. I know I have. But there are always seemingly small items that can change everything. Have checked a similar situation to what you're talking about recently, 6" PT slabs on PT beams spanning a bit further than you. But load on the shoring I've looked at is more like 13k after being shared between a couple floors, not 17k, and only 20" or so away from the edge of beam. Negative moment under shore force is a little bit lower than it would have been for just the live load distributed over the shore spacing. Doing a quick hand calc based on a two span condition, if you make that 36" away from beam instead of 20" then my negative moment increases 60%. Doesn't seem like much to a contractor, so he'll say he's done it this way before. Doesn't understand how taking a half step to his right means the difference between the shoring scheme working and the shoring scheme being nowhere close to working.

One thing I'd question a little further is this only getting reshored one floor as you've noted. If you're pouring a 5" slab, you're looking at 63 psf of normal weight concrete plus formwork/shoring and personnel loads. Would think all that would tend to come out at 2-3x higher than your typical 40 psf passenger car garage load that the floor would be expected to support in addition to its own weight. Unless garage was designed for a much heavier live or finish load or seriously overdesigned. Could be fine and I'm just missing something, but doesn't seem quite right at first glance.