Storage Tank Slab Opening 1

[wcl77]

I need a bit of guidance in tackling my most recent work assignment. I have a RC rectangular box storage tank (60-ft long and 20-ft wide and 13-ft deep) that contains sulfur. The storage tank is located in a refinery. I need to punch a hole (18-inches x 30-inches) in the top slab (13-inches thick with No. 6 @ 8" e.f. e.w.) to permit the installation of a submersible pump. There is no additional load on the top slab as top of slab is 6-inches above grade.

I am interested in methods of analysis (ACI 318?) to determine if the existing slab will support the load from the new pump (approx. 4.5 kips dead weight and 2.5 kips operating weight) and possible retrofits to the slab in the area of the new opening to ensure a safe and reliable design (already plan on sealing cut concrete with chemical resistant epoxy paint). My gut tells me there is plenty of flexural and shear capacity in the two way slab.

Any other pointers will also be appreciated.

 

[dik]

Posted this with your other query...

Keep the hole, if you can, approximately 10' from the exterior wall; this will take you out of regions of -ve moment. Have the long dimension of the new opening parallel to the 20' span.

You can check the reinforcing cut, likely a max of 3-#6 bars... The #6@8 T&BEW seems like real overkill but you can do the sums since I don't know what the loading is. If necessary to reinforce the slab because of the opening, then carbon fibre and epoxy might be in order. Check to see that there is no adverse reaction of the concrete and repair to sulphur... there shouldn't be... but check.

 

[miecz]

I really think this needs to be properly analyzed, and I know of no method short of finite elements that will do it. I've never seen a publication that says "when you poke a hole through a concrete slab, in lieu of adding framing, here's what to do...."

So I would model it up with finite elements. Follow dik's advice and keep the hole out of the center of the slab. Model the hole one or two bar spacings larger than the actual hole, and you will almost certainly not need to reinforce it.

First thing you need to do is determine is whether or not a truck could drive onto the slab. Whenever I have a tank cover 6 inches above grade, I design for HS20.

 

[dik]

Unless I had unusual loading, I'd just take this as a 'quick and dirty' problem and just cut the hole as noted; I wouldn't normally use a FE analysis. I'd try to keep the opening away from the middle of the slab in the 60' dimension, but place it in the middle of the slab in the 20' dimension to try to take advantage of limited support provided by the end walls.

Dik

 

[miecz]

wcl77

I agree with Dik, so long as you have enough experience with these situations that you trust your judgement. My advice was based on the assumption that this was the first time you were looking at this type of thing.

 

[dik]

Some people might think that it's best to tuck the opening into the corner, but corner 'levers' can develop due to the two way flexure. This puts the top of the slab into tension and might cause cracking. Putting the opening into an area of +ve moment at least the cracks occur on the bottom <G> and aren't as visible.

Dik

 

[chicopee]

I would instead build an external,self supporting steel frame work for the pump.

 

[JedClampett]

I would pour a "curb" around the opening (before cutting it) with enough capacity to make up the missing reinforcing when it acts like a beam. I would also use the curb to mount a hatch. Note that the curb will probably have to longer than the opening in one direction to to develop the reinforcing.
You can analyze an opening all you want with shiny tools, but you're at the mercy of as-built conditions. Plus when an opening is saw cut, the blade tends to overcut beyond your specified size.

 

[civilperson]

The analysis of the concrete top as a simple beam spanning 13 feet will show excess capacity over the existing dead load. Figure how many feet of width you must share the load over to equal the pump and accessories. Build a frame under the pump to spread the load over that amount of top, (ignoring the portion with the hole).