Field-Cast, Early Lift, Perforated Concrete Cap

[Skinnattittar]

Well this one is a doozey to me.

Attached, on the left, you'll see a rough diagram of a concrete well cap, 19ft exterior diameter, 16ft interior diameter. There are an assortment of different openings that have to be cast into the cap. Very heavy loads will be put over these openings, about 20,000# (total) plus additional 100psf live load for maintenance. That cap will be a total of 4ft 2in thick (50 inches).

My job? The engineer of record did not appreciate that this thing has to built! Due to site and use considerations, there is no actual ability to build the slab, in place, as they show it. At least, according to the contractor. So I have been tasked by my boss to design a slab that will be cast on site (but not in place), that will be lifted onto the well, then the rest of the concrete will be poured over.

The hurdles? All the holes shown in the diagram have to be cast into the cap. The contractor wants to wait as little as possible for the cap to cure before lifting and installing. The weight has to be kept to a bare minimum because of site access issues.

My approach; the right side diagram shows the cap with two hatched regions and some dotted lines. The dotted lines represent two beams, I would like to consider as the primary supporting structures in the cap. The hatched one on the right is just for clarity. The hatched blob of a zone on the left side is my assumed tributary area (I can probably dock the ends a bit, but we'll go with this diagram for now). Trying to get down to 12" thick.

My question; is this a reasonable approach? I've designed slabs before, with small perforations, but this one is pretty chopped up as far as I'm concerned. Another issue is that it may not be given 28 days to cure. Also, it will be done in the field, as in, not at a concrete plant, so I'm not sure how that effects things. The only concrete I've dealt with that is loaded prior to curing is as foundation walls, compression only, and then only very lightly loaded. Can you load un-cured concrete in flexure? Do I just dial back my concrete strength 50% for a 7 day cure? Are there any good resources for slabs with large holes?

Additional details; the project is for a municipal water supply, so there are special requirements for the concrete. The locations of the openings are not negotiable due to... I dunno, the contractor doesn't want to deal with it and the EoR is disinterested? Obviously there's good reason for some but I don't know them, so they're fixed in place.

 

[JoshPlumSE]

The contractor wants to wait as little as possible for the cap to cure before lifting and installing.

Of course he does. To me, this is "means and methods" issue. I wouldn't tell him too much about how to lift it and when. I would design it to be lifted, of course. But, I would assume it is sufficiently cured to achieve the specified concrete strength. If he wants that strength earlier, then he'll have to work that out with whoever designs the concrete mix.

I haven't done lifts of something this large before (I don't do tilt-up work). But, that's the type of details and engineering I'd start with if I were designing something like this.

You don't specify where your lift points are. Is this something that you're specifying or something the contractor has already worked out? I think that's pretty important to your design. Probably just as much as the openings. Assuming those beams take the load sounds conservative. I might prefer an FEM type of analysis so that I can see the 2D bending that occurs in the slab.

 

[HTURKAK]

Some points,
- What is the depth of caisson? is it possible to use prop in order to reduce the PC thk 12 in?
- Is this the only option to use 50 in. cap? imo, it is not reasonable to construct flat slab having wt 160000 lbs to support 45000 lb live load.
- What is the purpose of providing 55 eaches sleeve and drilling the caisson wall and anchoring with epoxy grout?

Assuming your option is the way to go, I will suggest,

- Analyze the slab with FEM .. The precast portion shall be designed for the temporary loads ( stripping from the form, lifting and erection, and LL from fresh concrete )..
- Common practice , the precast could be stripped with 0.7 fc. (approx. one week cure ).. You may speed up with steam curing etc..
- Provide hairpin , chair rebars etc. for bond with topping concrete. Look to web for ( filigree slab).
- Provide grout or dry mortar under the PC slab, imo, no need for drilling and epoxy anchor.


If i were in your shoes, i would prefer the use of composite slab. Provide two WF beams parallel to hatch openning short dim. and will be supported on the pockets provided at caisson wall and use composite deck. With this set up, total thk. of slab could be 10 in. not 50 in.

 

[Skinnattittar]

What is the depth of caisson? is it possible to use prop in order to reduce the PC thk 12 in?

I would have thought so, but apparently not. They don't seem confident that they could remove the forms in the space provided.

Is this the only option to use 50 in. cap? imo, it is not reasonable to construct flat slab having wt 160000 lbs to support 45000 lb live load.

I had a similar question. The caisson is existing, and they need to raise the top height to meet new flood requirements. I suggested raising the caisson walls, but they want the mass to deal with the vibration of the equipment? Seems more like the EoR said "bah! just pour a lot of concrete and that'll do it!" but I would also guess that the required parameters could also be close enough to 50" that the added complexity of additional wall is considered unnecessary.

What is the purpose of providing 55 eaches sleeve and drilling the caisson wall and anchoring with epoxy grout?

That is to match the EoR's detail of perimeter rebar drilled and epoxied into the existing caisson.

Provide hairpin , chair rebars etc. for bond with topping concrete. Look to web for ( filigree slab).

Oh! I like the detail for "filigree slab". I'll have to see if this contractor would be able to get them. I was going to go with 90d "loops" cast into the slab then running bar through that for the second pour, which is pretty much the same, but if its a product, that's even better!

If i were in your shoes, i would prefer the use of composite slab. Provide two WF beams parallel to hatch openning short dim. and will be supported on the pockets provided at caisson wall and use composite deck. With this set up, total thk. of slab could be 10 in. not 50 in.

Yeah, this would have been my solution, or something similar. When I asked, they said they did not like the idea of metal over the city water supply, but there's plenty of solutions to that; special paints, stainless steel, etc... I would have thought that a pair of beams across the caisson top with deck laid down across to the bottom flanges (and a series of details for those connections/supports) would be a simpler solution than just saying "yeah, 50 inches of concrete poured over top" with no access to remove the forms.

 

[HTURKAK]

...So the slab thick. 50 inches is for the need to raise the top height of the caisson to meet flood requirements...I am speechless.. Silence is gold.

 

[SlideRuleEra]

The caisson is existing, and they need to raise the top height to meet new flood requirements. That cap will be a total of 4ft 2in thick (50 inches).

Higher water level means the caisson (when empty) will have more uplift from buoyancy. Most cost effective way to resist buoyancy for an existing structure... add concrete to increase dead load, a lot of concrete.

http://www.SlideRuleEra.net

 

[Skinnattittar]

Higher water level means the caisson (when empty) will have more uplift from buoyancy. Most cost effective way to resist buoyancy for an existing structure... add concrete to increase dead load, a lot of concrete.

That's a good point, but that wasn't included in their response when we asked to design wall extensions and use a less thick cap, but on their plans, they point out that the top of the new cap is to meet the flood zone elevation requirement. There is nothing in their documents suggesting that adding weight is the objective. Additionally, the caisson is not water tight from from the surface or from below, so if there was a flood, either from below or across the ground, the caisson would flood as well.

But I could totally be missing something or they're just not including such information in the project documents. Still, you make a good point, that would definitely be a good, multipurpose solution to some project objectives.

 

[Skinnattittar]

...So the slab thick. 50 inches is for the need to raise the top height of the caisson to meet flood requirements...I am speechless.. Silence is gold.

They also mentioned that the mass is part of their considerations to deal with vibration? There are large water pumps set on top of the cap.

 

[SlideRuleEra]

With vertical pumps mounted on the top it's important that the cap be securely anchored to the walls of the existing caisson. There is a good chance that several of the perimeter #6 rebar dowels cannot be installed when the drilled hole encounters rebar in the caisson wall. Locations of the abandoned dowels will likely be random. I see three paths forward if (when) that happens:

1) Ignore the abandoned dowel holes; just fill the voids with grout. Depending how many and where dowels have to be abandoned, somewhat risky (vibration control risk).

2) Drill replacement holes through the cap and into the caisson wall. Since the cap will probably be heavily reinforced (500+ lb/ft[sup]2[/sup] live load for fresh concrete above the cap), drilling through the cap could be tricky, too.

3) Install a circle of additional pipe sleeves in the cap, on a different radius from the sleeves shown. When a dowel hole has to be abandoned, there are other nearby sleeves to try. Fill unused pipe sleeves with grout.

I would go with Option 3, cheap insurance that you get a "second chance" at anchorage when dowels cannot be installed as planned.

http://www.SlideRuleEra.net

 

[dhengr]

Skinnattittar:
I would make a strong pitch for casting that cover slab in place, in two lifts. The first lift 12-16” thk. (150 to 200lbs./sq.ft. conc. weight) and the second lift the remaining depth (400 to 500lbs./sq.ft.), both to act compositely. The contractor is thinking only of some difficulty (his difficulty and problem) of getting the forming materials out of the caisson, while ignoring all of the other complexities which he is forcing on the project, but they are someone else’s problems to solve, and his are not really that difficult. The 3’ x 5’ access should be big enough for 4’x8’ shts. of plywd. (8 shts. for slab forming), some main framing stl.bms., and some other misc. framing and matr’ls. I would build a maintenance/construction platform in the caisson, 7-8’ below the cap slab which would stay in place, (100+lbs./sq.ft. LL) and almost certainly be used for future maintenance needs, undersides of pumps, couplings, shafts, or down-hole work, etc. This maintenance level platform, with shoring, could take part of the load during the first slab pour and the early second pour. This is, of course, some extra framing and cost, but the benefits gained and when compared with a separate casting bed and early (weak conc.) lifting of a difficult/heavy conc. piece, should be a cost competitive improvement to the entire project. The biggest advantage is that you don’t have to lift that green cap slab to allow the contractor to continue on with his work on the second pour.

The contractor’s way, consider…: You need a casting bed which is nicely formed, flat, and well compacted, etc. You need formwork for that first pour, plus the hole/opening block-out forms. You need to hold all those 1” dia. pipe sleeves in place during that first pour, not to mention the difficulties that SRE brought up during final placement. And, finally you need a well thought out lifting plan and equipment, not just two slings. The contractor will want to lift that awkward conc. cover before the bleed water has dried. And, you should figure out which of the openings will start a major crack first, under some quick and dirty lifting operation.

Your way…: You drill and epoxy the #6’s into the top of caisson under better conditions; every other one bent over into the first pour, alternating with 4’ high straight bars into the second pour. One set of full height forming is used for both pours, including opening block-outs. The 25” dia. holes and the 3’ x 5’ access might be formed with light stl. and left in place for an improved finish. Two 14’+ stl. bms., 4’ either side of the vert. c.l. in your sketch, and one stl. bm. on the c.l., and headered off, to the other two bms., at the access opening is your major framing for both the slab formwork and for the work platform below. 8 shts. of plywd. and some misc. lumber and hardware, plus a rolled angle iron ring out at the caisson wall, forms the slab. This same general framing scheme, but grating replacing the plywd. makes up the work platform. You might consider a somewhat huskier work platform supporting some forming system on 4x4 posts and cribbing to support the first pour and then help supporting the second pour, thus saving the stl. bm. framing immediately uder the slab.

Consider the 3 – 25” dia. holes; draw a centerline (c.l.) through the left hole and the center hole and out over the caisson at 19’ dia., then do the same with the right and center holes. Parallel to each of these centerlines and just outside of their respective 25” dia. holes (25”/2 each side of the c.l.) you have about 28” of slab width for 4 crossing beams, an ‘X’ shaped beam layout. The 28” width is determined by the inner corners of the access hatch for the lower two beams. You must determine what the 25” dia. holes are for, I’ll bet they are for three large vert. axis standing pumps which make up most of the 20k loading, and they are the biggest final support problem, so you must know much more about these and their fixings. Now, you’ve confirmed the large pumps. The two beams which make up the two legs of the ‘X’ shaped beam layout, each carry one pump near their lower reaction point on the caisson, and half the pump on the c.l. They contain the tension stl. for the final composite design, and have closed stirrups extending up into the second pour. The rest of the reinforcing kinda radiates from this primary framing beam arrangement. You may not want to pour the 5’ width of the second pour, below the access hatch in your sketch, until after you have removed the slab forming, particularly the longer stl. bms.

 

[GC_Hopi]

I think dhengr's pitch for two lifts is good and as Josh pointed out a FEM analysis would be helpful for the slab design so you can take advantage of the two way action. You would definitely need a strong pitch since the contractor is likely dug in on their idea to have a precast slab.

Other items to consider is the curing and temperature control requirement of mass concrete since you are dealing with a thick concrete section. If you do two lifts with both under 3'-0" thick then generally you can stay away from mass concrete.