stair foundation precast concrete

[delagina]

we will be using precast concrete instead of cast in place for the stair foundation.

i just said precast concrete in the drawing.
is that right? the contractor will find a precast vendor for this foundation?

 

[jayrod12]

You can either specify pre-cast concrete designed by others, or show the reinforcing and dimensions. Not both.

If you are expecting the pre-cast to be designed by others you must provide the applied loads and the intended anchorage of the stair above. I would also ensure that the appropriate foundation design information is there (bearing capacity).

That being said, why? It's much tougher to hit proper elevations and leveling of the block when compared to a cast-in-place option. I fail to see how a pre-cast blob of concrete is a more economical and constructible.

 

[delagina]

Project manager wants precast and doesn't want any cast in place concrete on site.
Will my drawing work (as far as purchasing this precast concrete block)?
This is just a small stair foundation.
I already provided the rebar and concrete size for the precast vendor.
No need for them to re-design it.

 

[dhengr]

Delagina:
The 2' dimension, A.B’s. to front edge of ftg., in front of the lowest tread, is not enough length in the direction of travel for a landing or stoop. Make that dimension the same as the width of the stair treads. So, the ftg. will be about 2' wide, not 3'. Check with the local AHJ, they might hassle you about what you show in your sketch. I agree with Jayrod12. Making that ftg. precast and then trying to place it properly in orientation and elevation, seems difficult to me, in comparison to a cast in place flg./slab, with a couple A.B’s.

 

[jike]

How will the contractor achieve a level bearing surface so that it does not settle unevenly? I agree with others that it does not make sense to use precast for this item.

 

[Teguci]

Never heard of precast sidewalks. Is this one of those "the unions made me not do it" problems?

Anyway, I'd favor using something else for the landing and hide the precast grade beam (1'-0" x 4'-0" in plan view) behind the landing. With a nice long threaded rod epoxied in, you should be able to get all the tolerance you will need for final installation. Let the precast sidewalk guy figure out the landing!

Might not need any reinforcement for this foundation.

 

[delagina]

Lol @ replies keep repeating, you don't need a precast. I already said why it's precast. Project Manager doesn't want cast-in place because of the location making it difficult to bring the equipment necessary for cast in place concrete - according to him. I don't want to argue further with him and just want to make this precast work.

 

[Teguci]

Yeah, was actually thinking of piling or treated lumber, but precast is the right choice if you are required to be allergic to CIP.

 

[jayrod12]

Then heed the advice if it seems to be repeating itself. I understand the PMs position, but he likely isn't thinking about the difficulty in getting his compacted base accurate enough. How is he getting the material to provide the appropriate base? Surely there's truck access to the site.

There's also only like a third of a yard required to pour that. If designed properly (i.e. designed for a low concrete strength) they could probably mix it on site.

 

[Brad805]

This seems like an incidental item, so it will depend on the precast company. If you do not require them to issue a sealed drawing, I suspect the information you have shown is sufficient. If they are a PCI certified plant, and you require a sealed drawing, then they may ask for the design loads. The precast company we work for has built parts to the engineered drawings, but in those cases I have never sealed the drawing.

As for leveling, that can be done with sand or fill-crete. Since the stair has steel risers I would expect the contractor will make up the difference in grout under the stringer. Still seems like more work, but whatever the client wants.

 

[TehMightyEngineer]

I'd go with jayrod's suggestion, either show the rebar and have the precaster build it as you've drawn, or require the precaster to design it in-house. They're probably going to like it better to design it themselves than if you detail out the rebar. Another advantage of precast is you'll typically get better concrete than cast-in-place on grade.

We do a number of jobs like this for regions where there is poor access. Shouldn't cost much more for precast vs. CIP to make a difference to the budget.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[BUGGAR]

You could place this foundation on a prepared gravel bed. If you have the equipment to lift and place, you can lift and level as well. Keep a level instrument on hand. Place the footing a little low and put something on top of it (non-concrete, maybe precast pavers set in sand) to make a level landing.
Put more lifting inserts in it, though.

 

[RHTPE]

delagina - One stupid, simple question: Does this CM have equipment on the site to place a 3,600 lb (2' x 3' x 4' X 150 lb/cf) block of concrete in the correct location?

Seems to me to be a bit oversized at 2' thick.



Ralph
Structures Consulting
Northeast USA

 

[wannabeSE]

I am curious about the reinforcing shown. It looks more like column reinforcing rather than a slab or matt. If this is exposed to weather, I would specify galvanized or stainless steel anchor. Did an architect specify the plan dimensions? If there is no pavement or pavers around the concrete, it will be similar to a step with a 24" run and 3.5" rise.

The PM may be concerned about scheduling. No waiting for cast-in-place concrete to cure. (not that this is a good reason, just it is a reason)

 

[thaidavid40]

All those other design considerations aside, setting this chunk shouldn't be any more difficult than setting a precast concrete manhole to a defined elevation. Spread, level, and compact the gravel base and set the pad on top of it. If you need finer leveling control, leave the gravel an inch low, place some filter material on top, and top that off with sand. Set the whole thing about 1/4" low, and shim under the stair stringer bases. Easy as pie.

Dave

Thaidavid

 

[RHTPE]

thaidavid40 - I realize on most normal projects setting a 3600 lb precast element should be simple. But OP said:

Project Manager doesn't want cast-in place because of the location making it difficult to bring the equipment necessary for cast in place concrete - according to him.

If the PM can't get equipment to handle placing about 1 CY of readymix concrete, how will he set a 2 ton precast element? Since we can't see the site or the overall structure from here, it's awfully difficult to judge the logic involved.


Ralph
Structures Consulting
Northeast USA

 

[Ron]

I don't like this!

Have you considered the "pushing" load that will be exerted on this small precast slab? The bottom of the stair section will have a relatively high lateral load from the stair loading. This will tend to "push" the slab unless restrained by a walkway or other obstruction.

 

[delagina]

I changed to CIP and just put a note to use precast concrete if CIP is not possible. Let the contractor decide.

 

[thaidavid40]

@RHTPE -
Regardless of the relative sizes of the equipment needed to accomplish the two, competing concepts, it seemed apparent from the OP's statement that whatever equipment would be needed to set a precast pad was already acceptable. Otherwise, why the suggestion from the PM to use such? And by comparison, a single heavy fork truck would be much less disruptive than a concrete mixing truck. Therefore, my suggestions for how to handle setting such. I paid no heed to his logic in selecting the preferred option, and therefore judged it not, I only suggested a process to accomplish that.
Dave

Thaidavid

 

[TehMightyEngineer]

Our precast delivery trucks have 20 ton cranes on them that can span out 20 feet. Often places where you would have to have a pump truck to get concrete to a spot on a site, we can reach with our truck. Saves a pump truck expense and the hassles of placeing, finishing, and waiting for concrete to cure on site. There are a number of reasons precast can be just easier to build with regardless of what the cost difference is and this is often preferable for the contractor.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com