Reinforcement for large spread footing 3

[sentrifice]

Hi All,

I am working on foundation design for a Battery Storage enclosure and had some concerns. The design will go to a senior for review but for my own development I'm trying to take it to completion.

Background
The battery enclosure is a modified and braced shipping container, designed by others (8' x 30' x 9.5' tall). Total weight = 70 kips. Installation is in California so seismic loads are considered.

Current Design
Right now I have the foundation design as a 11' x 33' x 1.5' thick slab, with a bottom mat of #8 rebar 16" O.C. each way. The container will be set directly on the pad and anchored with 14 anchor brackets using epoxy anchors.

The bulk of my analysis was done by modeling the foundation as an isolated footing, where the pedestal is the width and length of the enclosure, and the footing is the size of the proposed foundation. I checked the following against code: soil bearing, overturning, sliding, flexure, one and two way shear, and minimum reinforcement.

Everything is checking out as far as code calculations go, and I've scoured the building codes for quite a while, but I feel like I'm missing something.

To double check myself I also modeled it as a "beam over elastic foundation" using the subgrade modulus in the geotech report and it showed max deflection of 0.124".

Question
Is there any reason to have a top mat of rebar? There is no calculated uplift or tension on the top of the slab. The anchor bolts also do not require any reinforcement since the anchor loads are low enough. One concern I have is of some kind of settlement related issues that might put the top into tension or cause cracking. The geotech report says that strip footings should be able to withstand 1/2" of differential settlement for a 30' footing. But I don't have a good feeling for if settlement is a concern for this type of installation.

Also, if anyone has any book recommendations on the topic I'd be happy to add to my growing collection.

 

[sentrifice]

@brad

I have come to agree that at this point its overkill. But my conclusion is if I want to change it any further, I need to understand the design and loading of the container. If I come to find out the load is supported at the edges, I could do perimeter beams and make the center much thinner.

But if the load is uniformly distributed like I originally assumed, do you think I could still get away with a thinner (~6") slab with perimeter beams? In other words, just because there is loading in the middle doesn't mean the foundation has to have a square cross section right? I would just have to make sure the 2-way slab (the thinner portion) can support the distributed loading with supports at the edges and elastic support from the soil in the middle?

I'm just thinking ahead until I get the information from the enclosure supplier.

 

[AskTooMuch]

Sentrice,

You need to look at the construction point of view on this. If you plan on excavating 5', how will put back compacted backfill if you would put perimenter beams. Also what kind of perimeter beam detail will you have. The savings in material may not compensate the more labor cost. This is only 11' wide.

I'd keep it simple, I'd just make it all same mat depth at maybe 1' thick.

I normally deal with construction contractors in the plant though that charge way more epxensive labor than in commercial projects.

 

[sentrifice]

@ask

Another good point. By going to 6" center with perimeter beams you're only saving ~6 yards of concrete (if i did my math right) vs the contractor having to do all the rebar details in the beams.

 

[SWComposites]

You mentioned this is in CA and on an island. Hmmm, not too many of those that are inhabited. Catalina?

 

[Brad221]

Assuming the floor and roof span the narrow dimension and the load is all delivered to the 30' sides, that's 35k/30'= 1,115 plf. That's a very light loading that could easily be supported on a small strip footing even if the bearing capacity of the soil is garbage. With the top 5' replaced, you'd have good bearing material, I'd assume. This assumes the walls deliver a uniform load and the 16 piers meant that the container walls are capable of spanning between.

Of the 16 piers, assuming 8 per side, that's 4.25' spacing. So if the loads are not delivered as a line load but that the container has 16 supporting points, you'd get about 5 kips per interior support and half that on the end supports. Again, that's not a very big load and would require small footings (and sonotube depending on the depth of footing). Or light helical piles, which might eliminate the need for the removal of the 5' of soils the geotech wants to avoid.

But I used "assuming" so many times above - we just don't have enough information to do anything more than suggest some possible solutions to this type of problem. And which of the possibilities is the best solution (least expensive since they can all probably satisfy the structural demands) depends on so many other things about the site, access, material availability, etc. Based on the limited information we have, in my location the helical piles would likely be the least expensive option.

 

[human909]

My revised design will reduce thickness down to 14", which will maintain a 12" embedment with room for variation in grade and will keep the surface above grade a little bit (please chime in if this is flawed logic).

I think you're logic is good there. It isn't the most 'efficient' design structurally but it is the simplest and that often what matters particularly for small constructions.

Its a fixed price job, and they certainly have had input about the costs of other parts of the job, so its interesting. From what I've heard there's not even a batch plant on the island. Yes, island. But anyways thats where my knowledge ends.

Whoa!

That changes things. There are probably more economical solutions possible, but without details I can't comment.