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.

 

[driftLimiter]

Seems reasonable to me. Only comment would be the #8 bars and single mat. A double mat and smaller bars would make the construction that much easier (less weight of bars, less lap lengths etc).

A top mat will help control cracks that are visible at the top as well.

 

[JoelTXCive]

What is the soil type under the slab?

In Houston we have a lot of fat clays, so soil heaving is just as likely as a soil shrinking condition.

The end result is that we don't know which face of the slab will be in tension, so we just match the top steel to the bottom steel.

 

[sentrifice]

@driftLimiter

Would that not take away from the flexural reinforcement? Seems like no matter what I will need the #8 bars on the bottom to take care of the minimum reinforcement.

@Joel

That makes sense, I didn't know about that possibility.

The geotech report says that the soil encountered in the upper 5 feet of the site consist of sandy clay and clay with sand with a low to medium expansion potential. The plan (according to the geotech) is to remove 5 feet and fill and compact with reused soil. It also recommends that the upper 12 inches of compacted fill below concrete slabs should consist of select non-expansive fill material.

Assuming that heaving is a problem, and tension may be encountered on the top, then I would just copy the minimum reinforcement from the bottom onto the top?

 

[driftLimiter]

Well you didn't really say whether you need the #8 @ 18" o.c. for flexure or just min. steel. If you need it for flexure (which I would find rather surprising) then you need it and a top mat would just be additional steel.

If its just T&S reinf. I just look at the gross cross section and am okay dividing the required amount between two mats.
If its just a footing similar to a 1-way slab ACI-14 Section 24.4.3.2 allows you to distribute it to the top and bottom of the slab.

If your looking at it like a beam then maybe the 9.6.1.2 section controls on each face.

The load as a line load along the perimeter of the container we should only expect 1 way flexure on each side of the line load. I would just use section 24.4.3.2

 

[PEinc]

To answer the question, wouldn't sentrifice need to indicate how the loads are being applied to the large spread footing? Is the load applied uniformly to the entire or partial footing area or is there some type of framing system in the bottom of the enclosure that might cause line loads and positive and negative bending?

http://www.PeirceEngineering.com

 

[sentrifice]

@driftLimiter

aha! I think I see where I was mistaken. I calculated "minimum flexural reinforcement" using 7.6.1.1. Which got me my #8s @ 16". If I use section 9.6.1.2, I get an As,min that is significantly less, which would make a bit more sense.

So would it be OK (with careful consideration) to disregard Chapter 7 for a design like this? Would I, for the sake of stress analysis, consider it just a beam and use chapter 9? Because it seems like 7.6.1.1 is pretty strict with what it considers flexural reinforcement.

@PEinc

I did the best I could throughout the analysis to consider the enclosure design as much as possible. What I do know is that the battery racks (the heaviest component by far) are distributed fairly uniformly throughout the interior of the enclosure. When I was calculating moments, punching shears, etc, I assumed that these were acting only on the footprint of the battery racks. For example, the total container enclosure has outside dimensions of 30' x 8', but the battery racks take up an approx. 26' x 6' area inside. So this is where I made all my forces act. I assumed that this is conservative and if there is extra bracing or anything it will just reduce the punching shear and bending on the foundation by being spread out more.

 

[BAretired]

Sounds like an overly thick footing to me. Why is it so thick? A load of 70,000# spread over 8x30 is less than 300 psf. Spread over, say 10x32, the soil pressure would be about 220 psf. Why so thick? Why not 8" or 10" thickness?

You might want top reinforcing because the footing is exposed to the weather, so it would be shrinkage and temperature steel.

 

[hokie66]

Your design looks like overkill to me. If the load from the batteries is applied reasonably uniformly, the pressure would be only about 300 PSF. So a gravel pad would likely suffice for just the gravity loading. But if you are required to provide a concrete footing or pavement for anchorage laterally, 18" is wasteful.

 

[driftLimiter]

No I would stick with 7.6.11 if you have tension on the top face then provide T&S there.

I thought that the storage containers have a stiff perimeter beam at the base and that the floor of the enclosure is actually elevated above the base of the perimeter beams.

So if this is true you have a line load around the perimeter of the enclosure. If you load the slab like that and look at it as one single element then its really more a mat foundation. In this situation like PEinc is pointing out, you could very well experience a tensile force on the top surface of the slab (in the middle).

This load is fairly light Im getting around 900 plf if uniform around the entire perimeter. One could reason that that load can be taken in bearing directly below the loads and that the slab outside of that region is not experiencing flexure.

I would probably do something like this. At the end of the day you have a pretty low bearing demand over the entire region of the footing ~ 200 psf.
Then any flexure that the slab resists would be miniscule and I would just use section 26 T&S reinf.

 

[human909]

Sounds like an overly thick footing to me. Why is it so thick? A load of 70,000# spread over 8x30 is less than 300 psf. Spread over, say 10x32, the soil pressure would be about 220 psf. Why so thick? Why not 8" or 10" thickness?

You might want top reinforcing because the footing is exposed to the weather, so it would be shrinkage and temperature steel.

Agreed and that was my first reaction when I read it. It seems crazy thick.

I'd generally consider myself a rookie in slab design but I've used slabs 1.8' thick for bulk material storage. Around 2000 psf.

Granted if it is a small area with minimal sub grade preparation, deeper excavation required to get out of fill or if there are overturning reasons then I might end up with thicker slabs.

 

[sentrifice]

@BAretired, hokie, and human

I appreciate the consensus. At this stage I certainly lack the gut-feel of what design should look like.

It appears though that the minimum thickness will be governed by the geotech report which states that 'footings should be embedded to at least 12 inches below lowest adjacent grade'. Since we (the geotechnical engineer and I) are considering this a spread footing I assumed this criteria applied.

 

[Tomfh]

It needs top steel.

What is the basis for assuming uniform loading of the slab? Shipping containers are usually supported on the corners, which are typically proud of the rails and cross members.

For designs like this supporting heavy equipment we often do a 200 or 250 slab, with edge beams to provide greater embedment.

 

[AskTooMuch]

I doubt an enclosure is very sensitive to settlement. Remove 5' depth of a 33'x11' area, then backfill and compact?. You need to consider the construction cost impact of that. Maybe cheaper to just put 6 drilled piers and put the 1' thick slab/mat on it.

If you really need to temove 5', backfill with flowable fill that's way cheaper than the labor cost of backfiling and compacting select fill.

 

[sentrifice]

An update for everyone.

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). The rebar will also be reduced to #6 @ 16"OC both top and bottom. This satisfies 7.6.1.1 for the bottom and the top to address any concerns for heaving/differential settlement and also for T&S.

Part of the reason this design is the way it is is because we have not received solid structural information from the enclosure supplier. Once I receive more information from the enclosure supplier I will revisit how the loads are applied and I can consider a thinner slab with perimeter beams. There are some project management issues with the client that is controlling the flow of the enclosure information and the submittal schedule, but I digress.

I appreciate everyone's continued input, this has been more educational than I could have possibly imagined.

Also @asktoomuch
That is an interesting approach, I didn't know about that as a possibility. I will say though, the area is quite remote and I've come to believe that labor is cheaper than importing materials so this may play a role in it. We are working closely with the contractor and they have not complained about the excavation requirements yet.

 

[AskTooMuch]

The contractor won't complain especially if they are paid T&M. More labor/work, the more they get paid normally. I'm speaking from owner's perspective. I work inside a plant as a Project Engineer and labor is very expensive compared to concrete or rebar.

Concrete should be at the site within 1 hour after leaving batch plant, so I guess you are not that remote that material/shipping is more expensive than labor. If you are, then you should consider alternative of mixing at the site.

 

[sentrifice]

@asktoomuch

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.

 

[dauwerda]

If it is a shipping container as you describe I am having a hard time understanding why you would need (or want to pay for) a mat slab under the entire thing. I would be looking at drilled piers (as AskTooMuch suggested) or spread footings at each support location (probably each corner if it truly is a shipping container) with grade beams tying them together around the perimeter.

 

[sentrifice]

@dauwerda

The shipper container supplier said that if piers were used that there would need to be 16 of them, so the client opted for a slab. I wasn't there for the conversation that's just what I heard.

 

[Brad221]

Even at the reduced 14" thickness, the mat slab weighs almost as much as the container and contents, effectively doubling the bearing stress. Not knowing how it's loaded or the particulars of the project, it seems overkill to me too.

What is the rationale for requiring 12" below grade? Is it for frost depth? If so, it seems you're just using concrete for fill (to get from 12" below grade to a few inches above grade). That's usually expensive fill. Might be better to insulate. Or use flowable fill or gravel to get below frost depth (removing the frost susceptible native soils).

Not enough information to determine the right solution but this doesn't sound like it.