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Point loads on a residential slab

Point loads on a residential slab

Point loads on a residential slab

(OP)
I end up on this forum periodically to read, but have never had a relevant question until now. I apologize if this is the wrong site or subforum for this sort of question, please direct me accordingly. A search for aquarium loads didn't turn much up, but if there is a more relevant set of search terms please also point me in that direction.

My education and PE are mechanical, not civil/structural. I am in the process of installing a large (126"x34"x32") aquarium into my house. I have designed and will be building the stand. For various reasons I do not want to go with the typical 10 leg stand design, and instead have designed a stand with 4 legs, 70" center to center sideways and 32" c-c front to back. All four legs will be equally loaded.I am not concerned about the suitability of the stand. What I am concerned about is the loading on the slab because that is outside my expertise. I'm continually amazed at the number of 3500 lbf machining centers being supported by 4 legs in residential garages without visible slab damage, but this is a bit heavier. Everything said and done I'm expecting a permanent load of up to 6,000 lbf equally distributed across four feet, with another 1,000 on a larger distribution underneath the stand. I can easily see this increasing by 500 pounds for short periods of time due to three people crawling around on/in it to do work. There are two basic questions:

1) Are there any guidelines on what loads do not require deeper analysis? IE "Put 4"x4"x1" steel plates under the feet and you'll be fine up to conservatively XXXX lbf.
2) If more analysis is needed, what information do I need?

General rule of thumb in the aquarium world is that a 55 gallon aquarium (450-500 lbf, 4 feet long) can be safely placed almost anywhere in a house or apartment. A 75 gallon (600-700 lbf, 4 feet long) can also be placed about anywhere if along a supporting wall or perpendicular to floor joists. A 300 gallon aquarium (2500 lbf either 8'x 2' or 6'x3' on 4 feet) is fine anywhere on the first floor of a slab home. I'm trying to figure out of my application also falls within "it will be fine" vs. "check the following things" vs. "go hire a structural engineer".

More background:
The house is in Murphy (Dallas), TX, was built in 1992, and is a two level house on slab. Obviously this is going on the first floor. My proposed location is along an outside wall in front of a window. This area is one tall level rather than two. Just outside the house at this area the slab is exposed for 10 inches below the level of the interior floor. I do not know how far down it goes beyond that. There are steel cables through the slab 5 inches below the level of the interior floor on 60 inch spacing. I am basing this spacing off of two partially exposed cables, and a few more places which have cement patches on the same spacing. One cable is exposed at the end, and is at least 1/2 inch diameter. In at least one place there is a pair of these cables less than a foot apart.

RE: Point loads on a residential slab

I don't have much experience with residential or construction in Texas, but since nobody else has answered, here's what I think:

Sounds like there's no basement, correct? This is going on the lowest level of your house?
The biggest questions that I have is how thick this concrete slab is. It's interesting to me that the cables are 5" down. Could it be a 10" thick slab? Maybe someone familiar with Texas will chime in.
The cables that you're seeing tension the concrete to help control cracking and settlement.

What's the consequence of failure? If the slab cracks, one leg settles, and the tank cracks - are you going to shrug your shoulders and say "at least I tried"? Or break down in tears because you've wasting thousands of dollars and weeks of your life?

If it was me, putting this in my basement, and I had a 4" thick slab on grade, I'd connect adjacent legs with an angle iron, hss, or something similar. This would turn your point loads into line loads, distribute the weight over a larger area, and reduce the possibility of having an issue.





RE: Point loads on a residential slab

In general, there's most likely a wall or turned down edge of slab at the perimeter, so putting the tank on the outside wall ,as you're considering, is probably best.

The limited experience I have in Texas is a project where the ground floor slab was poured over a void form so that the expansive clay didn't affect the slab. If you were to have someone check this out for you, then he or she would want to know what type of slab you have and its thickness. You might learn something by digging down to find the bottom of the exposed edge of concrete on the exterior.

RE: Point loads on a residential slab

(OP)
Thank you for the replies so far, here are the answers to the questions asked.

1) Correct, no basement.
2) I found this interesting as well. I've seen factory floors that aren't 10" thick, but it doesn't make a ton of sense for the cables to be in dirt (coming from a non structural guy of course). Are slabs normally constant thickness, or could there be a grid of thicker concrete beams? It looks like there are non-destructive ways to measure concrete thickness, I wonder if any of these tools are easy enough to use that they can be rented? If not there are a ton of foundation repair places around here, I may be able to hire one to come map out thickness. The problem is there are very few regulations on almost everything down here and a lot of these places aren't the most professional outfits.
3) If it fails I'll be a bit broken up, particularly over the loss of life in the tank, but it won't be the end of the world. At least I'll finally force myself to replace the laminate flooring I dislike so much in that room. That said, I'd just as soon not take on a several thousand dollar flood and foundation repair bill if I can plan around it. It sounds like you're in the "Consult as structural firm" camp rather than the "That is a normal and acceptable load" camp). If I go that route what do I look for? It's been a long time since I worked with any structural engineers. Is this the sort of thing that I just open the yellow pages (figuratively) and find a foundation place, or is there something specific I should be after? I don't want to do the engineering equivalent of going to a large corporate litigation firm for a marriage annulment just because I don't know the difference.
3b) The center of gravity will be a bit behind center, so in theory a front leg failing would unweight a leg, but not topple the stand. I'll have to run the analysis and see what that does to the top platform though. Short term the foam between the tank and stand may absorb the difference in deflection. The stand platform is on the overbuilt side to allow the small number of legs, but I do see that tank failure is a real possibility at that point. If using the stand as designed requires (for example) a $2000 change to the foundation of the house then I'll change the stand. If it requires a $0 to say $500 change, I'll probably keep the design. I could also make some additional removable legs and only remove them when the tank is say 50% drained. It is effort to remove and replace 300 gallons of salt water, but it does remove 2500 pounds and I am equipped to do this during the rare occasion that large components need to be removed from under the stand.
4) I have looked in to putting ribbing between the feed to spread the load, but the geometry required to make that work is causing some space issues for the equipment which goes under the tank. If I space the legs out a few more inches I would have space to run a beam front to back (the short dimension), but running anything across the front is going to be an issue. One option around this is to put the propagation tank (72"x24"x18", 1000 lbf) on rollers and actuators so that it can be moved out from under the stand. That however is a lot of water to have slosh around, and moving the legs out those few inches may actually be a significant change.

RE: Point loads on a residential slab

I can't imagine this load having an effect on a post tensioned slab on grade.

RE: Point loads on a residential slab

This aquarium weight seems no more than that of a fully dish-laden china cabinet in a typical formal dining room. Do you have any comparable loads on that slab already which would tell you about its performance under those loads? Experience is the best predictor of future performance.
Dave

Thaidavid

RE: Point loads on a residential slab

(OP)
I have a 2300 pound bridgeport mill on effectively four points (back is metal rim on slab, front is 1"x6"x6" thick oak boards under the feet in the garage.
Ten feet from that are two water tanks next to each other, something like 30" diameter, 1300 lbf each, but smooth bottom in contact with the floor.
Inside the house (and 6-8 feet from this intended location) is an aquarium with a total of four feet (3.5" squares) on a 96"x24" pattern. That one is probably 2600 lbf with 700 lbf of additional weight distributed under it.

No issues with any of the above, but all smaller loads.

RE: Point loads on a residential slab

(OP)
An update on this. Three local geo/structural firms were asked about this. One replied with "For $500 we'll come out and certify the entire first floor of the house". The other two replied with variants of "For a slab on grade in this area that does not warrant inspection, it will be fine", and were willing to provide their name as well.

This doesn't answer the curiosity as to what specific thickness the slab is through the house, but it does easy my concerns significantly. If anyone thinks I'm just hearing what I want to, please feel free to say so. Thank you all for your suggestions. I'll keep perusing these boards and maybe someday I'll come across a question that I have a clear answer for.

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