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Participant

Join Date: Mar 2008
Posts: 4

Max Load Possible for Slab?

03/07/2011 9:44 AM

Hi there,

I have a 7" (175mm) slab/raft, 37N concrete, A353 (10mm) mesh on the bottom layer, slab is on well compacted 804 on good ground, slab is 6mx4m. I want to know (or how to calculate) approximately how much load could be placed on any one square meter. Thanks for your help in advance.

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derek egan
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Associate

Join Date: Mar 2008
Location: Minnesota, USA
Posts: 46
Good Answers: 1
#1

Re: Max Load Possible for Slab?

03/08/2011 9:16 AM

I would have a qualified structural engineer make the calculations. It seems about the size of a small garage or maybe a patio, is it? Cost wise a structural engineer might or might not be cost effective depending on what you want to put on it. Since you are asking what amount of load you could put on it, I would assume you want to put something heavier than you would normally think would go on it. In that case you will need to find out the allowable bearing pressure of the soil the slab is sitting on.

It would also be easier to give a rule of thumb figure if you gave some indication of the type and mass of the load you wanted to put on it. Generally to find the capacity of a slab on grade, more information is needed. Since it is so small it probably would be easier to tell the forum what kind of mass you were going to put on it, and then let the forum discuss whether it will work for you particular application.

Cheers

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Guru

Join Date: Dec 2007
Location: California
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Good Answers: 63
#2
In reply to #1

Re: Max Load Possible for Slab?

03/08/2011 11:13 AM

Hmm, it is something more than a residential slab, or any patio slab. Concrete pavement on AB for truck loads is only maybe 6". He has a really thick slab at 7". This kind of makes me wonder what he intends to use it for, and what it was designed for. Sounds like some industrial slab with that thick concrete, though the steel seems awful light for an industrial/heavy commercial slab.

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Associate

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Location: Minnesota, USA
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#3
In reply to #2

Re: Max Load Possible for Slab?

03/08/2011 12:14 PM

The 7" slab was a little confusing since I have only used a 7" thick slab under heavy machinery with average to poor soil; ex. mining dump trucks, front loaders, etc. Equipment that was bigger than the standard construction size, but not as big as the really big stuff. With it being about 18' by 12' and having about the equivalent reinforcing as #3 bars on an unknown grid pattern, I figured it was some kind of parking pad. Yes, I would definitely agree that the reinforcing steel is smaller than normally used on a slab this size. It would certainly depend on the spacing I am a little familiar with 200x10, but that would still only be #3 bar at just under 8" on center each way.

I miss spoke on the patio pad. I should have realized it was not going to be a patio with it being 7" thick.

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Participant

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#4
In reply to #3

Re: Max Load Possible for Slab?

03/08/2011 12:38 PM

Hi there,

Thanks for your comments. The plan is to place a piece of plant machinery on the slab, the machine will be 12 ton on 8 legs, 1.5ton per leg, machine is 5m x 3m. I'm an engineer myself but not in structures and this query is coming from an uncle of mine who is a farmer, I'm not even too sure what sort of machine it will be! Cheers

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#5
In reply to #4

Re: Max Load Possible for Slab?

03/08/2011 1:00 PM

That's a horse of a different color. The governing failure mode would probably be punching shear. That will depend on the size of the base plates and columns support the machine and how that 1.5 tons is being spread out over the base plate area. Although I believe punching shear will govern, flexure and flexural shear would need to be checked as well as whether the amount and size of reinforcing is adequate. I suggest finding a copy of a reinforced concrete book. The one I have is "Reinforced Concrete Mechanics and Design" 3rd edition, by James Macgregor. Another great reference that I have is the "Structural Engineering Reference Manual" 5th edition by Alan Williams. It contains a process to calculate the strength of a concrete footing in the foundation section. This would probably be the closest to your problem.

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#6
In reply to #4

Re: Max Load Possible for Slab?

03/08/2011 3:36 PM

Actually, with machinery you have more than the dead load of the machine to consider. You should determine the estimated Live load, and consider any vibrations and asymetry in loading. 1.5 Ton per leg would be a symmetric dead load for the machine if always balanced and the floor contact loads were always equal. then there is the SF to consider. Typically 1.7LL+1.4DL+other is a common SF to use. The dead load of 12 tons really isn't that much spread over that 5m x 3m area, about 151 lbs/sqft. However, it isn't really spread but applied as 8 point loads, each at a bearing leg. Add in the live loads and vibrations you could end up knocking a hole in the concrete surface under some of the leg contact points. He might need some steel plates under the legs to spread the point loads onto the concrete.

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Associate

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#7
In reply to #4

Re: Max Load Possible for Slab?

03/08/2011 4:14 PM

RCE brings up a good point about vibration. Depending on what type of machinery it is, how high it is off the ground, and exactly how it is supported, vibration could make it more of a hassle to determine the loads on the concrete slab. The other considerations he mentioned, the proper load combinations and exactly what gravity loads and lateral loads are acting upon it, will most certainly make this a more complicated answer. I live in the Midwestern US, wind controls here since not a lot of seismic activity. I used to live in the western US where seismic controlled, and wind was less of a problem. 12 tons is a lot of weight to be moving around in an area that has seismic issues. I would assume it is not very high off the ground, but depending on the surface area that the wind impacts on, you may need to check that too. Unfortunately there is not a simple answer to the original question. I don't suppose you live in a low seismic area and the machine is compact, because that would simplify things a little. I wish you luck with it. It will certainly be a great opportunity to expand you engineering knowledge, and that's never a bad thing.

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Participant

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#8
In reply to #7

Re: Max Load Possible for Slab?

03/09/2011 4:06 PM

Thanks for the comments, as far as I can make out the vibration will be very little and I do think the main problem is the load at each leg with the chance of the slab failing locally, I'm not sure about the size of the base of each leg but I have told my uncle to get a structural eng to check it out but I also think that perhaps as he is not restricted by height another solution may be to put a steel frame under it to distribute the weight of the 8 legs. Thanks again for the help.

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derek egan
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#9
In reply to #8

Re: Max Load Possible for Slab?

03/09/2011 4:31 PM

Your welcome, I am glad I could help.

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