[camerontech]

Ok, I have a question and seem to be stuck at the moment. I'm a geologist, haven't done to much engineering stuff in a few years and all my books and notes are back home. I've been doing some research on the internet but I'm not getting all the answers.

here's the question.

I have a concrete slab 16' long and 8' wide. the slab is reinforced with rebar and is either 6", 9 or 12" thick. I want the slab as thin as possible (since it cost money to make it thicker) and I want to know how thick it can be while still being able to hold a large amount of weight. All I know is teh compression strength of concrete will be around 3000psi and the yield strength of rebar about 50,000psi

The slab will cover a large hole and overlap each side but for this particular example imagine it's only being supported at the edge of the 16' length. could this support 20 tons?

i'd get a diagram for you but my image hosting website isn't working

can someone give a science guy a hand with a physics question? If I had my books with me I could probably do it but I'm a little far from home at the moment.

it's appreciated,
thanks

[Blue_Nose]

How is the 20 tons distributed?

[ShepherdsDog]

better yet, how wide a lip to help distribute the weight

[camerontech]

well here's the thing. hopefully no weight will ever be put onto the slab. it's simply covering a hole in case an accident happens and machinery drives over it 60 years from now.

it's an open shaft from a mine in the 20's that we are covering. the 20 tons is just a conservative estimate of the largest piece of logging or mining equipment that could stumble in that bush and onto the slab in the future.

the slab will probably overlap by 3'.

thanks guys

[Blue_Nose]

I don't think you're going to be able to do it under those assumptions - with a 12" slab and using 20M bars (which are about 3/4" in diameter), you're looking at a spacing of only 4 inches. It'll be one hell of a slab, but I think you're better off being a little less conservative, specifically with your span length - what's the actual clear span that needs to be covered? Is the entrance soil or rock?

[Knoss]

If your refering to a truck it is usally best to pour cement on gravel or stone foundation rather then over a hole. The more rebar the better but rebar is more expensive then concrete. 1.25" plate would work as would 13" concrete with no rebar pushing on a soild surface. I'd think 12" with rebar would be your best bet. What are you building?

[Blue_Nose]

Actually, if you're not actually expecting any significant traffic over the entrance, you might be better off building a visually obvious barrier around the entrance instead...

[Knoss]

Thats true you could build a 2' wall around the hole and cover it with tin.

[camerontech]

to rephrase the question, we're using 8'x16' foot slabs to cover the hole and the actual clear span is 10' (so about 3' on each side will lay on a compacted rock floor) I just got the yield rebar strength as an estimate really, you can assume standard rebar spacing if you want, I'm not too familiar with it.

If I understand what you're saying, you are saying that to be able to hold 20 tons the 12"x8'x16' slab would have to have a 4" rebar spacing?

What I'm wondering is how thick I could make this without going way overboard and making something way too strong (or too weak).

[camerontech]

Blue_Nose wrote:

Actually, if you're not actually expecting any significant traffic over the entrance, you might be better off building a visually obvious barrier around the entrance instead...

well we already have a fence around it with signs and everything. It's just the legal aspect of things that are forcing us to cover the hole for good, company lawyers and higher ups have asked me to do this and it's not really my field of expertise. The method has already been determined, I'm just figuring out a way to make it work with the concrete slabs

[Blue_Nose]

camerontech wrote:

to rephrase the question, we're using 8'x16' foot slabs to cover the hole and the actual clear span is 10' (so about 3' on each side will lay on a compacted rock floor) I just got the yield rebar strength as an estimate really, you can assume standard rebar spacing if you want, I'm not too familiar with it.

Once you have a thickness, there's too things you have to consider - bar size and spacing (obviously, larger spacing means larger bar size). Your yield strength is reasonable for structural steel, but a little low for rebar - usually it's about 60000 psi.

If your clear span is only 10', you can get away with a 7" bar spacing, assuming you use the 20M bars. You also typically want to have some bars placed in the perpendicular direction - 10M at 6" or something like that.

That being said, if there's a significant possibility that the slab will be driven on, you should ask your higher ups to call up an engineer to do up a drawing for you - the only thing worse than an open pit is a poorly design cover that will fail when a logging vehicle operator thinks he can drive over it.

The specs I gave should only be considered a general guideline - don't use it without confirming it with someone you work with professionally.

[Knoss]

many ways I think 1.25" plate would be the easiest to work with.

[Blue_Nose]

Assuming the 20 tons is a single wheel load, 1 1/2" plate wouldn't be strong enough.

[camerontech]

Thanks Blue_nose,

There will be an engineer coming to look at it and get it finalized, what I'm doing is giving the higher ups an estimate of what we're going to have to work with and a general idea to see if it will work or not. Getting an engineer cost a lot of money so they want me to see if it's possible before we go any further.
I was basically asked "can a 6 inch thick slab of concrete hold up a big machine?, figure out the thickness we would need to make it safe in case it happens"

That was it, I won't be designing the final product just giving them my input, and with that they will get an estimate on the price. once we have a rough price estimate they will decide what to do and bring the engineer in.

So I guess to finalize this, with 7" bar spacing and perpendicular 6" spacing (assuming 60000psi now) could a 6" slab be recommended or a 9" or 12"?

thanks again, you've given me a lot of help in this. I've been asking around and have been everywhere on the net but finding good info on this is hard

[Knoss]

many ways I think 1.25" plate would be the easiest to work with.