water pressure on surface

[brian m]

I have a cement chamber with a 5' x 4' hole that I wish to cover with an aluminum cover. The hole is 21' below the surface of a lake. 21/2.31=9.09 psi. will the cover have 9 pounds of pressure against it or is that multiplied by the square inches of surface area? yeah, not an engineer...how do I go about ensuring the cover will be strong enough to hold at that depth?

[jboggs]

Yeah - pressure and force are two different things. Pressure is force per unit area. Pressure is multiplied by area to find total force. So: 5' x 4' = 60" x 48" = 2,880 sq.in. area. 9 psi x 2880 sq.in = 25,920 lbs total force against your plate.

As for your question - "how do I go about ensuring the cover will be strong enough to hold at that depth?" Answer - Do not attempt it yourself. Failure could be catastrophic. You get an engineer familiar with that type work to design it. I suspect that your local codes will require that the cover be designed by a LICENSED engineer. You should answer that question before you proceed. Could be a very costly mistake.