Water Pressure and Steel Yield Strength 4

[samond]

I am trying to figure out what thickness of steel plate and type of weld will be necessary in a aeration basin to block an opening. The water level will be 15' deep and the gate will be 62" squared.

I know that the hydrostatic pressure is the same in all directions at any given point and I plan to be conservative and use the full depth to determine the pressure for the entire plate.

Pressure = γH.
I get P =15*62.4= 936 lb/ft2 converted to 6.5 psi which is really small.
Do I need to multiply that by the area of the plate (3844 in2 ?)

Force = P/A = 24,986 lb
That would be in pounds and not pounds per square inch (psi) so how do I compare that to the steel yield strengths since those are in psi?

Can you please tell me what I am missing?

Thank you.

 

[Ron]

I am assuming that you are installing this gate in an existing aeration basin that has steel walls, since you are considering welding. If installing into concrete, you will need to fabricate an angle frame or similar to attach to the concrete.

The pressure on your plate (the load) is 6.5 psi with the water level you have given. When designing such, I usually consider the freeboard depth as well as the actual water depth, since you wouldn't want the gate to fail if the tank overflowed. To determine the stresses in the plate, you will have to analyze the plate as a structural element. The stress will depend on the restraint/support conditions of the plate. If you are blocking an opening, you can choose to make the plate slightly smaller than the opening and have a butt weld or fillet weld which will be designed for shear resistance across the throat of the weld. You can make the plate larger than the opening and seal the perimeter with a fillet weld such that there is minimal stress on the weld. In either case, you will have to determine the thickness of the plate based on the bending stresses and/or shear stresses in the plate.

The easiest way to do this is by FEA modeling. If you don't have that capability, you can gut it out manually using the plate analysis equations in Rourke, Timoshenko or other references. Look for von Mises stress analysis if you Google.

For something like what you have described,

 

[Ron]

...continuing.....a plate thickness of 1/4" steel is usually sufficient, so that gives you a place to start.

 

[JedClampett]

For a 15 ft. head, you'll need a very thick plate or it will have to be stiffened. And don't forget that all those forces need to be taken out in shear at the edges, so the stiffeners might have to be detailed to fit into into the frame.
You might want to talk to a structural engineer. While 6.5 psi is not a lot in piping design, for a 5'-0" square plate, it's pretty considerable.

 

[dicksewerrat]

What are yuo actually blocking? Air? Water? What will the internal pressure be?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[samond]

Thank you for your reply.
This steel plate will be welded to an existing steel frame. Instead of the 15' water depth I will use the actual depth of the the basin for the pressure on the plate.
I do not have access to FEA modeling and googled Von Mises equation. There is a sentence in wikipedia's explanation, that states that the equation when substitued with Cauchy stress tensor components defiend a circular cylinder and that it implies that the yield condition is independent of hydrostatic stresses. Since it is on the internet it must be true, right?! Which makes me question the importance of the equation.
I started looking at Von Mise's equation and sadly realize this is way over my level of expertise. When I was asked if a steel plate was going to be able to withstand the pressure of the water, I naively thought that I could just calculate the pressure and compare it to the yield strengths from a list of steel specifications. I am starting to think this is not the case.
Thank you again for your reply. This is a great forum for engineering questions.

 

[samond]

Thank you all for your replies.
We need to block water in the aeration basin.

 

[Ron]

Samond...you are confusing the loading with the stress induced by the loading. The hydrostatic pressure is the load on the plate that induces stress within the plate. For your application, you will be concerned about bending and shear stresses. If the plate were only attached on two sides of the frame, then the plate would act like a simple beam. Since it is attached (restained) on all four sides, the analysis of the stresses gets to be a little more complicated.

If you can get access to Roark's Formulas for Stress and Strain, you can closely approximate the stresses in the plate.

 

[samond]

Ron,
Thank you for all of your input. I can see how I am confusing my situation as a simply supported beam, since it will be welded on all four sides as opposed to two. I will look for Roark's formulas.

 

[dik]

I'd use a 1/4 or 3/16 stiffened plate using steel angles, else the plate has to be 5/8" for an L/100 deflection according to Alex Tomanovitch's plate spreadsheet.

Dik

 

[cvg]

you can buy bulkheads, stop logs and slide gates made specifically for your application. several companies including hydrogate, armtec, waterman all sell them. they come complete with the mounting frame, seals and custom designed for your water pressure. It might be a good option instead of trying to design your own.

http://www.armtec.com/en-ca/product...t/water-control-products/bulk-head-gates.aspx

 

[mes11]

Another source that may be useful is AISC's website and go to the steel tools link. This has multiple spreadsheets that have been developed and possibly one of those could help you out.