Flat Plate Design
Flat Plate Design
(OP)
I need to design a Rectangular tank that is holding water (sg=1). The tank dimensions are 28ft. long x 14ft. wide x 12ft high. When I analyze the tank wall without any stiffeners, I use a=144 and b=336 the a/b ratio =0.429. Using Roark Case 2d, the a/b ratio starts at 0.5, so my a/b ratio falls off the chart. What do you do in that situation.
Thanks for your help.
Thanks for your help.





RE: Flat Plate Design
RE: Flat Plate Design
RE: Flat Plate Design
RE: Flat Plate Design
Alternatively, extrapolate the curve, see how much difference it makes.
Or use FEA
Realistically, you need a stiffener, or a tie bar.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Flat Plate Design
RE: Flat Plate Design
RE: Flat Plate Design
Of course your aim of calculating that flat wall under hydrostatic pressure without stiffeners appears unreasonable...
prex
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RE: Flat Plate Design
RE: Flat Plate Design
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Flat Plate Design
Maximum moments are: -
* Top edge at side wall - horizontal - tension inside
* Centre of wall top edge - horizontal - tension outside
* Bottom centre of wall - vertical - tension inside
Coefficients for these three cases are: -
* 0.0715
* 0.0263
* 0.0984
Moment = coeffic x pressure x depth**2
Metric units with depth 3.7 m.
pressure = gamma x depth = 9.8 x 3.7 = 36.3 kN/m**2
moment = 0.0984 x 36.3 x 3.7**2 = 48.9 kNm
stress = M/Z
For a fy of 300 MPa & a stress of 2/3 fy = 200 MPa
t = 38 mm
That's 1.5 inches (in the old money).
As mechengdude said "use stiffeners".
Start with a square grid of stiffeners which is obvious for a uniform load but is very close even with a water (triangular) load. Near the bottom of the tank, the difference between the load at the top of the panel & the bottom of the panel, is becoming smaller.
Remember, that this analysis assumes that all edges of the plates are rigid (a good starting point & an extreme one). This concept is not acceptable in the 'real' world. There are rotations at the corners, the tanks sits on 'an elastic foundation' etc etc.
However, this concept design (rigid edges) is a safe design. It probably can't be any worse.
If you have access to a FEA model, use this but be very careful of your 'spring constants' (beam on an elastic foundation) that you use of soils or clay or rock etc.
RE: Flat Plate Design
RE: Flat Plate Design
RE: Flat Plate Design
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: Flat Plate Design
Otherwise, on the stiffeners, you can work it a lot of different ways. Vertical stiffeners framing into a large beam at top, a series of box-shaped stiffeners around the sides, etc. Stiffeners can be inside or out, although outside is preferred. Stiffeners welded to the plate can be considered as composite sections using part of the plate. When you get done, it will make you appreciate a circular tank.
RE: Flat Plate Design
Mark Hutton