Plate design

[adammilad]

I am designing weir in a cylinder. I want to make it out of steel plate. the diameter of the cylinder is 150" the height of the weir is 120". I know the thickness of the plate is 0.375". Assuming that one side of the plate filled to the top with water and on the other side is dry. How do I find out if the plate thickness is good and if not how would I go about finding stiffener thickness and spacing? Thanks in advance for the help.

 

[adammilad]

Attached is a simple sketch of what I am looking to design.

dhenger-
I was looking at equation 3 on page 2. According to this formula it shows that there is no need for spacing and we all know that is not true. I used 18.8 ksi for f which is what the material is rated for at that temp.

Going to page 4 I decided to come up with a thickness that allow for 50" spacing so I used the rectangle section with uniform p, supported and B=120" I came up with a 0.5" plate. Now that I am thinking about it I should probably go to pg.466 in Roarks "Formulas for Stress and Strain" (3 sides fixed 1 free).

I will attach the pages I am referencing. This is part of a liquid seal I am designing for those that asked previously.

 

[adammilad]

Also I forgot to add that for the uniform load I used 2.2psi.

Attached is page 2 and 3 from "Steel Plate Engineering Data - Vol. 2,"

 

[adammilad]

Attached is pg 4 and 5

 

[adammilad]

This is Pg. 466 from Roark's

 

[rb1957]

figure 1 shows a stiffener cantilevered at both ends, no?
you don't have this. [depends on which way your stiffeners run !]

"the uniform load I used 2.2psi." ... you don't have a uniform load, you could use the load from 1/2 the depth ... makes me think i automatically assumed vertical stiffeners, but maybe everyone else is thinking of horizontal ones ??
also, distributed load should be lbs/in ...

Roark's is a good source ... look into his reference, google something like "moody plates" to get to the original doc, it includes more loadings, more relevent to your case, and a lot more detail.

 

[adammilad]

Figure 1 is just showing the height of the weir. Sorry should have said a uniform pressure and yes I do plan on having vertical stiffeners with a piece of angle that runs horizontally along the span of the top of the weir.

 

[rb1957]

no, i meant fig 1 from your copied pages, pgs 2-3 ...

but if you have vertical stiffeners, then they don't have uniform load and probably aren't fixed at both ends. running an angle along the top makes sense, and you can probably could this as a pinned support [but i suspect that it'll see some pretty big loads]. with this, the base (connecting to the cyclinder) can be pinned or fixed.

horizontal stiffeners would work just as well. a grillage (horizontal and vertical stiffeners) would work too; in you case you could have continuous members running in both directions (on either side of the plate).

many ways to skin cats ...

 

[dhengr]

Four thousand man hours into this thread and we first learn that you are thinking of vertical stiffeners with a horiz. stiffening angle at the top of the weir plate. And, you also have Rb1957 on that bandwagon as the only other person who thought of vert. stiffeners for this problem. I couldn’t figure out the cantilevers Rb was talked about earlier, but now I see, and disagree. No offense guys, you can skin this cat either way, but since we can change the stiff. spacing to account for the varying water pressure, and make the pl. check out, easier than we can change the pl. thick. at various pressure levels, over the uniformly spaced stiffeners, which will each be different also; I would be inclined, as almost everyone else was, to run the stiffeners horiz. This is getting to complicated for me, and I’ve got some other fish to fry, so I’ll just watch for a while; warn my fryin oil and cool my jets.

 

[BAretired]

In an earlier post, I committed the worst sin of all...I mixed my units. dhengr will no doubt haul me over the coals for that transgression but, c'est la vie.

The stiffeners could be placed vertically or horizontally. If they are placed horizontally, the plate spans vertically with a variable pressure. Without introducing significant error, if the pressure is taken half way between two beams, the maximum moment is roughly wL^2/12.

Since fmax = 18,800 and S per inch = 0.375^2/6 = 0.0234 in^3, we have the relationship that wL^2/12 = fS.

L = (12*f*S/w)^1/2.

If H = 9', w = 562 psf or 3.9 psi and L = 36.8" (say 36")

If H = 6', w = 374 psf or 2.6 psi and L = 42.6" (say 42")

So use 3/8" plate throughout with three stiffeners at Elevation 36", 78" and 120" respectively.

Design the three beams for their tributary load.

BA

 

[dhengr]

BA:
Well, you did say ‘some rough approximations to start with,’ so, only Forty Lashes for you, or a few beers if we ever meet. Ah, yes, using ft.-lbs. when we should be using inch-lbs. might lead to an error on the order of (12)^½ to small a span length. We disagreed so seldom that I usually wouldn’t check your every calc., and you do such a nice job of laying it out that I don’t ever have much to add to your presentation. Actually, I generally worked with large structures and loads, so ft.-kips were the norm until I got to the stress calc. at which point I made the conversion to inch-kips with a 12 in./ft.; and kips and ft. were the norm and 1728 showed up in every deflection calc. Of course, this very feel for and comfort with the units you normally work in is the cause of the whole resistence to the conversion to SI units, isn’t it? I do hope the younger people make the change, but it’s still fraught with industry conversion problems. I actually have some trouble working in inch-lbs., just no feel for the magnitude of the numbers. I looked at the equations, they looked right, and the proportions of the spacings looked about right, and I really didn’t give the exact magnitudes a second thought because I hadn’t done the calcs. and tabulation.

Even so, and maybe more importantly, you showed the younger engineers that everyone makes a mistake from time to time, so we must check our own work. Otherwise another fine example of a top notch experienced engineering simplifying the problem, paying attention to his work, by checking his own work, always checking the units in your calcs. or having a gut feeling that something doesn’t look quite right, as he thinks through the problem. The difference is that you have the experience to do this, and I wonder if us older guys are the only ones who check their units in calcs. any longer, out of force of habit, since a slide rule wouldn’t do that for us. But, so many of these young people are floundering around understanding the very basics of the problem, to the extent that they wouldn’t know a wrong answer if it bit them in the butt. Your suggestion to make some quick first approximations on a problem like this, to start to get a feel for it, is certainly the approach I usually take. We may never have seen this problem before, so we have to hone in on it, to get a feel for sizes, thicknesses and proportions; then we can start to add the complexities of deflections, connections, welding and any stress concentrations btwn. the stiffeners, weir pl. and the cylinder plate, etc.

Just before 7JAN11 12:32, I was writing a piece extolling the virtues of your posts and approach, and I looked up and found that he forgot to mention that he was considering vert. stiffeners, so you and I weren’t even in the same discussion as he and Rb; at which point I dashed off the 12:32 bit in exasperation. And, now I learn that you went from virtuous to sinner in one fell swoop, so consider yourself taken to task, we don’t use much coal around here anymore, so no hauling over the.... I might add, like you, I’m continuously exasperated by how difficult it is to get the needed info. or a sketch out of these people. They want your help, and in many cases it is all to obvious that they really need help, but they appear not to understand their own problem well enough to even know what info. is needed to even begin to solve it.

 

[adammilad]

I have decided to go with 0.5" Plate to minimize fabrication thus requiring the need for only 2 supports. I am going with vertical supports water and air from being trapped in the support beams. I am giving the plate a uniform pressure for simplicity P=2.2psi. attached is a sketch with my calcs. Would you guys agree with these? I am using S for the section modulus of the beam I am designing.

 

[delagina]

how you looked into horizontal pressure vessel design. you should post this at mechanical forum, they must have designed a lot of this.

 

[adammilad]

No I have not I will look into that. Thanks