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.

 

[rb1957]

can you calculate the pressure the water is applying to the plate ? (something like rho*g*h) from this you can determine the force the water is applying to the plate, and so determine the reactions (between the plate and the cyclinder) and so the maximum moment in the plate.

the rest is QED ...

 

[BAretired]

I think you can safely assume that a 3/8" plate in a 12'-6" cylinder will need stiffening. Decide on a pattern of stiffeners and design each stiffener as a beam with unit load equal to water pressure times tributary width.

BA

 

[JStephen]

The flat-plate formulas in Roark's Formulas for Stress and Strain can be used for the plate itself.

 

[dik]

For the dimensions noted, as BARet noted the plate thickness will not be adequate unless stiffened. You also have to decide if the plate acts in flexure or as a membrane/diaphragm (essentially tension).

Dik

 

[adammilad]

I am aware that 3/8 plate will not suffice. i was just wondering if there was a standard way to calculate the required stiffener spacings and size.

 

[rb1957]

only if there was a standard stiffner !? (and a standard load, etc).

no, each stiffening is designed for the particular case ... you may want a few large stiffeners, or lots of small ones.

 

[dhengr]

Yes, there is a standard way of calculating and designing this weir plate and its stiffeners. It’s the stuff we should have learned in our first Statics and Strength of Materials courses in college, with a little dose of some design code allowable stresses and deflections thrown in.

The pressure on the plate (lbs./sq.ft.) increases with depth, as a function of the depth and the weight of water. Then the plate is designed as a continuous plate, spanning vertically, over a number of stiffeners; essentially acting in bending over the stiffeners and as a tension field btwn. the stiffeners. You can probably tolerate a fair deflection in the plate, except where it joins the main cylinder. And, you may want to add some sort of stiffener ring to the inside of the cylinder where it and the weir plate will join.

With the pressure increasing with depth, but the stiffener length decreasing as you move away from the 6.25' depth on the cylinder, I suspect you can pick one practical size stiffener and then maybe adjust the stiffener spacing a bit to stay within allowable stresses. Don’t forget earthquake loading, etc. if that’s appropriate. Make the end stiffeners out of channels with their legs down, so as not to collect junk. You might design these as simple beams, with a reduced center moment and a fixed moment at their ends. At the center height of the cylinder, the stiffener (12.5' + long) frames into the inside of a channel web at each end, and these end channels are toes into the cylinder and have a vertical web. These end channels should be long enough and lap the outside of the cylinder so as to distribute the end moments without stress concentrations in the cylinder plate. The other stiffeners have their length and end angles cut to fit end channels which still mate with the outside of the main cylinder, but now with sloped webs. These end details and the associated welding and weld designs are the important part of the project. Perfect fit-up may alter the exact stiffener spacing by a very small amount, but design for that in weir pl. and stiffener strength.

That’s one of several possibilities, the design is your’s, and if you can’t figure it out you should be talking to a local mentor or your boss, so he knows what you don’t know and can keep you out of trouble. Done wrong, this could cause some serious damage.

 

[BAretired]

I would start with a guess, then refine it. You have a height of 10'. Water pressure varies from 0 to 624 psf. How far can a 3/8" plate span as a one way plate under a pressure of 500, 400, 300, 200 or 100 psf? That should be quick and easy to do by hand.

Then space your beams such that the allowable span of the plate is not exceeded throughout the height. When you are satisfied that you have found a workable solution, check the beam with the highest moment. For simplicity, you may wish to use the same beam size for every location or you may opt to design each one separately, i.e. use your engineering judgment.

BA

 

[JedClampett]

Does this sound like a class project to anyone else?
A 3/8 inch plate spanning 12'6" with a 10'-0" liquid height?
It's going to have to be stiffened with wide flange shapes.
I guess I'm even more scared if it's not a class project.

 

[dhengr]

JedC:
Your “scared” comment is exactly my point, about half the time.

These are some fairly serious projects, but some fairly basic problems, unless the guy slept all the way through college. I’m all for helping young engineers learn the ropes, but at the same time I wonder if an answer, without every possible detail covered in even more detail, just makes them all the more dangerous. The basic design and analysis question should not need to be asked, or I wonder if the guy isn’t in way over his head; I wonder if the boss or client doesn’t give a damn or doesn’t know any better either; I wonder how these people have B.S’ed. their employer about their actual abilities; I wonder where all the senior engineers have gone at these companies and why there are no mentors around any longer, God knows we all needed them at the early stages of our careers too. Are they coming here because they don’t want to admit to the boss what they don’t know, when in fact most good bosses understand this situation with new young engineers, and are more than willing to help, if you didn’t sell yourself for something you are not. The boss should want to help if for no other reason than to keep his own tit out of the wringer. Then, experience here might step in and help, even outside their own company, to discuss the question ‘is there a better way to do this or is there a better detail than this?’

 

[adammilad]

It is not a school project and it has been a while since I have been in school and I might add I was a great student. It has been a while since I have tackled problems like this. I have asked my lead and he has been helping i just wanted to get a second opinion. I was looking through a book "Steel Plate Engineering Data- Volume 2" and there where a couple formulas in there that seemed contradictory so I thought I might come here and try to get some pointers I was planning on having my integrity picked apart. Yes I am a young new engineer without a lot of experience and there is plenty I don't and plenty I have forgotten so please forgive me.

 

[adammilad]

"rb,
only if there was a standard stiffner !? (and a standard load, etc).

no, each stiffening is designed for the particular case ... you may want a few large stiffeners, or lots of small ones."

I am not asking for a standard stiffener but a standard to calculate the distance required between stiffeners.

Also I am not having troubles calculating the load or pressure.

 

[BAretired]

adammilad,

In problems of this sort, it is best to start out with some rough approximations. For a steel plate spanning vertically between beams, assume that M = wL^2/12 where w is the water pressure halfway between beams.

Assume an allowable bending stress of f = 30,000psi.
The section modulus is approximately 12*0.375^2/6 = 0.281 so the allowable bending moment is f*S = 30,000*0.281 = 8,437"#.

Equating wL^2/12 t0 8437 gives the following maximum plate spans for various pressures:

w Lmax
psf in.

100 31.8
200 22.5
300 18.3
400 15.9
500 14.2
600 13.0

This gives you an idea about how to space your horizontal beams. So select a proposed pattern of beams and complete the analysis.

BA

 

[a2mfk]

adam- I am not piling on here... And out of curiosity, what the heck are you designing?

You got jumped on a bit because of how you phrased your question, and you are not the first nor will you be the last. Also, for future reference, a sketch will be demanded most of the time unless it is a code/spec type question. Probably not necessary for your situation but keep that in mind.

"How do I find out if the plate thickness is good and if not how would I go about finding stiffener thickness and spacing?"

"I am not asking for a standard stiffener but a standard to calculate the distance required between stiffeners."

It would be the same as saying how do I figure out if this W12x14 works or not, and how far do I space them, and what span is it good for...

Dhengr's point is you should be able to attack this problem, break it down into pieces, and figure it mostly out on your own (engineering); or have someone in your office you can ask to get you started. There are no standard beam, column, stiffener, etc. sizes or spaces or a lot of structural engineers would be looking for other work. This is not residential floor design where you can pick a joist out of a table for a given span and spacing. (The guys' points above...)

Now with your response in mind, if you had presented the two formulas and why they are confusing, then you may have gotten responses more to your liking.

But some of the guys on this board get pretty PO'd, and understandably, if they think someone is fishing for a design and hasn't at least done their own homework or research. Hardly anyone knows each other on this board, so you can get judged pretty quickly and harshly based on what you write.

But don't let it discourage you from posting, just come with a strong post, full of details and a sketch, and take the problem as far as you can on your own.

FWIW I have never designed anything like you are describing in my 13 years, so I would also want to discuss this with another SE who has designed something like this before I gave it a go!

 

[dhengr]

Asammilad:
Reread your OP, and imagine how it might have prompted JedC’s post; reread my posts and imagine how they might fit your OP, particularly my second post and if the shoe fits wear it. For the parts that don’t fit, I apologize, but that post is all too often very fitting, it just showed up again in your thread, as a reminder to all. If you had asked your question more like your 6JAN11,17:56 post, you might have gotten completely different responses, because it at least shows you have done some digging and research on the problem. However, the book you reference has many equations in it, so we are still in the dark about your exact problem.

Get out your Statics and Strength of Materials books, much of what you need should be there. Look at Roark’s book “Formulas for Stress and Strain,” these books should be right at your fingertips if you are going to learn to do this kind of engineering work; and/or they should be available in your office or you shouldn’t be doing this kind of work. Later you will want some advanced books on Mechanics of Materials and Theory of Elasticity: “Theory of Elasticity” by Timoshenko and Goodier, and “Theory of Plates and Shells” by Timoshenko and Woinowsky-Krieger, are two examples of the latter. You must, almost always, use some of your own basic reference materials to get started on a problem, and your own native abilities to formulate the problem, before you start asking question or you don’t come across looking very competent. That is also a very good reason to develop a meaningful professional relationship with a very competent local engineer, maybe a mentor, if it develops that way. You can both be looking at the same specs., plans, sketches, etc. and ask questions or getting correction and feedback instantaneously. He can also tell you ‘that’s a pretty dumb question, you should know better, look in this book, and then tell me why your assumption or comment was wrong’ and it won’t hurt nearly as much because the relationship you have developed has your best interest in mind, and you know it by the smile on his face as he’s calling you a dummy. And, you have to be brave enough to go find this senior engineer and ask him if he will be your mentor. It’s a wonderful experience when it works. You’ll be amazed at your progress and he’ll be happy to watch your development.

Now, to “Steel Plate Engineering Data - Vol. 2," what page and which formulas? Why are you stuck on 3/8" pl. for the weir pl.? A thicker pl. might mean wider stiffener spacing, and those stiffeners are expensive. My earlier comments about your weir pl. conform more to Fig. 2, pg. 3 in my copy of that book. What’s the thickness of the main cylinder pl. and where in the cylinder is the weir pl., at the end of the cylinder or at some distance interior from the end? There are probably many more questions like these which you must explain so that we have a full picture of your problem, and how we might analyze and detail a solution, if you want meaningful help. You can see it we can’t, so you have to give all the info. needed, and ask a fairly specific question, to start to develop a meaningful solution.

 

[dik]

BA... you're treating the plate as a flexural item and not a membrane...

Dik

 

[JedClampett]

Sorry if I came off as the hard guy, but in additon to the confusing question, I also looked at adammilad's posting history and it was his first one.
Why .375" plate? How does the weir work in a cylinder (how is one side isolated from the other)? What supports the bottom of the weir? And how are the stiffeners tied into the cylinder (very important to carry shear)?
Now some supervisors (or mentors) believe in throwing a young engineer into the deep end and see what he or she does. And if that is the case, the questions adammilad had might just be an alternate way of researching the answer. But this problem has disaster written all over it.

 

[BAretired]

dik,

Yes, I am treating the plate as a plate and not a membrane. If you wish to treat it as a membrane, you must consider large deflections. I do not think that is the proper way to treat the problem, but in the limiting case, it may be an acceptable solution to the problem provided the necessary connections can be provided at the boundaries.

BA

 

[rb1957]

the plate distributes the water pressure onto the stiffeners. you can add cross beams (intercostals) that help the plate carry the load to the stiffeners

the stiffeners act like cantilevered beams, carrying the water pressure to the cyclinder.

water pressure varies with depth (obviously), so the stiffeners have a triangular loading.

you could start with a stiffener size (something typical, "standard"), and a spacing, (1' ?). what's the stress in the plate, say a 1" width close to the bottom ? can the plate carry the load ?? (bending, yes?) can the stiffener carry the load (from a strip 1' wide, 1/2 the spacing either side) ? again, bending (compression is going to be critical)

if not, change ... spacing (maybe inter-pitch some stiffeners in the heavily loaded section of the plate ?), stiffener size, ... my 2c, the load increases significantly towards the bttm, maybe two sizes of stiffener (careful how you join them), maybe two spacings (inter-pitching).

i'd also look into the codes you're reading, what seems contradictory ?? ask your lead.

does this construction seem odd to anyone ... why would you build a weir in a cyclinder ? presumably water is flowing in and out ?? doesn't this change things, maybe not significantly ??