Plate section modulus
Plate section modulus
(OP)
I have a 4x8 plate that will span 4 feet across two beams. Then a load will be applied to the mid span. When calculating the section modulus to determine Mn=FyZ, how would you determine what b should be used in Z=bd^2/4?






RE: Plate section modulus
RE: Plate section modulus
d is the depth
Are you sure you are a structural engineer? This is a very basic question.
RE: Plate section modulus
BA
RE: Plate section modulus
RE: Plate section modulus
strlengr7 is that what you are asking?
RE: Plate section modulus
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Plate section modulus
RE: Plate section modulus
RE: Plate section modulus
Roark's has tables for this scenario. Please check with that reference.
RE: Plate section modulus
I agree with paddingtongreen it makes big differentce for line load and point load. For uniformly line load b=8', for concentrated point load at exactly center of the plate, I use 45 degree rule as is shown in attached diagram, therefore b=4'. I also think graybeach's b=1' is too much conservative.
RE: Plate section modulus
a point load on a plate is a very complicated problem and as far as I remember the cases in Roark do not include this...my sugestion would be to add an extra bm underneath the point load and move on...
RE: Plate section modulus
RE: Plate section modulus
"The plate is 4 feet x 8 feet and is fully supported by beams along both 4 foot sides. Therefore, it is spanning the 4 foot direction."
If the 4" edges are supported, it is spanning the 8'direction. Here's why you need Roarke, the corners of this plate will try to lift up if the effect of the point load exceeds the effect of the plate dead load. If I was winging it, I would calculate the effective width to be the smaller of; the width of the point load + one quarter of the span, or the width of the plate.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Plate section modulus
SAIL3:
I know it is primarily a very complicated plate theory problem. However, we know every theory has some kind of assumptions and which will leads to errors to cpmpare with "true" solution in engineering practice. My 45 degree approximation theory came from my long time research and I have built several very fine grid element computer models to do calculation verifications and finally I have made an Lab experiment which proved that the 45 degree approximation gives a very good result (a little bit around 5% --- 10% in conservative side).
The experiment is like this:
I take a 32"x32"x0.125" steel plate, put it on 2 long blocks with a span of 16", then apply a 250 lb load on center of the plate, the deflection on center point is 0.26".
Next, I take a 16"x32"x0.125" steel plate and put it on the same 2 long blocks with a span of 16" and with 16" in support width direction. Then I use a very rigid 16" long steel block to apply a 250 lb load on center of the steel plate to form a uniform line load, the deflection is measured around 0.28".
After above research and experiment I feel very comfortable with this 45 degree approximation. The most advantage is it is very easy and fast to apply in engineering practice.
On the other hand, even if you use Roark's or other more complicated and so called "accurate" theory, you still can not guarantee that it has no error to compare with engineering practice.
RE: Plate section modulus
I think you meant to say fully supported along both 8' sides. That means the ends are held down to prevent them lifting off the supports.
A point load is a singularity resulting in infinite stress at the load point, dying out very quickly as you move away. If the load is spread over a small rectangular area, the problem is addressed in Article 37 of "Theory of Plates and Shells" by Timoshenko and Woinowsky-Krieger for an infinitely long plate with two simply supported edges.
BA
RE: Plate section modulus
As Paddington suggested this is really confusing, and it isn't us who are confusing the issue. So, I am adding that you seem very confused about what you are doing. If you are an engineering professional, you are doing a really poor job of defining your problem: span length and support conditions, load magnitude and its footprint, plate thickness, etc, etc. With such a basic question, so ill expressed, you should not be coming to a forum like Eng-Tips for your fundamental engineering education. We generally don't do first and second year engineering education here. Given the nature of your question, you probably shouldn't be doing this problem without the help of a senior engineer from your own office. Your boss should know what you know and what you don't know, so he can help keep you out of trouble, and guide you in your learning process. If your point load is on some rectangular area in the middle of the plate, what you might be asking is what is an effective width of this plate for these spans and plate thickness and load, so that using simple beam theory, I can take a first long hand shot at this problem. But then, also study how simple beam theory might not be appropriate for a plate bending problem, and come back and tell us. BA's suggested reading will get you a better answer, once you figure out which edges are supported, and how, and what your span lengths are.
RE: Plate section modulus
RE: Plate section modulus
btw, b*d^2/4 ? ... /6, no?, from (b*d^3/12)/(d/2)
RE: Plate section modulus
if the pl is just supported along the 8' sides and not held down, the pl can not develop membrane stresses and so would act like a bm.....this results in a very thick pl.
anyway, your original question of what b to use..
sorry don't have a reasonable answer for that, check out what Chrisaope suggested even though I do not understand what theory it is based on..
RE: Plate section modulus
my 2c ... i think even if the plate is just resting on the two supports, that given enough deflection (to invalidate plate bending) it would develop membrane stresses, and react these as a contact force against the supports.
one way to visualise what's happening to the plate is to divide it up into strips (nominally) 1" wide. the central strip has the load applied to it, and reacts this at the supports and shear into the adjacent strips, maybe a sine wave dist'n ? the adjacent strips see this load and react it at the supports, and shear into the next strip; and so on. "play" with the amplitude of the sine waves, each strip would react a portion of the load. the deflection of the adjacent strips either side of the load point should be similar to the deflection of the central strip ... the plate will try to make a reasonably symmetric -ve mound around the load.
i wonder what our irish friend, Tim O'Shenk, would make of this ??
or you could use FE.
or you could simply test it ...
RE: Plate section modulus
But yeah, if it were an initial design (ie not retrofitting to something existing) and the thickness coming out of the calcs wasn't crazy I'd likely just assume a force distribution in the plate.
I'd also be concerned about deflection if you're spanning this far with plate. In this sort of situation it may make sense to go with a thinner plate and stiffeners.
RE: Plate section modulus
RE: Plate section modulus
RE: Plate section modulus
RE: Plate section modulus
Take b=12 times plate thickness may be more easily to be justified. However it is apparently toooooooooooo much conservative.
I have just repeated another test, I happen to have a 30.625"x20"x16g(measured t=0.059")plate at hand. I simply supported this plate along the two 30.625" edges (support width = 30.625") with a span=18" (no support along the 18" edge). Now I put 25.8 lb weight on center point of the plate with the center point area = 1.25" Diameter area, then I got the measured deflection = 0.27". From theoretical calculation, with b=18", the deflection will be calculated as 0.35". This repeated test again proves that even 45 degree rule is still conservative enough.
RE: Plate section modulus
I suspect for a point load that the two possible governing yield line patterns are those shown in the attached. The first one will only happen if your plate is narrow enough in the non-spanning direction to allow the other yield line mechanism to intersect with the side. You can see that you gain capacity from where your hinges develop on the sides.
There are a couple of extra patterns to check (yield lines intersecting back to the corners would be one I'd need to check because your outside edge is free to rotate), but I don't see
them being lower load failures.
RE: Plate section modulus
Stress in the immediate vicinity of the load will be much larger than that in more remote regions...in both directions.
BA
RE: Plate section modulus
With the load at the center, the greatest deflection will be at the same place. The deflection will be a little less ahead of, behind and to the left and right of the center. It will be more reduced a little more forward, back, left and right of the center. A cross section through the center, in both directions will be a curve with the low point at the center. Cross sections further forward and back, left and right will be similarly curved, but the low point will be higher than the center section. This implies that there will be high lines running diagonally from near the center, similar to an upside down vaulted cathedral, to perhaps the corners but perhaps not that far out. This is why I said earlier that the corners try to lift up. Anyone with Roarke can check me on this. If the corners lift, the ridges go towards the last contact point with the support. The point of this is that the load from the jack and the whole weight of the plate is carried by only the center part that is in contact with the support.
The point all of this is to give my reason for my conservative approximation. It is less generous than it sounds. In this case the width of the load plus 1'-0" (1/4 of the span).
Personally, I would be more concerned with the slope and whether the edge on the support would lift than I would the stress. Function must be catered for before we think of stress.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Plate section modulus
Good point, BAretired.
I actually also considered about this stress concentration issue, that is why I am not solely rely only on deflection measurement test. This 45 degree result actually comes from my Finite element analysis model. The reason we have done so many analysi is we actually had taken several industry projects before, which involves a lot of point load on center of a steel plate problem. If take effective b=load width+12*t, or b=load width + 1/4 span, then these projects become a mission impossible. This forced us to dig into more detailed analysis to find the more truthful, more realistic solution. One of our example FEA result showes that once the point load width is equal or larger than 1/9 span, then the maximum concentrated stress in the vicinity of the point load is already smaller than the stress calculated from effective b = 45 degree rule (21.4 ksi vs. 24.0 ksi). Of course if the point load width shrinks toward zero, then the concentrated load will be surely excessive, however in this situation, the punching shear will control the design.
As I stated before, I am not against effective b=load width+12*t, or b=load width + 1/4 span. I think it is more easier to get justified however not practical in many circumtances. On the other hand, our 45degree approximation, it is more complicated and troublesome to get justified (our FEA analysis model is our legal support), however it does effectively solved a lot of practical engineering problems in real world.
RE: Plate section modulus
If the load is displaced by unit distance, the External Work is P*1 or P. If the span is 'a' and the width is 'b' and the yield moment of the plate is 'm' foot pounds per foot, then Internal Work = (m*b*2/a)2 = 4mb/a. In our case, b/a = 2 so I.W. = 8m.
Equating E.W. to I.W. we get m = P/8.
In the second diagram, there are four yield lines, two diagonal lines and two vertical lines. The vertical lines are negative bending. If x represents the distance between negative yield lines, then:
E.W. = P
I.W. = m(4a*2/x+2x*2/a) = 4m(2a/x + x/a)
Setting the last expression equal to zero, we find x = √2*a and I.W. = P/11.3 which is less critical than the previous case, i.e. the plate requires a smaller yield moment.
If we consider a third case where the plate is not held down along the long edges but permitted to lift off the supports, then x = a and m = P/8, which is the same as we obtained in the first case.
BA
RE: Plate section modulus
Setting the derivative of the last expression equal to zero, we find x = √2*a and I.W. = P/11.3 which is less critical than the previous case, i.e. the plate requires a smaller yield moment.
BA
RE: Plate section modulus
By yield line analysis, it appears that the value of b to use in this case is 8', but if 4b/a > 11.3 (i.e. aspect ratio b/a > 2.825 the second yield line pattern shown by TLHS would govern.
BA
RE: Plate section modulus
From a comfort and confidence standpoint with plate structures, yield line theory is very much the best tool I've encountered. It's a good balance between simple and practical. While I may make more conservative assumptions, it gives you something to fall back on that lets you work out your actual failure state. It lets you use a reasonable hand method to check finite element, and it also gives you a good basis that you can use to justify working around small local stress peaks in a finite element solution.
It's the only major analysis type I regularly use that wasn't even touched on in my classes back in university. I think it's ridiculous that, while plastic analysis of frames was done, nobody ever took the extra few minutes necessary to explain that it can be extended to plate structures.
RE: Plate section modulus
Good job, BA, one more star for you, and one star for TLHS for suggesting yield line pattern.
Thanks.
RE: Plate section modulus
I agree that yield line theory is a useful method of tackling certain problems, particularly when checking steel plates but it must be used with caution.
Yield line theory always gives an upper bound to collapse load and there is no assurance of adequate strength in diagonal tension or deflection control.
The Hillerborg strip method gives a lower bound to collapse load and is deemed by many to be preferable to the yield line method for the design of concrete slabs.
BA