Large Steel Plate Analysis 2

[Mudar]

Hello,

I need to design a steel plate to distribute an axial point load coming down from a lifting jack onto a concrete floor. The steel plate is 7m in length, and 1.5m in width. The total load from the lifting jack is 400kN. The contact area between the steel plate and lifting jack is approximately 1x1m.

I am going to use FEA software to do stress analysis for the plate. Also, for the analysis purpose, I am considering the steel plate as a pad foundation with a column (lifting jack) on top of it.

As the steel plate is very large, I am wondering if I can assume the stress underneath it is uniformly distributed (like in the case of pad foundation analysis).

Any input is highly appreciated.

Mudar

 

[canwesteng]

It should be conservative for the case of stress in the plate to assume a uniform stress distribution under the plate, but unconservative for whatever is underneath it. Based on your loading (only 400kN) I doubt there will be an issue, but I'd try and find some rational way to check two way shear and bearing on the concrete surface below that doesn't assume a uniform bearing pressure under the plate.

 

[JStephen]

I would not be surprised if the corners/ends of the plate did not lift up under heavy loading, even.

 

[DETstru]

Is this a slab on grade?
If so, the behavior of the plate and concrete underneath is tied to the soil stiffness.

 

[Mudar]

Thanks for your replies guys.

At this stage, I am not concerned about the concrete and soil beneath the steel plate (will look into them later).

I have attached a sketch of plate configuration as well as a free body diagram of the plate, which illustrates how I am planning to model the plate in the FEA software. Can you have a look at it and tell me if there are any issues with it?

 

[canwesteng]

If you aren't concerned about the concrete and soil below, why even bother with the plate in the first place? But for the purposes of checking the plate your sketch looks fine.

 

[Mudar]

canwesteng

I am tackling one problem at a time. I will definitely need to check the concrete and soil, but at this stage I need to design the steel plate and determine its thickness assuming the concrete and soil are fine.

Thanks for your input.

 

[TLHS]

But you don't know how stiff the plate needs to be, or how far it needs to span, if you don't understand what you're trying to accomplish with the loads in the items below?

The only point of the plate is to make the structure below work out, presumably? The plate is almost certainly of a size where it definitely won't spread load over the full area of the plate before it deflects far enough that it can't push on the concrete below it. You need to know what area you need to spread the load over in the substructure so that you can determine how stiff that plate needs to be. If you legitimately need to spread it over that much area, you're going to need stiffeners or beams to increase the moment of inertia.

 

[SAIL3]

as drawn, the plate will definitely not distribute the load for it's full length...just to distribute the load for ,say 1m, on each side of jack I would expect a substantial thickness of pl...compare it to base pl on col....

 

[prex]

Repeating what others say, you definitely can't spread the load from the jack under the plate, so that it's uniform over the full surface of 10.5 sq m. The calculation you intend to perform is unrealistic: you need to account for grade stiffness or assume a partial length of plate as the effective one.

prex
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[SAIL3]

also, if stability of jack is a concern, the base pl must have sufficient stiffness to provide this and with the spacing of the anchor bolts, as shown, I doubt if it provides any significant stability to the jack.....

 

[ThomasH]

Mudar,

If the concrete and soil is strong you can possibly just remove the steel plate. 400 kN on 1.0 x 1.0 m is 400 kPa. The concrete will not be crushed so if the soil is good enough you'll be fine. Why do you have that huge steel plate?

If you want the plate to distribute the pressure uniformly over 1.5 x 7.0 m it has to be very stiff. Or the stiffness of the steel plate has to match the stiffness of the concrete and soil. They will be working together.

I would suggest that you start by looking at the full system. Then you can use FEA to solve parts if you feel a need to do so. But FEA will only solve the problem as you give it. For example, you probably need contact conditions between steel and concrete.

Good Luck

Thomas

 

[BAretired]

I don't think the soil will be able to resist 400kPa (8360 psf). You may want to consider using stiff beams and a plate to spread the load over such a large area.

BA

 

[271828]

"I don't think the soil will be able to resist 400kPa (8360 psf). You may want to consider using stiff beams and a plate to spread the load over such a large area."

I'd do something like that. At least a stiffened plate. As drawn, the L/t ratio of this plate is enormous.

 

[dhengr]

Mudar:
Where on earth did you come up with that plate size? Did you just happen to have that size sitting on your desk and want to use it up on the next project. You say..., “At this stage, I am not concerned about the concrete and soil beneath the steel plate (will look into them later).” and “I am tackling one problem at a time. I will definitely need to check the concrete and soil, but at this stage I need to design the steel plate and determine its thickness assuming the concrete and soil are fine. You are doing this bass-ackwords, as suggested by several of the posters above. Real world engineering problems don’t work like the one-topic, text book, to teach a specific topic or detail, all pertinent info. given, problems you saw in your first structural engineering Uni. class. You often have to gather extra/additional design info. and you certainly have to consider how several different systems interact with each other before you can start to solve the entire problem. There is no sense in designing the hell out of an improperly sized bearing/base plate, only to discover that the conc. slab or the soil are overloaded, and won’t work. So, you have to make some preliminary engineering judgements, quick prelim. design calcs. and determinations to see that all of the affected items are in the ballpark, and will probably work, with minor adjustments.

For starters, what’s the allowable soil bearing value, what settlement can you tolerate, and thus, approx. what soil bearing area do you need for your 90kip load, with what factor of safety (FoS)? Then, can the slab tolerate this load, what bending moments, shears and punching shear are o.k.? Now, what size bearing pl. do you need, so the soil and slab work? You pretty much can’t change them, so they have to be o.k. before you select and design your base pl. The base pl. on your jack is probably about 12" square, (90k)/(12 x 12) = .625ksi, so conc. bearing shouldn’t be an issue, but punching shear likely will be. One of the things we often do on a problem like this is provide a multi layer mat of oak (or some such) timbers between the jack base and the conc. slab. Above the conc. slab., a first layer of 6x6 timbers, maybe 8' long and 3 or 4' wide; atop these a cross layer of timbers 3 or 4' long and 18 or 24" wide; and then a base pl. 20 or 24" square under the jack base pl. The timbers distribute the load to the slab and soil to make them work. You will never make your steel pl. thick enough, economically, to make it work, FEA or not.

 

[Rabbit12]

I'm with dhengr. You need to check the slab and soil bearing first and then figure out how you can distribute the load to that required area. Minimizing the dimensions of the plate will be critical to having a plate that isn't crazy thick.

 

[Mudar]

Guys,

I am currently discussing with the client why they need a plate of these dimensions (as it was originally provided by them), and whether or not this size a structural requirement.

I will post my discussion outcome as soon as possible.

Thanks all for your contribution.

 

[ldeem]

I just did a problem similar to this. The plate will not spread the load very far unless it is stiffened. I would assume the load area to be same as the jack, assuming the jack base is sufficiently rigid.
The first step is to look at the problem as a concentrated load on a slab. There are several methods to this, I have attached an article that will help. Pay careful attention to the subgrade assumptions. If this doesn’t work out your next choice is to ignore the concrete and design a stiffened steel base sufficiently large to get the load down to the safe soil bearing.

As for the base I have good success using C6 or C8 as stiffeners. As others have pointed out for timbers or steel beam grillage are effective temporary methods to distribute jack loads.
As for your FEA analysis – you need to be careful with the support conditions. This problem requires the use of soil springs – depending on the program you have this may be something you will need to calculate. I personally use sub grade modulus in the 50 to 100 lbs per ft^2 per foot. The assumptions used here will have a huge effect on your results.

 

[PEinc]

Maybe install a round shaft, helical, bearing pile directly under the jack, beneath the slab?

http://www.PeirceEngineering.com

 

[SAIL3]

not knowing any details about the jack I would probably consider it a cantilevered col which would require some type of mom conn at base for stability, at least...the big question is what loading condition to consider when checking instability...probably, the most common one would be an eccentric load on the jack.....