Two-Way Concrete Slabs

[CowboyBill74]

Hello all -

I am afraid that i know that answer to this question already but this forum has been very helpful in the past. I have the pleasure of analyizing an existing two way slab that carries not only uniform loading but also has various pressure loads (uniform loading is dead and live load, pressue loading is from a tank that has 6 legs resting on the slab). My question is this, of course i am under a time crunch, i need a way to analyize this two way slab. The slab is two way continuous in one direction and 5 way continuous in the other. With this type of configuaration i am unable to use the simplified method of analyizing the slab according to code. I tried analyizing it as a one way slab (being conservative) but the slab does not have enough capacity looking at it this way. I tried putting this system into StaddPro but i do not think that i have correctly inputed the information because of the results that i am getting. Does anyone know of a program or a simplified hand calculation that i could use to determine the two way capacity of this slab? I have worked with engineers that showed me how to come up with the uniform load going in each direction by equating the wl^2 for each direction. However i have many tanks (4 in all with 6 legs a piece) do you think that this mythod is still applicable? It would still be a ruff hand calculation but i am running into my deadline quickly.

Thanks in advance for any information.

 

[DaveAtkins]

I would use the equivalent frame method--divide the slab into equivalent frames in each direction, and run each frame on STAAD PRO. Include the point loads on the appropriate equivalent frames in each direction. Then check the column strips and middle strips.

DaveAtkins

 

[civilperson]

The location of the tanks may negate any possible standard live loading for the area under the tank. Use correct load factor for fluid in the tank. Adding beams under existing slabs is common practice for over loaded slabs receiving new vessels.

 

[kslee1000]

For rough hand cal on bending, I will convert the tank load to a uniform load by dividing the weight over the area around the legs, then apply the resulting uniform load to the entire bay. After that, you can check punching shears at the legs for the concentrate loads. However, you may need to strengthen the slab locally to account for other effects, such as the tank is nearing an edge, or else.

 

[jike]

There needs to be some judgement used here as to how significant the concentrated loads are from the tanks relative to the tank locations, overall bay size and loading. If you are unsure, then the method that Dave Atkins suggested is probably your best bet.

ACI 318-63 has several quick methods for 2 way slabs (supported on 4 edges) with uniform loads in the appendix.

 

[CowboyBill74]

jike -

Yes that seems to be my biggest problem, all of the examples and codes that i have read show the way to analyize a two way slab is starting with a uniform pressure over the entire slab. I guess what i need to be asking is how to take the point loads that are located through out the slab to a uniform pressure over the entire slab. Even with the equivalent frame method you are only looking at a certain width of the slab. With the point loads coming down on the slab at various locations, even though i may look at a section of slab and it be ok with the loading on that portion some of the loading from other areas on the slab are going to load the section that i am looking at. (Does that make sense?) The slab will act more like a plate and any point loading on the slab will be distributed through out the slab, my concern is that i will look at a section and say it is ok but with addtional loading from else where it may not be. I dont know if i said this already but the slab is existing and is doing fine, and i have a hard time telling a client that something that he has had and has been working fine is already overloaded, which is what my hand calc's are showing so far.

 

[jike]

Have you added up all the concentrated loads in one bay and divided it by the bay? What do you get?

Have you taken the load of one tank and divided by its footprint? What do you get?

Have you taken the load of one tank and divided it by the area of its footprint plus half the distance to the adjacent tank all around? To this you might want to add some live load in the aisles around the tank for personnel, fork truck, storage, etc. (whichever might be applicable). What do you get?

These are just some approximations to get an idea what magnitude of loads you are dealing with.

 

[kslee1000]

If the tanks were added on after the original built, then this phenomenon (slab worked) could be attributed to that the real live load is lower than the original design, and the fact that the areas occupied by (direct under) the tanks have no live load. You may facing the delimma to lower the live load posting to reflect the facts.

If the tanks were in the original design, then I will have someone else to check my cal and assumptions, if the differences are large.

 

[asixth]

I wouldn't try to convert the point loads from the tanks to equivalent udls, because point loads alway magnify the moments in the slab (think area of the shear force diagram).

I have used a technique for distributing moments in both direction for slabs that are supported by four walls. That being:

Short_coefficient= L_long^4/(L_short^4+L_long^4)

and vice versa for the long span co-efficient. This approach cannot be used for flat slabs supported by columns, where column strips and middle strips must be used.

If you cannot satisfy the constraints to use a simplified approach, then you shouldn't use a simplified approach. I would perform an equivalent frame analysis. Remember to follow the point loads through the struture. If the point loads occur over a middle strip, the load must first span from the middle strip to column strip in one direction, and then from column strip to support in the other direction.

All the best.

 

[jike]

The approximations that I suggested are just that APPROXIMATIONS! Not something to be used for final analysis but just to get a feel for how much load is on the floor.

None of us know how large or how small these tanks are, nor do we know how strong or weak this floor is. This is for the OP to determine.

 

[kslee1000]

When perform approximation, try to picture how things would fail. An image of deformed shape/deflection will help to determine where needs more attention and how to distribute the loads.