two way slab design 3

[hx200]

I have a question about two way slab design.

ACI 318 use the area loading design column strips and middle strips in one direction and then use the same loading design the other direction. Are we designing for the same load twice?

Any thinking about this. Thanks a lot.

 

[rapt]

hx200,

No, we are designing for the load once. We are just using 2 separate analyses to get the design actions for each direction.

If you think about a one way beam and slab design, all of the load is carried in each direction, once by the slab and once by the beams to eventually get the load to the columns.

A correct finite element model for the flat slab will show exactly the same thing when the moments are converted to an orthogonal x/y arrangement and including in the design for Mx, My and Mxy.

 

[DaveAtkins]

I think of it this way. The middle strips in one direction carry the load to the column strips in the other direction. These column strips then carry the load to the columns.

DaveAtkins

 

[hx200]

Thanks for the reponse. But in one way slab design, we are desigining for one load path for the area distributed load. In two way slab design, seems we are designing for the same load for two load paths (two directions), 100% of the load each way.

 

[lkjh345]

hx200:

I've always thought the same thing. That you are basically designing for the same load twice. Once each in each direction. I'm probably wrong on this, and someone will probably show me the error in my ways, but when I design 2-way slabs, I always get this impression.

However, since this seems to be a conservative way of doing things, I am quite happy to comply as I like conservative designs.

 

[Lion06]

If you want to think of it as designing for the same loads twice, I suppose you can, but you can also think of it this way:
With a one way systemm, you design the slab for all of the moment in one direction, and then the beams for all of the moment in the other direction. In the two way system, you are designing the slab for all of the moment in both directions. The load needs to travel both directions to get to the columns.

 

[miecz]

Or think of it this way. Say you eliminated the strength in one direction. Would your slab still stand? I don't think so, as it could not carry the force from the middle strip to the columns.

 

[hx200]

Thank for all the replies again.

OK, I figured out that for the Direct design method (ACI 13.6), it's not design for twice. Since we have the column strips take a portion of the loading (say,75% ). if we have one way load transfer, we will have 100% on the column strip plus 100% from the middle strip. So it's clear that it's not designing the load twice.

But, for the equivalent frame method, which applies to more general cases with less limitations than the Direct design method , it analyzes the panels as one-way slabs in each direction, with each carrying the 100% area load . Any one can help me out on this.

 

[KCRatnayake]

Further to StructuralEIT, said moments in the direction of short and long spans depend on the aspect ratio of the slab panel. One other way to understand this behaviour easily is use of yield line analysis.

 

[DaveAtkins]

I think you are misunderstanding ACI 13.6. Whether you use the DD Method or the EF Method, you use ACI 13.6 to divide the moment into a column strip portion and a middle strip portion. And this explains why these slabs are designed this way (which is what I was trying to explain in my earlier response). The column strip takes more of the moment (75% of the negative moment and 60% of the positive moment at interior spans without beams) because it is picking up load from the slab spanning in the perpendicular direction. The middle strip takes less of the moment, because it is taking its load to the column strip in the perpendicular direction.

DaveAtkins

 

[KCRatnayake]

I'm bit confused. Are flat slab and two-way slab the same?

 

[Lion06]

There are several types of two way systems and, yes, flat slab is one of them.

 

[hx200]

Thanks DaveAtkins,

You are right, both the DD and EMF method are the same.

 

[rapt]

clefcon

2 way slabs
2way slabs have continuous supports in both directions, either walls or beams. In this case, only a portion of the lkoad is carried in each direction by the slabs so that a total of 100% is carried by the slab.

If the supports are beams, then the beams carry the loads to the columns, 100% split between the beams. In total, the total load is carried in both directions by a combination of the slab + beams. And the columns carry the load to the ground

If the supports are walls, the walls carry the laod to the ground.

Flat Slabs,

As DaveAitkens poointed out above, the column strips in this case are like the beams in the 2way slab case. The middle strips carry the load to the column strips (a total of 100% of the load split between the 2 middle strips, (similar to the slab in the 2 way case). Then the column strips carry those loas to the columns (similar to the beams in the 2way case).
And in total 100% is carried in each direction.

There is nothing conservative about it. As I said in an earlier post, FEM will give the same result.

 

[DaveAtkins]

In the ACI Code, the term "two way slabs" encompasses flat plates, flat slabs, and waffle slabs.

What rapt is calling a two way slab would be more accurately labelled a two way slab with beams.

DaveAtkins

 

[hokie66]

Dave is correct. Any slab which is not a one way slab is a two way slab.

 

[rapt]

Depends where you are.

In UK, a Two Way Slab is edge supported (by beams or walls) while a Flat Slab is column supported.

When over there, I get in trouble for calling a Flat Slab a Two way Slab.

 

[hokie66]

They do have some problems in the UK with the language. Oh well, when in London---. Rapt, I am sure you can get in enough trouble without arguing about their quirky terminology.

 

[DaveAtkins]

To paraphrase Henry Higgins from "My Fair Lady":

"Why can't the English teach their engineers how to speak?"



DaveAtkins

 

[KCRatnayake]

We're using BS codes in Sri Lanka. Agree with rapt.