two way slab design
two way slab design
(OP)
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.
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.






RE: two way slab design
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.
RE: two way slab design
DaveAtkins
RE: two way slab design
RE: two way slab design
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.
RE: two way slab design
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.
RE: two way slab design
RE: two way slab design
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.
RE: two way slab design
RE: two way slab design
DaveAtkins
RE: two way slab design
RE: two way slab design
RE: two way slab design
You are right, both the DD and EMF method are the same.
RE: two way slab design
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.
RE: two way slab design
What rapt is calling a two way slab would be more accurately labelled a two way slab with beams.
DaveAtkins
RE: two way slab design
RE: two way slab design
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.
RE: two way slab design
RE: two way slab design
"Why can't the English teach their engineers how to speak?"
DaveAtkins
RE: two way slab design
RE: two way slab design