ACI 318-99 two way slab design 2

[kronosconcrete]

When designing a two way slab, does the weight get apportioned according to the ratio of the length and width of the slab? It doesn't seem the longer dimension should be designed for the same weight as the shorter dimension since obviously the shorter carries more of the load. If one uses the same w on the longer dimension, the moments would seem out of proportion. I've read the actual delegation is the inverse fourth power of the length/width ratio. That seems correct to me. Is the shorter dimension designed for actual w and the longer dimension with a reduced w of that fourth power inverse ratio? I've never been clear on how the transverse (perpindicular) direction is treated.
Thanks

 

[JAE]

ACI Chapter 13 provides for two methods of determining the forces acting on the two-way slab: Direct Design and Equivalent Frame.

We've almost always use the equivalent frame method which requires you to analyze the total load in one direction and then turn 90 degrees and analyze the total load in the orthogonal direction. I've always questioned this as not really being a true "two-way" system because you are really designing two one-way systems perpendicular to each other.

For each direction, you are designing a strip of concrete of width "w" where w is the distance from center of bay to center of bay with the column line in the middle. You detail reinforcing for a column strip in the middle and two half-middle strips on either side.

With this method, there is really no need to proportion the loads to each orthogonal direction because each direction takes 100% of the load.

 

[breaks]

I have frequently designed two-way slabs using Rectangular Plate Design Aids. These aids provide charts for different plate side ratios, loading conditions (uniform pressure, varying pressure) and edge conditions (fixed, pinned, free edges). Using these chart coefficients, the loading and plate sizes, you can calculate the resulting moment and shear at almost any point in the slab.

One reference that contains such charts is "Rectangular Concrete Tanks" published by the Portland Cement Association (PCA). Another is titled "Moments & Reactions for Rectangular Plates" published by the U.S. Department of the Interior.

Hope this helps...

 

[hippo11]

It took a while for me to justify to myself why, in a two-way slab, you design for 100% of the load in the X direction and 100% in the Y direction, as opposed to proportioning it like 50/50 or 60/40.

It seems redundant to design for 100 percent each way, but it' not.

This is because the loading "picked up" by the X-span flows east and west to the Y-span, where the load turns a corner and follows the Y-span until it hits a support. And similarly with loading "picked up" by the Y-span, it flows north-south till it turns a corner and follows the X-span until it hits a support.


So basically what the X-span "sees" is the load it directly picks up as well as the load it "receives" from the Y-span, totalling 100% of the applied dead + live. And similarly for the Y-span.

 

[sdz]

On the question of whether you need to design for 100% of load to be carried in each direction; it depends on the support conditions.

If you have a slab supported on walls then you will only need to design for a portion of the load in each direction. This can be done using a grid analysis, slab design tables, yield line, strip method, or other suitable method.

If the slab is supported on columns then you need to design for the total load to be carried in both directions. Again by a variety of methods.