ACI 318-99 two way slab design
ACI 318-99 two way slab design
(OP)
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.
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RE: ACI 318-99 two way slab design
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.
RE: ACI 318-99 two way slab design
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...
RE: ACI 318-99 two way slab design
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.
RE: ACI 318-99 two way slab design
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.