design of two way slab 1

[fyaure]

I could not find ACI design specifications for two-way single-span slab simply supported on all sides.
Can anyone help? Specific design example would be very helpful. Or please tell me the books I can refer to.
1. how to calculate the maximum moment in this case?
2. how/where to find moment and shear cofficients?
The slab size is huge, square 23ft X 23ft and 6ft deep(clear span is 15ft).

 

[JJF]

fyaure,

This sounds more like a large equipment foundation.

Do you have more details?

JJF

 

[UcfSE]

With a slab that thick compared to its span you'll probably (not sure) have to account for shear deformation as well as bending (thick plate equations instead of thin plate). You can probably design it for the full moment in each direction instead of dividing the moment into each direction. That's what the ACI methods do. With single span simply supported obviously thats (1/8)wL^2. I don't know how you could take into account the fact that it's so thick, analagous to the extra provisions for deep beams compared to shallow beams.

 

[tngv752]

How come u have that very thick slab ? Be more specific.

 

[jheidt2543]

The thickness sounds more like it is a slab for radiation protection over an X-ray or other medical imaging machine.

 

[Zhuge]

I suggest you to use strut and tie method, or to use finite element method to calculate the reinforcement needed.

 

[fyaure]

Well, the slab size does look wierd. It is actually a cover a huge mine shaft which cannot be backfilled. The traditional method is to put a plug (unreinforced) into the shaft where the plug would be as thick as the width of the shaft. However, in this case, I feel more comfortable designing something with actual load conditions than just throwing in the concrete which may not be stable because the the shaft conditions are not known and investigation is way too expensive (tight budget here).
So what I am doing is digging the peripherial bedrock in the shape of the the slab and placing a slab on the shaft such that the top level of the shaft is even with the bed rock surface. Now, it looks like deep beam but does'nt satisfy the requirements. Also, since it is laterally restrained from all sides by bed rock and yet is a simply supported condition, I do not know how it is going to affect the design.
The load conditions show that such huge thickness is not required. However, taking into the consideration the seriousness of the problem (if the cap fails), I think it is wise to put a big chunk of concrete which even on breaking wouldn't collapse right away but get logged itself in the shaft giving time for remediation measures.
While designing the slab, I also realized that the minimum reinforcement for such a huge area comes out to be too large and the reinforcement seems conjested.
Can soemone tell me where I can find the details and requirements minimum reinforcement for such slabs? Also, will the reinforcement at the top and corners be necccessary? I could not not find any specification that could actually addrresses this kind of strcuture. I wish I could draw a picture. Is there way to do that in this forum page?
Please help!

 

[SlideRuleEra]

fyaure - For the situation that you describe their is probably no established requirement. At these times, I like to look for "outside the box" ways to approach the problem. Here is an interesting design concept from the bridge industry: ftp://imgs.ebuild.com/woc/C890023.pdf

Following this logic, sounds like reinforcing such as #14 @ 10" o.c., each way, top & bottom, could be a place to start.
As always, if you get better advice, take it.

 

[71corvette]

Out of curiousity, what type of formwork is used to support the slab during placement? At about 900 PSF it must be quite substantial...

 

[pcronin]

Is the PCA (Portland Cement Association) Manual for Concrete Tanks. It has the moment, shear and deflection coefficients for all types of end conditions, aspect ratios and loading conditions. If you don't have access to that, use the Roark book on plates and shells which has similar coefficients for stresses that you can then back calculate your moments. Good luck.

For mass concrete pours like you are talking about, the minimum reinforcement ratios you are used to from ACI do not apply. There is criteria for providing reinforcement in the outer faces only, but I forget where it is located. Min reinforcement is mainly to control shrinkage and in mass concrete, it will only occur at the faces that are exposed to the air. The center of the pour will probably take years to hydrate so it will be less likely to have shrinkage cracking.

 

[haynewp]

It may be a mass concrete problem. I have not dealt with a volume of concrete like this before and cannot offer much help.

I think you should look at ACI 207.1 to see if it meets the criteria for mass concrete. I think this month's issue of Concrete International also has a story on mass concrete as well and how it was approached in the construction process. You might think about requiring the specification procedures for mass concrete during construction so deep cracking and failure will not occur.

Just adding a whole lot of thickness of concrete may not necessarily be giving you a higher factor of safety.

http://www.mcgonagall-online.org.uk/poems/pgdisaster.htm