Two Way Slab on Ground 1

[Banks]

I have a situation where I have a client with a pool that is proposed to be supported on an 8" thick concrete slab. There is poor soil below the pool so the 8" thick slab has to be self supporting between rammed aggregate piers. Now, it seems like everything regarding slabs that we have at our office assumes the slabs are fixed to the columns. I don't have this situation and can't have this situation. Does anyone know of any way that I can perform an analysis without turning these into one way slabs (without a computer analysis program)?

 

[msquared48]

I would put an edge beam at the periphery of the slab, spanning between the piers and use Method 2 of the '63 ACI building code with the associated "m" coeffieient for Case 5 - four edges discontinuous - to get the relative positive moment distribution.

Sorry to be dating myself here...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SlideRuleEra]

In this worked-out example in a recent (2007) PCA Design Aid it looks like the 2-way slab is supported by, but not fixed to the columns. Perhaps this a reasonable approximation of your project and can be followed:

http://www.cement.org/buildings/Timesaving-2WaySlabs-IA.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[DaveAtkins]

You could use the Direct Design Method in the ACI Code.

DaveAtkins

 

[ATSE]

A non-cemented rammed aggregate pier (Geopier) acts like a large bed of soft springs (say, 3' x 3' effective square). So, like DaveAtkins said, the DDM is probably appropriate, with the exception that there's not a punching shear or unbalanced shear problem at the "columns", assuming the piers are spaced at less than 8'-0" on center each way.

 

[Tomfh]

What is the ACI DDM? Is that design-strip/column-strip/middle-strip etc etc approach?

 

[OHIOMatt]

ATSE mentions Geopier. If this in fact the company installing the rammed aggregate piers, they may be able to offer some assistance.

In past projects, I have provided them with loading and a proposed slab thickness. They have returned suggestions for reinforcing as well as slab stresses. Obvioulsy you check there output, but they have a FE analsysis program set up for there piers. They model the piers as spring with differing stiffnesses extending from something close to infinite rigid at pier center line.

Just a suggestion to help validate your calculations

 

[Banks]

Okay, so I think I have some of my confusion with the DDM answered. I have two points that I need clarification on though.

1. DDM says that live load must be no greater than 2x dead load. I have a fluid load (a pool filled with water) which isn't really either a dead or live load, but I would consider to be more in line with a dead load situation seeing that they are always (well not always) combined with each other.

2. This part may seem like the dumb question part, but my moment transferred into the column would be zero correct? That was one of the points hanging me up because every example that I've ever seen has fixed supports, which makes sense because it's all cast concrete. With DDM I'm not restricted to fixed supports am I?

Thanks for all the replies so far!

 

[DaveAtkins]

Correct--moments will not be introduced into the new piers.

DaveAtkins

 

[rapt]

Banks,

Surely you can work out a BM diagram for a slab (any member) supported on knife edge supports! You learn that in 1st year university!

Doesn't the DDM make some assumptions about moments into supports?

 

[csd72]

Banks,

1. the live versus dead load limitations in the code are because of pattern loading - not really applicable in your case.
2. treat the columns as knife edge supports and this will be conservative.