Design of Structural Slab-Over-Void-Box Foundation 1

[oengineer]

I am seeking design info to design a Structural Slab-Over-Void-Box Foundation to support equipment.

For the Structural Slab-Over-Void-Box Foundation, I have been trying to see what ACI has regarding design procedure for Structural Slabs. With this in mind, I have found "ACI 302.1R-15 Guide to Concrete Floor and Slab Construction " & "ACI 421.3R-15 Guide to Design of Reinforced Two-Way Slab Systems ".

I am glad to have located these documents, but it would be helpful to have a design calculation example of a Slab-Over-Void-Box Foundation.

In my research I have also found it said that " This type of foundation can be a mixture of both one and two way spanning slabs. So, there are different approaches to how one might design this type of slab. The most common procedures used is the CRSI design book or a RAM Concept finite element model."

I do not have access to the RAM Concept finite element model, but I do have access to the CRSI Design Handbook 2008 & CRSI Design Guide for Pile Caps.

If anyone is familiar with structural slab designs (and both CRSI books), please feel free to provide input on the design procedure.

Comments/suggestion are appreciated.

 

[GC_Hopi]

You should be able to use ACI 318. You just need to break the structural system into its elements and design each accordingly. You have piles, grade beams/piles caps, and slabs.

 

[jayrod12]

WE put collapsible void under the slabs around here as well. We generally accept beadboard void below gradebeams and walls as there's generally more load there to resist uplift from ground movement. However below slabs we don't accept anything but cardboard voidform or delegated engineered void like geospan.

 

[oengineer]

You should be able to use ACI 318. You just need to break the structural system into its elements and design each accordingly. You have piles, grade beams/piles caps, and slabs.

Do you happen to have access to a go-by plan/detail sheet of a structural slab, along with some example design calculations, that you can share? I am new to this type of foundation design.

 

[GC_Hopi]

Chapter 4 in Link is my go to for one-way slab design. You should consult a senior engineer in your office for a "go by"

 

[oengineer]

Chapter 4 in Link is my go to for one-way slab design. You should consult a senior engineer in your office for a "go by"

From the link you posted, it appears that your saying that I should design the Structural Slab-Over-Void-Box Foundation used to support my equipment like a regular typical one-way slab. Is this correct?

I say this, because from my original post (see quote below) about the design procedure, I thought the design/analysis would be more involved.

This type of foundation can be a mixture of both one and two way spanning slabs. So, there are different approaches to how one might design this type of slab. The most common procedures used is the CRSI design book or a RAM Concept finite element model.

If it is just using the basics of reinforced concrete design of a slab to carry out the Structural Slab-Over-Void-Box Foundation, that would be pretty simple. But is this accurate to obtain a structurally acceptable design?

 

[JoelTXCive]

OEngineer-

Post a plan view of your slab and we can tell you if it's 1-way or 2-way. If the length is twice the width, then it's a 1-way element. Anything less than that and you will have some 2-way action. 1-way slabs are significantly easier to design with hand calcs compared to 2-way slabs, so if you have freedom to move the support conditions around, then I would do that. (if you are doing hand calcs.)

Once a slab is lifted 1mm off the ground, it is no longer a slab on grade. It's an elevated slab spanning between supports. Conceptually, it could be 10ft off the ground and the math would be the same.

 

[GC_Hopi]

Yes, its basic RC design. You can follow ACI to determine whether you should be analyzing your slabs as 1 or 2 way. And you can have some that are 1 way and other that are 2 way (your quote states this as well). As Joel mentioned, it will all depend on the layout of piles and grade beams.

 

[oengineer]

In regards to one-way vs two-way slab design, I have come across the following (see images):

Based on the information provided in the images, would it be good practice to design the Structural Slab-Over-Void-Box Foundation as a one-way slab instead of a two-way slab?

Since loads are transferred mainly in the shorter direction for a one-way slab, once the main reinforcement steel area has been determined that same steel area could be applied in the other direction as well (min S&T steel area would still have to be verified of course).