turndown slab foundation 1

[steve1]

Does anyone know what is meant by "a shallow monolithically-placed turndown slab foundation". This phrase has turned up in a geotechnical report that I am currently reviewing. The report was written to provide geotechnical recommendations for design and construction of a natural gas station, which will include the use of precast concrete buildings (similar in size to a mobile home and/or construction trailer). The turndown slab foundation was recommeded for use with these buildings.

 

[Rowland33]

I think the writer means a slab on grade with thickened edges.

 

[Qshake]

I agree with ROWLAND33. Another detail to consider with these thickened edges is that they typically transition back into the slab underneath at a rate of 1 to 1.

 

[Taro]

A couple other considerations for this type of slab...

1. If there are heavy wall loads on the turned downed section of slab, the deflections of this part will likely be greater than that of the rest of the slab and significant cracking and differential settlement can occur.

2. The downturned section will act to restrain slab shrinkage and unacceptable cracking can occur unless proper control joints are detailed.

 

[ishvaaag]

Another consideration is that heavy point loads, such wheels of trucks inflict noticeable damage if the boundaries of the slab are not thick enough. A perimetral beam is used in those cases. To use ties at perimeters is in general good practice in construction, as long as potential inconveniences such those described by Taro are properly dealt with.

 

[Zulak]

The turn-down slab is more like a mat slab with edge beams. Proper reinforcement detailing will limit cracking. This method is fine for smaller structures. Make sure that the turn-downs satidfy your frost depth requirements of your Code.

 

[Ron]

A "turn-down" slab is just as Rowland33 and Qshake have stated. It is a slab with integral thickened edges. Typically, for a 4 or 5-inch thick slab, the edge will be about 12 inches thick with a bearing width of at least 12 inches, reinforced in the thickened edge much in the same manner as a strip footing. A transition is made on the inside edge of the bearing width upward and inward as Qshake described at a 1:1 ratio. Hooked bars are often used to tie the "slab" portion to the "footing" portion, usually extending a couple of feet past the transition area into the slab.

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Something like this!

Be aware that scouring can be a problem with this type of slab system.

 

[Ron]

Obviously, my "StarCAD" ability is lacking! The bottom line should not extend to the right past the diagonal.

 

[steve1]

Thanks for all the input. A follow up question. Is this type of foundation designed similiar to a mat or a strip footing? How are concentrated loads in the slab portion of the foundation handled?

 

[DMWWEngr]

I'm sorry in advance for this question...but what is meant by "scouring"?? I have not seen this term used before.

Thanks,
---
Andrew

 

[Ron]

The slab portion can be designed just as you would a pavement section. Handle concentrated loads as if a wheel load.

Regarding scour....this is when runoff from the roof or surface cross-flow removes the soil adjacent to and under the bearing edge of the thickened edge, causing a loss of bearing (cantilevered edge).

 

[DMWWEngr]

Thank you Ron!!

 

[JAE]

An extension to what Ron so "adequately" sketched....in some areas where you have expansive clays, many smaller structures such as residential homes, or small office buildings, are constructed using this kind of system.

Usually, the perimeter beams are supplemented with an interior grid of beams to form what is called a "stiffened slab-on-grade". The beams are spaced between 12 ft. and 16 ft. on center and are usually 12 to 18 inches wide and can get as deep as 36 inches (even in non-frost areas). The concept is to create a rigid mat on which the building rests.

A design methodology has been developed by the Concrete Reinforcing Institute in conjunction with The Wire Reinforcing Institute. It was prepared by Walter L. Snowden, P.E. in Austin, Texas back in the 70's (I think).

I have the document which is titled "Design of Slab-on-Ground Foundations, A Design, Construction, and Inspection Aid for Consulting Engineers", dated August, 1981. I would hope that there are copies available through a search on the internet.