Strip footings and residental slabs 1

[AnaAnt]

Hi folks!
Ive seen this to many times and I was always wondering wich system is "right".
Foundation system - strip footings under bearing masonry walls
Groundfloor slab - cast over strip footings or between them on compacted fill material.

Wich solution is right?
Maybe dumb thing to ask since it seems that contractors do this in both ways but I was always leaning toward the "in-between" solution.
Your thoughts and ideas...

 

[Ron]

Neither system is "right". Either would be acceptable under appropriate conditions. The more common for separately placed slab and footing is the "in filled" condition. For the 2nd condition where the slab is "sandwiched" between the footing and wall, it is usually done as a monolithic slab with a thickened edge condition that serves as the "footing".

 

[concretemasonry]

One many basements, single and muti-family are built with a strip footing poured and the back or concrete wall then is placed on the strip footing and the slab poured later after utilities and drainage is installed. It is generally a region method and some local codes require the basement slab to be poured on the strip footing and have 4" of the slab in contact with the wall.

The advantage is getting the footing in quickly and providing a hard, level base to build the wall on quickly. In a cold climate, the basement can be excavated, strip footing poured and walls up an ready (4 days) for capping the first deck. In cold weather, the furnace can be hung from the deck to provide heat when the framing is dome and closed up. It make the interior ready for all trades to work under good conditions efficiently.

The walls are braced until backfilled and the basement slab is poured later (usually at the same time as the garage slab and sidewalks.

A big factor is the speed of construction when there is variable weather (cold, sudden storms, etc.).

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[AnaAnt]

@Ron
WHat do you mean by "slab with a thickened edge condition that serves as the "footing"".
Can you post some sketch or drawing

@concretemasonry
Im not talking aboth basement slab. If this was the case I would probably choose to foundate all the structure on basement slab that would act as a raft.

 

[Ron]

Here is a typical section. Dimensions are in inches.

 

[msquared48]

Two advantages to Ron's detail:

1 There is only one pour instead of two, and

2. You only have to provide removable forms on one side of the footing.

These footings are found a lot wheere the bearing wall is at the very outside of the footing, but the reinforcing pattern is different.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Ron]

Mike...I picked a detail off the internet. My detail uses hooked bars back into the slab.

 

[AnaAnt]

But your drawing is more a raft system than it is a strip footing!
All cast at once with reinforcing steel!

 

[Ron]

AnaAnt....in most cases in my area, the slab is essentially unreinforced so it really is not a "raft" or mat system. The reinforcement is only carried into the slab area to hold the inevitable perimeter crack together.

 

[hokie66]

There are many solutions, but I think AnaAnt is right about the definition of a strip footing. Strip footings are not cast monolithically with a slab.

 

[AnaAnt]

@Ron
Ok, so you are providing minimum reinforcement steel to prevent shrinkge cracking.

@hokie66
Yes, this is why Im asking when "in-between or over" situation and never cast together if your having a strip foundation system.

When "cast over" you could say that strips act as end "supports" and at the same time slab is resting on the ground.
When cast "in-between" perimeter strips, its a clear situation - slab on grade, wich will hold partition walls, live load, dead loads (floor coverings).
But Im interested win your opinions, pros and cons...and which system to use and when....

 

[hokie66]

As I said, there are many solutions, none of which are perfect. One thing though...casting a strip footing right up to floor level as shown on the left of your first picture is not wise. The footing should stop below the floor, with either the wall or slab starting there. Matching the top of the footing and floor level is asking too much of the builder.

 

[wannabeSE]

Ron,
Do you use the thickened slab only on lightly loaded walls? Or, do you use the slab to resist the eccentricity caused by the wall being at the edge of the foundation. When we don't have the masonry wall directly on the footing, we have a thickened slab edge or concrete foundation wall sit on the continuous footing.

 

[AnaAnt]

@hokie66
Agree, but here the strips would be first casted.
Then later, ground slabs, between the strips. Strips would basicly serve as side formwork.
But here another thing that I see as a problem. Elastic settlement of strips.
If the settle eg.2cm, slabs wont follow and there could be hydroinsulation problems and crackings.

 

[msquared48]

The solution you end up with will not only include your engineering input and experience, but also that of the contractor and wnat he is used to doing.

If you are worried about that differentialo settlement, then go to a crawlspace and eliminate the problem.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

AnaAnt,
Yes, as I said, it is generally a poor detail, for many reasons. I can think of a couple of types of industrial facilities where that detail would be appropriate, but not for residential buildings.

 

[AnaAnt]

Hohie66 thanx for your response.
So you are in favour for "casted over strips" solution?

 

[TXStructural]

It is dependent upon many things. In my part of the country, highly plastic, expansive clays make some systems a poor choice. Slabs which are free to move allow with the soil can allow for expansion and settlement. Where stable soils underlie the slab, an integral method works well.
This is an older reference but cover the topic well:

http://www.nap.edu/openbook.php?record_id=9804&page=1

 

[AnaAnt]

@TXStructural
Thanx for the reference text.
This topic seems to be more oriented towards type I&II slabs (unreinforced and slightly reinforced).
Type III slabs are general raft systems where slab recives and transmits all load from superstucture to the ground.

Looking at drawings (pages 40,42 and 44) seems that all slabs are separated from foundations by an expansion joint.
Also on page 43, last sentense, its clearly stated that slab Type I&II should be "free of foundation walls, piers or footings, otherwise the slab is likely to experience flexural stresses wich it is not capable of resisting without cracking."
Questions to this quote: Is author saying this because of probable elastic settlement of footings, meaning it is better to keep footing and slab on ground /residentual slab)separated and allow them to behave independently in these cases?

Basicly footing and slabs should be cast as in the sketch attached.
What should be the width of the expansion joint or could the foundation and slab be casted seperatly (next to each other) but not connected by the reinforcement?

 

[AnaAnt]

Hi!
I found a very interesting topic here on slabs on grade

http://anc-tbquimby01.uaa.alaska.edu/courses/ce433/Quimby/ce43310.htm

Last topic coverd.
Heres the direct link for download:

http://anc-tbquimby01.uaa.alaska.edu/courses/ce433/Quimby/docs/2010/10CE433SlabsOnGrade.mp4