Monolithic footing question 1

[jonathanwilkins]

Working on a small town house project...30' deep. Typical IRC allows a 16" wide monolithic footer. We're in high wind so we're supposed to [prescriptively] go to 24". We're going to build a bunch of these and want to use 16" footers to save concrete so I'm trying to design them. Everything works except excessive bearing pressure at the edge and negative pressure at the inside. My question is how much of the slab can I use, if any, to do this. Adding some weight to the inside will solve my problem. Thanks; JEW

 

[a2mfk]

I'd make the footing a little wider, a couple of extra yards of concrete is a drop in the bucket of the total cost of the structure, much less the finishes. I am always concerned about mono-ftgs settling and then cracking at the slab transition. I deal with residential foundation problems every day, and I have to think pushing the envelope with the footing width pays zero dividends. Fixing cracks in walls and slabs a few years later will cost a whole lot more then the difference between 24" wide vs 16" wide...

My $0.02....

 

[concretemasonry]

30' deep or 30" deep for a small townhouse project? - How many stories?

The extra concrete is cheap. The real cost is the forming and placement time and not the materials.

Even a lowly basement is routinely built here has 20" or 24" wide strip footings even though the code may be narrower because it is easier, better and cheaper to do to a standard and have a little extra room if everything is not done correctly in layouts.

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

 

[paddingtongreen]

If the footing fails, almost the whole house has to be repaired. Skimp to save money where the damage will be less extensive if it fails.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[enko]

in the geotechnical designs, the things get multiplied by 2 or 3 (safety factor) so I would recomend the large size foot

 

[dcarr82775]

I agree with everybody else, just make the footing wider. To answer your question, I wouldn't count on the slab at all. Your insurance company will thank you for just making the footing wider

 

[rowingengineer]

I think you can use some of the slab, but I would like to see a sketch of the footing and layout before recommending an fuzy amount.

I assume the problem is in overturning, if you have internal footing and integral slab, I would have thought your torsions strength to the internal footings would also help.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[jonathanwilkins]

I made the footer bigger on the load-bearing walls. The end walls worked at 16". Not 24" but big enough to avoid overturning and excessive bearing pressue on the outer edge of the footer.

Another thing that I did was detail the exact location of conraction joints. I located a joint 3' inside each footer. Hopefully this will limit cracking just inside the footer. While I didn't use it, I feel this gives me 3' of slab helping.

The building is 30' deep, not the footer. Sorry for the confusion.

Comments on my thoughts?

 

[BAretired]

We're going to build a bunch of these and want to use 16" footers to save concrete so I'm trying to design them.

The above statement suggests to me that, sooner or later, you are going to run into trouble with your designs. Your approach to structural design is pathetic. I hope that you will seek the guidance of a mentor who will be able to steer you toward a more professional approach.

BA

 

[a2mfk]

CJs in residential slabs is very unusual (though it would avoid a lot of cracking I see everyday), check with the architect that you don't have tile going on that surface. I would expect most CJs in residential to align with interior non-brg walls if possible to hide them...

 

[jonathanwilkins]

Not sure why the design is pathetic? Making the footer the size it needs to be...not too small but not oversized so we can save some money. Same as optimizing a steel beam connection or similar. Just looking for some advise.

 

[Lion06]

I agree. Why skimp on a footing? That's really the last place you want to skimp - the fix is difficult and very expensive.

Assuming the footing is 12" thick, let's say 16" just to be a little more beneficial to your approach, you're saving 8"*16"=128in2/ft. I you assume plan dimension of 40'x40', that means you're saving 128/144*160 = 142ft3 = 5.3yd3 of concrete. Even at $250/yd3, that's less than $1500 that is being saved - likely on the order of 1/2 of 1% of construction cost. It doesn't seem prudent to sharpen the pencil too much there when there is so much risk associated with doing so.

You say it's designing the footing and not skimping. I say if you are tryng to count on the slab to hold down the footing to get soil pressures to work out then that is not a prudent design. I don't think that making it the size the it wants to be in order to work on its own without help from the slab-on-grade would be considered "over-sizing" it.

 

[rowingengineer]

I don’t like the idea of the CJ’s, I would suggest removing them from the slab and have them in the footing, vertical CJ’s are a no-no, however horizontal are ok.

Why bother to hire a structural engineer to design the foundations if you are not going to pick up your pencil, just read some table and implement. I see so many problems every day when people do this implantation without thought.

We are talking about the size of a foundation for wind loads, not only is this a small time event meaning the allowable bearing pressure is a conservative estimate at best. I see no problems with an engineer taking a look at other options to save a few $$, I like to see an engineer know just what the footing can and can’t do and the extreme limitations even if after you have done the cals you decide to run with the conservative option.

If you are to talk about the foundations from another design point: expansion of clays. A footing that has a large foot print will encourage more movement due to expansion pressure than a smaller width foundation. This was shown to be a huge problem with foundations in northern Queensland 20 years ago. The typical detail was a large width small depth foundation, while being strong enough the footing didn’t handle the movements. In my mind deeper is always better than width.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[BAretired]

jonathan,

Perhaps I was a bit harsh in my condemnation of your approach. I apologize if it was offensive. It seemed to me that you were trying to justify the use of a 16" wide strip footing even if the bearing stresses were a bit too high.

Why take a chance to save such a paltry sum? Check with your geotechnical consultant and determine what would be a safe bearing pressure. If in doubt, (and with soil, you are always in doubt), be conservative. It is much more expensive to underpin a foundation after experiencing excessive settlements than to use a slightly wider footing in the first place. Your design must be based on the results of a geotechnical investigation.

Everything works except excessive bearing pressure at the edge and negative pressure at the inside. My question is how much of the slab can I use, if any, to do this. Adding some weight to the inside will solve my problem.

In similar situations, I have not used the slab to do anything except carry live load. Usually, it is free floating and cannot be considered to contribute in any way to the capacity of the footing. If you have a different situation, then perhaps you should show us a cross section.

BA

 

[Lion06]

Are you designing the wall as fixed? That's the only way you should have moment at the ftg. Why not design the wall as pinned-pinned?

 

[jonathanwilkins]

UPDATE...

Don't think I'm going to do the job. Oh well. I did learn a lot, though. Our code specifies a minimum of 16" wide footer. There's a footnote, though, which allows this to be reduced to 12" if poured monolithically. (all of this doesn't apply to us as we're in a high wind zone) I talked to our department of insurance and was given the opinion (of the individual, not doi), that this is because in a practical application the slab does contribute to the "evening out" of the stresses and the footer can, as a result, be smaller. Interesting. No way that I can figure, though, to quantify that.

On to the next project.

 

[ron9876]

In South Florida mono footings are very common. The slab weight is required to offset the overturning moment. The slab is typically reinforced with #4's at 12" c/c. I don't see any problem with the concept if the analysis works. In the real world the footing has to rotate slightly to mobilize the slab.

My first house had a small crack at the top of the footing/slab which was likely a result of the rotation. That is the main problem that I see with the concept.