Discrete vs Combined Footings 4

[Althalus]

Way back when I took a foundation design course my instructor said that when you have vertical loads close together, it is often more cost effective to provide combined footings when the Aconc >= 0.35 x Agross. This was in the context of typical column spacings for typical building construction.

Since then I've found many people never heard of this. I'm wondering how common this rule of thumb is.

BTW, I've discovered that when a footing is especially deep (frost depth) other variables come into play and a modification to the 35% figure should be employed due to costs of excavation.

Any thoughts?

 

[BAretired]

Never heard of that rule. Please define Aconc and Agross.

When two separate square footings overlap, it is probably more economical to use a combined footing.

BA

 

[Althalus]

Aconc = Area of concrete footing.

Agross = Total area of footing plus void space where you only see soil, no concrete. Obviously for differing geometries, this can vary in definition as well.

 

[BAretired]

Althalus,

Have you tried testing the formula with one or two examples? Offhand, I can't see how it would work, but it might be helpful to try it in a real situation.

BA

 

[Althalus]

BA,

It isn't really that kind of question. But I believe you've answered it in that you haven't heard of using this rule of thumb.

Here's the idea:

1) Given a piece of equipment with two legs 15 ft apart.
2) Load calculations indicate that the appropriate footing size for each of the two legs is 5ft x 5ft.
3) 5x5 = 25 ft^2 each. This equals 50 ft^2 total.
4) If instead we used a single footing that was 20 ft long x 5 ft wide, this would equal 100 ft^2. Thus the footings = 50% of the combined footing area.
5) Since 50% > 35%, this single footing should produce a more cost effective design because the cost of rebar and concrete is minimal compared to the additional labor for the discreet footings.

This is with the assumption that whatever changes to the loading due to the combined footing, it would not significantly change rebar ratios in each direction. It may even lighten a bit due to greater concrete mass.

Note: I used these numbers to inidcate what I meant by the 35% rule. Don't get hung up on other alternative dimensions and how I could minimize further. This is a completely different issue.

The questions now are:

1) Has anyone ever heard of this rule before?
2) Does this actually produce a savings in TIC (Total Installed Cost)?

 

[bootlegend]

The savings would be that the contractor only has to form the one large foundation instead of the multiple smaller foundations. I've never heard of a specific number (35%) put too it, but it is something most people consider in design I assume. You just have to ask if the cost of the extra concrete is more than the contractor's labor for the extra forming.

 

[mtu1972]

To answer your questions:

1) No
2) The concept is correct, but this ratio may be an approximation and your application of it needs refining.

I will get hung up on other options. In the case you have described, I may choose to use a 3' x 18' combined footing, which will satisfy the allowable bearing conditions. As the footing thickness may then need to be increased, both the concrete and forming quantities will be increased, thereby probably increasing the total cost.

The rule I found in a textbook ages ago is: When the clear distance between adjacent individual footings is less than the length of their parallel sides, a combined footing should be more economical. I have used this to determine where combined and/or strip footings should result in construction cost savings.

In your case, you have 10' clear between the 5' square footings - so individual footings should still be more economical.

Combined footings can provide reduced Installed Costs where the conditions warrant that solution.

gjc

 

[BAretired]

If square footings are reinforced only on the bottom, then the use of a combined footing requires (a) more bottom steel and (b) top steel between the load points. Also, combining the footings probably results in more errors in steel placement because of the extra complication.

If two columns are 5' apart and each requires a 5'x5' square footing, I would prefer to use two separate footings notwithstanding the fact that Aconc/Agross = 1.0.

BA

 

[mtu1972]

Yes top steel will be required, but it does not have to be complicated. #X@12" o.c. SW Top and Bot. + #Y@12" o.c. LW Top and Bot. The reinforcing can be spec'd. so that even the least experienced installers would have trouble screwing it up.

gjc

 

[Althalus]

MTU,

That is essentially why I added the note. The 3' dimension won't work because of equipment geometry and load. The 10' clear vs the 5' width may apply in this case, but I tend to move them in a bit when combining, thus diminishing the 10' space (like I said I was trying to explain what was meant by the 35%).

And when dealing with multiple footings (like columns of a building) your rule would be difficult to apply. Imagine a grid of 10x10 columns each with a spread footing. The overall area of the rectangle created by the corner footings vs the total area of all the spread footings. I'm thinking the perimeter is shorter than the individual footings.

However, I see the logic in applying your rule for equipment foundations. I'll have to consider that a little longer. Thank you.

Since my foundation instructor was specifically talking about buildings, maybe it is only valid for buildings. And this rule you presented is more applicable for smaller numbers of load points for equipment.

 

[mtu1972]

Althalus -

Sorry to make assumptions that do not fit the criteria.

I have successfully used the method I describe for building columns where the grid spacing is 20' to 24' in one direction, with varying spans in the perpendicular direction (typical paper mill buildings). When I do use it, I generally design the individual (square) footings first and then look to see if a combined/strip footing results in less concrete and/or forming area. I generally don't worry about reinforcing steel quantities. My gut feel is they are a small portion of the entire cost.

I generally use square column footings because early in my career the field crew screwed up the orientation of a rectangular footing/pier/anchor bolts because true north was at an angle to plan north and somehow some of the individual footings were placed incorrectly.

As to the entire building, there are instances where the soil is so poor that only a mat foundation for the entire building will work out. Granted this is the last resort for my design choices, but there have been instances where that needed to be done.

gjc

 

[Althalus]

BA,

For the sizes of footings I usually deal with, we have 18" thick footings anyway. I've always used two mats of rebar once we reach 18" thickness on slabs or footings.

Is there another issue you were talking about when you described a "complication"?


MTU,

I just ran the calculations on what it would take for the individual footings to equal 35% of the area in a building. We'd probably need a building in the neighborhood of 10 stories high to required such loads to require footings that would equal 35% of the total footprint. I've never done a high rise. But that seems like it would be time to use piles. So, now I have reason to really wonder where the 35% rule came from. My instructor, I know. But...

I'm now thinking of some pump stations where heavy equipment is kept on upper floors of warehouse type structures. These could require bigger footings in tighter spaces. I'm really beginning to wonder about the applicability of this rule of thumb.

 

[BAretired]

Althalus,

When a building foundation consists of numerous square footings, bottom steel mats are usually tied together in a bundle and lifted to the proper location by crane when the formwork is ready or to the excavation when the footing is to be poured against the soil without forms. The workers get into a routine, tying bars, lifting mats, supporting mats and pouring concrete. Most of them do not look at a drawing throughout the entire process. The whole operation is done with an economy of movement.

If you introduce combined footings, there is the added complication of (a) getting the footing in the correct orientation, (b) placing top steel between columns and (c) supporting top steel on high chairs or similar supports so they don't trample them out of position when pouring and vibrating concrete.

BA

 

[mtu1972]

I reviewed my reference (Foundation Engineering Handbook - Winterkorn and Fang 1975; Chapter 16 - Combined and Special Footings by Joseph Bowles) and found that in addition to the figure that I recalled, it also stated "When spread footings for the columns begin to occupy a large percentage of the foundation site or the area between columns, the designer must weigh formwork costs against the extra footing materials required by using continuous footings or mat foundations".

The formula provided by your instructor provides a method of quantifying that concept.

For a 20' x 20' column grid spacing - Tributary Area (Agross) = 400 SF. 0.35 x's = 140 SF = Aconc. Square root of 140 = 11.83'. This would lead to a clear space between footings of 8.17' and would meet the guideline I have always followed.

Attached is a page from the reference noted above.



gjc

 

[JLNJ]

Footings are often earth-formed and not hard-formed. I think BA has it right for most applications.

 

[madmantrapper]

Yes I think BA is right also. I've never done what the OP suggest unless the edges were so close the earth fell in between the holes.

 

[hokie66]

I just look at it on a case by case basis, but don't see any reason to use more concrete than necessary.

One comment, Althalus. Why would you use top bars in an 18" thick footing unless there is tension in the top? Do you like to see plastic settlement cracking?

 

[Althalus]

Hokie,

I've left out a lot of the details as to why a lot of this is done. It would take too much time and add confusion to the central question. But as engineers we always tend to focus on what is missing rather than what is presented. So, that causes us to go off on a tangent. I thought I was the only one who did that. It's good to know I'm among like-minded individuals.

Due to pier geometry the bending loads on the concrete are fairly low but reversible. Hence two layers.

 

[BAretired]

If "bending loads" are large enough to produce negative moment on the top of square footings, then no rule of thumb can be given about discrete vs combined footings. This is an entirely different situation and would have to be examined on a case by case basis as hokie suggested.

BA

 

[Archie264]

Hokie66,

I'm intrigued by your comment, "Why would you use top bars in an 18" thick footing unless there is tension in the top? Do you like to see plastic settlement cracking?"

I infer from that that the top bars would artificially hold the concrete in place just enough to prevent proper settlement...for a while. Then as it cures settlement continues and at this point it's constrained, causing cracking? Is that correct, or on right path?

Thanks.