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Combined Footing Design

abdallah hamdan

abdallah hamdan

Structural
Oct 13, 2021
36
I was designing a Combined Footing for columns that are close to each other, and the columns are not at the same horizontal and vertical level, as shown
2.PNG



When I studied another appropriate shapes for the foundation, I found that the following shape require less area and smaller thickness , 65 cm instead of 75 cm. i have not seen this shape in the footings before , Is this shape acceptable and practical?

1.PNG
 
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Some footings are only needed to be 3000mm deep. Others might be 1600mm deep, others might be 4000mm deep.

I don't see why 650mm deep should be considered unacceptable or impractical compared 750mm.

Design the footing appropriate for the load and the ground conditions. If you have done your job properly accept you answer. If you are sufficiently confused by a difference of 100mm in you analysis outcome then it sounds like you are inexperienced and you should be seeking guidance from colleagues around. You should not be seeking advice from from an internet forum especially when you provide almost zero information regarding the loads, structure or ground conditions.
 
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Giving OP some grace: Anything can be made to work, but I still prefer the first detail. I haven't seen the second detail, either, and it presents a couple of questions. Are both columns stressed equally enough that a crack won't develop through the reentrant corners? Are the bearing pressures from the geotech still valid at that shape? Even without information about the loads, I'm skeptical that this small/thin footing can be made rigid enough considering that narrow bit.

It's already cheap as a shallow foundation, so I don't know how valuable it is to shave 10cm of thickness and a bit of area. Contractors are wheelbarrowing concrete into a hole in the ground. Stick to the big one; their diggers are more much likely to create that rectangle, anyway.
 
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The problem is that my colleagues and I have never seen the construction of a foundation in the second shape, and usually, the foundation is constructed in the first shape. I have done my job properly and checked the soil pressure under the footing, designed both for moment, one way shear, and punching ..... , My question is why, in this case, we don’ t use shape number 2 . I just want to hear opinions on whether there are any issues with using the second shape.
 
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I've seen combined footings like that before. I can't remember when, but I know I've seen it.

To be honest, the ones I saw were almost certainly each designed as single spread footings.... And, were just combined together for construction simplicity. That's probably how I would do with this footings. Design them as individual spread footings.... Demonstrate that each of them works on their own. Then, point out that the overlap is only something like 6 inches. Works by inspection.

That's way better than inventing new calcs for a strange shape.
 
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ANE91 said:
Giving OP some grace: Anything can be made to work, but I still prefer the first detail. I haven't seen the second detail, either, and it presents a couple of questions. Are both columns stressed equally enough that a crack won't develop through the reentrant corners? Are the bearing pressures from the geotech still valid at that shape? Even without information about the loads, I'm skeptical that this small/thin footing can be made rigid enough considering that narrow bit.

It's already cheap as a shallow foundation, so I don't know how valuable it is to shave 10cm of thickness and a bit of area. Contractors are wheelbarrowing concrete into a hole in the ground. Stick to the big one; their diggers are more much likely to create that rectangle, anyway.
Click to expand...
both columns stressed equally , and i think that is not valuable, but ,i want to know if its possible to use the second shape. I was thinking that one of the disadvantages of using the second shape is that it is harder in construction , but I wanted to know if there are any other issues with such shapes or any other unusual shapes. If I design any shape and check all the factors like soil pressure, design for moment, and shear forces, would that shape be suitable for implementation? Thank you very much.
JoshPlumSE said:
I've seen combined footings like that before. I can't remember when, but I know I've seen it.

To be honest, the ones I saw were almost certainly each designed as single spread footings.... And, were just combined together for construction simplicity. That's probably how I would do with this footings. Design them as individual spread footings.... Demonstrate that each of them works on their own. Then, point out that the overlap is only something like 6 inches. Works by inspection.

That's way better than inventing new calcs for a strange shape.
Click to expand...
Thank you . knowing someone has used this shape before and how they designed it is very helpful
 
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If you have satisfied yourself that concrete strength and bearing pressure distribution are okay, then there isn't a reason not to go with option 2 from that standpoint.

There are some downsides to number 2 though from a construction standpoint.

You have a jagged perimeter which requires more forming or trenching. Generally I'd expect better results with option 1 because digging a big rectangle is easier than digging two overlapping ones.

Another point is that the reinforcement layout is more complicated, its not impossible just more to think about in the field. A big rectangular hole with a series of straight reinforcement is plain and simple.

I'll also agree with JoshP in that two minorly overlapping spread footings can be easily calced and specified as such. But it sounds like you already went through the motions of treating them as a combined footing.

ANE's point about different loads in the columns is valid, and for either case I would want to look into that to be sure we have a relatively even distribution of bearing pressure under the entire combined footing.

I think what needs to be examined perhaps the most, is WHY does shape 2 require less thickness? is it punching shear? flexure? something feels strange that the bigger footing area needs more thickness. Have you investigated and validated this outcome?
 
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abdallah hamdan said:
I was designing a Combined Footing for columns that are close to each other, and the columns are not at the same horizontal and vertical level, as shown
View attachment 6920



When I studied another appropriate shapes for the foundation, I found that the following shape require less area and smaller thickness , 65 cm instead of 75 cm. i have not seen this shape in the footings before , Is this shape acceptable and practical?

View attachment 6921
Click to expand...
 
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I would prefer to use two separate rectangular footings as shown in orange below, rather than a combined footing. I would leave a small gap between them. I believe the reinforcement is simpler.

1742606907466.png
Having said that, your second footing could be used too, and if the columns were closer together, that may be best.
 
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I believe the second option will not be cheaper to build. As others have said, the formwork is more complicated and the rebar is more complex (different lengths, etc.).

Even if you are convinced Option 2 works, I bet the contractors would prefer Option 1 for the simplicity. I also like the solution BAretired presented, although the forms between the two pads might be difficult to achieve.
 
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driftLimiter ,one way shear value was larger for option one , also when calculating reinforcement needed option 2 was less , the shape maybe affected the distribution of loads and then the reinforcement and dimensions, i will investigate this outcome and will reply to you again​

for the situation i will try to separate them and if not worked i will go with option 1 due to simplicity in reinforcement and work, thank you all very much​

 
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