Eccentric footings and combined footings 5

[pattontom]

In our office, colleagues often use load reduction to design footings with the columns directly on the exterior portion of the lot (zero clearance) to avoid oversize footings. This often results in underdesigned exterior footings. Now I need to design one that needs to use the full commercial live load and dead load for 4 storeys. The footings that come out of SAFE are about 50% bigger. The lot has 12 meters distance end to end and 3 columns spanning them (or 6 meters beam span). I'm thinking of using combined footings for the 3 columns in a row. But haven't seen much book references that treat such for most combined footings often use 2 columns only. Do you recommend that I use 50% bigger spread footings for the eccentric footing or one combined footing for the 3 columns? Which would have better seismic resistance, for those with experience of familiar with them? I understand the combined footing is more sensitive to variable changes in loading affecting the shears and moments in the span. Thank you.

 

[hokie66]

Not sure I understand. When you say the lot is 12 metres, and divided into 2 spans, does that mean it is 12 metres wide or 12 metres deep? I assume 12 metres wide, so there is a column at each boundary and one in the middle. In that case, a combined footing for all three columns, or a strap beam between footings to resist the external eccentricity...either would work.

 

[BAretired]

There are several approaches to this problem. The combined strap footing for three columns is probably the safest and best.

The 50% bigger footing has no merit whatsoever as it has no basis.

One method which I have used is to provide a rectangular footing with the short dimension parallel to the beam. This minimizes the eccentricity between the column and the footing, but it requires that the column be designed for the eccentric moment for the height of the first story. It also requires that the top of the first story column be tied continuously across to the opposite side.

I have done this for one or two story buildings but never for four story buildings. It may be a bit of a challenge but it is worth looking into.

BA

 

[pattontom]

Please find attached the foundation layout. I haven't worked with combined footings before that encompassed 3 footings that is 5.8 meter apart (see picture). If I won't use any combined footings and just use isolated footings as as. Can anyone comment on the seismic behavior of it as far as settlement and lateral movement is concerned? The side footings are much larger than the middle (but loads one half of the middle). During vertical and lateral seismic shaking, would there be real tendencies for them to get separated? Hope those who have actual experience or have worked on this very irregular footings can comment as I have no experience working with 3 combined footings. And would horizontal combined footings for the 3 columns make it significantly smaller? Any who has worked on such problem before? Thanks.

 

[hokie66]

Making the side footings much larger than the centre footing is not logical, and does not accomplish what you want. I would use a combined footing of constant width all across the three columns. These are generally designed with the assumption that upward soil pressure on the footing is uniform. To help you visualise it better, just turn it upside down, and you have a two span beam with the three columns acting as the supports. So your main reinforcement in the span will be at the top, and at the columns in the bottom.

 

[BAretired]

I agree with hokie. Use combined footings across three columns.

BA

 

[BAretired]

And, by the way, pattontom, you should have seen this for yourself.

BA

 

[pattontom]

hokie66,

The foundation sizes come from SAFE. If I make it smaller, the end of the footing would tilt
and bearing capacity reduces considerably and only the footing portion below the columns would feel the full weight and even exceed the soil bearing capacity. Remember columns must be position with the kern region for all parts of the footings to be in contact with the soil. Go outside it and the end lifts up. How come the layout doesn't accomplish what I want? I don't understand. Please elaborate. Thanks.

 

[BAretired]

The foundation sizes come from SAFE. If I make it smaller, the end of the footing would tilt
and bearing capacity reduces considerably and only the footing portion below the columns would feel the full weight and even exceed the soil bearing capacity. Remember columns must be position with the kern region for all parts of the footings to be in contact with the soil. Go outside it and the end lifts up. How come the layout doesn't accomplish what I want? I don't understand. Please elaborate. Thanks.

What the heck is SAFE?

Use a combined footing with a length of 11.73m and a width of whatever you require. You will have three columns applying load to the combined footing. You must reinforce the footing for the resulting bending moments and shears.

Your present foundation plan is unacceptable because the exterior columns are outside the kern of the exterior footings.


BA

 

[pattontom]

I'm surprised. You mean in the United States you don't design any footing whose columns are outside the kern? But in other countries, it is quite popular. Has no one else here do it??

About SAFE. See:

http://www.csiberkeley.com/safe

"SAFE is the ultimate tool for designing concrete floor and foundation systems.".

Well. The footing sizes come from SAFE, which can model the column with zero clearance. The reason the program produces huge footings is precisely because it addresses the shear and moment issues.

Hope others can comment too.

 

[msquared48]

You have so much footing areawith respect to the building area, I think you should investigate a matt footing design here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

I cannot comment on the practice in the United States as I reside in Canada. As I stated in an earlier post, I have considered using a footing where the load falls outside of the kern. When I did, I made sure that the column had adequate strength and stiffness to deliver the load to the footing such that the soil pressure could be reasonably uniformly distributed over the footing area. Your foundation plan does not accomplish that.

I would not consider adopting your foundation plan in a real structure. If it comes from "SAFE", I would not consider using "SAFE" for future foundation design.

It should be self evident that, given the size of your footings and the small gap between them, that a combined footing incorporating three columns would be more efficient than three separate footings, the outside two of which are hugely eccentric to the applied load.

If that is not self evident to you, we need to talk a little more.

BA

 

[BAretired]

I agree with msquared48 that a mat footing would be well worth investigating.

BA

 

[dhengr]

I agree with Hokie and BA that the best solution is to use combined footings. Hokie suggests a good way visualize your problem. And, BA suggests spanning these in the 11.73m direction and suggests you must be mis-using or mis-understanding this SAFE program. It seems to me that the four corner columns will have the least gravity load, the center exterior columns will be next and the middle column will have the greatest load. To that extent, you might change the width of these combined footings, in the immediate area of the columns, in proportion to these load magnitudes, to try to equalize the soil bearing pressures a bit. But those forming changes may cost more than the added concrete of a constant width footing which was primarily based on the greater column load of the three. You also seem to show a foundation wall between the columns. This might be turned into an inverted Tee beam between the columns, in keeping with Hokie’s visualization.

 

[pattontom]

Combined footings that uses more than 2 columns is very sensitive to the loads. Since I don't know the future loads or exact positions of the furnitures or movable items in the commercial spaces in the 4 storey that would be put, I can't be certain of the exact positions of the shear and moments. How do you handle this in 3 combined footing?

dhengr, it's footing tie beam around the perimeter of the layout. Anyway.. guys.. For two combined columns.. to get resultant.. one uses x = (column load 1) Length/ (column load 1) x (column load 2), what formula do you use for 3 columns in a row? All examples I saw in references uses only 2 columns in combined footing except mat foundation. In my country.. many structural engineers put the columns at the edge of the footing to save previous land spaces especially in commercial establishments so one can't afford setback so we have few experiences with limited combined footing that uses 3 columns spaces. Is there no other country like mine that uses the layout often?

 

[hokie66]

I don't know what country you are in, but principles of structural analysis do not change. This is just a two span beam problem...what could be simpler? Everything can't be determinate. Placing columns at the edge of isolated pad footings is just not appropriate. You have three countries who have chimed in: US, Canada, Australia. We all agree that your scheme is poor, whether you think it is "popular" or not.

 

[rowingengineer]

Another vote for a combined footing. I know my geotech would be concerned about differential settlements with footing size that varied by about 50%. I would say that your office might use this layout often, would pull your whole country in on the statement. I would also suggest that this is on projects where having combined footing would be difficult and also the footing are not of the size being considered for this project. What I would say is most people would do these in a rectangle not square arrangement if possible.

http://data.bolton.ac.uk/staff/phm2...otechnics/Bearing Capacity v1.00 Sept2010.pdf

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[pattontom]

Thanks guys. So I'll use combined footings. Anyway. The rear and back of the ground floor is open and the second floor front and back cantilever 1.5 meters forward and back. Only the left and right side has full 6" thick wall. My tranverse side is weak. I rely solely on the column-beam joints to give strength to the open front and back. Anyone can comment about open story? Are you confident that joints moment frames are enough? I can't use shear walls because the front and back has to be opened. Any suggestions? Thanks.

 

[hokie66]

You could use moment frames. The footings continuous across will help with that. I would try to make the columns as deep as possible in that direction.

 

[BAretired]

Moment frames in the transverse direction appear to be the only option given the architectural limitations described above.

BA