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.

 

[pattontom]

The combined footings thing idea seems simple.. just upside down columns and beams and upper tension at midspan and lower tension at supports.. how many have actually designed such? any unexpected problems you encountered? the following is the idea.. note the lower combined footings can't be exactly rectangular.. any other suggestions? any torsions you think can happen? also note the vault position.. any torsion from this and how do you think overall can behave?

 

[hokie66]

Why can't the lower footing be straight? Just centre it along the line of the three columns. The vault just goes on it own footing, probably just a thick slab on grade.

These are elementary questions, which you should be asking of your mentor. I hope you have one, if you are designing 4 storey structures.

 

[pattontom]

I don't have mentor and this is not elementary question. I'll explain. We are trying to fix the problem done by another engineer who gave the design to our client. As I have said. 95% of engineers in my country use eccentric footings. The following is the original size of the footings (and hence actual building load) from Area = Service Load/bearing capacity. I'll continue after the image...

The engineer (that we are not connected with) did the footing size using Area=Service Load/bearing capacity and just put the column at the exterior side of footing. We feel the building may overturn because the formula used is for centrally uniform bearing. Now what we did was to increase the size of the footings to what I shared yesterday (the picture with very big side footings). Now when you guys suggested we do combined footings. It is still oversized.. that is.. the combined footings would have area 3 times the actual. If we do it with actual area corresponding to actual weight. It would become very narrow (just imagine how you would do combined footings of the above picture). You may suggest to use strap beam, but the contractor of the clients and most contractors can't do strap beam because it is quite complicated due to the fact that there must be no soil bearing underneath the beam and this is difficult to do in my country. So does anyone have other suggestion? The reason it's difficult to do the combined footing thing is because we can't make the width smaller anymore because the shear and moments would no longer be safe for the eccentric footings. So maybe I must use my original oversize isolated footings? My concern is the differential settlement. Or maybe just do mat foundation to solve it and just tell the client to make his story designed for 6 storey because of foundation issues?

 

[BAretired]

This is turning into a messy situation. I think you are going to get yourself into a bunch of trouble if you continue with this design. If you do not have a mentor, then team up with a more experienced structural engineer in your area. You cannot expect to get the entire design concept from a forum such as Eng-Tips and it is clear that you do not understand the design principles involved. So, be prudent and get the local help you need.



BA

 

[pattontom]

No. We have 3 engineers working with this. Ok. I live in the Philippines. 95% of engineers use full eccentric footings in buildings. Is there anyone here in the Philippines who can comment why we use them when this is not allowed in other countries? But if designed, it can really work. See the following:

It works, isn't it? Any comment? this is why most engineers in Philippines use it. Although you may agree that this is good only for very low rise up to 3 storey, right? If this is the only solution we have. Then I have to advice client his layout is optimum only for 2 storey or 3. I admit that for 4 storeys and up, the eccentric footing can get so big and create the problem I have now.

 

[hokie66]

Why have you assumed that the previous design was inadequate? Those beams shown FTB-1 look like strap beams to me, but I don't have any details. Using strap beams to rectify the eccentricity is a perfectly acceptable solution, but supporting the columns on the edge is not acceptable, even if 95% of Phillipine engineers would do it that way.

 

[pattontom]

Those are not strap beams.. FTB means Footing Tie Beam, as shown in the previous engineer plan above.. they are connected just below the floor slabs and above the footing. This is just to prevent settlement but not handle the eccentricty. Well, the reason his footings not adequate is because the loadings he used plus the weight of the footing is how he got the area.. but this is for centrally loaded footing. So he got it wrong. Anyway. I'll advice client that either I'll use oversized combined footing to make it 6 storey or he has to settle for 2 storey to just use zero clearance eccentric footing. This is allowed for 2-storey or residentials because the loads are only very small. The Philippines copy all the structural codes in the United States ACI but in the portions in footings.. the kern requirement is removed to save pages.. that is why most designs you can see use columns located at side or even at corner of footing in residential or low rise commercial.. but not in high rise which needs centrally loaded footing.

 

[hokie66]

Why not use strap beams of the required bending capacity on those grids? Strap beams work in basically the same manner as combined footings, and I fail to see why "there must be no soil bearing underneath the beam" is a requirement. Where did this talk of 6 storeys vs. 2 storeys come from? I thought you said the building is 4 storeys.

Your coloured sketches above are not valid. Just increasing the size does not make the pressure uniform.

 

[pattontom]

Then you missed this:

http://www.structuremag.org/article.aspx?articleid=1054

"Conclusion

Results indicate that when a strap footing is used as part of a foundation system, a detail that allows for pressure to be relieved from the strap footing is necessary on construction documents. Without it, a considerable unforeseen load path could be created that may result in the failure of the strap beam, followed by overstress of the soil/rock under the eccentric footing. It is also important to emphasize the need for field enforcement and control of these requirements."

It's dangerous. It's underground, one can't know. At least in beams above ceiling, you can see warning it's failing by the beam flexing. Underground no warning and sudden failure. Hence strap beams not used here and no contractors make them.

For eccentric footing that is oversize, the end can serve as counterweight to the eccentric end hence soil pressure can be fairly uniform.. but note even in actual, bearing of soil vs
clay are not uniform, so the counterweight can do the trick. We used it on millions of buildings and no one collapse.. but only for residential and low rise commercial.

The reason I suggest to make it 6 storey is because the size of the combined footings would
be larger than area of 4 storey so make as well make it 5-6 storey in design to avoid waste
of foundation construction. I talked with client. We will go for combined footings that
that would gap the entire lot. Hope calculations next week can support it. But we can go for 4-storey if the design can make it possible. 2 other engineers are also working on this same project and they will do the calculations.

 

[BAretired]

1. The article on "Heavily Loaded Strap Footings" is not applicable here. You do not have loads of 3000 or 4000 kips and you do not have rock with a subgrade reaction of 800 pounds per cubic inch.

2. If you use eccentric footings as first proposed, the exterior column must carry the entire eccentric moment of the exterior footing. It can be done but it is not efficient and it may be unacceptable architecturally (because the column is too large).

3. If you use combined footings, you will have three loads on each footing. The centroid of those three loads will be near the centroid of the footing. If the footing is sufficiently rigid, the pressure will be nearly uniform over its entire area. It will have positive and negative moments which may be reinforced with continuous top and bottom bars running the full 11.7m length of the footing.

4. I understand that wind forces in the Philippines are quite high, so the rigid frames in the transverse direction will benefit from rotational restraint at the base of each column. Such restraint will be provided by a combined footing but not with individual pad footings.

5. There is no need no create a void under any part of each of the combined footings. In fact, a void would be detrimental to the proper functioning of the foundation.

6. There is no need to make each combined footing wide enough for a six story building. Make it wide enough for a four story building because that is what you have. It should be deep enough so that it behaves as a rigid body.

7. Do not jog the south foundation as shown on an earlier plan. Make it straight so that your reinforcement can be made straight. Its outer perimeter will then be a parallelogram.



BA

 

[pattontom]

Thanks. I know gap is not necessariy for combined footings.. only for strap beam. My only concern for now before we meet the design team next week is the combined footings may be narrow if designed for 4-storey only. The designed column sizes are 0.5x0.4 meter with the 0.5 in the longitudinal direction with each supporting less than or about 300 kips. Can the width of the 11.7 meter combined footing be merely say 1.5 meter? For isolated footing. It is easy to calculate the length and width of the area by getting the square root of it. But in combined footing. If the total area is say 17.55 sq. meter and the length is 11.7 meter, then the width is only 1.5 meters. Is this sufficient, or must there be certain minimum? This is the only question for now before I meet the design team next week for integrating into the SAFE software. So please bear with me for this important and last question about the width. Thanks.

 

[slickdeals]

2. If you use eccentric footings as first proposed, the exterior column must carry the entire eccentric moment of the exterior footing. It can be done but it is not efficient and it may be unacceptable architecturally (because the column is too large).

Interesting point. Can the column base connection not be detailed to alleviate this?

 

[slickdeals]

See attached from Peck & Thornburn

 

[hokie66]

slickdeals,
The reference you attached shows examples of combined footings and also footing with strap beams, both as described above. Using these solutions, there is no moment (or at least no added moment due to eccentricity from the footing) on the columns. BA's statement was about eccentric pad footings, with no rectifying moments.

 

[pattontom]

I wonder if my combined footing is too narrow. Initial actual calculations show:

Middle combined columns

Service Load of Colume 1: 730 Kn
Service Load of Colume 1: 1200 Kn
Service Load of Colume 1: 900 Kn
Footing weight: 200 Kn
Total Service load: 3030 Kn.
Soil Bearing capacity: 150 kpa
Area of combined footings: 20.2
Length of combined footings: 11.7
Width of combined footings: 1.72 meters

Imagine footing that span 11.7 meters and only 1.72 meters in width. The column sizes are 0.5x0.5 on average. Won't the width be too narrow and the whole setup can overturn? If I make the width 3 meters. Then the volume and weight of footing would be about twice. Is this advisable in practice to use twice the area required? Has anyone designed 3 combined footings here? What do you say of the short width? Also I assume footing depth of 0.5 meters which is average for isolated footing. Are footing depths for combined footings deeper usually?

Also the resultant would not exactly be at the center of the 11.7 combined footing but more than the center on the right. Would this produce bearable soil pressure at all? Is combined footings in my case a bad or good idea?

Next week my partner would do the detail calculations of the punching shear, moments, etc. but the thickness concerns me and it's the only few thing I want to know as tips for now. Thanks.

 

[BAretired]

I wonder if my combined footing is too narrow. Initial actual calculations show:

Middle combined columns

Service Load of Column 1: 730 Kn
Service Load of Column 1: 1200 Kn
Service Load of Column 1: 900 Kn
Footing weight: 200 Kn
Total Service load: 3030 Kn.
Soil Bearing capacity: 150 kpa
Area of combined footings: 20.2
Length of combined footings: 11.7
Width of combined footings: 1.72 meters
This is not quite correct because the loads are not quite centered on the footing.
e = (900 -730)(5.87 - 0.25)/3030 = 0.315m
S = 1.72(11.7)^2/6 = 39.2 m[sup]3[/sup]
P*e/S = 3030*0.315/39.2 = 24.3 kPa
So P/A + M/S = 174 kPa (need wider footing or possibly tapered footing to meet stipulated soil bearing of 150 kPa).

Imagine footing that span 11.7 meters and only 1.72 meters in width. The column sizes are 0.5x0.5 on average. Won't the width be too narrow and the whole setup can overturn? It might, but it would be a result of N-S wind on the shearwalls. You have to provide a means to carry wind on the end walls down to the ground. The combined footing has not been sized for that.

If I make the width 3 meters. Then the volume and weight of footing would be about twice. Is this advisable in practice to use twice the area required? No, it is not an economical solution to the problem. You could investigate other ways of handling north-south wind. Has anyone designed 3 combined footings here? What do you say of the short width? Also I assume footing depth of 0.5 meters which is average for isolated footing. Are footing depths for combined footings deeper usually? Not necessarily, but you must check bending, shear and deflection to make sure you have a workable solution.

Also the resultant would not exactly be at the center of the 11.7 combined footing but more than the center on the right. Would this produce bearable soil pressure at all? Is combined footings in my case a bad or good idea?

Next week my partner would do the detail calculations of the punching shear, moments, etc. but the thickness concerns me and it's the only few thing I want to know as tips for now. Thanks.

BA

 

[BAretired]

I posted before I had finished commenting.

pattontom stated (in black, my comments in red):

Also the resultant would not exactly be at the center of the 11.7 combined footing but more than the center on the right. Would this produce bearable soil pressure at all? Yes, indeed. See my calculation above. Is combined footings in my case a bad or good idea? It is a good idea, but you must carry through with a complete analysis of how the various loads affect your foundation.

Next week my partner would do the detail calculations of the punching shear, moments, etc. but the thickness concerns me and it's the only few thing I want to know as tips for now. Thanks. I don't know what thickness you will need without doing the design. Try 500mm for a start and see how the reinforcement works out. If it is too crowded, you can always alter the depth, width or both.

BA

 

[paddingtongreen]

@pattontom, looking at your "soil bearing area" diagrams (24 Nov 12 20:36). The weight of the foundation plus the overburden modifies the location of the centroid of load but not enough to give you the picture on the right. That picture shows a footing not in equilibrium! In all of these, ΣF=0 and ΣM=0 must be true. Your third diagram is not in equilibrium and makes it look as though only the bottom is in tension while it is the top for at least part of the length.

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

 

[BAretired]

@paddingtongreen,

I think the idea in the third diagram is that the eccentricity of the load is designed to be carried by the column in bending. This means bending moments in both column and footing as well as horizontal forces at top and bottom of column.

Depending on the properties of the soil and rigidity of the column, the soil pressure under the footing would be variable.

BA

 

[pattontom]

paddingtongreen and BA, in the soil bearing diagram, all axial loads are the same. In diagram in the middle with eccentric load but normal size footing, the column at edge can produce tension in 90% of bottom. But in the rightmost picture with oversize footing and same axial load with respect to left diagram, the bending and shear and sheer weight of the footing can compensate for the lifting and make it stable. This is what SAFE shows. BA, what do you mean the column has to be designed for bending, it doesn't bend because the 2nd floor has beams that directly connect the left and right side of the lot and all columns connected continuously in upper floors (starting with second) and the floor diaphragms makes it act as one unit. This is the setup for millions of homes in our country. What do you think is wrong? Anyway. Is it specifically mentioned in the ACI that the column needs to be located inside the kern? Next year our structural code which is copied from the USA will be updated. Maybe I can convince the committee to include the kern thing and make all new building permit obey it? For that to be true, we need to master the art of combined footings which is quite rare here. For example, the setup I'm doing have not been done by more than 80% of engineers here and they haven't handled such thing before except mostly mat and piles foundation.

BA. What do you mean of North south wind on the shearwall? I don't have any shearwall, the left and right walls are 6 inches Concrete Hollow Block filled with cement. The back has wall that is 1.5 meters beyond the columns (cantilevered), the front is open as the place would be lease to banks in the future. Also remember the 3 strips of combined footings will be connected to the upper floors solidly. So how would smaller width of the combined footings in each of them overturn the building longitudinally (which is longer than the transverse side) when they are connected as a unit?