2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 1/18

Home » Forums » Structural Engineers » Activities » Structural engineering other technical topics Forum

Eccentric footings and combined footings thread507334406

Share

In our office, colleagues often use load reduction to design footings with the columns directly on the exterior portion ofthe lot (zero clearance) to avoid oversize footings. This often results in underdesigned exterior footings. Now I need todesign one that needs to use the full commercial live load and dead load for 4 storeys. The footings that come out ofSAFE are about 50% bigger. The lot has 12 meters distance end to end and 3 columns spanning them (or 6 metersbeam span). I'm thinking of using combined footings for the 3 columns in a row. But haven't seen much bookreferences 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 betterseismic resistance, for those with experience of familiar with them? I understand the combined footing is moresensitive to variable changes in loading affecting the shears and moments in the span. Thank you.

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

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

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. Thisminimizes the eccentricity between the column and the footing, but it requires that the column be designed for theeccentric moment for the height of the first story. It also requires that the top of the first story column be tiedcontinuously 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 isworth looking into.

BA

Please find attached the foundation layout. I haven't worked with combined footings before that encompassed 3footings 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 sidefootings 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 onthis very irregular footings can comment as I have no experience working with 3 combined footings. And wouldhorizontal combined footings for the 3 columns make it significantly smaller? Any who has worked on such problembefore? Thanks.

5

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 2/18

Making the side footings much larger than the centre footing is not logical, and does not accomplish what you want. Iwould use a combined footing of constant width all across the three columns. These are generally designed with theassumption 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 spanwill be at the top, and at the columns in the bottom.

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

BA

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

BA

hokie66,

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

Quote (pattontom)The foundation sizes come from SAFE. If I make it smaller, the end of the footing would tiltand bearing capacity reduces considerably and only the footing portion below the columns would feel the fullweight and even exceed the soil bearing capacity. Remember columns must be position with the kern regionfor all parts of the footings to be in contact with the soil. Go outside it and the end lifts up. How come thelayout 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 columnsapplying 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 exteriorfootings.

BA

I'm surprised. You mean in the United States you don't design any footing whose columns are outside the kern? But inother 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.".

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 3/18

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

Hope others can comment too.

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

Mike McCannMMC Engineeringhttp://mmcengineering.tripod.com

I cannot comment on the practice in the United States as I reside in Canada. As I stated in an earlier post, I haveconsidered using a footing where the load falls outside of the kern. When I did, I made sure that the column hadadequate strength and stiffness to deliver the load to the footing such that the soil pressure could be reasonablyuniformly 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 considerusing "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 footingincorporating three columns would be more efficient than three separate footings, the outside two of which are hugelyeccentric to the applied load.

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

BA

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

BA

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

Combined footings that uses more than 2 columns is very sensitive to the loads. Since I don't know the future loads orexact positions of the furnitures or movable items in the commercial spaces in the 4 storey that would be put, I can't becertain 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 getresultant.. one uses x = (column load 1) Length/ (column load 1) x (column load 2), what formula do you use for 3columns in a row? All examples I saw in references uses only 2 columns in combined footing except mat foundation. Inmy country.. many structural engineers put the columns at the edge of the footing to save previous land spacesespecially in commercial establishments so one can't afford setback so we have few experiences with limited combined

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 4/18

footing that uses 3 columns spaces. Is there no other country like mine that uses the layout often?

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

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

http://data.bolton.ac.uk/staff/phm2/files/Semester...

http://www.nceng.com.au/"Programming today is a race between software engineers striving to build bigger and better idiotproof programs, and theUniverse trying to produce bigger and better idiots. So far, the Universe is winning."

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

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

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

BA

The combined footings thing idea seems simple.. just upside down columns and beams and upper tension at midspanand 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? anytorsions you think can happen? also note the vault position.. any torsion from this and how do you think overall canbehave?

Why can't the lower footing be straight? Just centre it along the line of the three columns. The vault just goes on it ownfooting, 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 designing4 storey structures.

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 5/18

I don't have mentor and this is not elementary question. I'll explain. We are trying to fix the problem done by anotherengineer who gave the design to our client. As I have said. 95% of engineers in my country use eccentric footings. Thefollowing 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 justput the column at the exterior side of footing. We feel the building may overturn because the formula used is forcentrally uniform bearing. Now what we did was to increase the size of the footings to what I shared yesterday (thepicture with very big side footings). Now when you guys suggested we do combined footings. It is still oversized.. thatis.. the combined footings would have area 3 times the actual. If we do it with actual area corresponding to actualweight. It would become very narrow (just imagine how you would do combined footings of the above picture). Youmay suggest to use strap beam, but the contractor of the clients and most contractors can't do strap beam because itis quite complicated due to the fact that there must be no soil bearing underneath the beam and this is difficult to do inmy country. So does anyone have other suggestion? The reason it's difficult to do the combined footing thing is becausewe can't make the width smaller anymore because the shear and moments would no longer be safe for the eccentricfootings. So maybe I must use my original oversize isolated footings? My concern is the differential settlement. Ormaybe just do mat foundation to solve it and just tell the client to make his story designed for 6 storey because offoundation issues?

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

BA

No. We have 3 engineers working with this. Ok. I live in the Philippines. 95% of engineers use full eccentric footings inbuildings. Is there anyone here in the Philippines who can comment why we use them when this is not allowed in othercountries? 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 isgood only for very low rise up to 3 storey, right? If this is the only solution we have. Then I have to advice client hislayout is optimum only for 2 storey or 3. I admit that for 4 storeys and up, the eccentric footing can get so big andcreate the problem I have now.

Why have you assumed that the previous design was inadequate? Those beams shown FTB1 look like strap beams tome, but I don't have any details. Using strap beams to rectify the eccentricity is a perfectly acceptable solution, butsupporting the columns on the edge is not acceptable, even if 95% of Phillipine engineers would do it that way.

Those are not strap beams.. FTB means Footing Tie Beam, as shown in the previous engineer plan above.. they areconnected just below the floor slabs and above the footing. This is just to prevent settlement but not handle the

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 6/18

eccentricty. Well, the reason his footings not adequate is because the loadings he used plus the weight of the footing ishow he got the area.. but this is for centrally loaded footing. So he got it wrong. Anyway. I'll advice client that either I'lluse oversized combined footing to make it 6 storey or he has to settle for 2 storey to just use zero clearance eccentricfooting. This is allowed for 2storey or residentials because the loads are only very small. The Philippines copy all thestructural codes in the United States ACI but in the portions in footings.. the kern requirement is removed to savepages.. that is why most designs you can see use columns located at side or even at corner of footing in residential orlow rise commercial.. but not in high rise which needs centrally loaded footing.

Why not use strap beams of the required bending capacity on those grids? Strap beams work in basically the samemanner as combined footings, and I fail to see why "there must be no soil bearing underneath the beam" is arequirement. 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.

Then you missed this:

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

"Conclusion

Results indicate that when a strap footing is used as part of a foundation system, a detail that allows for pressure to berelieved from the strap footing is necessary on construction documents. Without it, a considerable unforeseen load pathcould be created that may result in the failure of the strap beam, followed by overstress of the soil/rock under theeccentric 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 bythe beam flexing. Underground no warning and sudden failure. Hence strap beams not used here and no contractorsmake them.

For eccentric footing that is oversize, the end can serve as counterweight to the eccentric end hence soil pressure canbe fairly uniform.. but note even in actual, bearing of soil vsclay 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 wouldbe larger than area of 4 storey so make as well make it 56 storey in design to avoid wasteof foundation construction. I talked with client. We will go for combined footings thatthat would gap the entire lot. Hope calculations next week can support it. But we can go for 4storey if the design canmake it possible. 2 other engineers are also working on this same project and they will do the calculations.

1. The article on "Heavily Loaded Strap Footings" is not applicable here. You do not have loads of 3000 or 4000 kipsand 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 theexterior footing. It can be done but it is not efficient and it may be unacceptable architecturally (because the column istoo large).

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

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 7/18

4. I understand that wind forces in the Philippines are quite high, so the rigid frames in the transverse direction willbenefit from rotational restraint at the base of each column. Such restraint will be provided by a combined footing butnot 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 bedetrimental 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 fourstory 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 bemade straight. Its outer perimeter will then be a parallelogram.

BA

Thanks. I know gap is not necessariy for combined footings.. only for strap beam. My only concern for now before wemeet the design team next week is the combined footings may be narrow if designed for 4storey only. The designedcolumn sizes are 0.5x0.4 meter with the 0.5 in the longitudinal direction with each supporting less than or about 300kips. Can the width of the 11.7 meter combined footing be merely say 1.5 meter? For isolated footing. It is easy tocalculate the length and width of the area by getting the square root of it. But in combined footing. If the total area issay 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 becertain minimum? This is the only question for now before I meet the design team next week for integrating into theSAFE software. So please bear with me for this important and last question about the width. Thanks.

Quote (BA)2. If you use eccentric footings as first proposed, the exterior column must carry the entire eccentric momentof 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?

See attached from Peck & Thornburn

slickdeals,The reference you attached shows examples of combined footings and also footing with strap beams, both as describedabove. 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.

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

Middle combined columns

Service Load of Colume 1: 730 KnService Load of Colume 1: 1200 KnService Load of Colume 1: 900 KnFooting weight: 200 KnTotal Service load: 3030 Kn.Soil Bearing capacity: 150 kpaArea of combined footings: 20.2Length of combined footings: 11.7

http://files.engineering.com/getfile.aspx?folder=a7f77d9fce96413ab17706

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 8/18

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'tthe width be too narrow and the whole setup can overturn? If I make the width 3 meters. Then the volume and weightof footing would be about twice. Is this advisable in practice to use twice the area required? Has anyone designed 3combined footings here? What do you say of the short width? Also I assume footing depth of 0.5 meters which isaverage 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 theright. 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 concernsme and it's the only few thing I want to know as tips for now. Thanks.

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

Middle combined columns

Service Load of Column 1: 730 KnService Load of Column 1: 1200 KnService Load of Column 1: 900 KnFooting weight: 200 KnTotal Service load: 3030 Kn.Soil Bearing capacity: 150 kpaArea of combined footings: 20.2Length of combined footings: 11.7Width of combined footings: 1.72 metersThis 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 m3

P*e/S = 3030*0.315/39.2 = 24.3 kPaSo 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'tthe width be too narrow and the whole setup can overturn? It might, but it would be a result of NS wind on theshearwalls. You have to provide a means to carry wind on the end walls down to the ground. The combined footing hasnot 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 practiceto use twice the area required? No, it is not an economical solution to the problem. You could investigate other ways ofhandling northsouth wind. Has anyone designed 3 combined footings here? What do you say of the short width? Also Iassume footing depth of 0.5 meters which is average for isolated footing. Are footing depths for combined footingsdeeper usually? Not necessarily, but you must check bending, shear and deflection to make sure you have a workablesolution.

Also the resultant would not exactly be at the center of the 11.7 combined footing but more than the center on theright. 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 concernsme and it's the only few thing I want to know as tips for now. Thanks.

BA

I posted before I had finished commenting.

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 9/18

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 theright. Would this produce bearable soil pressure at all? Yes, indeed. See my calculation above. Is combined footings inmy case a bad or good idea? It is a good idea, but you must carry through with a complete analysis of how the variousloads affect your foundation.

Next week my partner would do the detail calculations of the punching shear, moments, etc. but the thickness concernsme 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 withoutdoing the design. Try 500mm for a start and see how the reinforcement works out. If it is too crowded, you can alwaysalter the depth, width or both.

BA

@pattontom, looking at your "soil bearing area" diagrams (24 Nov 12 20:36). The weight of the foundation plus theoverburden modifies the location of the centroid of load but not enough to give you the picture on the right. Thatpicture shows a footing not in equilibrium! In all of these, ΣF=0 and ΣM=0 must be true. Your third diagram is not inequilibrium 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.

@paddingtongreen,

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

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

BA

paddingtongreen and BA, in the soil bearing diagram, all axial loads are the same. In diagram in the middle witheccentric load but normal size footing, the column at edge can produce tension in 90% of bottom. But in the rightmostpicture with oversize footing and same axial load with respect to left diagram, the bending and shear and sheer weightof the footing can compensate for the lifting and make it stable. This is what SAFE shows. BA, what do you mean thecolumn has to be designed for bending, it doesn't bend because the 2nd floor has beams that directly connect the leftand right side of the lot and all columns connected continuously in upper floors (starting with second) and the floordiaphragms 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 ourstructural code which is copied from the USA will be updated. Maybe I can convince the committee to include the kernthing and make all new building permit obey it? For that to be true, we need to master the art of combined footingswhich is quite rare here. For example, the setup I'm doing have not been done by more than 80% of engineers hereand 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 6inches 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 ofcombined footings will be connected to the upper floors solidly. So how would smaller width of the combined footings ineach of them overturn the building longitudinally (which is longer than the transverse side) when they are connected asa unit?

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 10/18

Pattontom:Are you a real Structural Engineer? Your questions and lack of basic understanding of structural principles and conceptsbrings this question into play. This sounds like a pretty big project to be cutting your teeth on, and with some typicalstructural complications too. And, now you say there are two other engineers helping you think they way you arethinking. Given the types of questions you are asking it would seem wise, to me, that you find a local mentor, who hasbeen through this type of construction and design before, who knows the local codes and methods of doing things, butalso understands basic Structural Engineering methods, and who could sit with all of you and talk some of this stuffover. You could be looking at the same set of plans, seeing each others sketches and facial expressions as youdiscussed these things, back and forth. He can generally see when you aren’t understanding what he is trying toexplain, and try a different approach. Several real smart guys here aren’t having much luck changing your thinking.That’s tougher to do from a distance and with someone who we have had no experience with. You can learn an awfullot from these older fellows who has been in the structural/construction wars for a longer time, and have seen some ofthese problems before. Contrary to your protestations, your country does not have different laws of physics and gravitythan our countries do, and the mechanics by which we design structures don’t really change from country to countryeither. There might be minor variations in code criteria, but even those are not appreciably different. No one, not evenin the Philippines, can put a large column load on the edge of a spread footing without providing some means ofaccounting for this eccentricity; either with a combined footing, or tie beams as one of your sketches shows, or bymoment capacity in the columns as BA suggested.

pattontom in black, BA in red.

paddingtongreen and BA, in the soil bearing diagram, all axial loads are the same. In diagram in the middle witheccentric load but normal size footing, the column at edge can produce tension in 90% of bottom. If the column atedge is hinged to the footing, it is an eccentric load. It produces variable compression on the soil near the edge, butcannot produce tension because soil cannot take tension. So it produces a triangular block of compression near theedge. But in the rightmost picture with oversize footing and same axial load with respect to left diagram, the bendingand shear and sheer weight of the footing can compensate for the lifting and make it stable. This is simply not true.This is what SAFE shows. I don't think so. BA, what do you mean the column has to be designed for bending, it doesn'tbend because the 2nd floor has beams that directly connect the left and right side of the lot and all columns connectedcontinuously in upper floors (starting with second) and the floor diaphragms makes it act as one unit. Draw a ShearForce and Bending Moment diagram for the footing. The moment taken by the column is the footing load multiplied bythe eccentricity to the center of column. This is the setup for millions of homes in our country. What do you think iswrong? I think what is wrong is your understanding of the situation. Anyway. Is it specifically mentioned in the ACI thatthe column needs to be located inside the kern? No, it is not nor does it need to be. Next year our structural code whichis copied from the USA will be updated. Maybe I can convince the committee to include the kern thing and make all newbuilding permit obey it? Don't even try because that would be wrong. For that to be true, we need to master the art ofcombined footings which is quite rare here. For example, the setup I'm doing have not been done by more than 80% ofengineers here and they haven't handled such thing before except mostly mat and piles foundation. No comment onthat as I do not know your local practice.

BA. What do you mean of North south wind on the shearwall? I don't have any shearwall, the left and right walls are 6inches Concrete Hollow Block filled with cement. An earlier comment of yours stated that the left and right walls are full6" and I assumed they were shear walls for lateral forces acting in the northsouth direction. I am assuming North atthe top of your sketch. The back has wall that is 1.5 meters beyond the columns (cantilevered), the front is open as theplace would be lease to banks in the future. Also remember the 3 strips of combined footings will be connected to theupper floors solidly. So how would smaller width of the combined footings in each of them overturn the buildinglongitudinally (which is longer than the transverse side) when they are connected as a unit? The shear walls must beeffectively carried down to footing level or some other bracing supplied to prevent overturning of the combined footingsabout an EastWest axis.

Quote (pattontom)Thanks guys. So I'll use combined footings. Anyway. The rear and back of the ground floor is open and thesecond 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 columnbeam joints to give strength to the openfront and back. Anyone can comment about open story? Are you confident that joints moment frames areenough? I can't use shear walls because the front and back has to be opened. Any suggestions? Thanks.

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 11/18

BA

Oh.. he was referring to the biaxial bending moments of the columns.. of course, it's taken care of. We mostly usestaad and etabs in getting the moments of the structures. We don't do manual calculations anymore here (so manystructural engrs. even forgot the formulas). After we got the moments in the software. We enter them in either PCAcolumns, PCA beams, SAFE or other software to get the reinforcements required for shears and moments.

It's just that we commonly use zero clearance eccentric footings even at corner of building. Our local codes don'tdisallow it. Maybe it's time we really update our codes. The reasons the building don't fail even with fully eccentricfooting is because the load combinations we used for the moments and shear is high at say 1.4DL + 1.7 LL or 1.2DL +0.5 LL + 1 EQ. Now with actual service loadings that is way down, the actual bearing pressures of the soil actuallyapplied is very low. This is how we get away with zero clearance eccentric loading.

BA,

We only used shear walls for buildings above 5 storey. We never use shear walls for 4 storey and below. The walls inthe pictures are all concrete hollow block that only needs wall footing and not shear wall foundation. The buildingseismic resisting members are all special moment frames in both the transverse and longitudinal directions. For mylayout and 4 storey. Do you think this is sufficient? In your country up to what storey can you use pure momentresisting frames as the complete lateral resisting system?

pattontom,He was not referring to biaxial moments, only the column moment which resists the footing moment.

One more try here. Are you familiar with the principle of superposition? Probably not, if you just trust computerprograms to do your job for you. But on the off chance that superposition rings a bell, try this: Take your big footingand place it on the ground. That gives you a uniform soil bearing pressure upwards, resisting just the mass of thefooting. Now supermpose a force (your column load) at the edge of the footing. What happens? If there is just anapplied force, the force goes straight down through the edge of the footing into the soil below. If you want to distributesome of that force to the rest of the footing, you have to provide a moment connection of the column to the footing. Doyou ever design cantilevered retaining walls? Do you have a moment connection between the wall and the footing? Ofcourse you do. I don't know why you fail to see that the same principle is in effect here.

Back to your computer programs...if you just believe everything that is output, you are just a technician, not anengineer. I imagine there are a lot of competent engineers in the Philippines, so won't take your comments aboutstandard practice there as accurate.

As to shear walls, in Australia we commonly use shear walls wherever they are available, even in single storeystructures.

Of course it's handled. The column bars are connected continous to the footings. I don't use dowels. For the originaloversized exterior footing (the middle left side) with column at the edge most portion of the footing. The specs are thefollowing:

Service Load: 730 KnSoil bearing capacity: 150 kpaLength of footing: 4.6 meters Fc' 20.8 mpaWidth of footing: 3.0 metersthickness: 0.650 metersBottom bars along length: 17 pcs of 20mm bars grade 60

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 12/18

Bottom bars along width: 11 pcs of 20 mm bars grade 60Top bars along length: 17 pcs of 20mm bars grade 60Top bars along width: 11 pcs of 20 mm bars grade 60

With this. There is uniform soil pressure at the bottom of the 4.6x3 meter footing withservice load of only 730 kn even a zero clearance completely eccentric column at edge of footing. Work of genius? Lol.

But I'll try combined footing because the sizes of this and other footings differ by 50% that I fear differential settlementcan occur. So I'll tell the other senior engineers to start working on the combined footing BA suggested. I admit I'mjust a junior member though. Thanks for the ideas and clarifications and tips.

The soil pressure would not be uniform, but let's say it is. Using your 730 kN service load, that would give a soilpressure on your size footing of 53 kPa. This would give you a service load bending moment at the face of the column(assuming a 500 column) of 730 kNm. This bending then has to be taken by the column as well as the footing. Can a500 x 500 column resist that much moment? No, it can't.

Those numbers come from the SAFE program and you are right that have to manually confirm everything the programsays. Hmm.. how would combined footing design help then in making the edge column resist the moment? Rememberin the combined footing version the column is still at the edge most location. A combined footing only distribute the soilpressure to the whole combined footing encompassing other columns.

In a combined footing, the columns just deliver forces to the top of the footing. The footing then distributes theseforces to the soil by bending. The model assumption would be that the columns are pinned to the footing, although ifrequired to resist lateral forces, they could be moment connected. In the pinned base model, the footing would havezero bending at the exterior columns.

But if you have an isolated, offset pad, then the column has to deliver both the force and a moment which balances theeccentricity of the soil loading. A pinned base doesn't work.

I suggest that you contact the SAFE technical staff for assistance in interpreting the program output.

Using a 0.5x0.5 meter column. Do you know the theoretical axial load limit, like maybe 100200 KN only, where a zeroclearance eccentric column on edge of footing can work (can take the bending moment of any size of footing)? Maybe730 KN would be so heavy.. so there must be a threshold where it can be done. Any idea what it is? Maybe the reasonsmillions of buildings in the Philippines use columns at edge of footing is because the loading is so low that it cantheoretically take it.

Well, take a 150 kN column load. With 150 kPa allowable pressure, you would need 1.0 m^2. So you could use a 500 x2000 footing. No eccentricity. If there is any eccentricity, the soil pressure is not uniform, and the column has somebending moment.

I'm asking of eccentric footing where the column is at edge of footing. Because if the combined footings very expensiveand client can't afford it. Then I'd advice him to just design building for 2 storey with metal roof. But since his columnsneed to be at edge of footing with maximum eccentricity. Then the design has to be isolated spread footing withmaximum eccentricity. I wonder what is the maximum axial load theoretically possible for this any size of column. I useEtabs and Safes all the time that I forgot how to compute manually. And I want to verify it with manual computationsfirst. So please bear with me this last questions. Thanks.

I believe you said the system is used on one and two story buildings. I suspect that the weight of the bigger foundationis moving the center of load away from the column, this will bring small relief. I also suspect that the column is taking

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 13/18

moment and that these effects plus eating into the safety factors is what is holding up the buildings. With the weight offour floors, the weight of the foundation is less significant, that is, it will move the center of load less than with just twofloors.

A problem for those of us who are professional engineers is that we cannot live with something we cannot cover withcalculations that we understand.

If you draw a free body diagram of your right hand drawing, take moment6s about the point where the column hits thefoundation, you will find a resultant moment that can only go into the column. If it is hinged, the middle diagram iswhat you get, and the top of the footing needs to be reinforced.

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

Eccentricity always has consequences. No exceptions. Maybe you think I am eccentric. Maybe I am.

Complication in the footings is no reason for reducing the scope of this project. You already had tie beams, just makethem big enough to work as rectifying strap beams.

No offence, but using computer aids without any knowledge of how or why the results are generated is a much moredangerous thing than eccentric footings.

How far below top of Ground Floor Slab is the underside of footing? Section 4/S5 "Elevation of Tied Column" is not toscale, but seems to indicate a substantial difference in elevation between slab and footing. In other words, how muchroom do you have for grade beams?

So far, we have been talking about using combined footings in the transverse direction. But there are lateral forces inthe northsouth direction as well which must be transferred to the foundation. Foundation design must take intoaccount all forces.

The three pad footings on Grid X2 can be centered under the columns with no difficulty. Perhaps a strip footing could beused on Grids X1 and X2 with a grade beam over it to distribute the column loads. In this way, the eccentricity would besmall and the column may be adequate to handle the resulting bending moment.

Alternatively, msquare48's idea of a mat foundation could be explored.

BA

Plenty of experienced engineers have responded here to the technical aspects of the original question so I will stay outof that conversation.

Pattontom my only comment is that you have stated that you do not have a mentor yet you are using a very powerfulprogram like SAFE. To use that type of program without knowledge and experience in performing the design by manualcalculations is a major concern. This is exactly what worries some of us gray hairs that have seen the next generationof structural engineers come along that simply trust the output from a program. I agree with the recommendationsabove that you find a local engineer that has the knowledge and experience to design the structure. Good luck.

Ron,

Other engineers handling this and the calculations. I'm just one of them and just offering suggestions esp wheneccentric loadings are normally done by anyone here.

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 14/18

BA,

The grade beam or tie beam are actually just below the slabs where the beam supports the slab.

About forces in the lateral direction. I thought the larger width of the combined horizontal footings can take care ofthat. Remember in isolated spread footing, it can take tranverse and longitudinal forces well.

When you mentioned "The three pad footings on Grid X2 can be centered under the columns with no difficulty. Perhapsa strip footing could be used on Grids X1 and X2 with a grade beam over it to distribute the column loads. In this way,the eccentricity would be small and the column may be adequate to handle the resulting bending moment." Are youtalking about separate strip footing and grade beam in X1 and X2 in addition to the 3 combined horizontal footings orseparate?

Mat foundation may be overkill. Anyway making the width of the 3 horizontal combined footings can take care of thelateral forces in the northsouth direction? No? I'm still waiting for the engineer in charge of it to do the calculations. Ican only offer suggestions.

Hokie66 you said "The soil pressure would not be uniform, but let's say it is. Using your 730 kN service load, that wouldgive a soil pressure on your size footing of 53 kPa. This would give you a service load bending moment at the face ofthe column (assuming a 500 column) of 730 kNm. This bending then has to be taken by the column as well as thefooting. Can a 500 x 500 column resist that much moment? No, it can't."

How did you get the value of the 730 KNm moment?

Your column load was stated to be 730 kN. Using your assumption that the soil pressure is uniform, then the upwardsoil pressure is also 730 kN (ignoring little things like the mass of the footing). So you have a force upwards of 730 kN,centred at a distance of 1.0 metre from the face of the column. 730 kN x 1 m = 730 kNm. Simplified and not correct, asthe soil pressure would vary, but illustrative of the problem you have in developing such a large moment without a bigcolumn.

To get design soil pressure, formula is qu=Pu/AThe service load is 730knDead load is 500 kn. Live load is 230 kn.So Pu=1.2DL + 1.6 LL = 1.2(500) + 1.6 (230) = 600 + 368 = 968 Knqu = 968 Kn/(footing area)^2= 968 Kn/13.8^2 sq.m= 70. 14 kn/m^2 or kpa

Now. Moment = WuL^2/2L=(4.60.5)/2 = 2.05wu = qu (1 meter ) = 70 kn/m

Mu= WuL^2/2= 70kn/m (2.05)^2/ 2 = 147 kn.m

so moment of footing is only 147 kn.m and not the 730 kn you mentioned.

Quote (pattontom)Are you talking about separate strip footing and grade beam in X1 and X2 in addition to the 3 combinedhorizontal footings or separate?

I was trying to suggest an alternative foundation system consisting of a strip footing on Grids X1 and X3 (I typed X2 inerror) and three square footings, each centered under a column on Grid X2. The advantage is that the strip footingwould have a small width, hence a small eccentricity which, perhaps could be carried by bending in the exteriorcolumns.

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 15/18

A second possibility is to provide a strip footing all around the entire perimeter plus one single square footing under thecentral column on Grid X2.

The strip footing would have to be stiff enough to distribute the column loads over the length of footing. I don't knowwhether this would be practical or not but it is worth exploring.

BA

Something looks funny there. Is "13.8" the area of the footing?

Mike McCannMMC Engineeringhttp://mmcengineering.tripod.com

Actually we tried the first idea already with strip footing in grid X1 and X3. But the improvement is only 0.5 metersmaller width. So instead of 3 meter strip footing in sides. We have 2.5 meter. And there is still eccentricity in theexterior columns. I think the best is your main proposed horizontal combined footing plan where there are 3 horizontalcombined footing in rear, middle and front.. Don't you think the width of the 3 horizontal combined footings is enoughto resist the northsouth lateral forces? The dead load of the front and back would be sufficient to resist anyoverturning, isn't it so why do you worry about the northsouth direction. I wonder whether to add grade beams to allthe footings to make the moments distribute. Or would combined footings (horizontal) be sufficient to do it??

msquared, yes. 13.8 is the area of the footing of the left middle. See the original pic. Although the footing only supportsload of 730 kn. The reason it is oversize is to handle the fact the column is put at the extreme exterior end with zeroclearance. The oversize is to avoid the footing tilting and many bars top and bottom to distribute the moments to eacharea of the footing. So this is really feasible, isn't it. But our problem is different sizes of footings can cause differentialsettlement so we the combined footings where the left, mid and right footing is connected as one.

pattontom,

Your calculation gives the ultimate moment/metre in the NorthSouth direction.

My calculation was for the total service load moment for the EastWest direction, where you have the footingcantilevered off one side of the column. The footing is 4.6 metres wide in that direction, but the column would have totake the same bending moment, so it won't work.

hokie, if one uses very strong grade beam or tie beam above the footing and the below the slabs holding all thecolumns together. this can take care of the bending moments of the columns, isn't it. I wonder who use this techniqueand what is the advantage, disadvantage.. remember tie beam can redistribute moments of all columns.

anyway. if combined footings are used, it can redistribute the moments and tie beams not needed, right?

Quote (pattontom)Actually we tried the first idea already with strip footing in grid X1 and X3. But the improvement is only 0.5meter smaller width. So instead of 3 meter strip footing in sides. We have 2.5 meter. And there is stilleccentricity in the exterior columns.

Why do you need 2.5m width of strip footing? A load of 730 kN requires an area of 4.9 m2 which could be supplied by astrip footing approximately 1 m in width which boils down to an eccentricity of 250mm assuming a 500 wide column.Where do you come up with these numbers?

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 16/18

If you do not start looking seriously and intelligently at our suggestions, I am going to depart from this thread with onelast recommendation...find someone in your area who knows what the hell he is doing because you clearly do not.

BA

4.9 m2 above should read 4.9 m2.

BA

The problem with 1 meter string footing at the sides is the end of the footing opposite to the columns can lift up, this isbecause we have to put the columns on the left side of 1 meter footing so the walls can be at edge most location. Thatis why 2.5 meter was calculated to decrease the lift up of the other side of the footing with heavily reinforcementmoment bars.

Of course I'm seriously considering your suggestion of combined footings that is why I told the seniors engineers to goahead do the design of 3 horizontal combined footings at back, middle and front and they say load combination andcalculations can support it and it is stable. So thanks to all your useful suggestions. You guys convinced me combinedfootings is the way to go for completely eccentric column positions.

Quote (pattontom)... I told the seniors engineers to go ahead do the design of 3 horizontal combined footings at back, middleand front and they say load combination and calculations can support it and it is stable.

I was just wondering why your senior engineers need to be told what to do? Aren't they experienced in doing this kindof work? Based on this thread, it seems you are designing a commercial building in your country that you are notcapable of doing so. Sorry for the words. I hope that you find a good mentor to teach you not just relying oncommercial softwares. A master's degree in Structural Engineering in a local university may also help.

I'm just a new graduate and the building is supposed to be my first design. Also I happeedn to be the one assigned tocommunicate with the client. This is why with the project getting complicated. I let the seniors handle them and I'm themiddleman or communication channel between what the client and architect wants and the senior engineers. Theydon't directly contact the client because they are working full time on structural design.

You seem to get along with the client, ask him/her to buy a small land for centric foundation.

You should also consider the effect of shear forces at the top of the footings to the total bending moment.

Analysis and Design of arbitrary cross sections Reinforcement design to all major codesMoment Curvature analysishttp://www.engissol.com/crosssectionanalysisdes...

When I've encountered difficult combined footing with eccentricity I've built a FEMmodel of the foundation, on a springarea support to validate stability and allowed soil pressure. This also lets you see if it's stable for multiple loadcombinations..

Disclaimer: Didn't read the whole thread.

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 17/18

Without reading the whole of the thread, I think its very important to do appropriate designs of eccentic footings. Theycan be very dangerous so consider combined or strap footings to suit.

Quote (pattontom)Oh.. he was referring to the biaxial bending moments of the columns.. of course, it's taken care of. We mostlyuse staad and etabs in getting the moments of the structures. We don't do manual calculations anymorehere (so many structural engrs. even forgot the formulas). After we got the moments in the software.We enter them in either PCA columns, PCA beams, SAFE or other software to get the reinforcements requiredfor shears and moments.

This is one of the most reckless assertions that I have read on these forums in sometime. I only skimmed this post,and I do not mean to preach or soapbox, but I think it's useful to address it.

Tom, as a selfdescribed recent graduate, you need to recognize this statement as a huge red flag. If you pursueengineering as a career which will require dozens of calculations a day, for approx. 6500 days in your 25 year career you are guaranteed to make mistakes. If you follow the lead of your peer engineers who don't "do manualcalculations anymore" and "forget the formulas", you are guaranteed not to catch them. And that's bad.

Strap footings, combined footings, moment frames, or whatever these are just terms used to describe some thing thattries to balance some thing else. And balance is the same on that side of the Pacific as it is the other.

Equilibrium isn't a formula (even though you can develop formulas to describe equilibrium). So I would strongly suggestnot following the path of your peers and forgetting it won't serve you well over time and in the end.

"We shape our buildings, thereafter they shape us." WSC

Use a Pile

MJB315, your contibution was very useful.

Analysis and Design of arbitrary cross sections Reinforcement design to all major codesMoment Curvature analysishttp://www.engissol.com/crosssectionanalysisdes...

John,

I know and it retrospect my post was a little too precious and heavyhanded. But after I read through the thread forthe first time, I felt like ringing the bell.

Tom,

Sorry for the brusqueness of my past response. I could have been more helpful. But everything starts with the truththat: if you don't want your structure to move, it has to be in equilibrium. Sketching it out and putting a hand calculationto it will get you at least 90% of your way there. It's how the world was built.

I think that designing the right sized combined footing is the answer to your issue. I wouldn't try to resolve anyunbalanced moments by trying to engage the anchor bolts and bending the column. It's too dicey in my opinion,especially when the foundation solution seems the most straight forward and timetested.

"We shape our buildings, thereafter they shape us." WSC

2/23/2016 Eccentric footings and combined footings Structural engineering other technical topics EngTips

http://www.engtips.com/viewthread.cfm?qid=334406 18/18

Quote (pattontom)

hokie, if one uses very strong grade beam or tie beam above the footing and the below the slabs holding allthe columns together. this can take care of the bending moments of the columns, isn't it. I wonder who usethis technique and what is the advantage, disadvantage.. remember tie beam can redistribute moments of allcolumns.

anyway. if combined footings are used, it can redistribute the moments and tie beams not needed, right?

tom,

Using grade beams to take the lateral loads is common practice, it will provide additional lateral support at the sametime it could carry your brick walls. for the columns I release the support moment parallel to where the beam will span,so if i have grade beams at both directions that will hit my column then that column will be pinned supported.

Anyway, it is always a good practice to do some hand calculations from time to time. And i usually prefer to do somehand calculations first before using a computer model to generate the complete calculations, that way i get to practiceand at the same time check the computer generated results.

combined footing will have more resistance to seismic unless you tie the isolated footings with beams

chris magadia

www.chrismagadia.com

ChrisMagadia.Com The Structural Engineers' Forum and Resources Website. Civilizations owe its existence to StructuralEngineering. Do you Agree?

Join | Indeed Jobs | Advertise

Copyright © 19982016 ENGINEERING.com, Inc. All rights reserved. Unauthorized reproduction or linking forbidden without expressed written permission. Registration on or use of this site constitutes acceptance of our Privacy

Policy.