Eccentric Footing
Eccentric Footing
(OP)
I am dropping a column down 1/2" from the face of a wall for a renovation project. There is an existing strip footing under the masonry wall that I am pouring my isolated column's eccentric footing atop. Basically, the bottom of the bulk of the footing will be level with the bottom of the existing strip footing, but there will be a portion of the footing that is to be poured atop the strip footing. This portion will have a styrofoam strip to keep the new footing from loading the strip footing. Does anyone have a design example for this exact condition? I have attached a link to a pdf image.






RE: Eccentric Footing
RE: Eccentric Footing
RE: Eccentric Footing
-Can you drill and epoxy into the existing footing and analyze it as a "T" shaped footing by utilizing a certain effective width of the existing footing?
-Can you move the column away from the existing building and cantilever the joists over the top of the beam with joist seat extensions or bottom chord bearing joists?
-Provide a strap beam like SteelPE said to the next interior footing.
My fear with your approach is that you will be so eccentric that you will have rotation in the footing causing long term settlment.
RE: Eccentric Footing
I really don't want to load the existing footing, thus the "T" shaped footing is not ideal.
This is a retrofit to a building....the joist seats were anchored into concrete beams with (2) 2" sleeve anchors, which failed due to concrete break out. I am proposing to put in a beam and column line to support the existing joists. The columns need to be as snug to the wall as possible so that they do not interfere with the building use.
There are no interior footings...this is a clear span condition. Even if there was an interior column, I'm not following the strap beam idea.
I understand your concern about rotation.
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I can't see your current detail working at all. Too much eccentricity--the bearing pressure under the footing would be enormous.
DaveAtkins
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What about using (2) helical piers or concrete sonotubes. One would be placed near the existing footing and another would be placed approximately 4 to 6 feet away from the existing footing (both perpendicular to the existing wall). You could then cast a concrete beam on top of the helical piers/sonotubes and cantilever it out to pick up the new column. The reason for using the piers/sonotubes is to be able to take resist tension by using the pier or sufficent weight to counterbalance the overturning force in the case of the sonotube.
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BA
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Thank you for the detail. I'm really ok with mine. We've done this type of "diving board" eccentric footing several times and basically wanted to double check my design.
azcats,
I see where you are going with this, but I'd rather not undermine the existing footing.
Thanks everyone for the feedback
RE: Eccentric Footing
You are using the weight of the new footing plus soil plus slab to counterbalance the applied column load, but it is clear from your drawing that you do not really know the dimensions of the existing footing. This could be a problem.
I'm not sure why you do not want to tie the new structure into the old. Is the bearing pressure very high in the existing footing? I would seek to engage the existing footing with the new footing by means of dowels embedded into the existing and epoxied.
Alternatively, if you are adamant about not tying the two together, why not use one or two screw piles at the outside edge of the new footing to provide a tiedown, balancing the column load. If you do that, you might consider a rectangular footing instead of square...or even better, a T shape.
BA
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You guys have talked me in to doweling into the existing footing. We really don't have much load on the existing footing. I typically don't like to load existing footings unless I know for sure what load the footing.
For those who suggested strap footings...I now see what you are talking about and that would be a good option too, but tying in to the existing would be less construction.
For those who suggested helical piles or sonotubes, I just think the owner is looking for something more simple and as cheap as possible. They are going to bid it out to some really small local contractors.
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Good luck.
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That is the existing condition. The only thing new is the column, beam, and footing. This was definitely a strange building. Concrete beam high, grade beam low, and masonry/brick infill between. It was an exterior wall at one time.
RE: Eccentric Footing
RE: Eccentric Footing
A projection of 3'-0" produces a substantial moment at the face of the existing footing. This is carried by 5-#4 bottom bars epoxied into the footing. If the existing footing is plain concrete (unreinforced) there is a danger of cracking it under the existing wall. Perhaps a lesser projection would be advisable.
BA
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1) The largest moment is going to be located near the new to existing footing interface. You need to embed the #4 bars deep enough to carry the moment into the strip footing.
2) I don't think it is practical to have the width of the existing footing field verified. The contractor would have to undermine the footing or core to determine the width. Make a conservative assumption on the footing width and go with it.
3) I would recommend encasing the base plate in concrete to protect it rather than exposing it to the soil.
4) You should verify that the 3/4" base plate is adequate since the column is very eccentric.
5) I believe you need a minimum of (4) anchor bolts to meet the OSHA requirements for erection.
6) I recommend detailing a 1/4" set plate or leveling nuts or shim stacks for setting the column.
7) You should grease the end of the dowels cast in to the new slab on grade to allow shrinkage of the new concrete.
8) Show the new vapor barrier and how it laps with the existing.
9) Detail expansion felt and bent horizontal bars at the column to slab interface.
10) Consider how the contractor will need to shore the existing joists and transfer them to the new beams. Will shim plates be necessary to connect the joist seat to the beam flange? Has the eccentricity of the joists been considered in the column/base plate/ footing design?
Anyway, here are my thoughts and now I need to get back to work. Good luck...
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BA has given you some great advice, and just make sure you are OK with the tension capacity of those bottom rebar due to the soil pressure induced moment...
Existing footings present some unknowns for sure, but the good thing is they have preloaded and compacted the soil, sometimes for decades, so you should get a reasonably high bearing capacity (for your area or whatever your geotech says) with not much concern about additional settlement. Just make sure they compact the new footing area well.
One thing, I believe OSHA requires four bolts for column base plates, but there may be some exceptions (it is an erection safety feature which may not apply to an existing structure such as yours).
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Let me see if I have this right... The existing roof trusses on the right of your drawings were originally supported by the large, funny shaped, conc. beam up at the top of the masonry wall; and there are reinf'd. conc. pilasters every 18 or 20' or so, as part of the top conc. beam and bot. conc. grade beam framing arrangement; all of which was carried by the existing strip ftg. under this wall arrangement. And, the reason you are going to all your work (trouble), what with the eccentric stl. beam, column, base plate and ftg., and shoring of the trusses too, is because the two 2" long expansion bolts which fasten the stl. jst. bearings seat angles to the top conc. beam are failing due to cracking of the conc. beam. With the 2" bolts and the edge distances you show, that's not real surprising, is it? You may have done these "diving board" eccentric ftgs. before, but I'll bet they have never been loaded to their intended design cap'y. because they don't work so good. And, your detailing of the 3'x4' ftg. and its reinf'g. don't show a very good understanding of how they might work.
The top conc. beam carried the roof trusses on the right btwn. the reinf'd. conc. pilasters until you found the failing stl. jst. bearing seat angles. And, you claim you don't want too big a stl. column projecting too far out into the room on the right. Why don't you just eliminate the stl. col. and the ftg. altogether; and look at a slightly larger steel beam spanning btwn. the reinf'd. conc. pilasters, and corbeled to the pilasters? Maybe you don't even remove the existing seat angles, just jack your new stl. beam up under them and weld the seat angle to the stl. beam. The way you've drawn it, if you leave the seat angles in place the new stl. beam's web can just move closer the face of the pilaster, if you cope the back flange tips around the pilasters. You might try (think about) removing the existing expansion bolts at each truss, drilling the holes deeper, and epoxy new bolts in the old seal angle holes, for some lateral tie. You could use a W36 beam up under stl. jst. seats for considerably less money than what you are proposing.
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Trust me, I thought of this. I don't feel comfortable anchoring in to the pilasters due to their strange shape and the agency doesn't really feel comfortable with it either. I can send you a sketch later. The concrete completely failed due to break out and the adjacent two joists are failing as well.
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Does an existing joist align with each of the new columns? If so, you should make them tie joists by adding bottom chord extensions attached to a clip angle on each column. If not, you should provide stiffeners in the beam above each column, effectively continuing the column through the beam. The beam web is only 1/4" thick and cannot be considered a lateral brace for the column.
BA
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larsacious - I agree with your list. I ask for them to verify the width, but really I just want to know that the length extending from the face of the wall is what I assumed. If it is greater, the base plate will need to be modified. Your mention of the vapor barrier makes me wonder if it is not just easier for them to pour back concrete above the footing and avoid having to compact the soil above the footing.
larsacious, a2mfk, BAretired - the Hilti HIT HY-150 will provide sufficient tension capacity in the HAS Rods. BA - if I am following you, you are afraid that if there isn't any reinf. in the existing footing that it will create tension cracks due to the tension on the embedded rods. I'm not sure what the magic embedment length would be to ensure that the concrete does not crack.
BAretired and PUengineer - I agree with your comments on the stiffening of the beam web at the columns. Since on all the interior columns, the ends of the beams bear atop the columns, would you put the stiffeners at the very end of beams the beams?
I will post pictures of the failure of the joists tomorrow.
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The stiffeners should be placed where they will do the most good. Probably the best stiffener is one which extends the column through the beam. This may be accomplished by using a section cut from a 4x4HSS so that it fits neatly over the column below.
BA
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If you are relying on a stiffener in the beam, it would be better to separate the bolts a bit more. At present, they are going to be only 2" apart. You may want to use a baseplate matching the column cap plate so that the bolts can be separated. To permit as much shop welding as possible, you might want to consider splicing the beams off the column.
On the issue of cracking the existing footing, assume it is unreinforced concrete and govern the projection of the new footing such that the permissible stresses in plain concrete are not exceeded.
BA
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dhengr - you can see what I would be bolting to...the 2 faces are about 5" wide and recess about 5" deep. Essentially, I'd have a 5"x5" section.
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I'm sure you already thought of this, but is it possible to come up with a better attachment detail from the joist to the wall and avoid all the foundation/column work? Perhaps a plate or angle extending above the joist (on each side) and getting into the 'meat' of the concrete beam?
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BA
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As you know, there is always a "better" way of doing something. However, my fee was only $1400 for this fix. That means I have just over a day and a half to design, detail, and provide construction administration for this fix. Anchoring to the beams would require more analysis and testing of the concrete beam to find out the compressive strength and existing reinforcing. I just don't have all the time for that. I also believe that the column and beam line will provide the client with a better overall "feeling" if you will than anchoring to the same concrete beam that failed. Structurally, we know why it failed...the anchors were not embedded properly, but all the owner sees is that they attached to a concrete wall that failed.
It should be noted that the original renovation took place in 1975...it took 35 years for the concrete section to fail. As poorly as this connection was designed and implemented, it still took 35 years to fail....I find that interesting.
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BA
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I agree with BA, and my original suggestion implied nothing less. I would not blindly assume the top conc. beam and the funny shaped columns could carry the new roof load. But, the earlier engineer thought they could and 35 years has not suggested otherwise. The roof truss bearing seat detail was poorly handled and that's what finally failed, not the conc. beam or column or strip footing. Can you get your hands on the original bldg. plans to see what the conc. beam and column details were, to give you some assurance regarding their strength? Your drawings show the stl. jst. bearing elevation lower than it appears to be in the photos, as this relates to the top conc. beam shape and your final detailing; and the existing bearing seat angle is vert. leg down, unlike your drawing, so maybe you can't leave the seat angles in place. If you opt for the stl. columns, I'd put them right at the conc. columns, and make them 8" channels (or some such), with the channel toes to the conc., with the flanges bolted to the conc. column 5" outstanding legs every 4-5', to reduce kl/r. These epoxied bolts would transfer part of the column load and you would be left with a lighter punching shear problem down at the ftg. base plate.
It's a shame that we are always stuck with a small fee to fix a problem on which we never have all of the details until we have burned through that fee. That's been happening to BA and I for 50 years now. They have a 100k problem and they won't pay an engineer a few k to fix it right. I would go to the client and tell him you want a cut of what you can save him in construction costs and disruptions, by giving him a cleaner and more aesthetically pleasing solution, now that you really understand the whole extent of the problem and the existing conditions. I'd cut a 1'x2' long (parallel to the strip ftg.) slab opening and cast a pedestal from the top of the ftg. to the existing slab elev., with some anchor bolts in it, and call it good. You don't say anything about the roof truss lengths or spacings, roof loads, conc. column spacing, etc., but 4x4 HSS columns at 18' max. and not aligning with the existing conc. columns won't be pretty either.
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BA
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The original drawings are not available. I like your idea about the channels. We do this on new openings in masonry walls pretty regularly. However, the layout of the concrete pilasters does not work well with this option. The room is a 37' clear spanning room. The roof dead is 30 psf and live is 20 psf (reducible). The loading on the column is 15.76 kips at 18' column spacings. Thanks for pointing out that I drew the angle seat upside down.
BA,
Thanks for your input - I have given a viable solution to the fix, didn't I?