Eccentric Footing 2

[sponcyv]

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.

 

[larsacious]

Couple of comments regarding your detail:

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...

 

[a2mfk]

Wow, that is one odd existing wall section. Not having run any calcs, your detail looks good and is how I have done it dozens of time in the past on existing buildings. I would not avoid using existing footings, do so cautiously but know this is a very common procedure as you have experienced the alternatives, uhmm, suck... Eccentrically loaded footings are just highly inefficient and should be avoided.

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).

 

[PUEngineer]

Am I the only one who dislikes the W12x19 beam sitting on top of the column eccentrically without some kind of bracing for the top of the column or full depth bearing stiffeners at least? I know it's a minor eccentricity at the top (1.5"), but the beam has a pretty thin web and the column is only a 4x4, almost 18' long from top of footing to bottom of beam. Just a gut feel, but it seems somewhat unstable.

 

[dhengr]

Sponcyv:
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.

 

[sponcyv]

PU - The column was sized for the axial eccentricity and passes at 0.65. The dead + live deflection of the HSS4x4 is 0.237 (L/911).

 

[sponcyv]

dhengr,

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.

 

[BAretired]

sponcvv,

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

 

[sponcyv]

Thanks everybody for your input!

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.

 

[BAretired]

sponcvv,

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

 

[BAretired]

sponcvv,

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

 

[sponcyv]

Here is a pic of the failed bearing seat...

 

[sponcyv]

Another pic...

 

[sponcyv]

Here's a pic of the adjacent seat...you can see the cracks propagating at 45 degrees...

 

[sponcyv]

Here is a piece of concrete that fell and hit me in the head when I was using a broom to remove the ceiling tile. The dull side hit me and glanced across my eyebrow...minor cut. I saved it to remind me how fortunate I was....notice the sharp ends. Note to self - wear hard hat even if not a construction site.

 

[sponcyv]

This picture shows what the wall looked like before the expansion. This shows the high grade beam and pilasters.

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.

 

[azcats]

Man...that original bearing seat detail is terrible. I wonder if it was originally intended to have the angle reversed so the leg was up and switched in the field because the fascia shown in pic 5076 would have been in the way. That would have gained a few (4?) inches and likely not have had the same type of failure.

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?

 

[BAretired]

I agree that the anchorage detail is bad, but unless the reinforcement in the existing beam is known to be capable of carrying the joists, it is not advisable to consider using it. The section indicates that the beam was not carrying any roof load prior to the addition being built.

BA

 

[sponcyv]

This is the original detail - notice that no embedment was specified and the contractor used sleeve anchors instead of the self drilling red heads.

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.

 

[BAretired]

It might be prudent to anchor the bottom of the steel column into the face of the existing grade beam as there is some doubt about the adequacy of the existing footing to carry the moment resulting from the proposed new footing projection.

BA

 

[dhengr]

Sponcyv:
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.