Help. Need help with Footing in the wrong location 4

[Berniedog]

We are the EOR on an office building and we did the stair design also for local fabricator. We located the footing for the single column for the monumental stair. The stair was revised but we didn't catch the revised location for the single column. Well the Architect says we cannot cut the floor. We have a 3'x3' footing with a 4" concrete slab. The new post is 14 1/2" off of the centerline of the footing. Allowable bearing is 2500 psf.

So P/A = 1.53 psf
M/S =3.71
P/A +M/S = 5.24

 

[miningman]

so what exactly is your question???

 

[Berniedog]

I am sorry. The Architect said we cannot cut the floor. Is there a field fix? Over sized baseplate with a rod going back under the stair to take thrust?

 

[JStephen]

One thing I'd do is run the problem by the geotechnical engineer and see if there's any extra that be gleaned from the allowable bearing given the circumstances.

 

[bones206]

5.24 psf or ksf?

 

[Berniedog]

5.24 ksf

 

[CANPRO]

1st thing I'd do is sharpen the pencil and go through my design to make sure I wasn't too conservative with the loads. 1.53 ksf x 3' x 3' = 13,770 lbs - probably mostly live load?

Agree with JStephen, talk to the geotech. If you're using the typical allowable load for the job, its probably based on limiting long term settlements. If you have 10,000 lbs of live load on your stairs long term, I suspect you have other problems. Is the footing sitting on native soil, or at a higher elevation maybe on some better prepared sub-base?

Are the stairs fabricated yet? If your post/baseplate can handle some additional load, you could always look at providing a fixed connection to the footing - realistically probably not going to do squat, but theoretically it should counter some of the eccentricity.

 

[BAretired]

Unfortunately, the load is outside the kern of the footing, so M/S is much more than 3.71 ksf.



BA

 

[oldestguy]

If need be and if the soil will take it, some form of injection, such as two-component cements may improve things enough to do the job.

 

[BAretired]

I think the Architect is going to have to accept some penetration of the floor slab. Perhaps a small diameter pile could be used to reduce the area to be cut.

Alternatively, perhaps the stair framing could be modified to permit the support to align with the existing footing.

BA

 

[miningman]

It might be cheaper in the long run to redraft all relevant drawings and reimburse the owner / client for all resultant remedial corrective actions / fabrications etc.

 

[oldestguy]

What is between top of footing and the bottom of floor slab? If compacted fill manybe enlarge column to sit partly on the floor slab. If there is a question still, pump grout under the slab to compact the soil there and put very small lift on the floor slab Then, with enlarged column sufficient puts significant load on the floor slab in the direction of footing larger area. When all is complete take careful elevations and keep records. Later if there is any settlement, the same injection of grout can bring things back to where they started.

 

[TEDstruc]

I agree with BA. Architect is going to have to accept some form of modification to the slab. Maybe a cored hole in slab under or very close to stair column, then install some form of pile through the cored hole.

 

[JLNJ]

Your actual maximum soil pressure is more like 10 ksf, not 5.24. It's not a simple summation.

 

[JAE]

Agree with JLNJ - I got about 7300 psf with a 14 1/2" eccentricity - assuming a concentrated load of 12.5 kips to get to the 1530 value for P/A.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[bones206]

I like TEDstruc’s idea. Hide your repair under the column and everyone walks away happy.

 

[oldestguy]

On the use of piling, consider cutting out a section of the floor at the present column location. Arrange to temporarily holdup the existing column from beyond the cut out floor section. Cut off the bottom of the column if necessary. Use drills and jack hammers to make holes through the existing footing. Drive extendable mini piles through the cored holes and don't stop extending them until a given load can be taken with no noticeable settlement for one hour of load. How to confirm that load capacity can be by several different methods, including a calibrated jack. Then construct a reinforced pile cap in the space created by the sawed slab removal. Use no-shrink grout between existing column and the new pile cap, then release the temporary column supports. Using several mini-piles, a uniform support situation is created for the new pile cap. Under low headroom conditions mini-piles can do wonders. Top of pile cap can be flush with floor grade and finished accordingly. Of course modifications of this idea are possible, but due to the goof-up to begin with this had to be done right.

 

[dik]

Even the installation of a couple of micropiles on either side of the column will require penetration of the floor slab.

Dik

 

[dik]

Agree with JLNJ - I got about 7300 psf with a 14 1/2" eccentricity

. The loading has to be revisited as well as a discussion with the geotekkie.

you mean you cannot rely on tension? <G>

Dik

 

[BAretired]

Has the stair and single column been installed? If so, installing a pile is not feasible in which case, the footing would need to be replaced. If not, the area should be clear for installing a single pile under the column.

BA