What is the better way to locate new RC footings - retrofitting of an existing steel structure 3

[D.Jaya]

Dear Structural Engineers

We are carrying out an upgrading of existing steel frame (having 2 levels) with a existing RC foundation & to convert it to a Warehouse space at the upper level.

Initially the Steel frame & foundation(footings) has been designed for a office space at upper level and now the client want to convert it to a warahouse to store some materials. storage loadings are not much high and the stored material weight to be not exceeding 500 Kg/m2 and also the cloumn/beam structure arrangement is 3m apart (in Plan) and the width(long span) of the building to be 9 meters.We worked out the factored design load for the upper level proposed composite steel deck slab and it to be around 10 KN/m2

The main beams were analysed and observed their capacity was OK but the Columns and the RCC footing sizes were not adequate for the additional load & BM.

Therefore we suggested the clinet to have an extra column in front of the existing columns with new footing and the combination of the 2 columns and the (exsiting+additonal) footing can safely carry the Load + BM. This arrangement will give a clear space without center columns as per the client's requirement at the ground level.

The Bearing capacity of soil not checked but by visual inspection of the excavated test pit and the prior knowledege of the bearing capacity of the surrounding sites, it seems that the B.C. to be approximately in between 150 to 200 KN/m2.Accordingly new footing.sizes worked out

Now I need to know that, when placing the new fotting, what will be the better option(As per the attached sketch)

Option (A) - new footing to be adjoined to the existing footing and reinforcement will be connected through chemical anchoring.Here the new column will located eccentric to the new footing.

or

Option (B) - New footing to be cast partially on top of the existing footing as shown

 

[JLNJ]

The chemically anchored bars are not likely to be sufficient.

Just because you pin two pieces of concrete together with a little glue doesn't make them act as one footing.

 

[JLNJ]

I don't know what building code you are subject to, if any, but most US codes will require a minimum live load of 125 psf for "storage". This is 610 kg/m^2. This is for "light" storage. Items on pallets or in racks can easily exceed this.

"Heavy" storage is 250 psf.

 

[dik]

Something like this?

Dik

 

[r13]

For extension of the footing bottom bars, I would jack up the corner to relieve load in the existing column. Dig a hole to the backfill depth required, break existing footing concrete, thread and install tension coupler to the existing bars. Then, backfill, add bar mat, and place new footing concrete.

 

[r13]

dik,

It will work, depends how the (2nd floor) column above was connected. Also, a corner post maybe required if the exterior wall consists of girts and metal panels.

 

[NOLAscience]

I was thinking of dik's solution as I read through this thread for the first time today. I would not bother pinning the new foundation to the old. Run some numbers and see how this works out for the 125 psf or 250 psf storage, and keep in mind that the storage can be highly eccentric (i.e., all along one wall, empty on the other wall). Put the assumed loading (125 psf, for example) on the drawings. If you are planning to have loading in certain areas and aisles elsewhere, paint that on the floor.

 

[structSU10]

reinforcing the existing column and creating new grade beams to spread footing extensions is not an option? I envision plates / WT's welded to the existing column and then run new grade beams each side of the existing pier that runs back to new spread footings - or something like that.

 

[BAretired]

I imagine one of the reasons for the offset columns is to reduce the steel beam span and restrain the ends to make it more capable of carrying a heavier live load.

BA

 

[r13]

For the original layout (3m spacing), the beam is adequate to take the additional load, but the column and footing are inadequate. Furthermore, the client wants to eliminate the middle column for better space utilization, thus the idea of adding additional columns. I think the original exterior column needs to stay, and SRE's framing plus the modified (full depth) foundation is the simplest way to go.

 

[dik]

I didn't connect the two... just sat the new one on top.

Dik

 

[dik]

The cantilever helps with the beam moment, too.

Dik