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

 

[Lomarandil]

I would choose not to set a new footing above the existing footing (your option B). Lots of potential issues there and few benefits.

You could choose to use the chemical anchors, and take advantage of the combined footing. Or simply design the new footing as eccentric. Which choice is more economical will vary.

Not the point of your post, but do you feel confident in your method to determine the load split between the existing and new columns?

----
just call me Lo.

 

[r13]

Lo got good point, the existing column will see uplift rather than gravity load. Maybe you should consider a built-up column and enlarge the footing.

Think a little further, the present arrangement may be good for the footing, as the center of reaction will shift inward.

 

[BAretired]

Neither option is good. The original design is questionable too.

The steel beam will tend to load the new column and lift up the old column and footing.

The first thing to do is get a soil report so that you know what bearing pressure you will be able to use. The new footing should bear at the same level as the existing, i.e. 1.0 m below floor level. It should be more centralized under the column and should be of a size corresponding to the bearing values given in the soil report. I showed 600 x 1800 which is approximately the same area as 1000x1000.

Use a grade beam about 400 wide x 600 high tied into the existing stem wall. It's purpose is to spread the load over the existing footing, making use of it in a way that was not used before.

BA

 

[D.Jaya]

Thanks for both Lo and retired13 for your valuable comments.
Lo : I thought to model and analyse the structural frame by providing the actual member/section sizes,section properties and loadings etc.. (in ETABS program which is more familiar to me) and then I can get the load on new column and existing column.
...by that way I thought I can determine the load split between the existing and new columns...
Is this approach correct?

 

[D.Jaya]

Thanks BAretired(Structural)for your valuable opinion.

 

[r13]

I thought I can determine the load split between the existing and new columns...

I suggest to perform a frame analysis with real loadings. Approximation may work for one load case, but not another.

 

[BAretired]

Whether you use ETABS or hand calculations, the split between new and old columns will be difficult to determine with any precision. It depends on relative settlement of the new vs the old footings as well as the gap (if any) between new column and existing steel beam. It may be advantageous to leave a slight gap on purpose in order to reduce the load on the new column.

If you assume that everything is built tight and that the foundations have no relative settlement, the beam moment at the new column will not be much less than the fixed end moment of a 7m long beam, but that is likely not what you want. The best arrangement would be to try to end up with new and old column each carrying roughly the same load.

BA

 

[SlideRuleEra]

Here is my proposal:

A pair of new footings , one on each side the existing footing.
A concrete beam, with pier for a new column on the beam. The beam distributes load to the two new footings.
A single structural steel support for the existing 450x200 beam.

Features:
1) Existing footing remains unchanged, no new loads applied to the existing footing.
2) No connection between existing and new footings, (reasonable) differential settlement not a problem for footings.
3) Single support for existing beam allows predictable load sharing between existing and new footings. (Reasonable) differential settlement not a problem for structural steel.

http://www.SlideRuleEra.net

 

[Lomarandil]

DJ -- unfortunately, a straightforward analysis is not likely to represent this problem well. You need to account for three factors in some way:

[ul]
[li]staged/sequential loading[/li]
[li]settlement of the foundations (which governs vertical stiffness)[/li]
[li]how the new column is fit to the existing beam (affecting vertical stiffness)[/li]
[/ul]

I quite like SRE's solution.

----
just call me Lo.

 

[BAretired]

The beauty of SRE's solution is that it solves the problem of how the load is shared between existing and new columns by simply adjusting the reaction points, but it means the existing steel beam must be adequate to span between reactions as a simple span with very short cantilevers on each end.




BA

 

[D.Jaya]

Thanks for the valuable opinion received from SlideRuleEra(Structural)

 

[D.Jaya]

Dear BARetired(Structural)
I want an know that as I indicated in my post, if we connect the two footings at the base level by Chemically anchored R/F, will it act as a combined footing & will it create a negative moment at the top (in between the 2 columns)? where it needs to provide top reinforcement.

The problem here is now we cannot provide top reinforcement for the existing footing.
In that case can we keep the 2 footings separately at the same level without connecting with reinforcement (without chemially anchored)?
Also your idea of connecting the footing with a Grade beam anchored to existing stem wall is appreciated.

 

[dhengr]

D.Jayawardena:
My rules for footings…, 1.) they should generally be under the columns which they are intended to support; 2.) they should be above the soil which is intended to support them; 3.) you should kick the soil twice and pinch it btwn. you fingers to be sure that you have selected the correct soil bearing strength.

 

[D.Jaya]

Thanks dhengr

 

[dik]

What type of soil? Can you plate the existing column and use micropiles for added foundation capacity?

Dik

 

[Ron]

@SRE...nice solution.

 

[D.Jaya]

@dik(Structural)
The soil is sandy gravel mix.
plate the existing column is a good idea for increasing the column capacity.
But for the added foundation capacity -using micropiles is bit difficult due to the site location and also seems to be expensive.
Anyway thanks for your opinion.

 

[dik]

Do you need to use the original column? Can you add a new one and use a cantilever?

Dik

 

[r13]

The problem here is now we cannot provide top reinforcement for the existing footing.

Make your footing as shown, and provide the frame as suggested by SRE.