Column footing not to project beyond boundary causes large base 2

[LiloSA]

I am designing a column footing which must not project beyond the property line. The base is loaded with an eccentric column (1500x500mm) such that the edge of the column lines up with the edge of the base. To counter the large moment due to the eccentricity of the column, the base size required is too big.
Seeing that the column is so huge, does the column absorb some of that moment?
I have had some advice to design the base as if the column is centrically placed, because the size of the column resists the base from wanting to overturn. I would like to hear your opinions.

 

[hokie66]

In most cases, that is poor advice. Suggest you look at a strap footing over to the next internal column. By tying the two footings together, you can more evenly distribute the bearing on the soil and reduce the moments.

 

[csd72]

need more info, what are the loads, what is thesituation at the top of the column.

 

[LiloSA]

Hokie - a proper strap footing is a bit difficult, since the internal column is on another level due to existing building constraints. The internal column is also eccentric due to the adjacent building.

Csd - I have attached a sketch of the cross-section of the basement - please note that the sizing of the elements are preliminary.

 

[csd72]

What is the purpose of the foundation beam?

Personally I would make this 700 deep and lower it to the level of the footings then use it to balance the eccentricities from the two footings (often called a tie beam or grade beam).

You would also need to put the two footings at the same level (which isusually a good idea to avoid differential settlement).

 

[LiloSA]

The purpose of the foundation beam is to counter the horizontal forces generated in the arch beam wanting to push out the columns at the base.

I have also recommended the footings to be at the same level, but several things make it difficult, eg hard rock in-situ material; inside column level fixed as it has to be below the existing building strip foundations (Client wants to keep existing building for some reason... making it extremely difficult for everybody...); and the exterior column bases are awfully close to the adjacent property, so we don't want to go any deeper than is absolutely necessary.

 

[csd72]

I would suggest a sloping foundation beam then which is also designed for the additional bending moment from the eccentricities.

 

[ishvaaag]

I suggest that you make a 3D model of the foundation, its loads and a elastic halfspace. The notional tie on grade and moment equilibrating schemes are but notional and not a 10% informing about the behaviour than this kind of model. It also diverges greatly from both schemes.

I have put this kind of model in use many times when ties at the foundation level were not wanted to advantage; some walls may need to be retouched but in general the behaviour is reasonable and economical.

In this csase you would also acquire the behaviour of what happens of the incidence of push above footing level.

This is easily done today with 3D modeling tools like autocad, inventor, spaceclaim, and many 3D able FEM are able to produce meaningful results even in the absence of the complicated aspects of geotechnical behaviour; if such is the soil, use some specialized geotechnical FEM tool.

 

[FixedEarth]

Consider drilled pier foundation. The geotechnical firm will be able to provide you soils parameters.

 

[hokie66]

Can't you make the footings longer parallel to the boundaries, so not as wide? You can't just ignore the eccentricity.

 

[LiloSA]

Thanks for your input guys.

Hokie. There is a retaining wall in between the columns, so I increased the retaining wall footing for a specific dimension - creating a T-shape base. Problem is, it's still very large - the top of the T will almost be continuous up to the next base (5.36m c/c column spacing). If it has to be this way, it's fine, but I just want to propose the most economical solution.

So my question: "Seeing that the column is so huge, does the column absorb some of that moment?"

Csd. I haven't thought of sloping the foundation beam. Will have a look at that option.

 

[hokie66]

Sure, you could design it so the footing is cantilevered and the column absorbs that moment. Since you have a big column which is part of a rigid frame, that could be a practical solution.

 

[BAretired]

A little eccentricity is probably a good thing, but if you want the most economical solution, you may be better to modify the footing dimensions. At present, you are cantilevering 3500-1500 = 2000 parallel to the arch and (3000-500)/2 = 1250 normal to the arch.

The area of your footing is now 10.5m², so maybe a dimension of 2600x4000 would be a better choice (with the 2600 parallel to the arch). That would give you an eccentricity of 550 which you could easily reinforce the column to resist.

BA

 

[LiloSA]

Hi BA,

I think I reached a solution. Considering the base fixed, the moment working in on the base as result of the arch action (wanting to push the base out), counters the moment created by the eccentric axial load to some degree. And if I then play with the dimensions of the base to decrease the eccentricity as you suggested, the moments can be balanced out.

Thanks for your advice.

 

[csd72]

Just make sure that all the loads that you are using to balance things out are still going to be there when the unbalancing load is applied.