Column footing not to project beyond boundary causes large base
Column footing not to project beyond boundary causes large base
(OP)
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.
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.






RE: Column footing not to project beyond boundary causes large base
RE: Column footing not to project beyond boundary causes large base
RE: Column footing not to project beyond boundary causes large base
Csd - I have attached a sketch of the cross-section of the basement - please note that the sizing of the elements are preliminary.
RE: Column footing not to project beyond boundary causes large base
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).
RE: Column footing not to project beyond boundary causes large 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.
RE: Column footing not to project beyond boundary causes large base
RE: Column footing not to project beyond boundary causes large base
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.
RE: Column footing not to project beyond boundary causes large base
RE: Column footing not to project beyond boundary causes large base
RE: Column footing not to project beyond boundary causes large base
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.
RE: Column footing not to project beyond boundary causes large base
RE: Column footing not to project beyond boundary causes large base
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
RE: Column footing not to project beyond boundary causes large base
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.
RE: Column footing not to project beyond boundary causes large base