Foundations or footings for walls and new building structures or dwellings guide.
Getting a foundation or footings right is paramount for the structure of the new building. If you build a shed, an easy way to do it, is to employ the pillars or pier foundation technique. You must keep in mind that the technique is not design or recommended for very heavy shed structures. If you use light weight materials, light blocks instead of the solid brick walls, flat roof instead of tiles roof, it will do just fine. In combination with concrete lintels, you will save a lot of digging work, soil to dispose , skips and concrete.
You might notice in the picture above some square looking holes, that is where the concrete goes. It should be deep enough to be stable, depends on the quality of the soil and the weight of the structure. If you want to go hi tech and engineer style :
Foundation design is a complex process. It depends on the total load on a particular foundation as well as the Safe Bearing Capacity of the soil.
For example, a single storey building foundation with hard soil, where total load on footing does not exceed 300kN, a footing pit of 1M X 1M at a suitable depth up to hard strata would be sufficient. The pit would have a 6″ layer of PCC. On top of that, you would place a mesh of steel consisting of 8 rods each of 10mm diameter on both sides. Then place the column steel frame on top of it and tie the column bars to the steel mesh. Pour 6″ of concrete and let it harden. Then you can go for either tapered footing or block footing. Total thickness of block should be 15″, including the 6″ you of RCC poured earlier. So, for a single storey building whose load does not exceed 300 kN, and the soil strata is hard, the above given design should be sufficient.
Four loads are to be considered in order to measure total load on the footing:
Self load of the column x Number of floors
Self load of beams x Number of floors
Load of walls coming onto the column
Total Load on slab (Dead load + Live load)
If you get well versed with load calculations, then calculating the size of the footing and following the procedure for foundation design wouldn’t be a problem.
To keep everything simple, the holes – pillar foundation should be at least 600mm deep and width depends on the size of the wall, if you have a cavity wall, make sure the footings are wide enough to accommodate the walls easily , as the wider it is, the better at spreading the dead weight. ( in London you don’t get much snow and if its a flat roof, the wind won’t affect the live weight as much )
Then you use pre-stressed concrete lintels to bridge them, and in this way, the savings in labour and material becomes obvious.
I don’t have a real picture but its fairly simple to understand the process.
The most common type of foundation is the traditional type, basically you dig a trench and fill it with concrete. If you build a house or a house extension, the Building Control will ask you to do it this way, unless you can afford a good structural engineer and some expensive drilling equipment if the soil – ground is really bad. Remember that Venice is built on wooden piers.
In order to keep the Building Inspector happy, remember to keep the foundation clean, all the muck from the bottom of the trench must be cleared before pouring the concrete in.
If the water table is high, it is very probable that the trench will fill up with water overnight, or if its raining. You can use a bucket if there is not a high volume of water or you could rent a water pipe pump.
The concrete can be mixed on site, you could buy a concrete mixers and all the sand, gravel and cement and do the mixing yourself. If its a D.I.Y project and the foundation is not more than 3 to 4 square meters, it will be a good physical exercise as once you start mixing the concrete, you will have to complete the foundation before the concrete sets.
I would rather recommend C25 concrete for larger foundations, on a 1:5 ratio of cement to ballast. The strength of a concrete mix is measured in grades. There are different concrete grades for example C15, C20, C25.
And if the foundation is really large and it can not be completed in one go, it will have to done on segments, using metal reinforcement rods/bars between the segments of the footing in order to connect them.