Published on March 6, 2014
SHALLOW FOUNDATION PREPEARED BY MOHAMMAD SHAHID B.ARCH 3RD YR
INTRODUCTION TO FOUNDATION The lowest part of a structure is generally referred to as foundation. FUNCTION OF FOUNDATION To transfer load of the superstructure to the soil on which it is resting REQUIREMENTS A properly designed foundation is one that transfers the structural load throughout the soil without overstressing of soil which can result in either excessive settlement or shear failure, both of which can damage the structure.
LOADS AND SETTLEMENT OF FOUNDATIONS Types of loads on foundations Dead, live, wind, inclined thrusts and uplift, water table and earthquake forces Types of settlements Uniform and differential - Differential settlement must be minimized, depends on site soil conditions and distribution of loads on columns supporting the building
TYPES OF FOUNDATION 1) Shallow Foundation System – these types of foundations are used where the soil bearing capacity is high , i) Combined and isolated footing 2) Deep Foundation System i) Pile ii) Diaphragham wall iii) Pile walls iv) Caissons
SPREAD FOOTING FOUNDATION Also known as a footer or footing It’s an enlargement at the bottom of a column/ bearing wall that spreads the applied structural loads over a sufficiently large soil area. Each column & each bearing wall has its own spread footing, so each structure may include dozens of individual footings. SPREAD FOOTING
The spread foundation consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls. For the spread foundation system the structural load is literally spread out over a broad area under the building It Most common type of foundation used due to their low cost & ease of construction. Spread footings may be built in different shapes & sizes to accommodate individual needs such as the following:
SQUARE SPREAD FOOTINGS / PAD FOUNDATION support a single centrally located column use concrete mix 1:2:4 and reinforcement the reinforcement in both axes are to resist/carry tension loads.
RECTANGULAR SPREAD FOOTINGS Useful when obstructions prevent construction of a square footing with a sufficiently large base area and when large moment loads are present
CIRCULAR SPREAD FOOTINGS are round in plan view most frequently used as foundation for light standards, flagpoles and power transmission lines
CONTINUOUS SPREAD FOOTINGS / STRIP FOUNDATION Used to support bearing walls
COMBINED FOOTINGS support more than one column useful when columns are located too close together for each to have its own footing
RING SPREAD FOOTINGS continuous footings that have been wrapped into a circle commonly used to support the walls above-ground circular storage tanks. The contents of these tanks are spread evenly across the total base area and this weight is probably greater that the tank itself Therefore the geotechnical analyses of tanks usually treat them as circular foundations with diameters equal to the diameter of the tank
DEPTH AND LOCATION OF FOUNDATION Depth and location of foundation depends on 1. Zone of significant volume changes in soil. 2. Adjacent structures and property lines. 3. Ground water 4. Underground defects
Adjacent structures and property lines. Structures may be damaged by the construction of new foundations, as a result of vibrations, undermining by excavation or lowering of the water table. After new foundations have been constructed, the (new) loads they place on the soil may cause settlement of previously existing structures as a result of new stress pattern in the surrounding soil. In general, deeper the foundations and closer to the old structure, greater will be the potential for damage to old structures. Part existing property line Property line
Ground water Presence of water reduces soil bearing capacity, larger footing size more cost. During construction pumping is necessary – adds to the cost of construction.
RAFT FOUNDATION A foundation system in which essentially the entire building is placed on a large continuous footing. It is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls. Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. It is normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation. Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.
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