Shallow foundation

NIDA FATIMA ANSARIB.ARCH III YEAR

A foundation is the lowest and supporting layer of a structure and  a building component whichtransfers building loads to the soil.

There are two basic types of foundations:

SHALLOW - Shallow foundations transfer the load to soil atthe base of the substructure

DEEP - Deep foundations transfer loads far below thesubstructure. These foundations penetrate incompetent soil until asatisfactory bearing stratum is reached.

CHARACTERISTICS• The objective of shallow foundation is to distribute the structural concentrated load over a 

wide horizontal area at a little depth rather than a range of the depths.

• Shallow foundation is often selected when the soil has good bearing capacity and the structural load will not cause excessive settlement of the underlying soil layers.

• shallow foundations are more simple and cost effective to construct than deep foundations because little soil is removed or disturbed. 

• This foundation is usually utilized in residential and light commercial buildings.

• Shallow foundations are generally constructed using in-situ concrete but some substructure elements can alternatively be constructed in precast concrete in part or whole to improve speed of construction on site.

TYPES OF FOUNDATIONShallow Foundation System

   isolated foundation   Combined foundation  Mat / Raft Foundation

ISOLATED 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 Isolated footing, so each structure may include dozens of individual footings.

FOOTING FOR WALL. FOOTING FOR COLUMN

EXAMPLES FOR ISOLATED FOOTING

• The foundation consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls.

• For the isolated foundation system the structural load is literally spread out over a broad area under the building

• Most common type of foundation used due to their low cost & ease of construction.• Most often used in small to medium size structure with moderate to good soil

condition.

• isolated footings may be built in different shapes & sizes to accommodate individual needs such as the following: a) Square Spread Footings / Square Footings b) Rectangular Spread Footings c) Circular Spread Footings d) Continuous Spread Footings

a) Square Spread Footings / Pad Foundation

- support a single centrally located column

- the reinforcement in both axes are to

resist/carry tension loads.

b) 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

Square spread footing

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c) Circular Spread Footings

- are round in plan view

- most frequently used as foundation for

light standards, flagpoles and power

transmission lines.

Combined footing

This type of footing is adopted when the space between two columns is so small that the foundation for individual columns will overlap.

Combined footings are proportioned in such a way that the centre of gravity of the loads coincides with the center of gravity of the foundation. Hence these footings have either a trapezoidal or a rectangular shape.

Cantilever or Strap Footings.

• A strap footing is used to connect an eccentrically loaded column footing to an

interior column.• The strap is used to transmit the moment caused from an eccentricity to the interior

column footing so that a uniform soil pressure is generated beneath both footings.• The strap footing may be used instead of a rectangular or trapezoidal combined

footing if the distance between columns is large and / or the allowable soil pressure is relatively large so that the additional footing area is not needed.

MAT FOUNDATION• FOR VERY LOW BEARING CAPACITY OF SOIL, THE STRIP OR ISOLATED FOOTING

MERGE RESULTING IN THE MAT FOUNDATION .• IT IS A SINGLE LARGE CONTINUOUS FOOTING SUPPORTING THE ENTIRE

STRUCTURE.• IT ALSO REDUCES THE DIFFERENTIAL SETTLEMENT BETWEEN THE COLUMNS.

PLATE THICKENED UNDER COLUMN AND FLAT PLATEBEAM AND SLAB SYSTEM AMD PLATE WITH PEDESTAL

Mat-slab foundations

Mat Foundation often considered to be used when dealing with the following

conditions:

• The structural loads are so high or the soil condition so poor that spread footings would be exceptionally large

• The lateral loads are not uniformly distributed through the structure and thus may cause differential horizontal movements in spread footings and pile caps.

The continuity of a mat will resist such movement.

• The uplift loads are larger than spread footings can accommodate. The greater weight and continuity of a mat may provide sufficient resistance.

• The bottom of the structure is located below the groundwater table, so waterproofing is an important concern. Because mats are monolithic, they are much easier to waterproof. The weight of the mat also helps resist hydrostatic uplift forces from the groundwater.

In ground reinforced concrete foundation in cyclonic area, Northern Australia.

ADVANTAGES OF USING SHALLOW FOUNDATION1- Cost (affordable)2- Construction Procedure (simple) 3- Materials (mostly concrete)4- Labor (does not need expertise)

DISADVANTAGES OF USING SHALLOW FOUNDATION1- Settlement2- Limit Capacity * Soil * Structure3- Irregular ground surface (slope, retaining wall)4- Foundation subjected to pullout, torsion, moment.

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