Foundation Structure: Footings

Foundation Structure: Classification of Footings

Generally, In an RCC structure the applied load, moments & other forces are transmitted through the column to the foundation structure below ground level such as pile via pile cap or footings, without exceeding the safe bearing capacity. The settlement of the structure shall be kept as per tolerable limit and nearly uniform as possible.

In a footing, the plan area is much larger than the cross-sectional area of the column, the load is transferred from the slabs via beam to the columns and the total load is distributed throughout the area of the footing via reinforcements & concrete. The effect of the load is transferred to the soil by bearing pressure, normally the bearing pressure of the soil is much lower than concrete & steel. In generally in India the safe bearing capacity of a particular soil ranges between 100 – 400 kN/Sq.m whereas, the permissible compressive strength of concrete is approximately 5-15 N/mm² & for steel it ranges between 130 – 190 N/mm² in reinforced concrete columns under working loads. These types of supporting structure are termed as foundation structure and sometimes sub structure. 

 Foundation structure is broadly classified into two categories:

• Shallow foundations – Isolated footing, Combined footing, Eccentric footing, Trapezoidal footing & Strap footing.

• Deep foundations – Piles & Caissons.

Different types of footings are:

In general footings are designed to resist the bending moment & shear force developed due to soil reaction.

• Isolated footing – In framed RCC structure where the total load is not huge (generally less than 3-4 storied buildings) it is sufficient to provide separate independent footings connected via tie beam. This type of footing is termed as isolated footing. Generally, its shape varies depending upon the shape of the column’s cross-sectional it can be rectangle, square, circle etc. It consists of a thick slab which can be pad, sloped or stepped. 

Types of Isolated Footing

• Combined footing – When two heavily loaded column are placed close to each other and the isolated footings overlap each other it is much more sensible & effective to merge two different footing, thus creating combined footing. The use of combined footing also improves the integral behavior of the columns & the footing. The combined footing contains a beam between the two columns known as the connecting beam.

• Eccentric footing & Trapezoidal footing – When the column is located near the end of the footing due to some restriction of column/ footing placement (such as near the property line of a plot), Special type of footings are designed to overcome these hindrances. Isolated footings with eccentrically loaded column are termed as eccentric footings whereas, in combined footing it is termed as trapezoidal footing, generally in combined footing near the property line the size of the footing is reduced as requirement, thus giving it a trapezoidal shape.

• Strap footing – Strap footing are two isolated footing connected with a connecting beam through the footing. The footing’s thick slab acts a cantilever slab flanged on both side of the connecting beam. These footings are also used where there is problem of column placing in the centroid axis of the column.

Types of  Footings.