The different types of footings used in construction are described below:
- Continuous Wall Footing. The footing which supports a long masonry or RCC wall is known as a continuous footing.
- Isolated Footing.
- Combined Footing.
- Strip Footing.
- Strap Footing.
- Raft Footing.
- Pile Footing.
Strip FootingsA Strip Footing is a relatively small strip of concrete placed into a trench and reinforced with steel. The footing supports the load of the exterior walls and any interior wall that is load bearing or supports a slab such as for a bathroom.
Column Footing is also famous as independent footing. An independent footing is one which is provided under a column or either similar member for distributing concentrated load in the form of uniformly loads on the soil below. column footings may be circular, rectangular or square in plan.
When the column loads are heavy or when the safe bearing capacity of soil is very low, The required footing area become very large. So in order to spread the load over large area with less depth then we have to increase the footing area.
Different Types of Shallow FoundationsStrip footing. Spread or isolated footing. Combined footing Strap or cantilever footing. Mat or raft Foundation.
Tie Beam (Straight beam) is a beam which connects the two footings in the substructure. Tie beam is provided when the two footings are in the same line. Strap Beam(inclined beam) is similar to tie beam but it connects two footings at a certain angle. Strap beam is laid when two footings are in different levels.
We provide this type of beam when the two footings are in the same line. It connects two or more than two columns and makes a structure as a frame. It is used in the construction of buildings to avoid the settlement of columns or foundation. It helps to carry the horizontal load of the column.
Depending on the shape of the combined footing, there are three types of combined footing, namely, rectangular, trapezoidal and T-shaped combined footing. Consider the below figure for different types of combined footing.
When two or three columns are located very near to each other, we provide a single substructure for them and the footing is known as a combined footing. Combined footings are also usually made of reinforced concrete. A combined footing may be rectangle or trapezoidal in plan.
Mat foundations are popular in areas where basements are common. Mat foundation is also used for low bearing capacity soil in order to spread the load of a building and hence construct a stable foundation. It is used to reduce differential settlement of buildings.
Explanation: When the two columns load are unequal, with the outer column carrying a heavier load, and when there is space limitation beyond the outer column, a trapezoidal footing is provided.
Explanation: A spread footing for a single column is either known as the isolated footing or pad footing. The base area of such a footing is given by A = P/q where P is the total load transmitted by the column, including that of the footing and q is the safe bearing pressure for the soil.
The floor of a building immediately above the ground is known as the ground floor. Explanation: Substructure or foundation is the lower portion of the building which lies below the plinth level. Substructure transmits loads of the superstructure to the supporting soil.
The weight of the raft is not considered in the structural design because it is assumed to be carried directly by the subsoil. The raft may be ribbed where the column spacing is irregular or for economy in using a relatively thin slab over most of the area.
Isolated footings (also known as Pad or Spread footings) are commonly used for shallow foundations in order to carry and spread concentrated loads, caused for example by columns or pillars. Isolated footings can consist either of reinforced or non-reinforced material.
The proximity of building a line or existing building or sewer, adjacent to a building column. When the bearing capacity of the soil is low due to which the dimensions to be provided for each of the isolated footings come out to be high and hence they overlap, the combined footing is provided.
How to Calculate Footing Size
- Determine the width and length of the cement slab in inches.
- Divide the width by 12 to convert it to feet.
- Divide the length by 12 to convert it to feet.
- Determine the depth or thickness that is required for the footing in inches.
- Multiply the width by the length and then by the depth.
1.1 To calculate the footing base area, divide the service load by the allowable soil pressure. Assuming a square footing. Therefore, provide 12 x 12 ft square footing. Because there is no overturning moment, overall footing stability is assumed.
Design of footing as per IS 456-2000.
A = The area of the bottom of the footing. M = The total overturning moment measured at the bottom of the footing, including horizontal loads times the vertical distance from the load application location to the bottom of the footing plus any overturning moments.
Design Calculation of an Isolated Footing
- INTRODUCTION. Footings are the bases laid on the soil on top of which a structure is erected.
- PROPORTIONING OF FOOTING FOR COLUMN:
- Column B:
- Check for Bending Moment:
- Check for One-way shear.
- Check for two-way shear.
- Check for Bearing Stress.
- Check for Development Length.
