What is Foundation
It can be described in the following points;
- Foundation is provided to transfer any structure’s weight to the ground safely.
- The Foundation can be made of many types. Selection of the type of foundation depends on the sub-soil bearing capacity and the weight and nature of the structure.
- Spread Foundation is designed for Ordinary Soil and Lightweight building.
- If the weight of the building is in the form of columns instead of a wall, a separate foundation is designed for each column.
- A typical footing is provided instead of separate columns if the columns are located very close together and the weight is too high. This is called a raft or mat. This type of foundation is called a shallow foundation.
- To design a shallow foundation, the total weight on the wall or column of the building is spread according to the bearing capacity of the soil.
- The weight of the building should not exceed the land’s carrying capacity per unit area.
If the bearing capacity of the soil is low and the weight of the building is very high, then the weight of the building is transferred to the depth of the earth. Such a foundation is called Deep Foundation. Such foundations include the Pile Foundation, the Well Foundation, and the Caisson footing. Such a foundation is made for multi-storeyed buildings and bridges etc.
Trenches are dug deeper into the soil until a hard stratum is reached to build a footing. Concrete is poured into this trench to strengthen the footing. These trenches are reinforced with a cage to increase the foundation’s strength. Steel rods projecting outwards serve as the bones and must be connected to the substructure above. After the footing has been properly packed, the building’s construction can begin. The footing can be built with concrete, steel, stones, bricks, and other materials. The design loads and the underlying soil determine the material and type of foundation chosen for the desired structure.
Design of Wall Foundation
Wall footing design involves determining the footing’s width, the foundation, depth of the footing, and thickness of the concrete block. The following is the procedure for designing a wall footing for a standard soil type.
Width of Foundation
The width of the wall’s footing is kept so that the total load can be distributed according to the bearing capacity under the ground. The following formula (Thumb rule) is used to estimate the width of a footing for the construction of a typical building on a sway.
- Width = 2T + 30…cm
- Width = 2T + 12….inches
Where T = Thickness of wall
The following formula determines the width of the foundation for a building with more than two storeys.
- Width = P/Pb
Where P = Total load acting on the foundation.
Pb= Safe bearing capacity of the soil.
Depth of foundation
The depth of the standard type of foundation on ordinary soil is determined according to the following formula of Rankine.
Depth.
Where, Pb = Safe bearing capacity of soil
W = Unit weight of the soil
Ф = Angle of repose of the soil
Concrete Block Thickness
The following formula is commonly used to determine the thickness of the concrete block provided at the base of the wall.
d = J x √3p/m
Where, J = Offset of base concrete [15 cm or 6 inches]
P = Safe bearing capacity of soil
m = Modulus of rupture of concrete [2.46 – 5.27 kg/cm²]
Example
Data
Total Load per meter Length = P = 6500 Kg
Bearing Compacity of soil = Pb = 7500 Kg/m2 or 0.75 Kg/cm2
Density of soil = w = 1500 Kg/m3
Modulus of rupture = m = 3.5 Kg/cm2
Angle of repose = f = 30o
Wodth of foundation = P/Pb = 6500/7500 =0.867 say 0.90
Solution
1. By thumb rule, Width = 2T + 30
By thumb rule, Width = 2×3+30 = 90cm or 0.9m
2. Depth of foundation = D = Pb/w *( 1-sinФ/1+sinФ )2
Depth of foundation = D = 7500/1500 * ( 1-sin30/1+sin30 )2
D = 0.55m Say 0.60m
D = 60m
3. Depth of concrete block = d = J x √3p/m
d = 15√3×0.75/3.5
d = 12.027cm Say 15cm
Purpose of Foundation
- The footing is the primary reason for a structure’s stability. The more stable the structure, the stronger the footing.
- Proper footing design and construction provide a good surface for developing the substructure at an appropriate level and over a firm bed.
- A footing that has been specially designed aids in avoiding lateral movements of the supporting material.
- A proper footing evenly distributes the load on the bed’s surface. This uniform transfer aids in preventing the building from settling unevenly. Differential settlement is an unfavorable construction effect.
- The footing distributes the load from the structure across a large base area to the soil beneath. This load transferred to the ground should be within the soil’s allowable bearing capacity.
Functions of Foundation in Construction
- Ensure the structure’s overall lateral stability
- A footing’s purpose is to provide a level surface on which to build a substructure.
The distribution of the load is uniform. - To stay within the soil’s safe bearing capacity, the load intensity is decreased.
- The effect of soil movement is resisted and avoided
- The construction of a footing addresses the problems with scouring and undermining.
Load Bearing Wall
- Construction of the load-bearing wall is made with cement mortar.
- The ratio of cement to sand in this mortar can range from (1: 3) to (1:6). In standard types of work; this ratio is kept as (1:6).
- The joints of the bricks should be of equal thickness.
- The standard thickness of this joint should be 6 mm (1/4 inch).
- The vertical joints of the continuous layers of bricks should not overlap.
- The thickness of the wall is adjusted according to its load, which can be a full brick length or half a brick thickness.
That is, the wall can be 20 cm (9 inches), 30 cm (13-1/2 inches), 40 cm (18 inches), etc.
The footing of this type of wall is laid wide according to the bearing capacity of the soil. The wall thickness is gradually spread over the width of the footing. For this, the step of the wall is brought equal to the width of the footing by providing offsets equal to the width of the brick 10 cm (4-1/2 inches). The height of each brick step is at least 20 cm (6 inches).
A concrete block is usually provided under the base of the wall. Offsets of 15cm (6 inches) are provided in this concrete block.
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FAQ’s
What is a solid wall foundation?
A solid wall foundation is a type of building footing that consists of a continuous masonry or concrete wall that bears the weight of a structure. This type of footing is often used in areas with stable soil conditions where there is little risk of settling or shifting.
What is the standard wall footing size?
The standard wall footing size depends on soil type, load-bearing capacity, and building type. The width of a single-story building is usually 16-20 inches, and the depth is 8-12 inches. The footing size may need to be larger for multi-story or commercial buildings. It’s best to consult a building professional to determine the appropriate footing size for your project.
What is the spacing for foundation?
The spacing for a footing typically depends on several factors, such as the type of foundation, soil condition, and building codes. The spacing between footings should be at least the width of the wall they will support. For example, if the wall is 12-inch thick, the spacing between the footings should be at least 12 inches
What is basic footing for wall?
A wall’s basic footing is a horizontal layer of concrete or masonry that evenly distributes the weight of the wall and its contents across the soil beneath it. The footing also helps to prevent the wall from settling or shifting over time.
What is the size of brick for footing?
Bricks used to build foundation walls can be any size, but the standard dimensions are 8 inches long, 4 inches wide, and 2 ¼ inches thick. (8″ x 4″ x 2 ¼”). However, the size of the bricks can also be determined by the footing’s specific requirements and design.
