Common Foundation Types
Masonry Foundation (clause 7.2.8 of NBC 2005)
Unreinforced foundation may be of concrete or masonry (stone or brick) provided that angular spread of load from the base of column/wall or bedplate to the outer edge of the ground bearing is not more than 1 vertical to 1/2 horizontal for masonry or 1 vertical to 1 horizontal for cement concrete and 1 vertical to 2/3 horizontal for lime concrete. The minimum thickness of the foundation of the edge should not be less than 150 mm. In case the depth to transfer the load to the ground bearing is less than the permissible angle of spread, the foundations should be reinforced.
Main two types of masonry used in buildings are:
- Stone Masonry.
- Brick Masonry.
Stone masonry: Masonry units constructed with stones.
(a) Rubble masonry.
(b) Ashlars Masonry / Coursed Rubble masonry.
(a) Random Rubble Masonry
- Random Rubble masonry, in CM 1:5 may be adopted in hilly areas as also in plains where the cost of Random Rubble masonry is economical compared to 340 mm thick brick masonry.
- For both RR & CR Masonry, bond stone or through stone should be provided at every 1.5 m to 1.8 m, in each course.
- A set of 2 or more bond stone overlapping each other by at least 15 cm shall be provided in a line from face to back.
- Each bond stone or a set of bond stone shall be provided for every 0.5 sqm of the wall face.
(b) Brick Masonry
DESIGN EXAMPLE: Foundation for a single storey Building
|Maximum width required`||= 2 x width of the wall + 0. 3m|
|(Assuming 23cm thick brick wall)||= 2 x 0.23 + 0.3 m. = 0.76m.|
|Minimum depth required||= 2|
|P- Intensity of loading||= 125 KN/m2|
|W-Unit weight of soil||= 18 KN/m3|
|Ø-Angle of repose of the soil||= 30|
|Minimum depth of footing||= (125/18) x 0.111 = 0.77m.|
|Assuming bearing capacity of soil||= 200 Kn/sqm|
|Width required||= 125/200 = 0.625m|
Thus provide a foundation width of 0.76m and depth 0.77m.
The purpose of a masonry foundation is to support the weight of a structure while distributing the weight across subsurface strata and to act as an anchor to keep the structure in place.
Table from NBC 2005 Thickness of Footings
|Sl No.||Type of Footings||The thickness of Footings (Min)||Remarks|
|i)||Masonry||a) 250 mm||Select the greater of the two values|
|b) Twice the maximum Projection from the face of the wall|
|ii)||Plain concrete For normal structures||a) 200 mm|
|b) Twice the maximum offset in a stepped footing||For footings resting on top of the pile|
|c) 300 mm|
|For lightly loaded structures|
|For footings resting on soil Resting on soil|
|a) 150 mm||Resting on pile|
|b) 200 mm|
|iii)||Reinforced Concrete||a) 150 mm||Resting on soil Resting on pile|
|b) 300 mm|
RCC Isolated Foundation
Isolated foundations are used to support an individual point load from columns. They may be circular, square or rectangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched, if they are required to spread the load from a heavy column. Isolated foundations are usually shallow, but deep pad foundations can also be used.
Tags: Common foundation types, stone masonry foundation, Brick masonry foundation, RCC isolated foundation, Masonry foundation, Foundation types & Design, Details of foundation.