Types of Building Foundations

Foundations are basically of two (02) types:

1) Shallow Foundation

2) Deep Foundation

1) Shallow Foundation: According to Terzaghi, if the depth of foundation is less than or equal to its width, then it is called a Shallow Foundation. This type of foundation transfer building load to the earth very near to the surface.

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Shallow Foundations are:

i) Isolated Footing: This is the most common type of foundation used for building construction and also most economical one. They are most commonly square and rectangular is size.

ii) Combined Footing: Combined footing support more than one column in a row. Combined footings are constructed when two or more columns are close enough and their isolated footing overlaps one-another.

iii) Strap Footing: A strap footing consists of two isolated footings connected with a structural strap or a lever. The strap connects the two footing such that they behave as one unit. The strap simply act as a connected beam and does not take any soil reaction.

iv) Mat or Raft Foundation: A mat or a raft is a large slab supporting a number of columns and wall under the entire or large part of a structure. Mat foundation required when the allowable soil pressure is low or where the columns or walls are so closed that individual footings overlap or touch each-other.

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v) Wall Footing: This type of foundation runs along the direction of the wall and helps to transmit the load of the wall into the ground.

2) Deep Foundation: According to Tarzaghi, if the depth of foundation is greater than its width. then it is called Deep Foundation. A deep foundation is generally much more expensive than a shallow foundation.

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Deep Foundations are:

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i) Pile Foundation : A pile is a slender structural member made of steel, concrete or wood. A pile is either driven into the soil or formed in-situ by excavating a hole a filling it with concrete.

For details :Classification of Piles

ii) Pier Foundation: A pier is a large diameter concrete cylinder build in the ground. The difference between a drilled pier and a bored pile is basically of size.

iii) Well Foundation: A well foundation is similar to an open caisson. Well foundation can be constructed on a dry bed or after making a sand island.

Lapping: In RCC structure, placing of a single bar may not meet the required length of the section. So it becomes necessary to join two bars, to meet the desire length. To join two bars, one bar should overlap another and it is called lapping of bar and the length overlap each other is caller lap length or overlap length.

Lap length is different for different concrete section.

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Calculation of Lapping Length:

Lapping is not permitted for reinforcement greater than No.11

lapping length for longitudinal reinforcements are as follows:

for Class A Splice: Lapping length will be the greater of 1.0Ld and 12 inch.

for Class B Splice: Lapping length will be the greater of 1.3 Ld and 12 inch.

Table: Splice Type

* Here Ld is the development length.

Lapping length for Tension Reinforcement (same as Longitudinal Reinforcement):

for Class A splice: Lapping length will be the greater value of

1.0 Ld and 12 inch.

for Class B splice: Lapping length will be the greater value of 1.3 Ld and 12 inch.

Here Ld is the development length.

Compression Lapping length of No.11 or smaller deformed bar

Shall be calculated as :

(a) for fy </= 60,000 psi lapping length is greater of 0.0005 fy*db and 12inch.

(b) for fy > 60,000 psi, lapping length is greater of (0.0009 fy-24)db and 12 inch.

Note: for fc'<3000 psi the length of lapping will be increased by one-third.

compression lapping should not permitted for bar greater than No.11 except permitted at code.

all informations are from ACI Code

Recalling Civil Engineering