FOUNDATIONS

• Components of a building• Superstructure – placed on top of substructure• Substructure – placed below GL

Need for Foundations

• The elements of the superstructure transfer the loads and

moments to its adjacent element below it.

• Finally all loads and moments come to the foundation

structure.

• Foundation transfers them to the underlying soil or rock

Requirements of Foundations

• To distribute the load of the structure over a large bearing

area so as to bring the intensity of load within the safe

bearing capacity of soil.

• To load the bearing surface at a uniform rate to avoid

differential settlement.

• To prevent lateral movement of supporting soil

• To attain a level and firm bed for building operations

• To increase the ability of the structure as a whole.

Types of Foundation Structures • Two basic types( Acc. To Terzaghi)

• Shallow Foundations ( Depth < Width)• Deep Foundations (Depth > Width )

Shallow foundations

• Shallow foundations are used when the soil has sufficient

strength within a short depth below the ground level

• They need sufficient plan area to transfer the heavy loads

to the base soil.

Types of Shallow foundations

• Spread footing

• Combined footing

• Strap footing

• Grillage Foundation

• Raft/Mat foundation

Types of Shallow foundations• Spread Footing

• Spread the superimposed load over a large area.

• Support either a column or a wall

• Different types of spread footing

• Simple footing

• Sloped footing

• Stepped footing

Types of Shallow Foundations

Combined Footings

• Combined footing is preferred when

• The columns are spaced too closely that if isolated

footing is provided the soil beneath may have a part of

common influence zone.

• The bearing capacity of soil is such that isolated footing

design will require extent of the column foundation to go

beyond the property line.

Types of Combined Footing

• Types of combined footings

• Rectangular combined footing

• Trapezoidal combined footing

• Strap beam combined footing

Rectangular combined footing

• If two or more columns are carrying almost equal loads,

rectangular combined footing is provided

• C.g of column loads should coincide with centroid of

footing area

Trapezoidal combined footing

• If one of the columns is carrying much larger load than the other one, trapezoidal combined footing is provided

Combined Footings

Strap footing

• Strap footing is used to connect an eccentrically loaded

footing to an interior column so that the moment can be

transferred through the beam and have uniform stress

distribution beneath both the foundations.

• preferred over the rectangular or trapezoidal footing if

distance between the columns is relatively large.

Strap Footing

Grillage Footing

• Type of foundation used at the base of column.

• It consist of one, two ore more tier of steel beams

superimposed on layer of concrete.

• Adjacent tiers being placed at right angles to each other

while all tiers are encased in concrete

• Generally used for heavy structure columns piers and

steel stanchions

Grillage Footing

Grillage Footing

Mat/Raft Foundation

• Combined footing provided for all the columns.

• For soft soils strata or site with pockets of weak soil.

• For foundations where differential settlement can be a

major concern.

• In situations where individual footings may touch or overlap

each other.

• Structures like chimneys, silos, cooling towers, buildings

with basements where continuous water proofing is needed

Mat/Raft Foundation

Deep Foundations

• Foundation that transfers load of the structure to a harder

strata deep inside the earth.

• Types of Deep Foundations

• Deep strip( rectangular or square footings)

• Pile foundations

• Pier foundation

• Well foundation

Pile foundations

• Vertical structural element that transfers load to a

hard strata at a lower level

• Adopted in cases when:

• Firm bearing strata is not available at reasonable depth.

• Spread/strip footings become uneconomical.

• Pumping of subsoil water is too costly to permit the

construction of normal foundations.

Classification of Piles

• Two basic types:

• Bored Piles

• Driven Piles

• Based on mechanism of load transfer:

• End bearing pile

• Friction pile

• Combined end bearing and friction pile

• Compaction piles

End Bearing Piles• Used to transfer load through water or soft soil to hard strata.

• Commonly used for multi storey buildings to avoid differential

settlement.

Friction Piles• Transfer loads to a depth by means of skin friction along the length

of pile.• Used in granular soils where hard stratum is at great depths

Compaction Piles

• Used to compact loose granular soils to increase bearing capacity

• Do not carry any load

• May be of timber, bamboo etc.

Classification of Piles

• Based on material used:

• Steel Piles

• Concrete Piles

• Precast piles

• Cast in situ piles

• Wooden Piles

• Composite Piles