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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