prepared by : prof. alka m. shah civil engineering department institute of technology nirma...
TRANSCRIPT
FOUNDATION Engineering
Design of shallow foundation
Prepared by :
Prof. Alka M. ShahCivil Engineering Department
Institute of TechnologyNirma University
WHAT IS FOUNDATION ?
Lowest part of a structure directly in contact with the ground and transmits all the loads to the sub-soil below it
Majority of structure fails due to failure of their foundation
Major purpose is to transmission of load in such a way soil is not over-stressed and does not undergo deformation
PURPOSE
To support structure Distributes the loads over a larger area Minimizes the differential settlements Increases stability & prevents overturning Distribute non-uniform load uniformly to the soil
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SHALLOW FOUNDATION
Bearing capacity of soil is more. ground water table(W.T) is low. dewatering of foundation is not required. top layers of soil are uniform and stable. Load on the structure is less.
DEEP FOUNDATION Bearing capacity of soil is low. ground water table(W.T) is high. dewatering of foundation is costly and difficult. top layers of soil are non uniform and unstable. Load on the structure is more.
SUITABILITY OF FOUNDATION
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SHALLOW FOUNDATION
Advantages:
a) Cost (affordable)b) Construction Procedure (simple)c) Material (mostly concrete)d) Labour (doesn’t need expertise)
Disadvantages:
e) Settlementb) Foundation gets subjected to pullout, torsion
etcc) Irregular ground surface(slope, retaining
wall)
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TYPES OF SHALLOW FOUNDATION
1) SPREAD FOOTING i) Continuous footing (strip or wall) ii) Isolated column footing a) Square b) Circular c) Rectangular iii) Reinforced concrete footings
2) STRAP FOOTING
3) COMBINED FOOTING i) Rectangular ii) Trapezoidal
4) RAFT FOUNDATION 6
1) SPREAD FOOTING
Carries a single column
Used to spread out loads from walls of columns over a
wider area
Consists of concrete slabs located under each structural
column and a continuous slab under load-bearing walls
Commonly used due to their low cost & ease of
construction
Used in small to medium size structures with moderate to
good soil condition
For high rise buildings it is not efficient 7
Wall Footing
Wall footing with masonry offsets (Stepped
wall footing) (SBC is low)
Simple wall footing without
masonry offsets (SBC is high)
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WALL FOOTING
A spread footing for a continuous wall is called strip footing. (Used to support load bearing walls)
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REINFORCED CONCRETE FOOTING
o Used where the walls are subjected to heavy loads & soil bearing capacity is low
2) COMBINED FOOTING
Supports two columns the two columns are so close to each other that
their individual footings would overlap one column is placed right at the property line C.G. of column load and centroid of the footing
should coincide
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3) STRAP FOOTING
Two isolated footings connected with a strap or beam Connects in such a way that they behave as one unit The strap acts as a connecting beam and does not take any
soil reaction. The strap is designed as rigid beam. It does not provide bearing
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to connect an eccentrically loaded column footing to an interior column
Used to transmit the moment caused from an eccentricity to the interior
Can be used instead of combined footing if the distance between two columns is large
More economical than combined footings
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4) RAFT FOUNDATION
Large reinforced concrete slab supporting no. of columns & walls
area of spread footings or combined footings exceeds about 50 percent of the gross area of the building
Greater weight & continuity of a mat provides sufficient resistance
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Raft/Mat Foundation is suitable under following conditions
Structural loads are heavy or the soil condition is poor Soft or loose soils having less bearing capacity Foundation soil is non-homogeneous and prone to excessive
differential settlements Lateral loads are non-uniform Columns are so close that their individual footings would
overlap To resist water pressures when groundwater extends above
the floor level of the lowest basement of a building There is a large variation in the loads on individual columns
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FACTORS AFFECTING FOUNDATION SELECTIONFACTORS AFFECTING FOUNDATION CHOICE:
Sub-surface soil Ground water table conditions Type of structure Magnitude of load Tolerances of structure i.e. permissible values of settlement and tilt etc.
BASIC REQUIREMENT OF FOUNDATION
Foundation must be properly located considering any future influence which could adversely affect its performance
Soil supporting must be safe against shear failure Foundation must not settle or deflect Should safe against sliding and overturning
Geotechnical Shallow Foundation Design
Soil Design of foundation Include:
Loads coming on foundation W (DL, LL, etc.)
Determination of Allowable Bearing pressure of soil, qa
Determination of area of foundation
Area = [W + Self weight of foundation(10% of W)] / qa
Layout of foundation: such a way that C.G. of foundation coincides
with C.G. of load
Upward soil pressure on footing p = W/A < A. B. P