prepared by : prof. alka m. shah civil engineering department institute of technology nirma...

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FOUNDATION Engineering Design of shallow foundation Prepared by : Prof. Alka M. Shah Civil Engineering Department Institute of Technology Nirma University

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Prepared by : Prof. Alka M. Shah Civil Engineering Department Institute of Technology Nirma University Slide 2 W HAT IS F OUNDATION ? 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 2 Slide 3 FOUNDATIONS SHALLOW FOUNDATION D B DEEP FOUNDATIONS D > B T YPES OF F OUNDATION 3 Slide 4 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. D EEP 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. S UITABILITY OF F OUNDATION 4 Slide 5 SHALLOW FOUNDATION Advantages: a) Cost (affordable) b) Construction Procedure (simple) c) Material (mostly concrete) d) Labour (doesnt need expertise) Disadvantages: a) Settlement b) Foundation gets subjected to pullout, torsion etc c) Irregular ground surface(slope, retaining wall) 5 Slide 6 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 Slide 7 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 Slide 8 Wall Footing Wall footing with masonry offsets (Stepped wall footing) (SBC is low) Simple wall footing without masonry offsets (SBC is high) 8 Slide 9 W ALL FOOTING A spread footing for a continuous wall is called strip footing. (Used to support load bearing walls) 9 Slide 10 10 R EINFORCED C ONCRETE FOOTING o Used where the walls are subjected to heavy loads & soil bearing capacity is low Slide 11 ISOLATED COLUMN FOOTING Used to support individual columns. 11 Slide 12 ISOLATED COLUMN FOOTING 12 Slide 13 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 13 Slide 14 COMBINED FOOTING Rectangular footingTrapezoidal footing 14 Slide 15 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 15 Slide 16 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 16 Slide 17 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 17 Slide 18 RAFT / MAT FOUNDATION 18 Slide 19 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 19 Slide 20 F ACTORS A FFECTING F OUNDATION S ELECTION FACTORS 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. Slide 21 21 Slide 22 22 Slide 23 23 PERMISSIBLE DIFPERENTUL SRTTLEMENTS AND TILT IS:1904 Slide 24 B ASIC 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 Slide 25 D ESIGN OF FOUNDATION INCLUDE 1. Soil Design 2. Structural Design Slide 26 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 Slide 27 THANK YOU 27