Page 1 Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Objectives In this section you will learn the following introduction Basic definitions Presumptive bearing capacity Page 2 Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Objectives In this section you will learn the following introduction Basic definitions Presumptive bearing capacity Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] 17. Bearing capacity : It is the load carrying capacity of the soil. Basic definitions Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will cause shear failure of the supporting soil immediately below the footing. Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of the footing (concrete) and soil ( ) are close enough to be considered equal, then where, is the depth of the footing, Ref. fig. 4.7 Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two types, Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil considering only shear failure. It is given by, Page 3 Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Objectives In this section you will learn the following introduction Basic definitions Presumptive bearing capacity Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] 17. Bearing capacity : It is the load carrying capacity of the soil. Basic definitions Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will cause shear failure of the supporting soil immediately below the footing. Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of the footing (concrete) and soil ( ) are close enough to be considered equal, then where, is the depth of the footing, Ref. fig. 4.7 Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two types, Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil considering only shear failure. It is given by, Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Safe gross bearing capacity ( ): It is the maximum gross pressure which the soil can carry safely without shear failure. It is given by, Allowable Bearing Pressure: It is the maximum soil pressure without any shear failure or settlement failure. Fig. 4.7 Bearing capacity of footing Page 4 Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Objectives In this section you will learn the following introduction Basic definitions Presumptive bearing capacity Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] 17. Bearing capacity : It is the load carrying capacity of the soil. Basic definitions Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will cause shear failure of the supporting soil immediately below the footing. Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of the footing (concrete) and soil ( ) are close enough to be considered equal, then where, is the depth of the footing, Ref. fig. 4.7 Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two types, Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil considering only shear failure. It is given by, Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Safe gross bearing capacity ( ): It is the maximum gross pressure which the soil can carry safely without shear failure. It is given by, Allowable Bearing Pressure: It is the maximum soil pressure without any shear failure or settlement failure. Fig. 4.7 Bearing capacity of footing Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Presumptive bearing capacity : Building codes of various organizations in different countries gives the allowable bearing capacity that can be used for proportioning footings. These are â€œPresumptive bearing capacity values based on experience with other structures already built. As presumptive values are based only on visual classification of surface soils, they are not reliable. These values don't consider important factors affecting the bearing capacity such as the shape, width, depth of footing, location of water table, strength and compressibility of the soil. Generally these values are conservative and can be used for preliminary design or even for final design of small unimportant structure. IS1904-1978 recommends that the safe bearing capacity should be calculated on the basis of the soil test data. But, in absence of such data, the values of safe bearing capacity can be taken equal to the presumptive bearing capacity values given in table 4.1, for different types of soils and rocks. It is further recommended that for non-cohesive soils, the values should be reduced by 50% if the water table is above or near base of footing. Table 4.1 Presumptive bearing capacity values as per IS1904-1978. Type of soil/rock Safe/allowable bearing capacity (KN/ m 2 ) Rock 3240 Soft rock 440 Coarse sand 440 Medium sand 245 Fine sand 440 Soft shell / stiff clay 100 Soft clay 100 Very soft caly 50 Page 5 Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Objectives In this section you will learn the following introduction Basic definitions Presumptive bearing capacity Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] 17. Bearing capacity : It is the load carrying capacity of the soil. Basic definitions Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will cause shear failure of the supporting soil immediately below the footing. Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of the footing (concrete) and soil ( ) are close enough to be considered equal, then where, is the depth of the footing, Ref. fig. 4.7 Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two types, Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil considering only shear failure. It is given by, Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Safe gross bearing capacity ( ): It is the maximum gross pressure which the soil can carry safely without shear failure. It is given by, Allowable Bearing Pressure: It is the maximum soil pressure without any shear failure or settlement failure. Fig. 4.7 Bearing capacity of footing Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Presumptive bearing capacity : Building codes of various organizations in different countries gives the allowable bearing capacity that can be used for proportioning footings. These are â€œPresumptive bearing capacity values based on experience with other structures already built. As presumptive values are based only on visual classification of surface soils, they are not reliable. These values don't consider important factors affecting the bearing capacity such as the shape, width, depth of footing, location of water table, strength and compressibility of the soil. Generally these values are conservative and can be used for preliminary design or even for final design of small unimportant structure. IS1904-1978 recommends that the safe bearing capacity should be calculated on the basis of the soil test data. But, in absence of such data, the values of safe bearing capacity can be taken equal to the presumptive bearing capacity values given in table 4.1, for different types of soils and rocks. It is further recommended that for non-cohesive soils, the values should be reduced by 50% if the water table is above or near base of footing. Table 4.1 Presumptive bearing capacity values as per IS1904-1978. Type of soil/rock Safe/allowable bearing capacity (KN/ m 2 ) Rock 3240 Soft rock 440 Coarse sand 440 Medium sand 245 Fine sand 440 Soft shell / stiff clay 100 Soft clay 100 Very soft caly 50 Module 4 : Design of Shallow Foundations Lecture 17 : Bearing capacity [ Section17.1 : Introduction ] Recap In this section you have learnt the following Introduction Basic definitions Presumptive bearing capacityRead More

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