TERZHAGI’ BEARING CAPACITY

ULTIMATE COHESION EFFECTIVE OVERBURDEN UNIT WEIGHT AT THEPRESSURE BOTTOM OF THE FOOTING

q q q q

A. GENERAL SHEAR FAILURE

SQUARE FOOTING

1.3 ' 0.4u c qq c N qN BN

CIRCULAR FOOTING

1.3 ' 0.3u c qq c N qN DN

CONTINUOUS FOOTING

' 0.5u c qq c N qN BN

B. LOCAL SHEAR FAILURE

1

2' '

3

2' tan tan '

3

c c

Problem 1

What will be the gross and net allowable bearing pressure of

sand having Ø’ = 35°, GS = 2.70, e = 0.65, and w = 20% under

the following cases: (a) size of footing 1.2 m square, (b)

circular footing of 1 m diameter and (c) 1.5 m wide strip footing.

The footing is placed at the depth of 1.5 m below NGL and the

water table is at a great depth. Use FS = 3. Compute the by

Terzaghi’s general shear failure theory.

EFFECT OF GROUNDWATER

Case I: Water table is above the bottom of the footing (a)

'

f

sat water

q D D D

Case II: Water table is at the bottom of the footing (b)

f

sat water

q D

Case III: Water table is below the bottom of the footing ©

fq D

1

'ave

ave

D B D D BB

D B

DESIGN OF ISOLATED SQUARE FOOTING

Problem 2

Design an interior isolated square footing that carries the

following load as transmitted by the 400 x 400 mm column

pedestal with 8 – 16 mm diameter bars distributed equally in all

sides:

D = 70 kN L = 58 kN

E = 34 kN W = 49 kN.

The footing is placed 1.5 m below the NGL. Use γsoil = 18.2

kN/m3, γconcrete = 23.54 kN/m

3, f’C = 28 MPa, and fy = 414 MPa.

Use 20 mm diameter for main reinforcements. The allowable

soil bearing capacity is 150 kPa.

Problem 3

Design a 300 mm thick interior isolated square footing that

carries the capacity of a 350 mm x 350 mm first floor concrete

column reinforced with 12 – 16 mm diameter distributed all

sides. Assume that the thickness is adequate to resist one and

two way shear. Assume that footing carries 100% axial loads

only

The footing is placed 1.8 m below the NGL. Use γsoil = 17.5

kN/m3, γconcrete = 23.54 kN/m

3, f’C = 21 MPa, and fy = 414 MPa.

Use 20 mm diameter for main reinforcements. The allowable

soil bearing capacity is 180 kPa

DESIGN OF ISOLATED RECTANGULAR FOOTING

Problem 4

The center of one of the column of MIT Intramuros is 1.3 m

from the property line. Design the footing for that particular

column given the following parameters: The 400 x 400 mm

column carries a dead load of 650 kN and a live load of 700

kN. The footing is placed 1.7 m below the NGL. The allowable

soil bearing capacity is 200 kPa. Use γsoil = 17.5 kN/m3, γconcrete

= 23.54 kN/m3, f’C = 21 MPa, and fy = 414 MPa. Use 25 mm

diameter for main reinforcements.