UNCERTAINTY AND RELIABILITY - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 130

of the resistance (see equations 12.3 and 12.10): R d = R/ R and R = B 2 (( + 2) c ud s c
i c + q 0 d ) = B 2 (5.14 c ud (1.2)(1) + q 0 d ), and c ud = cu/ M and q 0 d = q 0 / R . Next, the
safety condition S d < R d is checked for the various design approaches.

DA1/1
S d = 1.35(1027) + 1.5(180) = 1656 (kN)
c ud = 30 / 1.0 = 30 kPa

q 0 d = (18 / 1.0) (0.5 + 0.5) = 18 kPa
R d = R / 1.0 = (65.1(30) + 10.65(18) = 2145 kN
The requirement S d < R d is amply satisfied (over-design of 29.5%).

DA2/2
S d = 1.0(1027) + 1.3(180) = 1261 kN
c ud = 30 / 1.4 = 21.4 kPa

q 0 d = (18 / 1.4) (0.5 + 0.5) = 12.8 kPa
R d = R / 1.0 = 65.1(21.4) + 10.65(12.8) = 1529 kN
The requirement S d < R d is amply satisfied (over-design of 21.3%).

DA2
S d = 1.35(1027) + 1.5(180) = 1656 (kN)
c ud = 30 / 1.0 = 30 kPa

q 0 d = (18 / 1.0) (0.5 + 0.5) = 18 kPa
R d = R / 1.4 = (65.1(30) + 10.65(18)) / 1.4 = 1532 kN
The requirement S d < R d is not satisfied (under-design of 7.5%).

DA3
S d = 1.35(1027) + 1.5(180) = 1656 (kN)
c ud = 30 / 1.4 = 21.4 kPa

q 0 d = (18 / 1.0) (0.5 + 0.5) = 18 kPa
R d = R / 1.0 = 65.1(21.4) + 10.65(18) = 1585 kN
The requirement S d < R d is not satisfied (under-design of 4.3%).

The design does not satisfy the DA2 and DA3. A wider footing will do. Which
one?