Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.113
FIGURE 9.46 Design conditions for combined spread footing of trapezoidal shape.
moment about one axis, the factored soil pressure q s can be assumed to have a
linear distribution (Fig. 9.46b) with a maximum at the more heavily loaded edge:
and a minimum at the opposite edge:
Pu Puec qs1 � � (9.93)
A I
ƒ ƒ
Pu Puec qs2 � � (9.94)
A I
ƒ ƒ
where P u � factored vertical load
e � eccentricity of load
c � distance from center of gravity to section for which pressure is being
computed
I ƒ � moment of inertia of footing area
If the resultant falls outside the kern of the footing, the net factored pressure q s can
be assumed to have a linear distribution with a maximum value at the more heavily
loaded edge and a lengthwise distribution of 3 times the distance between the
resultant and the pressed edge. The balance of the footing will have no net factored
pressure.
With the net factored soil-pressure distribution known, the factored shears and
moments can be determined (Fig. 9.46d and e). Critical sections for shear and
moment are shown in Fig. 9.46a. The critical section for two-way shear is at a
distance d/2 from the columns, where d is the effective depth of footing. The
critical section for one-way shear in both the longitudinal and transverse direction
is at a distance d from the column face.