266 PRACTICAL STRUCTURAL DESIGN 



WL Wa 2WL Wa __ W(L - a) _ Wy 

 48 88 8 4 ' 



since (L - a) = 2y. 



Not all authorities agree that the above formula for the design 

 of footings is correct, the contention being that the maximum 

 moment is at the face of the wall, and not under the center. The 

 following formula is used for this assumption : 



- "V 



2y + a " 2 2(2y + a) 



To design a wall footing let w = weight per lineal foot of wall 

 and divide this by the safe soil load per square foot. This will 

 give the width L of the footing. The bending moment being ob- 

 tained the total thickness 

 will be t and the total off- 

 set is o, in the formula for 

 stepped footings. This will 

 ;he thickness at the 



, . y face of the wall and it may 



be stepped off by dividing 

 Fig. 172 Design of Stepped Footings ., . , , , . 



it into any number of steps 



each with a thickness, t, and solving for o by the formula for 

 each step. 



Such footings are seldom so designed. The usual way is to 

 ascertain the width and then from the edge of the bottom of the 

 wall draw a line at an angle of 45 degrees, or 60 degrees, with 

 the horizontal until the horizontal distance separating the lines 

 is equal to the spread of the footing. Steps are then drawn to 

 touch this line, as shown in Fig. 172. 



The real use made of the formulas for bending moment on 

 footings is to determine the size of I beams to use in a grillage 

 foundation, or the thickness and reinforcement for a reinforced 

 concrete footing. The design of grillage footings is given in the 

 steel handbooks. A reinforced concrete footing is designed as a 

 slab one foot wide, the thickness being fixed both by shear and 

 bending. 



Column footings differ from wall footings in being square or 

 rectangular. If the footing is square the area is found by dividing 

 the total load by the allowable soil pressure. The square root of 

 this area gives the length of each side. If the side of the column 



