COLUMNS AND STRUCTURES 291 



base and bring the toe pressure within the safe maximum pres- 

 sure. This projecting toe is designed as a cantilever beam resist- 

 ing the upward pressure. The slab in the rear is designed as a 

 cantilever beam to carry the weight of the earth on it which 

 forms a part of the wall. 



Sometimes the wall has counterforts along the back at regular 

 intervals, acting as ties. The spacing of these counterforts varies 

 from an interval equal to the height of the wall for walls under 

 fifteen feet in height to one-third the height for walls thirty feet 

 in height, and proportionately for walls more than fifteen and 

 less than thirty feet high. These counterforts have rods running 

 in them from the top of the wall to the back edge of the bottom 

 slab, to reinforce them as cantilever girders carrying the front 

 slab. The front slab is designed as a slab with a span equal to 

 the distance center to center of counterforts, 

 and is reinforced horizontally. 



The wall designed as a cantilever has vertical 

 reinforcement in the front wall and the rein- 

 forcement in the bottom slab and toe is normal 





to the face of the wall. The counterforted 

 wall has vertical and sloping steel in the coun- 

 terforts, but the slab reinforcement is all para- 

 llel with the length of the wall. RestSiJd Wall 



In Fig. 184 is shown, diagrammatically, a 

 retaining wall tied at the top and bottom. This may occur in the 

 case of an area wall pressing against a sidewalk at the top and 

 against a heavy floor at the bottom. It may occur as a wall 

 pressing against a foundation, or floor at the bottom, and having 

 a long girder, or waling, along the top held by ties to deadmen. 



The maximum bending moment is at a point = 0.58 h. It is 



then M 0.64p/i, 



in which p - pressure in pounds per square foot of a fluid. For 

 grain, p - 42; for stone, p - 48; for bituminous coal, p - 24; 

 for anthracite coal, p - 36; for ashes, p - 16; for sand, p - 48; 

 for earth, p - 32 (average). 



The maximum bending moment caused by water is 



M - 4A. 



The fluid weights are not actual weights but merely represent 

 the weight that must be possessed by a fluid which would exert 

 the same pressure against a wall as the material to which it cor- 



