Design of Reinforoed-Oonorete Retaining Walls

If this wall were founded upon the soil and not upon piles, the maximum pressure on the soil, by equation 22, page 473, would be: and the minimum pressure would be 42 lb. per sq. ft.

The tendency to slide is 55,770 lb., and the resistance to sliding, irrespective of the piles, for a coefficient of friction of 0.50 is: 306,590 X 0.50 = 153,295 lb. Hence the wall is abundantly safe against this method of failure.

The Reinforcement. The curtain wall is really a slab sup ported at its two vertical edges and at the bottom; but it is customary to design it as being composed of independent horizontal beams fixed at their ends, the error being on the safe side. Rods will be needed near the middle of the bay on the front of the wall to take the direct moment, and at the back of the wall near the counterfort to take the contrary moment. The former are usually made con tinuous, but the latter may be comparatively short. The reinforcing rods are sometimes so bent that a single rod serves both purposes, its end being near the back of the wall next to the counterfort and its middle portion being near the face of the wall midway between the counterforts.

The footing or floor of the bay may be regarded as being made up of horizontal beams fixed at the ends, carrying the downward weight of the earth upon their upper face and the upward reaction of the soil on their lower face. The reaction of the soil below the footing in creases from D toward C, and is computed as in f 1041.

The reinforcement in the counterforts which ties together the face wall and the footing may be placed either vertically and horizontally, or diagonally. The first is the more common, but the latter is the more scientific and the more economical of material.

According to the first method, the counterfort may be regarded as a cantilever anchored to the footing and also as a T-beam the flange of which is the curtain wall. Under this assumption the horizontal rods bind together the curtain wall and the counterfort, and the vertical rods connect the footing and the counterfort, and the reinforcement parallel to the long side of the counterfort resists the bending of the counterfort. For an example of this method of

construction, see Fig. 124, page 531. The amount of steel required at any point of the free edge of the cantilever can be determined approximately by erecting a perpendicular at the point and taking moments about the point where this perpendicular intersects the center line of the front wall.

The second method is to regard the counterfort

as being made up of diagonal beams, each carrying one or more longitudinal rods which tie the vertical curtain wall to the footing, these beams being wider at their junction with the front wall than at their connection with the footing. For example, for each foot of width on the footing, these tie-beams will be 30 _ 9 = 3.3 ft. deep at the front wail. The equivalent liquid pressure against each successive 3.3-foot section of the front wall can readily be computed, and from that the area of the steel required to resist this stress can easily be determined. The rod at its connection with the curtain wall may be diagonal, or may start horizontal and be curved to a direction parallel to the outer edge of the counterfort on a radius of about 20 times its diameter since with this curvature the side pressure of the rod upon the con crete will not exceed a safe limit. At the lower end, the rod may be diagonal or may start vertically and be curved to a line parallel to the outer edge of the counterfort to the same radius as above. The vertical pressure is always greater than the horizontal, and hence there is plenty of resistance to hold the lower end of these rods. The ends of these rods should be looped around the reinforcing rods in the footing and the face, or, better, should be passed through a plate or an angle to give sufficient anchorage. For an example of this form of construction, see Fig. 125, page 532.

Both horizontal and vertical temperature reinforcement should be placed in the face of the curtain wall (see i 1050).