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Reinforced Concrete Walls 129

wall, earth, base and cantilever

REINFORCED CONCRETE WALLS 129. Types of Reinforced Concrete Retaining Walls.—There are two types of reinforced concrete retaining walls in common use: 1. The cantilever type and 2, the counterforted type.

Both of these depend upon the weight of earth carried by the base of the wall to prevent overturning. They differ in the way in which the face wall is attached to the base.

A cantilever wall is shown in Fig. 65, consisting of a vertical stern attached to a base, ACFB. The weight of the mass of earth BFEG, rests upon the base of the wall BIB' and serves to assist the wall in resisting the overturning moment of the earth thrust. The hori zontal pressure of earth on EF is carried by the vertical stem CDEF acting as a cantilever beau. The projecting bases FB and AC are also cantilever berms, the one supporting the weight of earth resting upon it, the other resisting the upward thrust of the foundation at the toe of the wall.

A coun-tcrfortcd wall is shown in Fig. 66. The face wall CDEF is connected with the base ACFB by narrow counterforts EFB, spaced several feet apart. The counterforts are cantilever beams, each carrying the horizontal earth thrust on the face wall EF for a panel length of wall. The face walls CDEF are slabs holding the earth pressure between counterforts and transferring the pressure to the counterforts. The base FB is a slab carrying the weight of earth FEGB boween counterforts and holding down the ends of the counterforts. The base AC at the front of the wall is a cantilever carrying the upward thrust of the foundation at the toe of the watt.

The cantilever type is commonly used for moderate heights of wall. For walls more than 20 or 25 feet high, the counterforted wall is usually more economical. The quantities of materials required for a counterforted wall are less and the amount of form work more than for a cantilever wall.

130. Design of Cantilever Wall.—In designing reinforced con crete walls, the thrust in the vertical section of earth passing through the inner edge of the base may be computed by lianlcine's formula, as given in Section 123: Values of K may he taken from Table XVIII. This thrust is parallel to the upper surface of the earth and its horizontal and vertical components are H=E cos K cos i, and =K sin i.

The method of design will be illustrated by numerical Example 3.—I)esign a retaining wall to hold a level hank of earth 16 feet high. The base of footing is to be :3 feet below surface of ground and the pressure on the soil is limited to 4000 lb. The hacking is ordinary soil with angle of friction of Earth weighs 100 pounds and concrete 150 pounds per cubic foot. Unit stresses will be based upon use of 2000 pounds concrete and plain bars of medium steel.

Solulion.—Assume the base under the wall 12 inches thick. The height of the wall above the base is then 18 feet, and the hori zontal thrust, taking K from Table XVIII,