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,ar <br /> t ` <br /> Footings 489 <br /> - i <br /> In general, rock is considc red the best foundation material; graded - <br /> sand and gravel are good materials; fine particles of sand and silt are <br /> 18 generally questionable; and clay should be studied carefully. To give an <br /> idea of the relative bearing capacity which might typically be expected, a <br /> table of values given by the Wisconsin Building Code, Sec. 53.02, 1956, <br /> is presented in Table 18.2.1. <br /> TABLE 18.2.1 <br /> Footings ! soil (tons <br /> Capacity <br /> t (tons per sq tt) <br /> Quicksand and alluvial soils t <br /> Soft clay j <br /> Ordinary clay and sand together in <br /> layers,wet and spongy 2 <br /> Clay or fine sand,firm and dry 3 <br /> umn loads, wall Sand,compact and well cemented 4 <br /> test loads are to 4 Gravel and coarse sand,well packed <br /> irevent excessive Hardpan or shale 6 <br /> and to provide Rock Not more than <br />' 20%of ultimate <br /> crushing strength <br /> 18.3. TYPES OF FOOTINGS 1 ` <br /> safe bearing ca- i Most building footings may be classified as one of the following types <br /> s not within the (Fig. 18.3.1): <br /> bearing capacity 1) Isolated spread footings under individual columns. These may be `d <br />:termined by the square, rectangular, or occasionally circular in plan. " ? <br />)n with existing 2 Wall footings,either flat or -stepped,Peed, which support bearing walls. <br /> examination of 3) Combined footings supporting two or more column loads. These <br />)n of the science may be continuous with a rectangular or trapezoidal plan or they may be <br /> various sources isolated footings joined by a beam. . The latter case is referred to as a <br /> strap,or cantilever, footing. i <br /> concerning soil i 4) A mat foundation, which is one large continuous footing sup- <br /> e most common porting all the columns of the structure. This is used when soil condi- <br /> tions are poor but piles are not used. <br /> ch depend upon 5) Pile caps, structural elements which tie a group of piles together. <br /> I sedimentation These may support bearing walls, isolated columns, or groups of several <br /> columns. <br /> under load are <br /> c <br /> 18.4. TYPES OF FAILURE OF FOOTINGS [ <br /> The procedures used for the design of footings in the United States <br /> material which are based primarily on the work of Talbot in 1907 (I], Richart in 1946 <br /> (21,and Moe in 1957-59[3). <br /> h as rock heaps, 1 The work of Moe is the most recent complete study of the shear J <br /> 1 strength of slabs. He clearly defines the several types of failure which it <br />