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9 <br /> r <br /> Footings 437 ✓ <br /> umn of 0.250f.1, offsets Preliminary design often precedes field tests and in this case the t <br /> ite strength design the designer needs some guide to the probable bearing values of the soil. <br />'ACI Code, Sec. 2306f) This can be found in the allowable bearing values given in local build- <br /> All of the compressive ing codes or in the American Standard Building Code Regitirenzents <br /> arried to the footing by for Excavations and FowndationS3 of the ASA. Presumptive Unit <br /> M the bearing value of Soil Bearing Values, as given in the New fork State Building Code r <br /> '. <br />:comer excessive, a pile Manual,* are shown in Appendix C-5. They differ only slightly from <br /> the values of the ASA Code. Rock of various types is listed with '. <br /> Then the outer row of bearing values ranging from 15 to 100 tons per sq ft (tsf) ; sands run ' <br /> a single footing cannot from 2 tsf for fine sand to 6 tsf for gravel or sand-gravel combina- <br /> Ching on the adjoining tions; clay runs from 1 tsf for soft clay to 4 tsf for hard clay, and silt <br /> ting.so that the center is allowed a value of 0. Is the clay medium or soft? How fine is { <br /> atroid of the footing. fine sand,.and what percentage of it is present? These and similar <br /> ill be best depends on questions show that, although the Code values may be very useful, <br /> it is difficult to determine with accuracy what soil pressure to use. <br /> The presumptive values of soil strength are for undisturbed soils <br /> loaded at the surface or where there is no lateral support, as at the <br /> right-hand side of the footing shown in Fig. 15-1a. When a footing <br /> for different types of at or near the surface is loaded and pressed downwards, the adjacent <br /> mane combinations of soil flows outward and up, a surface bulge appearing near the founda- # <br /> tent of the soil has an tion. If the footing is located at the bottom of a 6-ft pit, the tendency <br /> foes the location of the of the soil adjacent to the footing to bulge upwards is resisted by the <br /> Le footing. When the 6-ft column of soil directly above it; an increase in the bearing <br /> capacity of the soil results. When computing the m m aximu pressure <br /> zdth of the footing, a =f <br /> Point to be considered ' on the soil, the weight of the backfill above the footing is often <br /> dry season. An en- omitted, the assumption being that the presence of the backfill will s <br />.an determine bearing increase the allowable soil pressure above that at the surface by an <br /> samples which have amount at least equal to the weight of the backfill. <br /> s may also be used to - <br /> pensive and could not 15-3 Settlement <br /> nlso be determined by <br /> to drive a standard The settlement of a loaded footing resting on sand or on gravel <br /> and penetration tests, takes place quickly; when resting on clay, settlement may continue for Fey <br /> quipment, are not ez-_ many months. Placing a load on clay forces out the water held ,_. <br /> point, on the job site i between the very small, flake-shaped mineral particles composing <br /> ons. These tests dis- the clay, and it is this slow flow that causes settlement. The design <br /> ing several feet below + of a foundation that may be expected to have a limiting total settle- ~' <br /> the relation between ment is a problem for a foundation engineer and is beyond our <br /> Ld the allowable bear- present scope. The differential settlement will be more important <br />;o Karl Terzaghi and than total settlement in most cases. When all footings carry the j <br /> Id T. H. Thornburn.2 same load intensity, the differential settlement will be very small. <br /> inexperienced. To keep the differential settlements small the footings should be <br /> a <br /> 1 <br /> r <br /> MWFOR—M <br /> Y P NEW <br /> J� <br /> _ J <br />