1983-11-29_REVISION - C1981017 - Laserfiche WebLink

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'rag MIAMI& 1 436 Essentials of Structural Design ' compared with the allowable stress in the column of 0.250f ' offsets Preliminary design ofte, ' the difference in concrete strengths; in ultimate strength design the designer needs some guide allowable bearing stress in the footing is 1.0 (ACI Code, See. 2306f) 'fliis can be found in the a } times that used in «orlon tress deli« ing codes or in the _Intern -s n. If all of the compressive or Excavations and Fou s load in,the concrete of the column cannot be carried to the footing by f { bearing, extra dowels must be provided. When the bearing value of Soil Bearing �'ahtes, as in A p the soil is so low that the size of the footing becomes excessive, a pile lllantcal,' are shown `; `• footing (Fig. 15-1d) is used. the values of the A?A G i A combined footing (Fig. 15-1c) is used when the outer row of bearing values canning fry t columns is so close to the property line, tIiat. a single footing cannot from 2 tsf for fine sand ' be built for the outer columns «•ithout encroaching on the adjoining tions; clay runs from i is property. It is desirable to proportion the footing, so that the center is allowed a value of 0. of gravity of the loads will frill at the centroid of the footing. fine sand, and what pore Whether a rectangular or trapezoidal shape will be best depends on questicns show that. altl. the relative size of the column loads. it is difficult to determine ' The presumptive value loaded at the surface or 15-2 Bearing Strength of Soils right-hand side of the fo The allowable bearing strength varies widely for different types of at or near the surface is soil, such as silt, clay, sand, gravel, and the many combinations of soil flows out"-ard and ul these found at the job site. The moisture content of the soil has an tion. If the footing is lo- ' of the soil adjacent to tl. important influence on its Learing strength, as does the location of the t;_ft column of soil dire water table with respectto the bottom of the footing. When the water table is closer to the bottom than the width of the footing, a ; capacity of the soil resin material reduction in load capacity results—a point to be considered on the soil, the weight when making a sub-surface investi ration in a dry season. An en- omitted, the assumption ' t gineer trained in the field of soil mechanics can determine bearing increase the allocable s4 capacities by making laboratory tests of soil samples which have amount at least equal to t s been properly taken in the field. Loading tests may also be used to : . determine soil capacity, but these are quite expensive and could not 15-3 Settlement be afforded on many jobs. Load capacity can also be determined by The settlement of a finding the number of blows per foot needed to drive a standard _ takes place quickly:whc - sampling tool through the soil. These standard penetration tests, I many months. Placing made with the use of sub-surface exploration equipment, are not ex- pensive, so that tests can be made at several points on the job site between the very sma' t ' in order to check the uniformity of soil conditions. These tests dis- the clay, -nd it is this > of a foundation that m, close the presence of pockets of soft material lying several feet below ` the surface of the ground. For information on the relation between ment is a problem di. ;. blows per foot observed in a penetration te--ts and the allowable bear- present scope. The d d ing strength of the soil, the reader, is referred to Karl Terza hi and than total settlement i same load intensity. the R. B. Peck, and R. B. Peck, 11 E. Hanson, and T. II. Thornburn.s To keep the ciificrei� ,a Interpretation of penetration tests is not for the inexperienced. ' 9 � i t 7 f