1983-11-29_REVISION - C1981017 - Laserfiche WebLink

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F i 436 Essentials of Structural Design compared with the allowable stress in the column of 0.250E', offsets Preliminary design often ' the difference in concrete strengths; in ultimate strength design the designer needs some guide allowable bearing stress in the footing is 1.9 (ACI Code, Sec. 2306E) This can be found in the at s times that used in working-stress design. If all of the compressive ing codes or in the Amer' �. load in,the concrete of the column cannot be carried to the footing by for Excavations and Four T bearing, extra dowels must be provided. When the bearing value of Soil Bearing Values, as gi• } the soil is so low that the size of the footing becomes excessive, a pile lylaniaal,* are shown in ApF '• footing (Fig. 15-1d) is used. the values of the ASA C s s A combined footing (Fig. 15-1c) is used when the outer row of bearing values ranging froi columns is so close to the property line, that a single footing cannot from 2 tsf for fine sand b be built for the outer columns without encroaching on the adjoining tions; clay runs from i tsf property. It is desirable to proportion the footing, so that the center is allowed a value of 0. of gravity of the loads will fall at the centroid of the footing. fine sand, and what perce Whether a rectangular or trapezoidal shape will be best depends on questions 'show that, althc the relative size of the column loads. it is difficult to determine ' The presumptive value: s loaded at the surface or 15-2 Bearing Strength of Soils z right-hand side of the foe The allowable bearing strength varies widely for different types of at or near the surface is l soil, such as silt, clay, sand, gravel, and the many combinations of soil flows outward and up; these found at the job site. The moisture content of the soil has an tion. If the footing is loc: of the soil adjacent to the important influence on its bearing strength, as does the location of the g_ft column of soil dire water table with respect to the bottom of the footing. When the x water table is closer to the bottom than the width of the footing, a t capacity of the soil result a material reduction in load capacity results—a point to be considered on the soil, the weight when making a sub-surface investigation in a dry season. An en- omitted, the assumption { ► gineer trained in the field of soil mechanics can determine bearing increase the allowable so capacities by making laboratory tests of soil samples which have amount at least equal to t: i been properly taken in the field. Loading tests may also be used to r £ eg _ !• 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 I i finding the number of blows per foot needed to drive a standard takes place quickly;whey - sampling tool through the soil. These standard penetration tests t I � I made with the use of sub-surface exploration equipment, are not ex� � many months. Placing -= pensive, so that tests can be made at several points on the job site It between the very small in order to check the uniformity of soil conditions. These tests dis- 3 the clay, and it is this s t close the presence of pockets of soft material lying several feet below ; of a foundation that ma the surface of the ground. For information on the relation between ment is a problem for blows per foot observed in a penetration tests and the allowable bear- present scope. The At s ing strength of the soil, the reader is referred to Karl Terzaghi and than total settlement iz R. B. Peck,' and R. B. Peck, W. F. Hanson, and T. H. Thornburn.' same load intensit}, the Interpretation of penetration tests is not for the inexperienced. To keep the different 3 I . • •