1983-11-29_REVISION - C1981017 - Laserfiche WebLink

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la�ni.wao,:.rlwdr'�. w.•...:.a,. -�...si��+.,::a.,.d:r.s' '_isthA:.au.+"�t,.ac�.+L�i�.i��S4ri,..:T�'� Y v Cc Footings 437 !turn of 0.250f�, offsets Preliminary design often precedes field tests and in this case the t ate strength design the designer needs some guide to the probable hearing values of the soil. ,'ACI Code, Sec. 230Gf) This can be found in the allowable bearing values given in local build- all of the compressive ing codes or in the American Standard Buitdir,7 Code Requirentents arried to the footing by for Excavations and Foundations' of the aSA. Presumptive Unit ._ m the bearing value of Soil Bearing Values, as given in the 'New York State Building Code r Vomes excessive, a pile Manual,* are shown in Appendix C-5. They differ only slightly from the values of the ASA Code. Rock of various types is listed «-ith - - rhen the outer row of bearing values ranging from 15 to 100 tons per -;q ft (tsf); sands run a single footing cannot from 2 tsf for fine sand to 6 tsf for gravel or sand-gravel combing- ? Ching on the adjoining tions; clay runs from 1 tsf for soft clay to 4 tsf for hard clay. and silt Ling. so that the center is allowed a value of 0. Is the clay medium or soft? How fine is rAtroid of the footing. fine sand, and what percentage of it is present? These and similar ill be best depends on questions show that, although the Codc values ,nay be very useful, it is difficult to determine with accuracy what soil pressure to use. The presumptive values of soil strength are for undisturbed soils loaded at the surface or where there is no lateral support, as at the right-hand side of the footing shown in Fig. 15--1a. When a footing for different types of at or near the surface is loaded and pressed dov:nivards, the adjacent mane combinations of soil flows outward and up, a surface bulge appearing near the founda- tent of the soil has an tion. If the footing is located at the bottom of a 6-ft pit, the tendency oes the location of the of the soil adjacent to the footing to bulge upwards is resisted by the e footing. When the G-ft column of soil directly above it; an increase in the bearing capacity of the soil results. When computing the maximum pressure r idth of the footing, a _ Joint to be considered i on the soil, the weight of th- backfill above the footing is often dry season. An en- omitted, the assun:;,tion heint; that the presence of the backfill will an determine bearing increase the allowable soil pressure above that at the surface by an samples which have amount at least equal to the weight of the baps•:;li. may also be used to ►ensive and could not 15-3 Settlement v Ilw be determined by to drive a standard The settlement of a loaded footing resting on sand or on gravel trd penetration tests, SS takes place quickly; when resting on clay, Lettlenient may continue for �Y luipment, are not ex-_ f many months. Placing a load on clay forces out the water held .. 'Dints on the job site between the very small, flake-sliaped mineral particles composing . ins. These tests dis- ! the clay, and it is this slow flow that causes settlement. The design . ng several feet below of a foundation that may be expected to have a limiting total settle- the relation between ment is a problein for a foundation engineer and is beyond our d the allowable bear- present scope: The di,fJerential settlement «ill be more inipc rtant u Karl Terzaghi and than total settlement in most cases. When all footings carry the ,d T. H. Thornburn.2 same load intensity, the differential settlement«-ill be very small. inexperienced. To keep the differential settlements small the footings should be 77: .