Footing Design.

      a. Allowable bearing pressures.   Procedures for determining allowable bearing pressures are
presented previously.  In many instances,  the allowable bearing pressure will be governed by the
allowable settlement.  Criteria for determining allowable settlement are discussed previously.  The maximum bearing pressure causing settlement consists of dead load plus normal live load for clays, and dead load plus maximum live loads for sands.  Subsoil profiles should be examined carefully to determine soil strata contributing to settlement.

      b. Footings on cohesive soils.

          (1) If most of the settlement is anticipated to occur in strata beneath the footings to a depth equal to the distance between footings, a settlement analysis should be made assuming the footings are independent
of each other.  Compute settlements for the maximum bearing pressure and for lesser values.  An example of such an analysis is shown in figure 10-1.  If significant settlements can occur in strata below a depth equal to
the distance between footings,  the settlement analysis should consider all footings to determine the settlement at selected footings.  Determine the vertical stresses beneath individual footings from the influence charts presented previously.  The footing size should be selected on the basis of the maximum bearing pressure as a first trial.  Depending on the nature of soil conditions,  it may or may not be possible to proportion footings to equalize settlements.  The possibility of reducing differential settlements by proportioning footing areas can be determined only on the basis of successive settlement analyses.  If the differential settlements between footings are excessive,  change the layout of the foundation,  employ a mat foundation, or use piles.


Example of method for selecting allowable bearing pressure.
Figure 10-1.  Example of method for selecting allowable bearing pressure.

          (2) If foundation soils are nonuniform in a horizontal direction, the settlement analysis should be
made for the largest footing, assuming that it will be founded on the most unfavorable soils disclosed by the borings and for the smallest adjacent footing.  Structural design is facilitated if results of settlement analyses are presented in charts (fig 10-1) which relate settlement, footing size, bearing pressures,  and column loads.

Proper footing sizes can be readily determined from such charts when the allowable settlement is known.  After a footing size has been selected,  compute the factor of safety with respect to bearing capacity for dead load plus maximum live load condition.

      c. Footings on cohesionless soils.   The settlement of footings on cohesionless soils is generally small and will take place mostly during construction.  A procedure for proportioning footings on sands to restrict the differential settlement to within tolerable limits for most structures is given in figure 10-2.

      d. Foundation pressures.   Assume a planar distribution of foundation pressure for the structural analysis of a footing.  This assumption is generally conservative.  For eccentrically loaded footings, the distribution of the bearing pressure should be determined by equating the downward load to the total upward bearing pressure and equating the moments of these forces about the center line in accordance with requirements of static equilibrium.  Examples of the bearing pressure distribution beneath footings are shown in figure 10-3.

Proportioning footings on cohesionless soils.
Figure 10-2.  Proportioning footings on cohesionless soils.

Distribution of bearing pressures.
Figure 10-3.  Distribution of bearing pressures.

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