Monday, November 19, 2012

Factored Loads - Deep Foundations.

The driven pile or drilled shaft in a group carrying the maximum factored load will be checked for structural failure.

a. Criterion.  Calculation of the factored load from the dead and uve loads on a pile or drilled shaft is given by equation 2-1:

b. Calculation of maximum load. The maximum load on a single pile in a group or on a drilled shaft can be detemiined from computer or hand solutions.

(1) Computer solutions. The pile or drilled shaft carrying the maxiinum axial load in a group can be determined from computer program CPGA (U.S. Army Engineer Waterways Expcrimcnt Station Technical Report ITL-89-3), which computes the distribution of axial loads in a pile group for a rigid pile cap. The maximum inclined and eccentric load in a group can be determined from computer program BENT1 (U.S. Army Engineer Waterways Experiment Station Misccllaneous Paper K-75-2). The total vertical and lateral loads are input into program BENT 1.

(2) Hand solutions. If all piles in a group have similar geometry, the axial load on any pile Qxy of an eccentrically loaded pile group may be calculated by hand (Scott 1969)

c. Buckling resistance. Driven piles or drilled shafts that extend aboye the ground surface through air or water, or when the soil is too weak to provide lateral support, may buckle under axial loads. Buckling failure may control axial load capacity of the pile.

(1) Buckling capacity. The critical buckling load Qcb of partially embedded piles or drilled shafts may be estimated by (Davisson and Robinson 1965) as follows: 

 The safe design load  Qcd wi11 be calculated using normal design procedures for columns and beam-columns when end moments and eccentricity are considered or by equation 2-1. This load will be less than Qcb.

(2) Equivalent length. The equivalent length Leq for long piles (where buckling may occur) can be calculated using the modulus of horizontal subgrade reaction, Els

Refer to paragraph 1 -7c (4)(d) for methods of estimating Els.

(3) Group effects. Adjaoent piles at spacings greater than eight times the pile width or diameter have no influence on the soil modulus or buckling capacity. The Els decreases to onefourth of that applicable for single piles when the spacing decreases to three times the pile diameter. The Els can be estimated by interpolation for intermediate spacings between piles in a group.

 Figure 2-1.  Eccentric load on a pile group

Table 2-2
Performance and Eccentricity Factors (Barker et al. 1991) (Copyright permission, National Cooperative Highway Research Program)

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