## Tuesday, November 20, 2012

### Design Example - Driven Piles.

A 30-foot length L of A-36 unbraced H-pile, HP14 x 73, has a cross section area Ay = 21.4 in.^2, Table 1-3. This pile is made of A36 steel with fy = 36 ksi, Ep = 29,000 ksi, and pI = 729 in.^4 for the X-X axis and 261 in.^4 for the Y-Y axis. Dead load QDL = 40 kips and live load QLL =  60 kips. Load factors FDL = 1.3 and FLL = 2.17. Free-standing length above the ground surface is 10 feet. The soil is a clay with a constant modulus of horizontal subgrade reaction Els = 1 ksi. Spacing between piles is three times the pile width.

(1) Design load. The applied design load per pile Qd from equation 2-1 is

(2) Structural capacity. The unfactored structural capacity Qcap is fy Ay = 36 x 21.4 = 770.4 kips. From Table 2-1 Φpf = 0.85 and Fe = 0.78. The factored structural capacity is FeΦpfQcap = 0.78 x 0.85 x 770.4  = 508.4 kips > Qd = 182.2 kips. Table 2-3 indicates that the allowable stress  qa = 10 ksi qa Ay = 10 x 21.4 kips > Qd = 182.2 kips.

(3) Buckling capacity. A flexible cap unbraced at the top is to be constructed for the pile group. The Els = (1/4) x 1 ksi = 0.25 ksi because the spacing is three times the pile width. The equivalent length Leq for the constant Els = 0.25 Ksi from equation 2-4a for the minimum Ip of the X-X axis is

The minimum critical buckling load Qcb from equation 2-3d is The Qcb = 201 kips > Qd = 182.2 kips. Buckling capacity is adequate.

Table 1-3 Standard H-Piles; Dimensions and Properties (AISC 1969)

Table 2-1 Tolerances in Drilled Shaft Construction.