Chance Technical Design Manual

DESIGN EXAMPLE 16: BUCKLING EXAMPLE USING THE DAVISSON METHOD

PROJECT A three-helix Chance® Type SS150 1-1/2” square shaft helical pile is to be installed into the soil profile as shown in Figure 8-33. The top three feet is uncontrolled fill and is assumed to be soft clay. The majority of the shaft length (12 feet) is confined by soft clay with a k h = 15 pci. The helix plates will be located in stiff clay below 15 feet. The buckling model assumes a pinned pinned end condition for the helical pile head and tip. Determine the critical buckling load using the Davisson method. ASSUMPTIONS • k h is constant, i.e., it does not vary with depth. This is a conservative assumption because k h usually varies with depth, and in most cases increases with depth. • Pinned-pinned end conditions are assumed. In reality, end conditions are more nearly fixed than pinned, thus the re sults are generally conservative. • From Figure 8-32, U cr ≈ 2

EQUATION 8-85

= 4 √ (30 x 10 6 x 0.396) / (15 x 1.5) = 26.96

R

l max

= (15 x 12) / 26.96 = 6.7

= (2 x 30 x 106 x 0.396) / 26.96 2 = 32.69 kip

P cr

CHANCE TYPE SS150 SQUARE SHAFT FOUNDATIONS PHYSICAL PROPERTIES, TABLE 8-10 MODULUS of ELASTICITY (E p ) MOMENT of INERTIA (I p ) SHAFT DIAMETER (D)

30 x 10 6 psi

0.396 in 4

1.5 in

DESIGN EXAMPLES

Model as

Foundation

Soft Clay N = 3 K h = 15 pci

K h = 15 pci

Sti Clay N ≥ 5

FOUNDATION DETAILS FIGURE 8-33

POULOS AND DAVIS (1980) FIGURE 8-32

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