Transmission And Substation Foundations - Technical Design Manual (TD06088E)

DESIGN EXAMPLE 11 BUCKLING EXAMPLE USING THE DAVISSON METHOD SYMBOLS USED IN THIS DESIGN EXAMPLE

k h .............................................................. Empirical Torque Factor for Helix 7-52 U cr ..............................................................................................Critical Capacity 7-52 R.............................................................................................................Resistance 7-53 I max .................................................................... Maximum Moment of Inertia 7-53 P cr .............................................................................................. Critical Pressure 7-53 E p ...................................................................................... Modulus of Elasticity 7-53 I p ..............................................................................................Moment of Inertia 7-53 D....................................................................................................Shaft Diameter 7-53 kip.......................................................................................................... Kilopound 7-53

Project A three-helix CHANCE ® helical 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.

DESIGN EXAMPLES

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 results are generally conservative. • From Figure 7-35, U cr ≈ 2

Poulos and Davis (1980) Figure 7-32

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