Transmission And Substation Foundations - Technical Design Manual (TD06088E)

DESIGN METHODOLOGY

Figure 5-24 Poulos and Davis (1980)

conditions and for the cases of constant modulus of sub grade reaction (K h ) with depth. One of these solutions is the Davisson (1963) Method as described below. Solutions for various boundary conditions are presented by Davisson in Figure 4-24. The axial load is assumed to be constant in the pile – that is no load transfer due to skin friction occurs and the pile initially is perfectly straight. The solutions shown in Figure 4-24 are in dimensionless form, as a plot of U cr versus I max .

where U cr = P cr R 2 /E p I p or P cr = U cr E p I p /R 2

Equation 4-56

4 √ E p I p /k h d

Equation 4-57

where R =

Equation 4-58

I max

= L/R

where

P cr E p

= Critical buckling load

= Modulus of elasticity of foundation shaft = Moment of inertia of foundation shaft

I p

K h

= Modulus of sub grade reaction

d = Foundation shaft diameter L

= Foundation shaft length over which k h is taken as constant

U cr = Dimensionless ratio By assuming a constant modulus of sub grade reaction (kh) for a given soil profile to determine R, and using Figure 4-24 to determine U cr , Equation 4-56 can be solved for the critical buckling load. Typical values for k h are shown in Table 4-16.

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