Under ultimate state, the star finned screw piles are designed to avoid catastrophic failure under the ultimate design wind load. Fatigue failure of the pile is considered in the design.
Highway lighting columns incorporating a slip base are connected to a specifically designed slip plate attached to the series 3B screw pile. This is designed to allow slip failure at the columns base/ slip pate interface to occur with minimal or no load transferred to the screw pile foundation.
Design Model
The method of analysis proposed for the design model was developed from the results of numerous field tests.
Upon lateral loading of the lighting columns, the star finned screw pile is subjected to an instantaneous wind load applied bending moment at the ground surface.
The moment is then transferred into the composite section of the pile which comprises the fins, hold down bolts, and the centre shaft. The stiffness of the composite section is approximately 50% more than the centre shaft stiffness alone. The applied load is a transient load and the soil can be assumed to be an undrained state.
This results in high passive resistance around the mobilised fins, as the passive resistance is a function of the undrained shear strength (rather than a function of the overburden pressure as in the drained case). The magnitude of this pressure resistance is also a function of the effective width of the star finned screw pile.
This effective width is six (6) times larger than the projected fin width. This larger passive restraint also provides overall
stability to the system. The applied moment is rapidly dissipated through this composite section by passive resistance of the soil surrounding the fins.