Wednesday, 6 March 2013

Lateral thoughts

With the Earthquakes in Christchurch, seismic design is increasingly becoming part of everyday language. One component of this is lateral load.

There are four ways of dissipating a lateral load being applied to a foundation:
  1. Through passive restraint of the ground beams or base friction
  2. Through passive restraint of the piles
  3. Shear keys
  4. Through raked piles

 We typically recommend that these are treated as mutually exclusive; differential responses will typically mean that the stiffest component will initially attract the majority of load, rather than share as a system.

Screw piles, although up to 450mm in diameter, do not have a significant lateral capacity when compared with bored piles. 

We have two ways of addressing this:
  1. The bored over screw – where a shallow bored pile is installed over the screw pile.
  2.  The step up pile – where the upper section of the screw pile is super-sized.

Others aspects to consider:
  • Ductility factor of the pile
  • Period of building oscillation
  • Composite pile shaft design (concrete filled steel shaft)
  • Liquefaction effects on passive restraint capacity 

When all this has been considered the system is modelled. We typically employ the tried and trusted Broms method for preliminary designs, followed by the use of L-Pile for detailed designs.  A robust design process can be achieved by using these tools in conjunction with software developed in-house.  Our software is based on Eurocode 4 to model moment capacities of steel/concrete composite tubes.

This is one of the key areas where the screw pile Engineer can add value to a project; the earlier the interaction begins with the Structural Engineer, the greater the optimisation of the foundation system and the greater the certainty of costs and performance for the Client. This is far more effectively delivered through closer relationships with the consultant and by moving away from the typical tender model of procurement to more collaborative methods of early contractor involvement (ECI) or nomination. Around 50% of our customers take advantage of this value and continue to return for their next project having witnessed the benefits.