Bearing Up
Base-isolation bearings
Southern columns (click to enlarge)
Suspended floor junctions and southern columns (click to enlarge)
The props are designed to carry 1000 tonnes
COP Outcome development and evaluation
Petone-based Robinson Seismic designed, tested, and manufactured eighteen 950mm square by 478mm high, 1.3-tonne base-isolating bearings. The steel and rubber layered blocks were vulcanised in a hot mould for 36 hours. Pure lead plugs were then pressed into cylindrical holes in the rubber laminate. Two prototypes were tested to their limits in Robinson Seismic's test rig. The 16 final bearings were stiffened slightly by altering the rubber formulation, to match them precisely to the building's dynamics. They were then tested to one third of the deflection of the prototypes. Base-isolating bearings do not need to be replaced, even after enduring severe displacement in a major earthquake.
Head contractors McKee Fehl Constructors Ltd devised a novel way of inserting the base isolators into the columns, which has the project two months ahead of schedule. Each column in turn, starting in the centre and working north and south, is propped up using 12 hollow steel sections over the lower three floors (three either side of each column on level 0, two either side on level 1, and one on level 2). [4] The props are designed to carry 1000 tonnes (although 800 tonnes gives tension in each column), a third of the weight being picked up at each level.
Once the column is in tension, two cuts are made through the base using a wire saw with diamond nubs; and a segment, about the size of a base-isolating bearing, is removed on a trolley. A steel shoe is fitted round the upper stub of the column [5] and bolted onto the top of the bearing, which is then grouted in place. Steel angle cleats are fitted to the bottom plate of the bearing, and anchor-bolted to the column-bearing pad. Once the grout is cured the props are removed and the load goes back onto the column and bearing, which squashes down about 4mm. The cutting and insertion process takes four days per bearing – instead of the expected two weeks.
