COP Brief development

The Wellington fault runs 900m from the Rankine Brown building, which houses Victoria University's main library, and with the Wairarapa fault to the southeast, the Ohariu, Shepard's Gully and Wairau faults to the northwest, the whole of Wellington is on shaky ground. Some years ago the Wellington office of Sydney-based Sinclair Knight Merz (SKM) reviewed Victoria's on-campus buildings, and identified the library, built in the early 1960s, as one of the more vulnerable buildings.

The Rankine Brown Building which houses the library is a rectangular tower block with a podium at ground level. Built from reinforced concrete, its precast long-span waffle-slab floors [1] create a very strong diaphragm. Sixteen vertical columns in two rows of eight support the ten floors. The regular form means that the way the building would react in an earthquake can be predicted with some certainty.

SKM determined that the frame action bending is carried by the waffle ribs on the two major axes between columns. SKM's deputy structural engineer manager Win Clark explains that under horizontal earthquake loading the ductile capacity of the rib hinge zones, adjacent to the column heads, would be suspect because they are not confined. Also the transverse pre-stressing cables in the waffle ribs pass through the column heads at the top of the section and drape down within the waffle ribbing. There is no reinforcing within the ribs to confine the stressing strands. Mr Clark says this means under a swaying action the stressing strands could pull out of the waffle slab as it slab sways and then sags down from each column head.

More modern construction requires transverse reinforcing in the beam hinge sections adjacent to columns, to carry the shear loads and confine the longitudinal bars or strands. However incorporating base-isolating bearings into the library's 16 main columns is significantly reducing the forces acting on those elements and ductile demand.