Sinclair Knight Merz are fitting the Victoria University's Rankine Brown building with base-isolating bearings similar to those protecting Parliament Buildings, Te Papa and other important structures in New Zealand and around the world. Jude Barlow examines the Rankine Brown structural upgrade, which has a value in the order of $3.8 million.
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.
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Analysing Options
3 New reinforcing cages on the bases of the southerm columns prepared them to meet their lower counterparts .
4 The props are designed to carry 1000 tonnes .
The library is pivotal to the university's functioning. It didn't want to be put in the position of California University, where the library was out of action for about eight years after the 1994 Long Ridge earthquake. So although there was no legal requirement for seismic upgrading, the university engaged SKM to investigate strengthening options. SKM had access to the original construction photographs and drawings, as the designer – Jones, Adams, Kingston and Reynolds (later KRTA) – was incorporated into SKM after a series of mergers.
The university placed certain provisos on the work. First, the library could not be closed during the academic year, so the contractor had to seismically upgrade the building while it was in use – according to Robinson Seismic, the first time this has been done in New Zealand. Secondly, the boilers had to be operational by 1 April. And thirdly, the library's collections had to be protected.
Some compromise was needed between what the university wanted to achieve and what it could afford, says Mr Clark. Several options were quickly ruled out on the grounds of operational disruption and expense. One was strengthening the 1,760 hinge zones between the column heads over 10 floors and the concrete ribs of the waffle slabs, by confining the vulnerable sections with steel plates. The second option, to tie the building to four steel towers rising through the podium to the top of the building, was also judged more disruptive and marginally more expensive than base isolation. Base isolation also provided a higher level of protection to the contents of the building, as it would reduce horizontal accelerations and velocities.
Development
Extra floor space
A third of the weight is picked up at each level .
Having chosen this option SKM, with the help of Robinson Seismic and the Institute of Geological and Nuclear Sciences, analysed the likely response of the superstructure using records of Turkish, Mexican and American earthquakes similar to what could be expected from movement of the Wellington fault or a very large earthquake in the Wellington area.
The records were modified to allow for "near fault" effects, and used with 3-D computer-modelling of the structure to predict how the library would move every fraction of a second in the various earthquake records. Engineers then worked out how to eliminate or minimise the forces acting on structural members and the building's responses, with and without base isolation, working out a response envelope within which the building had to be kept. Without base-isolating bearings the building would not collapse, but might have to be rebuilt and damage would be extensive.
The design criteria specified that the building should withstand a Code design-level earthquake with a 450-year return period, remaining operable with only minor damage. Movement on the Wellington Fault with a mean return period of 600 years is likely to generate a magnitude 7.3 earthquake on the Richter scale. Mr Clark says such an earthquake would cause "quite a bit" of damage to the building particularly to the lift shafts and block walls, but the rest of it "should be quite operable".
The Rankine Brown building's southern-end foundations were stepped in from level 2, ground level, to level 0. The university was keen to expand the library space; so the second-floor slab, which was sitting on fill, was demolished and fill removed to create 1000sqm more floor area. [1]
Meanwhile the above-ground weight of the southern end of the building was supported by racking steel struts [2] between the suspended floor junctions and the southern columns – which ended abruptly in midair at what was previously ground level. [3] New reinforcing cages on the bases of the southern columns prepared them to meet their lower counterparts, which were being built up from level 0. The horizontal beams through the first and second levels were modified to take the new suspended floors, which will extend to match the podium “footprint" above.
Before excavation began, the boilers were temporarily removed from the level 1 slab-on-grade plant room, and hot water provided from a container-mounted boiler at the northern end of the building. The new level 0 plant room with articulated pipes started operating on time on 1 April. New stairs linking levels 1 and 2 are to be installed at the southern end, with the potential for extension to level 0. The stairwell area has been designed by Athfield Architects to include harbour views and conversation areas.
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.
