Base-isolating New Zealand Parliament Buildings

Fig 1. General Assembly Building, with the library on the right.

Introduction

In 1989, the New Zealand Government decided to strengthen and refurbish two of the three main buildings in the Parliament complex. Parliament House (built in 1922) and the Parliament Library (built in two stages, 1883 and 1899) were chosen for upgrade because of earthquake risk and inadequate existing facilities. The third building, the Beehive or Executive Wing, completed in 1969, was deemed adequate.

Proposed strengthening schemes were required to treat the two old buildings first in seismic terms as National Monuments and then at a seismic level that would ensure occupant survival. These design levels were interpreted to be greater than Modified Mercalli (MM) X intensity and MM IX intensity, respectively. The proximity of the Wellington Fault and its associated hazard were also considered.

Holmes Consulting Group and Warren & Mahoney, Architects, were independently commissioned to develop a conventional shear-wall scheme and a base-isolated shear-wall scheme. The premium attached to the base-isolated scheme was approximately 3% of building cost. The Government accepted the recommendation to adopt the base-isolated scheme on the basis of demonstrated superior seismic performance.

Base isolation is a technique for seismic-resistant building design that introduces a flexible level within a building, greatly reducing the ground motions transmitted into the building. Seismic forces on the superstructure of a base-isolated building are often up to 8 times less than if conventional strengthening approaches are used. Significantly less superstructure strengthening is therefore required, with proportionately less disruption of the building's historic finishes.

Conservation

Both Parliament House and the Parliament Library have New Zealand Historic Places Trust A" classifications, which means that permanent preservation is essential. A critical aspect that the design team had to address early in the project was the conflicting parameters of conservation and restoration and the intrusive, sometimes destructive, requirements of strengthening.

Over a period of two years, the design team, together with the Parliament Service Commission and the Historic Places Trust, explored design options. Their efforts led to a solution that called for inserting strengthening skin walls of reinforced concrete into selected areas; tying these walls together using the in-plane strength of the floors; isolating the building from the ground on which it stood; and constructing new reinforced concrete walls within the light well-all the while retaining the essential historic spaces and corridors.

Structure of Original Buildings

Parliament House has five floor levels. The existing plan consisted of the central House of Representatives block, perimeter wings, and an extensive courtyard and light-well area between the two.

The existing structure was essentially a masonry bearing-wall structure, where the roof and floors were supported by masonry walls. The floors were concrete, simply reinforced with a metal lath mesh between a grid of steel beams. The steel beams were supported on exterior and light-well masonry walls and steel stanchions, which were generally built into internal brick walls. The exterior walls were brick with stone facing on the east and west facades and a plastered finish on the north facade.

Construction of the Parliament Library is similar to that of Parliament House, so this discussion will focus only on Parliament House.

New Structural System

Effectiveness of the base-isolated shear-wall system chosen for Parliament House was evaluated. Analysis of stiffnesses and strengths revealed that, where the existing walls were composite with new seismic walls, in-plane actions of the existing walls were not significant and could be neglected. Further, seismic forces accumulated in the various in-plane walls at each upper-floor level, were carried down to the ground floor, and were then redistributed to the various base isolators, which in turn transferred them to the foundations and ground. Where possible, the new floors were used as the major horizontal structural members, and existing floors, strengthened where necessary, were tied into them. Elsewhere, strengthened existing floors supplied all horizontal structural action.

Extensive strengthening of existing basement walls, and in some cases foundations, was required to redistribute vertical and horizontal foundation loads.

Seismic Loads

These buildings were assessed as having to resist very large seismic-induced loads, because their importance to the community required high resistance to extreme events (long earthquake return periods) and because they are proximate to an active geological fault. Dr. J. Berrill, of the University of Canterbury, helped to assess the risk.

Maximum Probable Event

The Uniform Risk Hazard analysis procedure was used to assess the Maximum Probable Event (MPE) for the Wellington region. A return period of 450 years was proposed and accepted by the Government-the time frame being influenced by and taking advantage of the fact that a base-isolation system was proposed as opposed to a more conventional system. The maximum ground acceleration anticipated at the site was 0.5g. This was equated to ground shaking of MM IX intensity. It was anticipated that an event of Richter magnitude 6.5 centred within 5 km of Parliament would produce this shaking or, alternatively, a larger quake at a greater distance would produce such shaking.

Maximum Credible Event

The site is within 400 m of the Wellington Fault, Class I (an extremely active fault), and is therefore subject to specific risk associated with a rupture on this fault. This risk, the Maximum Credible Event (MCE), is estimated to be an earthquake of approximate magnitude 7.0 to 7.5 with ground shaking of MM X intensity. Maximum ground accelerations of 0.85g are predicted.

Dr. J. Berrill and Dr. K. Berryman, of the then Department of Scientific and Industrial Research, collaborated to predict the likelihood of this event, which, in the 150-year design life of the building, was predicted to be between 10% and 50%. The recurrence interval of this event is believed to be 500 to 600 years, and the Geological Survey has advised that the fault has not moved for 350 years. The risk is therefore very real.

Analysis and Design of Bearings

There are a number of factors that influence the analytical approach and hence the design of the bearings in a structure such as Parliament House: (1) gravity loads and geometry of the structure; (2) bearing flexibility compared to the foundations and superstructure; and (3) concentration of mass in the unsymmetrical wall layout rather than in the floors, making the structure susceptible to twisting.

Analysis of the isolation systems during the documentation phase was done in three stages, each increasing in order of complexity.

Selected Bearings

In the final design, 80% of the bearings were of high-damping rubber (HDR), and the balance of lead-rubber. HDR offered the advantages of low height (approximately 280 mm overall, including integral endplates), providing excellent stability at maximum deflection and a high degree of resistance to rollover. It also had a high vertical stiffness-vertical deflection was less than 2 mm under design vertical loads. Maximum calculated displacement of the bearings in the MCE was 291 mm.

A total of 417 bearings were required, typically formed with alternating layers of HDR and thin steel shim plates sandwiched between thicker endplates top and bottom. All the bearings were round, with diameters ranging from 480 to 580 mm, and those with lead cores containing full-height plugs of lead with diameters of 155 to 190 mm, respectively. A few bearings in lightly loaded locations were sliding bearings formed with Teflon/stainless steel surfaces mounted on HDR backing.

Actions in Superstructure

In Parliament House, the analysis indicated very low interstorey deflections and very little yielding of the superstructure, which was desirable for old masonry buildings. The building was detailed with a 400-mm clearance all around at ground-floor level. In some cases, separations between noncritical architectural elements were reduced to 25 mm in order to make detailing practical. Repair will be required after moderate earthquakes.

Conclusions

Considering the historic significance of the old New Zealand Parliament buildings and the findings of site-specific seismic studies, the only practical method of strengthening to resist seismic forces was to base-isolate the buildings. This method reduced the forces induced into the superstructure by a factor of 8, and hence reduced intrusive strengthening elements to the maximum possible extent. Any such elements that were necessary were placed in areas of less historical significance. In addition to preservation of historic features, the construction cost of the isolated structural system was estimated to exceed that of a conventional system by only 3%, while providing considerably superior seismic performance.

By the end of July 1994, the old New Zealand Parliament buildings had been successfully base-isolated. All the foundation strengthening had been completed, and all the bearings had been inserted. Before strengthening, the old Parliament buildings-of traditional masonry construction, like numerous others in New Zealand and Australia-would have been potentially hazardous in the event of a significant earthquake.