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147 N.Z. PARLIAMENT BUILDINGS SEISMIC PROTECTION BY BASE ISOLATION R. A. Poole1 and J. E. Clendon2 This paper was presented at the Pacific Conference on Earthquake Engineering, Auckland, November 1991 SUMMARY Parliament House is to be partially demolished and rebuilt, extended within the existing perimeter envelope, refurbished and replanned except for the major public spaces, seismically upgraded by means of base isolation and enhancement of existing foundations, basement walls, ground floor, upper floor walls and floors. This paper describes the assessment of appropriate seismic loads, the structural system, the analysis and design of the retrofitted structure. Anticipated construction procedures and difficulties are also addressed. INTRODUCTION The New Zealand Parliament complex consists of three brief, although the proximity of the Wellington Fault and its permanent buildings: The Beehive or Executive Wing, associated hazard was noted. The selection process was completed in 1977; Parliament House, completed in 1922; The conducted by an independent committee of New Zealand and Parliamentary Library, which was completed in two stages, in international experts appointed by the Parliament Services 1883 and 1899. A fourth building Bowen House is temporarily Commission. fulfilling the functions of Parliament House and Parliament Library. Refer to the Site Plan, Figure 1. Development of Brief Selection of Consultants for Upgrading Holmes Consulting Group and Warren & Mahoney, Architects were commissioned in October 1989 to develop their moderate In 1989 the N.Z.Government took the decision to strengthen and conservation, National monument earthquake scheme with the refurbish Parliament House and the Parliamentary Library recommendation that base isolation of the proposed shear wall because of earthquake risk and the inadequacy of the existing structure be investigated. Structural schemes were developed as facilities. Eight teams of Architects and Engineers were invited part of the overall development of the brief for a conventional to submit six schemes for each building. Essentially the brief shear wall scheme and secondly a base isolated shear wall was open ended and asked the question "What would you scheme. The premium attached to the base isolated scheme was propose to provide the accommodation required at three approximately 3 % of building cost and the client accepted our different levels of conservation viz maximum, moderate and recommendation to adopt the base isolated scheme on the basis minimum. Maximum conservation involved minimum intrusion that the 3 % premium was well worth paying for the superior into the existing structure and refurbishment. Minimum protection achieved. Subsequent development of the design has conservation involved demolition and rebuilding within the confirmed this as the correct decision. It would have been very existing walls and moderate conservation involved adding new difficult to ensure adequate ductile behaviour in the conventional structure to the maximum conservation option. Further, for shear wall solution because of the geometry of the walls. In these three different levels of conservation, a proposal was particular special detailing would have been necessary at the required firstly treating the building in seismic terms as a base of the "columns" to allow them to yield. Consequently this National monument and secondly at a seismic level to ensure structure would have attracted very large seismic responses. survival of the occupants. These were interpreted by our team Working drawings were completed in mid-1991 and construction to be equivalent to greater than MM IX intensity and MM VIII is planned to start in mid 1992 and be completed in 1995. intensity respectively. Earthquake levels were not defined in the 1Holmes Consulting Group, Christchurch (Fellow) 2Holmes Consulting Group, Christchurch. BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vol. 25, No. 3, September 1992 148 /lui I 'Oil~ BEEHIVE (19",ll ) ) E [ [ Figure 1 Site Plan of Parliament House, Wellington, New Zealand. Existing Parliament House Parliament House has five floor levels designated basement, concrete topping overlays and steel strips and plates. A new ground, first (the main public floor) second and third. Some of lightweight steel and timber roof will be constructed over the the upper floor areas are additions to the original building. third floor. They are generally of a poor standard of construction and are to be demolished and rebuilt. A 1960's addition on the south side Parliament Library of the building is also to be demolished and rebuilt. The exist­ ing plan consists of the central House of Representatives block, The Parliamentary Library, consists of the west wing, which perimeter wings and an extensive courtyard/lightwell area be­ was built as an extension to an early General Assembly building tween the two. This lightwell area is to be infilled. Note the in 1883, and the east wing which was built in 1899. The east first floor plan, Figure 2 and the east-west cross section, wing is to be strengthened and refurbished by installing a base Figure 3. isolation system in the basement and strengthening the upper floor walls and floors, essentially the same system as Parliament Parliament House Strengthening Scheme House. The west wing does not have a layout suited to a modern library and is to be demolished and replaced with a Parliament House is to be base isolated by inserting a system of modem base isolated shear wall structure. The north west walls lead rubber bearings into the basement walls and columns. This of the existing west wing plus part of the grand Victorian men's requires extensive strengthening of existing basement walls and toilet are to be retained and integrated with the new building in the ground floor. The new structures in the lightwell areas and the interests of conservation. This is illustrated on Figure 1. on the south side have perimeter walls and new walls on grids 4 and 8. In addition various internal walls have concrete facings added to them principally to add strength to the building. Existing floors are to be strengthened to ensure adequate dia­ phragm strength. This strengthening will be a mixture of 149 0 ~ ,:a) ?f ® ' I 6 6 I I ~ I I ')II I I I I Q}-- I 0--- I ii !!!!!!1111 ' LEGEND , lndiG"otr.i ni:,w R'lSitu cone. wt1ll~ or rniw irl!;itu cone. ~k~ on l H Q)(t!iting wolh> Figure 2 Parliament House, Level 1 Floor Plan. Figure 3 Section through Parliament Building showing proposed strengthening and refurbishment. 150 New Carpark SEISMIC FORCES A new underground carpark building is to be built between In the original submissions design teams were asked to propose Parliament House and the Parliamentary Library with a link to seismic forces appropriate to the brief described earlier. We an existing underground carpark as indicated on the site plan, sought the advice of Dr J Berrill of the University of Canterbury Figure 1. This carpark is at the level of the Parliament House and he has assessed the risk in two ways: basement which is 7 metres below the existing basement of Library East. This has been a challenging retaining/under­ Maximum Probable Event pinning exercise. Firstly the Uniform Risk Hazard analysis was used to assess the This paper does not discuss these Library buildings or the Car Most Probable Event (MPE) for the Wellington region. A park in detail but does refer to them with respect to aspects of return period of 350 years was proposed and confirmed by the particular interest which arise in the Parliament House clients consultant selection committee, the time frame being discourse. influenced by the fact that a base isolation system was proposed as opposed to a more conventional system. The maximum PARLIAMENT HOUSE STRUCTURAL SYSTEM ground acceleration anticipated at the site is 0.5g. This is equated with ground shaking of MM IX on the Modified Description of Structure Mercalli scale. It is anticipated that an event of Richter Magnitude M = 6.5 centred within 5 km of Parliament would The existing structure is essentially a masonry bearing wall produce this shaking or alternatively a larger 'quake at a greater structure. The floors are concrete, simply bottom reinforced distance. with a metal lath mesh between a 2-way system of steel beams. The steel beams are supported on exterior and lightwell masonry walls and steel stanchions which are generally built into internal Maximum Credible Event brick walls. The exterior walls are brick with stone facing on the east and west facades and a plastered finish on the north Secondly the site is within 400 metres of the Wellington Fault, facade. Refer to Figure 3. Class I, and is therefore subject to specific risk associated with a rupture on this fault. The site is thus described as being The new lightwell and south wing structures consist of subject to near fault effects. This risk, the Maximum Credible proprietary precast flooring supported on reinforced concrete Event (MCE), is estimated to be an earthquake of approximate wall/frames. Richter Magnitude 7.0 to 7.5 and to produce ground shaking of MM X on the Modified Mercalli scale. Maximum ground accelerations of 0.85g are predicted. Dr J. Berrill and Dr System to Resist Earthquakes Berryman of the DSIR collaborated to predict the likelihood of this event. A field investigation on the fault was completed by Seismically the system is a base isolated shear wall structure. DSIR and the likelihood of this event, in the 150 year design For the purposes of analysis and design existing masonry walls life of the building, was predicted at between 10% and 50%. have been discounted. Seismic shears are accumulated in the The recurrence interval of this event is believed to be 500 to various "shear walls" at each upper floor level and carried down 600 years and Geological Survey have advised that the fault has to the ground floor where they are redistributed to the various not moved for 350 years.

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