Built heritage can be considered as three dimensional historic documents. The past can be read≠ in these buildings through their design, use of materials, technology, and use of the building over time, among many other values. Therefore everything about the place, including its fabric and structure, is precious and should be conserved for continued use, education, and enjoyment of present and future generations.

Conservation in New Zealand is guided by the NZ ICOMOS Charter for the Conservation of Places of Cultural Heritage Value. The charter requires that conservation of built heritage must be carried out with full knowledge of the heritage values of a place. This involves complete historical and physical surveys of all spaces and fabric. Relative levels of heritage value can be assigned to each, which will help in deciding on appropriate levels of intervention. An useful methodology of assessing heritage values and deciding on conservation actions to retain values is by writing conservation plans, using the New Zealand Historic Places Trust≠s guide or Jim Kerr≠s The Conservation Plan.

The range of possible acceptable interventions include non-intervention, maintenance, stabilisation, repair, restoration, reconstruction and adaptation. Each intervention should thoroughly documented, the minimum necessary, reversible where technically possible, and undertaken by appropriately qualified conservation professionals. Reversibility is a particularly important, but problematic concept of conservation in seismic zones. It may be that, in the future, a miraculous new technique is invented which respects the fabric and structure much more than existing techniques, and an allowance should be made for this possibility.

The Charter also recommends an assessment of potential risk from natural processes and to mitigate that risk by appropriate actions. As New Zealand is in one of the most active seismic zones in the world, built heritage is at considerable risk from the immediate and longer term effects of earthquakes. Invariably the only means considered to reduce the effects of an earthquake involve strengthening the fabric and structure to a specific Œdesign earthquake≠. The options are discussed below from a conservation and engineering viewpoint, however, builder owners can take actions themselves to reduce the long term impact of an earthquake through good records, good maintenance and appropriate emergency planning for the big event.

The preparation of accurate records of a heritage building will allow for restoration of damaged or missing elements after an earthquake. These can include measured drawings, rectified photography, photogrammetry, and a photographic survey of as much detail as possible. The greater the level of the detail of the documentation, the more accurate the restoration is able to be. The documentation should be stored in a suitable location so that it is safe from damage and can be easily retrieved.

Regular maintenance is probably the most effective action a heritage building owner can take to limit the damage in an earthquake. Poorly maintained buildings are most at risk and at least 50 per cent of damage to historic buildings in earthquakes is attributable to improper maintenance. Earthquakes find the weakest point in the structure or fabric of a building, and its effect will be most marked at that point. Well maintained unstrengthened buildings have survived large earthquakes in a much better condition than even strengthened buildings.

Good maintenance involves regular inspections of buildings, which should be carried out at least yearly. Inspections determine building condition, and can discover potential weaknesses in material, workmanship, inappropriate details and structure. Designs to rectify the weaknesses or deterioration visible would be programmed as part of the maintenance schedule. Examples of inadequate design include inadequate sub floor bracing which, in the Edgecumbe earthquake left houses with piles or piles puncturing the floor and rusted or inadequate ties to slate or tiled roofs. Both forms of roofing are heavy and if dislodged in an earthquake can cause damage to other parts of the house and well as injury to people nearby. Maintenance of pointing of masonry buildings is essential maintenance which prevents moisture into the structure, and consequent points of weakness.

A log of all work, including an inventory of all moveable property, should be kept for future reference.

Immediate measures after an earthquake should be designed to ensure the protection of the structure and fabric from further damage. Contact addresses for experienced conservation architects and engineers who are familiar with the heritage values of the place should be on hand for immediate inspections to be carried out. Authorities should be informed immediately of the condition of the heritage building from the inspection, and steps being taken to mitigate problems. Where authorities cannot be informed immediately, a large, weatherproof notice should be made fixed to the place to it as having high heritage values. These actions should avoid hasty decisions being made by authorities which could result in demolition. Often, in the heat of the moment, damaged but redeemable built heritage may be demolished, or valuable fabric which has fallen may be swept away to clear roadways as quickly as possible.

Any damaged moveable cultural fabric should be identified, labelled and removed to secure storage for later installation. Any damaged structure or fabric should be stored and protected in such a manner that it does not prohibit movement around the site, but can be accessed easily for rebuilding. This would ensure original fabric is reinstated as opposed to new material which is unlikely to match.

Emergency equipment to facilitate temporary shoring and weather protection should be readily available. This would include timber for shoring buildings, or bracing openings, acrow props for propping floors/ceilings, and tarpaulins for covering buildings from further damage from rain. Portable fire extinguishers and petrol operated generators would overcome problems of cuts to water mains and electical supplies.

An appropriate strengthening system assumes an adequate assessment of the strength of the existing building. This is often difficult as the building materials and quality of construction may not be uniform, so that testing of parts of the building may not give an accurate picture of the whole building. This is especially so with heritage buildings as their method of construction and materials may be archaic for which there are no means of assessment.

An approximate indication of the seismic resistance is the configuration of the building. Short symmetrical buildings perform better than asymmetrical tall buildings, as do those which have a cellular construction compared with those which have an open plan. Fortunately, heritage buildings often fall into the short, symmetrical and cellular forms

One indication is how the building has survived in the past. This is a pragmatic approach favoured by one international expert, and one for which there must be sympathy.

Conservation of historic buildings in seismic zones presents a dilemma. In order to protect the building against earthquakes by strengthening, a great deal of the structure and fabric may be lost. The extent of strengthening and the technique selected will determine the success of the strengthening in conservation terms.

While the range of engineering solutions are described elsewhere in this bulletin, from an architectural conservation point of view, there are several common approaches to strengthening. These include:

"dealing with elements"

supporting of floors,

a new structure

"Dealing with elements" historically has meant the demolition of decoration such as parapets, gables, chimneys, or even whole towers. This happened with the Wellington Town Hall and Dunedin Municipal Chambers, the latter having its tower reinstated in concrete. As it is likely that these elements are intrinsic to the design values of the structure, their retention is essential to retain heritage values. Tying these elements back into the structure is relatively easy and can be achieved will relatively low cost. The alternative of replacement with plastered polystyrene is not acceptable as this will replace an original, authentic element with a replica, losing heritage values. Where elements such as these have been lost over time, reconstruction in lightweight materials is acceptable where the design can be accurately verified, and the external texture and finish is the same as the original.

To a lesser extent, the securing of objects which can move easily can be undertaken as soon as possible. Large or heavy objects can pose a danger where these are not properly fixed to the structure of the building. Hot water cylinders and header tanks, when full are extremely heavy and should be secured. Solid fuel heaters should similarly be securely connected to the floor structure.

Supporting of walls and floors involves the introduction of independent structure which supports the floors, usually located on the inside. This method assumes life safety only and does not protect heritage fabric. There is often a visual intervention in the spaces from the introduced structure. The additional structure is often steel framing which can be removed with comparatively little damage, which goes a considerable way towards fulfilling the requirement for reversibility. The addition of supporting structure can be acceptable in heritage buildings where such elements do not detract from the interior design, such as in industrial buildings, or in spaces of low heritage value. Where this would not be acceptable is in buildings of an ornate and decorative aesthetic such as town hall council chambers, or spaces of high heritage value.

A new structure is often seen as the ultimate in strengthening. This usually involves the introduction of new shear walls and/or ductile frames. Damage from earthquake movement to the original fabric is often designed into this system which absorbs energy from the earthquake before the new structure comes into play. The frames can be steel or reinforced concrete and, as they are designed for the whole structure rather than just supporting walls, are consequently greater in dimension, and potentially have greater visual impact. The new structure can be obvious visually, such as structural steel framing and the same conditions for acceptability apply as above. For a greater level of intervention but less visual impact, the frames can be let into the existing walls.

Shear walls are less immediately obvious, however as they increase the size of wall, they can have an impact in increasing window reveals, and to decorative elements at the wall, floor and ceiling junctions. Relocation of cornices, skirtings, and ceiling decoration can result. The space is also smaller than originally designed. The impact on heritage fabric and spaces can be great, reducing heritage values, as well as being irreversible.

In the past the complete gutting of the building and spraying the remaining exterior walls with concrete has been seen as a good solution. A new interior is then added. This approach is clearly not consistent with maintaining heritage values and is not recommended at all.

Base isolation is a new procedure invented in New Zealand but first adopted for historic building strengthening over-seas. It possibly involves the least intervention of all, above ground anyway. The first retro-fitting of a heritage building with base isolators was in the City and County building in Salt Lake City, Utah. The ground floor is reinforced to act as a diaphragm with rubber isolators inserted into reinforced slots in the foundation, after which the . The isolators modify the frequency of the earthquake waves to a frequency which the building can more easily resist, and the isolators themselves can absorb energy from the earthquake. Apart from securing elements, often little other strengthening is required and the structure above ground retains its function. While a great deal of additional structure is added to the basement which is not reversible, the isolators themselves are designed to be removed and replaced over time, making this system inherently reversible above the cut. Parliament Buildings has be retro fitted with base isolators and this system of reducing the effects of earthquakes is proposed for Wellington≠s Maritime Museum.

An assessment of heritage values will determine the most significance spaces and fabric which should be adapted the least. Generally these are the main front elevations of the exterior and the principle spaces of the interior, such as entrance foyers, lobbies, stairs, corridors and large public spaces. Secondary spaces, or existing unused areas such as roof spaces, would generally be appropriate for the location of structural strengthening.

Each of the solutions described above is an "all-out" "once-and-for-all" solution. Building conservation agencies recommend a gradual approach to strengthening so that the efficacy of the solution can be tested and improved over time.

A further recommendation is that strengthening should be designed to withstand a 100 year return period earthquake. Measures to mitigate less frequent return period earthquakes which are of greater magnitude can result in interventions which greatly reduce heritage values of the building. This is an important recommendation which all who practice building conservation in New Zealand should be aware of.

Checklist

Whether or not an earthquake risk:

Prepare a maintenance plan to an appropriate level with reference to heritage values, and carry out appropriate maintenance;

prepare accurate documentation of the building by measured drawing, photogrammetry, or rectified photography and take a good photographic record;

have an engineer assess the strength level with respect to the Building Act, or higher if required;

prepare an emergency plan to protect the building immediately after an earthquake;

have, or have ready access to, items such as acrow props and tarpaulins to protect the building after an earthquake;

appoint an engineer and conservation architect to quickly inspect the building after an earthquake;

advise Civil Defence and other emergency authorities that yours is a heritage building;

Where an earthquake risk:

where the building is not strong enough to withstand a minimum level required by the Building Act, the building should be strengthened;

to ensure retention of heritage values and strengthen, a conservation plan is recommended, or at least a heritage assessment prepared by a conservation architect of all spaces and fabric to determine heritage values;

appoint an engineer and conservation architect to carry out the strengthen in such a manner that the most significant spaces and fabric remain undisturbed from the strengthening.

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