Modernising a building by digging a moat? Sounds strange, but this is what happened during the refurbishing of Parliament Buildings – the biggest building conservation project undertaken in New Zealand.
Parliament House and the Parliamentary Library have a Historic Places Trust classification, which means they must be preserved. However, they are located only 400 metres from Wellington’s main fault line, so when refurbishment began in 1992, the priority was earthquake protection.
Scientists studying the geology of Wellington have predicted an earthquake between 7 and 7.5 on the Richter scale is likely to occur about every 600 years. While there is up to a 50% chance of this happening, survey work shows that the main fault line has not moved in the past 350 years.
These masonry (stone) buildings were built before the introduction of earthquake building codes, and structural engineers considered they were at risk of collapse in a major earthquake. While flexible wooden buildings generally survive a moderate quake, brittle structures will not, as they have little tolerance for the movement an earthquake creates.
Before earthquakes could cause damage, the engineers beat them to it! The building had to be cut from the foundations to allow a base isolation system to be installed. Developed by Bill Robinson of Robinson Seismic, a base isolator is like a spring between the building and its foundation (like a spring between a car wheel and its body).
What did they do?
First a moat was dug around the original foundations and large blocks of foundation were cut out, section by section. As the weight of the building still needed to be carried by the foundation, strengthening was added to the basement walls to help spread out the foundation load.
Then 417 bearings were placed into load-bearing locations – each base isolator was made to stand a vertical weight of over 150 tonnes. The base isolators consist of alternate layers of high density rubber and steel plate, surrounding an inner core of lead. They are designed to move up to 30 centimetres in any horizontal direction during an earthquake up to 7.5 on the Richter scale. This motion stretches the rubber ‘outer’ of the isolator, which counters the movement by returning to its resting state.
During installation, flat jacks were moved under the building and filled with an epoxy grout, which would place the bearings under pressure to support the weight above.
At the next stage, a cylindrical hole was cut through the original foundations to allow access for a hydraulic arm with a large horizontal saw attached. These saws removed slices of the original foundation, effectively separating the building from its base and leaving the entire structure resting on the base isolators.
If putting the base isolators into position was not hard enough, the building itself had to be strengthened – this was done by adding supporting walls. Reinforced concrete beams were also inserted to prevent any twisting during an earthquake.
Ground floor beams also required major strengthening to redistribute wall loads to the base isolator bearings, which were then put into position.
The effect of all this work was to create an extensively strengthened building supported by very solid foundations yet isolated from most of the ground’s movements during an earthquake.
The strengthened Parliament Building has a design life of 150 years.
Other buildings have also been fitted with base isolators, such as Wellington Hospital.
Find out more about Building for earthquake resistance in this article from Te Ara – The Encyclopedia of New Zealand.