Braced for impact: preparing heritage icons for seismic shock
Three experts from Lovell Chen discuss the complexity of retrofitting heritage buildings to withstand an earthquake.
Earthquakes are infrequent in Australia but they are unpredictable and can have devastating consequences. All new buildings in Australia are required to have measures to mitigate the loss of life during an earthquake. However, these requirements were only introduced in the late 1990s. Therefore, what happens to heritage buildings during earthquakes? What complications do heritage buildings pose to those in charge of strengthening the structure against seismic events? Lovell Chen’s Milica Tumbas, Principal Architect, Deirdre Heffernan, Senior Associate, and Christophe Loustau, Senior Associate, discuss these considerations.
Two building codes govern seismic strengthening in Australia. The first is AS1170.4 (introduced in 2007), to ensure a higher standard required for all new buildings and larger, more at-risk heritage structures. For smaller heritage buildings, the code AS3826, introduced in 1998, is a minimum requirement. Seismic strengthening codes are triggered in heritage structures if their use changes, or if over 50 percent of the structure changes. “The primary reason for these codes is to protect the occupants and prevent the loss of life in the event of an earthquake,” says Tumbas, “and not necessarily to prevent damage to the building”.
When it comes to retrofitting buildings built before 1998, particularly heritage buildings from the 19th century, each building requires its own treatment to bring it up to standard says the team from Lovell Chen. They often work alongside structural engineers and building teams to realise seismic strengthening plans. The process begins with a virtual examination of the building, noting potential weaknesses. Although there are never any ‘typical’ problems, recurring precarious elements include skinny chimneys and inadequate lateral bracing within tall towers. “A building’s capability to withstand seismic events depends on the type of material used,” says Loustau. “The veneer on a building or a two-skin brick wall may not be tied to the internal structure”. And it’s not just 19th century buildings that are at risk, says Tumbas. Buildings from the 20th century can easily be at risk as well, including modern buildings with poorly-prepared materials that will need seismic strengthening in the future.
Finding innovative solutions among many constraints characterises seismic strengthening work. Often typical go-to mechanisms for strengthening aren’t appropriate. External steel trusses, for instance, would interfere with a heritage-protected façade.
Development Victoria is managing a range of works at Flinders Street Station. Most recently, the former administration building of the 1910 iconic railway station has undergone essential repair and conservation works alongside seismic strengthening of the upper levels of the complex to meet the more stringent requirements of AS1170. Working in close collaboration with structural engineers Bonacci Group, and managing contractor Built and its subcontractors, Lovell Chen’s focus was on managing the impact of the proposed seismic measures on the interior of the building, including the former offices, copper domed roof pavilions, and the famous ballroom.
In terms of Flinders Street Station’s capability to withstand a seismic event, “one of the fundamental problems is the lack of lateral cohesion along the building’s length,” says Tumbas. “This means that the front and back walls might fall over. Generally, we addressed this by adding another layer of concrete to the existing flat roof to tie the front and back walls together. Where this was not possible, for example to the steel and timber vaulted roof of the ballroom, additional steel sections were fixed to the historic steel trusses instead”.
In the event of an earthquake, the stability of the station’s clock tower was also addressed. “The clock tower is very robust in character,” says Heffernan, “so interventions visible from inside the building were acceptable in this case. Tension cables were installed within the tower, clamping into the structure at Level 10 and extending down to Level 2, where they anchor into the solid brick walls. While there is a visual impact internally, the functional character of the space can accommodate it and the reading of the architecture from the exterior is not affected”.
As Loustau says, “Understanding the significance of the building, its external façade, and internal space is vital to working out the most appropriate way for seismic strengthening. Sometimes the appropriate solution is not the most technologically advanced or the most sophisticated. We are always pushing for the best solution”.