Earthquake strengthening solutions in New Zealand
In early September 2010, Christchurch, New Zealand’s third largest city, was struck by a magnitude 7.1 earthquake. As violent as that quake was, the city miraculously escaped without any direct deaths or major damage.
But just six months later, on 22 February 2011 a second large earthquake struck. Even though this one was smaller (6.3 magnitude) than the earlier one, it was far more damaging because there was very high “ground acceleration”. Ground acceleration measures how much and how fast the ground, and the buildings on it, move during an earthquake.
185 people died in this second earthquake and 164 people were seriously injured. Many of Christchurch’s stone and brick heritage buildings were badly damaged. In total up to 100,000 buildings were damaged and 10,000 of them needed to be demolished. The total cost to insurers of rebuilding is estimated to be $40 billion.
A “real world” test for Sika structural strengthening systems
Prior to the earthquakes, the Christchurch Arts Centre was in the process of being restored. Considered the cultural centre of Christchurch, the Arts Centre is one of the city’s most significant heritage sites. It was established in 1873 as the University of Canterbury and is one of the New Zealand’s best examples of Gothic Revival architecture.
With a Category 1 rating from the New Zealand Historic Places Trust, the Arts Centre’s restoration plan required meticulous attention to detail and compliance with the highest standards of conservation practice. This extended to the earthquake strengthening project at the former Arts School building.
Working closely with main contractor Fletcher Construction and structural engineer Holmes Consulting Group, Sika New Zealand’s approved installer BBR Contech strengthened five of the Arts School building’s key internal walls with Sika’s fibre reinforced polymer (FRP) technology.
FRP offers a number of significant advantages to restoration projects such as this. Lightweight and with thicknesses of just 0.5 mm to 1.5 mm, it’s easy to apply while offering an impressive strength-to- weight ratio. It’s also corrosion resistant and can be covered with a variety of plaster finishes and coatings making it ideal for heritage building structural strengthening projects where the goal is to achieve a close-to-original finish.
Replacing original wall linings
“Because this is an historic building, we were required to remove then replace the original wall linings,” says John Hare, Director of Holmes Consulting. “Using FRP with an applied thickness of just 3mm enabled us to do this with little impact on the existing linings.”
After some rebuilding of the underlying original volcanic basalt stone walls, BBR Contech applied about 200 m2 of SikaWrap®. The internal linings were then reinstated to return the rooms to near-original condition with no visible sign of the restoration work beneath.
The big earthquake of 2010
Then came the big earthquakes. The September 2010 7.1 earthquake and the devastating 6.3 earthquake in February 2011 subjected the strengthening work at the Arts Centre to some dramatic real life testing under extreme seismic loads. The strengthened buildings performed very well and to expectations. The former Arts School was one of the few buildings on site to suffer only minimal damage.
- New Zealand has an estimated 15,000 to 25,000 “at risk” buildings
- 8% to 15% of building stock is earthquake prone
- Most of these structures were built between 1880 and 1935
- The New Zealand Government is proposing a tough new strengthening regime
- The Government wants 193,000 major structures assessed by 2018
- Affordability is a huge issue for building owners
Earthquake strengthening makes the front page
Earthquakes are not uncommon in New Zealand, yet many of the country’s heritage buildings lack “earthquake resistant” structural strengthening.
The New Zealand Government estimates there are 15,000 to 25,000 at-risk buildings in the country. A national register of these buildings is being compiled and building owners have been negotiating with the Government to establish a deadline for when those buildings will have to be either structurally strengthened or demolished.
Over 10,000 aftershocks
The risk will not diminish either - there have been over 13,000 aftershocks in the Christchurch area alone since the first major quake in 2010.
In July and August 2013, Wellington, the nation’s capital city, was hit by two strong (6.2 and 6.3) quakes. They were the most significant earthquakes Wellington had experienced in decades. Even though damage was minimal, these quakes were a sharp reminder to commercial building owners and politicians of the need for urgency in seismic strengthening.
Urgency of seismic strengthening
Not all commercial building owners were waiting for the Government’s lead however.
In December 2013 Wellington’s main newspaper the Dominion Post ran a lead story on Sika’s strengthening system being installed in a heritage building in the city.
Newspaper Story (edited): “Like many Wellington buildings, 61 Thorndon Quay is below what is considered safe in a quake, meeting about 50 per cent of the new government building standard. The company which recently bought the building, wants to convert it into a new apartment hotel complex, and aims to get it up to 70 per cent of the standard. The building's columns however don't have enough ductility, which means they could crack in a quake.”
Project structural upgrading engineer Marc Stewart says steel is what provides the flexibility for columns to survive a quake. ‘But the ones at 61 Thorndon Quay don't have enough of it. In a quake they would stay rigid, and seismic force would shake and crack them. Until the 1990s, the most common way to strengthen columns such as these was by adding more steel and concrete to them. You lose some of your inside space, and you're adding mass to the building, so it's strengthening for the sake of strengthening,’ Mr Stewart said.
Now, steel is often replaced with something different - a carbon-fibre weave wrapped around columns and stuck in place with epoxy glue. The wrap works in the same way as steel, by allowing the column to move with the quake, absorb its energy and dissipate it.
BBR Contech installed the carbon fibre, developed by building technology company Sika, on four floors of the building. It will also be used on some of the ceiling beams, which need work after the interior is remodeled.
The wrap can also be inserted into unreinforced masonry, or used on the exterior of a building and covered with facade - a possibility for some of Wellington's heritage buildings needing work.”
Heritage bridge strengthening using CarboDur
When it was completed in 1910, Grafton Bridge was at the forefront of construction technology, boasting the world’s largest single span of reinforced concrete.
The 296-metre-long bridge, which soars 43 meters above Auckland’s central city Grafton Gully, has attracted international acclaim as an engineering structure of unique value.
Almost 100 years after it was built, the bridge was strengthened as part of the Auckland Central Connector transport project. This has provided it with essential seismic resistance (enabling it to withstand a one-in-1000-year earthquake) as well as the capacity to carry increased bus traffic and cope with future transport innovations such as light rail – all without altering the bridge’s appearance or changing its heritage status.
Which material was needed to strengthen the bridge?
As part of the strengthening project, over 600 metres of Sika CarboDur® FRP strips and 830 CarboShear L plates were attached to the bridge structure. The CarboDur® strips were applied to the underside of the beams to provide additional mid-span moment resistance.
The CarboShear L plates were installed in pairs around the beams and up into the deck slab to improve shear performance.
Completed well ahead of schedule, the work has once again placed this iconic structure at the forefront of concrete construction technology, ensuring that the grand old bridge will continue to serve Auckland for many generations to come.
Experience the strength of Sika’s FRP solutions on You Tube!
A panel of unreinforced fibreboard breaks easily when 50 kg is placed on top. But after being reinforced with CarboDur®, a same sized panel will support 150 kgs plus 80 kgs of Navindra.
Using two thin concrete beams, one reinforced with a single strip of CarboDur®. While the unreinforced beam won’t support the weight of one person, the Carbodur® strengthened beam supports seven people easily.
“One key strength is that the core message of how exceptional our FRP products are under load can be easily understood in just a few minutes by anyone from a commercial building owners to architects or structural engineers.” says Peter Withell General manager of Sika New Zealand.