Following the 2010 and 2011 Christchurch NZ earthquakes, engineering scholars mobilised to try to devise solutions to the utter devastation that was seen in so many buildings that crumbled to the ground. The human cost of the 2011 quake was 185 lives but could have stretched into the thousands, according to Professor Andy Buchanan of the University of Canterbury in Christchurch.
Since then and a new product has been developed at the University in collaboration with PreStressed Timber Limited (PTL). Pres-Lam is pre-stressed laminated timber. It comprises glulam beams, strengthened by inserting compressed wood blocks into pre-cut rectangular cavities, with one third of the beam depth from the top of the beams. Professor Buchanan and his cohort of Italian colleagues found that the rocking system constructed out of wood produced results that were as good, if not better, than with reinforced concrete.
A steel rod is threaded into the cavity, stressed up and anchored. Pres-Lam relies on straight or draped tendons in the same way as pre-stressed concrete does. Inside the beam is much like a suspension bridge. The first building in the world to use this system is a three-story Arts building at Nelson Marlborough Institute of Technology in NZ. Construction features large LVL (Laminated Veneer Lumber) walls in both directions and a timber/concrete composite floor. This kind of construction is appropriate for any kind of ‘big box’ industrial or retail building with a lot of open space.
In earthquake zones, internal or external steel reinforcement can be coupled with the steel cables to provide extra strength and to dissipate energy. This adds up to a structural system that mitigates the kind of damage caused by earthquakes. Furthermore, Pres-Lam buildings in earthquake-vulnerable regions can be designed with a damage-avoiding structural system by making these additional elements replaceable in the case of a serious earthquake.
Professor Buchanan says the world is moving towards low damage buildings. “It’s not good enough to save lives,” he says. “You have to design buildings so you also save property. A low damage building is one that has controlled damage that’s in a place that can be repaired.”
Though originally designed with earthquakes in mind, Pres-Lam construction is fast and economical which is appealing even outside of areas prone to seismic activity. Pres-Lam can be used in conjunction with GLT, CLT and LVL products.
Onward and upward with damage-avoiding CLT structures
In late 2018, the world’s largest outdoor earthquake simulator, operated by structural engineers at the University of California San Diego received a multi-million-dollar grant to expand its testing capabilities. “It will be able to reproduce earthquake motions with the most accuracy of any shake table in the world,” according to the institution’s Professor Joel Conte. A full-scale 10-story building constructed from cross-laminated timber will be the first structure to undergo testing on the upgraded shake table. It’s believed that the resulting data will facilitate the design of wood buildings up to 20 stories, that will not suffer significant damage during earthquakes. The ultimate hope is that occupants can exit the building unharmed and the building can be re-entered and occupied again shortly following the event. The shake table will be out of action from February 2020 until July 2021 while the upgrades are underway.
