The early stages of the project involved assessing the cost and benefits of using cross-laminated timber (CLT) combined with Laminated Veneer Lumber (LVL) beams against conventional steel and concrete construction. “The pricing came back very similar for both options so, based on Otago Polytechnic's preference, we went with the timber option, given all the sustainable aspects of it.” Rui Li, Kirk Roberts' Structural Engineer explains.
“Generally you would've expected the timber option to be more expensive, but with our technical capability we were able to design it in a way that would be competitive against conventional construction. We don't believe in providing anything stock-standard and are passionate about pushing boundaries to deliver leading engineering solutions.”
Kirk Roberts has subsequently gained feedback from other developments that, based on the Otago Polytechnic project, the build cost was “significantly less” than comparable projects of timber construction.
Key to the project's efficiency was the ductile design for earthquake resilience instead of an elastic design; these are different means of dissipating the energy generated in an earthquake. “You basically have a defined point in the building which acts like a fuse where you get rid of all of the earthquake energy at that particular point.”
While engineering this system involved much greater technical expertise, the result was cost savings on materials because of the ability to use smaller structural members than if the ductile system had not been incorporated.
CLT panels are up to 80% lighter than tilt slab concrete panels so made it faster and easier for builders and sub-trades, while the prefabrication of the CLT panels streamlined the construction programme. The use of light timber also meant smaller foundations were able to be specified.
The building's foundation comprises concrete beams supported by 700 columns of stone rammed into the ground three to four metres deep. “Because of the light-weight nature of the building we were able to do that option instead of deep piles because of the ground being on reclaimed land.” Rui says a timber structure of the scale of Otago Polytechnic was a first for Kirk Roberts and, from the outset, the company's engineers were “definitely keen to be involved.
The build cost was “significantly less” than comparable projects of timber construction.