A New Zealand First

YHH applies innovative new engineering principles that were originally designed for concrete to NZ manufactured Laminated Veneer Lumber. Consent was gained and construction commenced late 2012, immediately outside the red zone, despite the uncertain and frequently changing regulatory landscape.

The structural engineering of YHH necessitated engineering vision and an investment of time and resource at a time when conservatism and risk aversion was the order of the day. As a landmark building for the Canterbury Rebuild, YHH also highlighted the value of engineering excellence across such criteria as; cost efficiency – due to its rapid erection and lighter weight than comparable buildings, sustainability in terms of a locally sourced and manufactured building material with known fire-resistant properties and building aesthetics derived from exposing the building frame due to the highly visual and tactile use of LVL.

Innovation

Post-tensioning technology initially overcame the challenge of connecting large concrete structural elements. Its adaption and application to relatively new laminated veneer lumber provided advantages by reducing construction time (concrete curing time avoided) and reducing construction cost (a consequence of faster erection and the lighter weight).

The beam-column connection is designed as a rocking mechanism, allowing gap opening at the beam column interface. The post tensioned steel tendons within the beam are rather like a large elastic band – they elongate as the frame moves and the joint opens up. As the post-tensioned strands stretch they work to pull the frame back together removing all residual displacement in the building.

At the out tension and compression zones, externally mounted mild steel energy dissipaters are installed. These have been designed to hit first yield at a 1 in 500 year seismic event. Control of the stiffness and displacement of the frames is achieved by opening and closing the joint interface enabling the structural elements to be protected from damage.

Kirk Roberts designed multiple iterations of the joint connections, targeting economy in design, ease of construction, and architectural expression. One of the key challenges posed by the LVL over concrete is its low compressive strength perpendicular to the grain and its lower stiffness. Being more flexible than timber, the beams can deform more in the elastic range than concrete , meaning larger drifts are required before gap opening can occur.

The gap opening was targeted through stiffening up the beams by making it composite with the concrete slab, although a zone around the columns was allowed to remain non-composite. The columns were protected by steel plates to protect the member from the high perpendicular compression resulting from the moment at the beam face.

Continuing with the timber theme, the flooring system on levels one and two was timber-composite concrete. This consisted of LVL joists (every other joist doubled for fire), with plywood infill and a concrete topping.