Timber and wood products used in buildings come in all shapes and sizes, with those products often divided into structural and non-structural categories.
Non-structural often comprises things such as:
• Cladding panels, and
• Decorative lining products of a variety of timber species.
While these products are thought of as conventional, there are some innovative products now on the market, including curved 215mm timber weatherboards manufactured by Woodform.
Several producers have developed and obtained New Zealand Building Code (NZBC) approval for conventional weatherboard systems which have improved detailing and fixing systems.
As well, weatherboards have now been approved for use under the NZBC which have been heat treated, rather than the more common treatment processes involving infusion of anti-decay chemicals.
For products used internally, there have been developments in the “reaction to fire” area where changes to the Fire Clause of the NZBC had restricted the use of clear coated timber products in public spaces.
By allowing the European Single Burning item test to be used to assign materials groups under the Code, a range of clear coatings with materials group ratings of up to 1S have been made available on the New Zealand market and at an apparently reasonable cost.
Structural products comprise the largest use of wood in buildings. Structural products range from smaller dimension sawn timber, through LVL and I beams, to glue laminated timber and cross laminated timber.
Sawn sizes of structural timber are being increasingly used in complex truss layouts, where the advanced engineering software, the increased reliability of graded structural timber and the ability to place Laminated Veneer Lumber (LVL) components in particularly highly loaded areas has extended the options available to building designers.
Overseas, applied research into fire-resistant systems has allowed the use of “light” timber framing systems to be extended to six-storey buildings. Work is progressing on this front in Australia and New Zealand.
Above the light timber frame systems come the heavy engineered wood components such as LVL and glue laminated timber (glulam).
Separately or together, these materials are extending timber construction into areas where it has not competed before.
Glulam comprises layers of sawn timber (usually) face bonded into beams of larger sizes than can be obtained by other methods.
The utility of glulam, LVL and Cross Laminated Timber (CLT) has been extended by the large investments made in CNC machinery which will saw, rout, bore and groove the timber products into components ready to be lifted, placed and fixed on site.
For example, details of one of the machines used by Timberlab Solutions Ltd show that it enables the processing of Glulam, LVL, CLT and solid timber elements up to 30m long, 4m wide and 600mm deep.
The speed and accuracy of this manufacturing process significantly decreases construction time and the time required on site for heavy lifting machinery.
All of this makes the production of extremely complex structures significantly more cost effective.
LVL is a mature engineered wood product, and engineers and architects are now learning how to best use its high structural and aesthetic properties.
LVL can be manufactured to required strength, stiffness, shear, bearing and connector properties, so for particular jobs LVL components can be manufactured to perform the job at optimum cost and use of resource.
The very high strength-to-weight ratio means loads are lower and, therefore, other structures such as foundations and connections can be smaller.
LVL is made in New Zealand by Nelson Pine, Carter Holt Harvey and Juken New Zealand.
CLT is the latest addition to the engineered wood construction armoury. While it has been used in Europe for some time, a New Zealand factory, Nelson-based X-lam, is now producing cross laminated timber panels, and has developed design and construction methodologies for local conditions.
The structural properties of CLT mean that the whole design envelope for wood structures has changed and, worldwide, larger and taller structures are being planned and built.
For example, a nine-storey CLT apartment building has been built in Melbourne Docklands.
A 300-bed backpackers building in Christchurch is under construction comprising CLT floors, walls and ceiling structure — approximately 5500sq m of CLT in total.
The building was modelled in 3D on CAD/CAM software, and all panels were CNC-cut to millimetre precision which allowed for a rapid build programme.
All these technologies can come together in ExPan timber buildings which were researched and developed at the University of Canterbury School of Engineering.
ExPan uses engineered wood beams which have steel tendons running along them, locking them to engineered wood columns.
The result is buildings which will move predictably in an earthquake but sustain no permanent damage.
An example is the Kaikoura District Council building. This post and beam gravity structure is made of Nelson Pine LVL, while the walls, or the lateral structure, are CLT and LVL. They are post-tensioned to create “rocking shear walls”.
This is the first time in the world a CLT and LVL combination has been used to make shear walls which are 70% CLT and 30% LVL.
The LVL is significantly stronger and was added to areas where there was maximum strain to provide added strength — in each end of the walls and within the core of the walls where the largest post-tensioning strain is.
A Potius stressed skin panel flooring system, fabricated using LVL, is used. The floor acts as a diaphragm between the shear walls and, unlike most commercial buildings, there is no concrete in the flooring systems.
Potius roofing panels made from a similar system — pre-insulated with polyurethane spray foam — and the internal finish, which provides the ceiling lining, is a pre-primed MDF surface.