Insights on timber building systems

This is a part of the Net-zero carbon buildings feature

By David Carradine - 1 February 2021, Build 182

As interest grows in New Zealand for timber in mid-rise and high-rise buildings, a BRANZ survey has identified what engineered wood products are being used and whether there are barriers limiting uptake.

AROUND THE WORLD and throughout New Zealand, the desire for larger and more-innovative timber buildings continues to grow.

Benefits and innovation broaden use

There are good reasons to utilise timber for multi-storey buildings, including a strength to weight ratio that results in lighter structures. This means they can have smaller foundations and potentially be constructed on ground that would be difficult for heavier structural systems.

Innovations in timber over recent decades have led to effective systems for creating high-performing multi-storey timber structures that can survive significant seismic events and be reoccupied quickly afterwards.

Timber is also able to store carbon for the life of the building. This suggests larger timber buildings will be sequestering more carbon and start their service life with potentially less embodied energy needs to be offset.

What systems are used in New Zealand?

For these reasons, it is valuable to know more about timber building systems currently used in New Zealand and the resources available to help facilitate their construction. This article describes the learnings from a survey-based research project designed to better understand the use of timber in New Zealand buildings.

A wide range of options exist these days for structural timber due to the development of engineered wood products (EWPs) such as cross-laminated timber (CLT), laminated veneer lumber (LVL) and glue-laminated timber (glulam) as well as various panel products such as plywood and other proprietary timber-based panels. Graded solid timber is also widely used for 1 and 2-storey houses as well as multi-storey applications.

BRANZ guidelines published in October 2019 (see www.branz.co.nz/LTFBuildings) provide guidance for designing resilient multi-storey light timber-framed (LTF) buildings in New Zealand.

BRANZ survey into EWP use

If using more timber in buildings and creating larger timber buildings is a potential way of reducing greenhouse gas emissions and sequestering more carbon, are there barriers to incorporating these innovative EWPs within buildings? Are there ways of increasing the uptake of timber-based building materials while still ensuring that all New Zealand Building Code requirements are met?

These questions formed the basis for conducting a BRANZ survey during 2019 to understand the appetite for EWPs in buildings and the perceptions that exist about them.

The online survey included 17 questions, and a total of 474 surveys were completed. A wide range of occupations were included so that multiple perspectives throughout the building industry could be gathered and understood. Information was also sought on perceived barriers to the use of EWPs and how the uptake of these materials could be increased.

Barriers identified

The main themes that became apparent from the survey responses included cost, availability, regulation, information on available products and education about how to use EWPs effectively for suitable applications. These were apparent from the responses, but the rationales behind them were complex.

Greater upfront cost and the limited availability of EWPs were related and frequently noted as barriers to using EWPs. The most significant regulatory concerns were around compliance pathways and increased prescriptive design methods.

Having Acceptable Solutions was seen as helpful, and they should include a better understanding of how EWPs could be used as replacements for other materials without compromising the integrity of the building. The need for more information and awareness raising were the most commonly cited ways of reducing adoption hurdles and increasing the use and uptake of EWPs throughout the New Zealand building landscape.

Resources widely available

Fortunately, there are numerous resources available as well as research projects at BRANZ that can help raise awareness and inform the building sector about using EWPs and the implications of their use.

From manufacturers …

EWP manufacturer information on mechanical properties, durability and the requirements for Code-compliant installation and maintenance exists. Many manufacturers provide this information, and having it easily accessible and understandable can help to increase exposure and acceptance of EWPs.

Designers, builders, engineers, building owners and compliance officials also need adequate information about EWPs so that substitutions and informed decisions can be made when choosing between different materials and products.

The Wood Processors and Manufacturers Association of New Zealand (WPMA) in conjunction with NZ Wood have created a series of guides (see www.wpma.org.nz) aimed at providing additional information for the use of timber in buildings. These include a significant amount of information on EWPs as well as other multi-storey timber applications.

Information on the environmental impact of different EWPs is also available through WPMA as environmental product declarations (EPDs) for a range of products and treatment options for use in New Zealand (see https://bit.ly/2KuzGz8).

Survey respondents want more

Increased education about EWPs was suggested across the building sector survey participants, from the EWP suppliers to the owners and developers of buildings who are making decisions about materials to use.

Case studies and specific data on potential cost increases and potential savings from designing and building using EWPs were seen as a very helpful tools for informed decision making. Better understanding of EWP durability and treatments used within New Zealand was also noted as important information to have when comparing building materials.

Engineers and designers should be made aware of existing online design aids that result in efficient and Code-compliant designs. Consenting officials would also need to understand the outputs from manufacturer supplied tools to be able to evaluate the use of EWPs and their applications and substitutions for other materials.

Timber-related research at BRANZ

BRANZ also has a role in providing information on EWPs and their use in New Zealand buildings. As noted previously, there is guidance on designing multi-storey LTF buildings, but research is also under way on a range of timber-related topics. These include:

timber durability
the use of adhesives for EWPs
fire resistance of timber buildings including mass timber such as CLT
design of hybrid structures that include LTF in conjunction with other structural systems
timber-based structural insulated panels (SIPs)
scoping projects on the use of CLT
specific construction methods used with timber buildings.

Research on building sustainability, while not specific to timber, has resulted in tools that help with conducting life cycle analyses so that different material combinations can be evaluated for carbon impacts.

Knowledge will continue to grow

There are clear indications that more timber is being used for larger building applications.

Work by BRANZ has identified some potential ways of helping link the building sector with the information needed to effectively make decisions about using timber. Currently, there are resources that can help, and research continues to provide knowledge and recommendations on timber usage in buildings throughout New Zealand.