On November 2nd, Timber Unlimited hosted the first Hybrid Buildings Seminar in Auckland in collaboration with Timber Design Society (TDS) aimed at improving best practice in the industry especially as it relates to engineered timber.
Hybrid buildings typically include those that have timber as structural elements or systems, in combination with one or more other structural materials such as steel and reinforced concrete. The hybrid system enables efficient use of each of the structural materials to advantage and results in building designs that are commercially competitive, with reduced embodied carbon relative to buildings that use only steel and or reinforced concrete. They provide a lower resistance path for industry to get more timber into the built environment in the short and medium term.
In terms of cost, structural integrity and aesthetics, mass timber compares very favourably with concrete or steel structures.
Speaking at the seminar, Andrea Stocchero from the Ministry of Primary Industries (MPI) agreed the overall performance of hybrid structures that include mass timber structural members can provide additional benefits to the performance of structural steel or concrete structures alone, while noting that there is an appropriate place for all. MPI has an interest in supporting timber and wood products because of their forestry industry remit.
When projects include timber, they not only add visual warmth to the finished build, they can optimise strength and flexibility, provide improved durability, fire performance, and support, and improve lower carbon emissions to address sustainability criteria. To achieve these potential improvements, though, optimising procedures at the early concept design stage is critical.
Considering all elements of the structure from its earliest design and costing stages was something every speaker called for.
As Principal Chris Scott from Jasmax pointed out, not only can timber’s potential for end-of-life reuse deliver sustainability credits, timber flooring can sit on concrete rafts above Potius beams to offer significant carbon savings. With a carbon lens over the whole project, timber comes into its own he believes.
Designing for manufacturing and assembly (DfMA) is critical for engineered timber components, explained Andrew Hewitt of Red Stag TimberLab. Local manufacturing capacity is limited at the moment, sometimes leading to longer lead times for mass timber elements that can interfere with smooth construction timelines - so an understanding of such challenges and designing efficient solutions and involving the manufacturing suppliers early is critical to mitigate this potential outcome.
Accurate costing of all timber elements at the design stage is critical, he believes. Specifying the right type of engineered timber for a given purpose is also important, as it likely will drop the costs.
In New Zealand, Cross-laminated timber (CLT) in floors is typically a very cost-effective flooring option due to the low quantity of required joints and connectors and the rapid speed of erection, reducing overall construction costs. Also, considering surface finish, if an element won’t be seen when the project is completed, it need not be finished to the same standard as visible elements. This offers substantial cost savings for the client, while performance under load isn’t affected.
Auckland Council representatives Ed Claridge and Jeff Fahrensohn have been dealing with the applications of a large, number of hybrid and timber buildings recently. They encourage developers, designers, and all interested parties to attend pre-construction meetings with Building Inspectors at concept stage, before any plans are finalised, or at least use the Council’s online portal for help at the beginning of a project.
Common mistakes can include, not planning for water access for firefighting. (sprinklers are only effective once the building is complete and the system is activated); lack of demonstrating an understanding of fire performance data beyond the standard tests, since fire resistance testing can be highly problematic; and the oft-repeated plea for structural and fire engineers to work together.
Initial costings should also allow for something as “simple” as specifying the correct fastenings for each material, and understanding the often-unexpected ways in which materials can interact with each other through these connections. Chipped or incorrectly applied protective coatings can also significantly weaken a building’s performance in high-risk events likes fires or even long-term durability and they explained how comprehensive documentation will help with maintenance issues including moisture control
There is always pressure from asset owners to keep expenditure as low as possible, and Alain McKinney, Precinct Properties’ Project Director for the Wynyard Quarter in Auckland, described some of the procurement lessons from using timber for the Flowers/Pipiri Lane project in Auckland’s Wynyard Quarter.
The Flowers Building is a mass timber, three-level annexe to 124 Halsey Street. Sustainable building practice is central to its design, and consequently the Flowers building is designed as a seismic resilient system with a 6 GreenStar rating and a 5-star NABERS (National Australian Built Environment Rating System NZ) rating. As such, 100 percent of the carbon emissions from construction will be offset.
Despite some unexpected cost increases, mostly due to the company’s unfamiliarity with mass timber construction and associated timelines, Alain is keen to pursue timber and hybrid building options wherever it makes sense, and he loves the Flowers project. On a recent visit to the USA for a timber conference, he saw the way timber is used there, and was very complimentary of the clarity of building regulations for such projects in California. He believes hybrid timber buildings will be more widely specified in New Zealand when their potential is better understood, and local regulations have caught up with technology.
He also believes in designing a timber building from scratch, rather than “squeezing timber into a steel and/or concrete building setting.”
Dunning Thornton Consultants are one of New Zealand’s most experienced structural engineering companies, and Nikki Vance presented a case study using 90 Devonport Road, Tauranga as an example of mass timber hybrid construction.
When completed, this will be an eight-storey curtain wall structure with lateral steel bracing and CLT core. It will achieve a 6-star GreenStar rating. A hybrid structure was chosen because it’s believed this is the most efficient type of structure for the intended purpose and location, especially for soils where structural loads need to be minimised. Timber weighs 4-5 times less than steel or concrete, so foundation systems and depths can be significantly reduced - something that is becoming increasingly recognised across the country.
This building will have a 60-minute fire rating, taking known solutions for code compliance and refining them upwards for a more robust structural system. Timber moisture is managed carefully, especially since the site is reasonably close to water. Any vibration issues are resolved by working closely with the specialist consultants to ensure an optimal spread of high axial loads.
Experimental work being carried out by BRANZ acknowledges hybrid buildings will be the future. Current testing is more focussed on the whole system performance of light timber framing in a wide range of situations, and in combinations with SIPs (structural insulated panels) and plasterboard.
Clearwater Quays Apartments won the 2023 NZ Timber Design Awards category for Mid-Rise Building Design. It was built by contractor Blair (Blu) Tipler’s company Kobe Construction, the first construction company to build a mass timber, mid-rise (five storey) residential project in New Zealand.
He quickly learned that if the drawings didn’t contain enough detail, it would cause problems on site - which is why Blu believes sharing education and knowledge is so important.
One of the advantages of using mass timber as part of a hybrid structure is its ability to be prefabricated to extremely tight tolerances, leading to significant shorter construction sequences and rapid installation on site. He commented that structural steel must also be specified with extremely tight tolerances, matching those of mass timber to avoid disruptions on site.
Low precision welds can warp steel members by millimetres, for example, which means contractors must spend additional time adjusting panels, beams and/or columns to ensure correct jointing for high demands.
Managing moisture content across all utilised timbers delivered to site was very important. For example, keeping the LVL under cover, but with good airflow was crucial until it was installed, he found – warehousing with good site access was ideal, since damp timber won’t dry quickly, especially when there’s a lot of it.
Timber ensures carbon sequestration. Clearwater Quays’ reporting showed that timber construction saved 87,400kg of carbon dioxide compared to steel or concrete construction, thereby rendering the project carbon negative.
The hosts, Dr. Robert Finch, Director of Timber Unlimited (TU) and Dr. Daniel Moroder, President of the Timber Design Society (TDS), pointed out that using timber as part of a hybrid structure can solve design challenges and create structurally efficient, robust, low-carbon solutions that will assist New Zealand reduce its greenhouse gas emissions and transition to a low emissions economy.