TIMBER BRIDGES
HOFFCON.PRO
We see a future where modern engineered timber bridges play a role in New Zealand’s bridge stock and we are driving this change in the industry.
Strong | Light | Durable | Sustainable
Timber is a material that has been designed by nature to build trees, its tallest structures. It's been used for millennia to cross rivers and span chasms. Trees take sunlight, water and CO2 and turn it into the most sustainable building material available to us today.
Modern engineered timber takes this material, which nature has refined over eons, and tailors it for our purposes. ​Hoffcon are part of the Waka Kotahi NZ Transport Agency working group for the development of the next generation of sustainable, durable, and cost-effective timber bridges in New Zealand.
These are bridges that carry trucks, trains and people, but don’t cost the earth.​ Speak to us if you would like to know more about what modern timber bridges have to offer to you.
Strong
Modern engineered timber can be as strong as concrete
Light
Modern engineered timber can achieve the strength to weight ratio of steel
Durable
When properly treated and detailed, timber bridges can last over 100 years
Sustainable
Sustainably sourced timber bridges have a negative carbon footprint and are grown in our back yard
Abundant in our backyard
Native timbers were originally used for bridges across New Zealand. However, today New Zealand grown, exotic, fast growing, easily treated trees supply timber within New Zealand, along with the rest of the world.
New Zealand provides ideal conditions for growing Radiata Pine, and because of this, we grow a lot of it! We grow so much that the majority of it (~75%) is sent overseas.
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Unlike steel and concrete alternatives, timber for bridges in New Zealand is able to be grown, felled, processed, and fabricated here in New Zealand. This saves on transportation which is costly for the planet, and costly for your back pocket.
It also provides jobs for New Zealanders and allows us to add value to the raw logs that would otherwise be shipped overseas. This also minimises the risk around supply chains which we have seen are susceptible to disruption from global events, like Covid-19.
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Strength to weight
Timber has an excellent strength to weight ratio. This means that timber bridges are generally lighter than their equivalents in concrete and even steel. This provides benefits under gravity loads, as the bridge doesn’t have to use as much of its strength to hold itself up, also minimising foundations. While it also provides further benefits in areas of high seismicity (like much of New Zealand) where this reduced weight in turn reduces the earthquake loads that need to be resisted
Timber bridges at Hoffcon
In 2021, Hoffcon came to NZ Transport Agency and presented a future for bridges in New Zealand where modern engineered timber played a significant part in the bridging stock. The vision included a suite of standardised timber superstructure designs and details that could be adapted for specific instances, in a similar way to what is available for prestressed concrete girders. Out of this Hoffcon worked with the Transport Agency to set up the Timber Bridges Working Group.
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In 2023, Hoffcon was engaged by the Waitaki District Council to undertake the design of the 162m long Kakanui Replacement Bridge. This bridge carries a single lane of vehicle traffic along with a footpath across the Kakanui River, south of Oamaru. The replacement design uses glulam beams and plywood decking to cross the river in 11 spans. Timber was selected as the preferred material for the design due to its cost advantage. The light weight of the structure meant that foundations were minimised and the requirements for large cranes to lift bridge elements was reduced. The bridge is currently being tendered for construction.
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Timber bridges are one of our core passions at Hoffcon. We believe that there are many bridge applications where timber is the right solution. However, our biggest love is for bridge. So, where timber is not the right fit, we are more than happy to design an efficient steel or concrete structure that meets, and may even exceed your expectations.
Designing bridges with timber in Aotearoa
If you need a bridge for cars, trucks, trains, cyclists or pedestrians, here is some information that will help you to understand the things that cost-effective, high performance, sustainable timber bridge can do.
If you are looking for design guidance for timber bridges the best places to start are the Australian Bridge Design standard, AS 5100 which has a section AS 5100.9 dedicated to timber bridges. NZS1720 also provides great NZ specific guidance on the general design of timber structures, or there are the NZ Wood Design Guides which are available online here.
Durability
Over this period there have been significant advances in timber treatment, which allows timber that might otherwise decay over 10-20 years, to be structurally sound after 50 years even when in the worst conditions for decay. Generally, structural elements that are in contact with the ground will be protected by concrete or other materials, but with well-considered detailing, bridge decks are able to be kept relatively dry, and if they do get wet, are able to dry out naturally. This detailing, combined with suitable treatment allows for a 100-year design life to be achieved in accordance with the Waka Kotahi Bridge Manual, through the Australian timber bridge design standard AS 5100.9. One significant benefit of New Zealand timber is that Radiata Pine is easy to treat. While many foreign timbers will only take an envelope (perimeter) treatment, leaving the interior of the timber section untreated, Radiata Pine is able to absorb treatment systems throughout its section, reducing the risk of interior rot. Cutting of many foreign timbers would expose untreated surfaces and require secondary treatment, but cutting of treated Radiata Pine does not require such secondary treatment (though it is common to touch up such surfaces). With the proper treatment and consideration of detailing, timber bridges can match the durability performance of their steel and concrete counterparts.
Modern Engineered Timber
The term ‘Engineered timber’ covers a wide range of timber products. The most common of these would be: •Plywood – where thin (3mm thick) veneers are peeled from a log and then glued and laid on top of each other in perpendicular orientations to form a sheet. •Glue laminated Lumber (Glulam) – where 90mm x 45/35mm planks are glued together while staked on top or beside each other to form a beam. •Laminated Veneer Lumber (LVL) – Similar to plywood, where thin (3mm) veneers are peeled from a log, but are then laid and glued on top of each other in the same orientation before being cut to form beams. Engineered timbers all follow a similar philosophy, which is to take timber logs, process them into smaller planks or sheets, grade them, and then reconstruct them into the required geometry using adhesives. This process assists in reducing the natural variability in the timber that comes from using an organic product. Where one knot in a natural timber beam could reduce its capacity by 50%, by slicing that timber up and redistributing the knot through the beam, rather than being concentrated at one location, the effect of the know may only result in a 5% reduction in capacity. Through this process engineered timber becomes a far more consistent material. Historically, the size of timber beam that you could create was limited by the size of the tree that you cut down. For bridges this was a serious limitation. However, with engineered timber, this is no longer the case. Engineered timber producers are able to manufacture beams that are 2m deep and 30m+ long, and the limitation now comes from the transportation requirements and the size of the processing equipment. Engineered timber has been in used in New Zealand in the building industry since the mid 1900’s, and the first patent for a glue laminated timber construction was lodged in 1901 in Switzerland. The science had been refined over the last 120 years, and these days designers are looking to include other material such are carbon fibre and other modern materials into their beams to improve their performance even further.
Density
One of the greatest advantages that timber has over other materials is its light weight (low density). New Zealand Pinus Radiata has a design density of 550kg/m3 (AS 5100.9 Table A7) which is four times less than concrete and 14 times less than steel (note that 700kg/m3 is often used in design to account for wet conditions). For bridges, where the bridge self-weight if often as much, if not more than the weight of what is it supporting, this is important. When comparing properties with other materials, the density becomes a key consideration.
