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Structural timber: Technical growth pays off

Engineers are responding to advances in structural timber with some imaginative designs, as Dave Parker reports.

The first two examples of innovative earthquake-resistant post tensioned timber construction have opened in Christchurch, New Zealand, three years after the destructive earthquakes that destroyed much of the city. The three-storey Merritt building and the two-storey Trimble building feature engineered timber load bearing structures designed to absorb seismic movement and “self centre”, minimising damage and allowing the buildings to be back in use very quickly.

New Zealand innovation: The timber structure of the Merritt building in Christchurch

The timber structure of the Merritt building in Christchurch

More buildings using a similar design philosophy are under construction. Post tensioned timber construction is now a government-backed design option that is seen as a key factor in the city’s recovery from the devastating quakes of 2010 and 2011, in which 185 people died.

“A lot of conventional buildings in Christchurch have to be demolished even though they didn’t actually collapse during the earthquakes because the cost of repairing damage to the structural frame is too high,” says UK-based Smith and Wallwork Engineers senior engineer Katie Symons.

“These new systems take advantage of timber’s inherent seismic-resistant properties - light weight and high strain to failure. And there is a strong local tradition of timber construction, and ample timber resources, mainly Radiata pine.”

Timber options

Timber’s emergence as a mainstream structural alternative to steel and concrete was triggered by the development of effective adhesives based on synthetic resins.

Post-WWII, these enabled the creation of an entire new family of materials, collectively dubbed engineered timber.

Until then, timber was seen as a naturally variable material almost impossible to analyse and design with the same degree of rigour as other structural materials.

Glulam (glue-laminated) timber was one of the first engineered timbers to be accepted as a viable structural material.

Thin strips of wood from which all flaws have been cut out are glued together and built up into larger sections in a relatively low technology process.

Both hard and softwoods can be used, and curved or arch members are simple to fabricate.

Like plywood, laminated veneer lumber (LVL) is based on thin veneers peeled from logs and bonded together in sheets under high temperature and pressure.

Unlike plywood, the grains of the veneers run in the same direction. LVL has many advantages, but it does require high capital investment and high demand to keep costs down.

An increasingly popular option in Europe depends on the availability of high quality softwoods.

Cross laminated timber (CLT) is built up from up to seven layers of solid boards 10mm to 15mm thick and up to 240mm wide, glued and stacked together orthogonally and bonded under pressure.

All three options lend themselves to offsite prefabrication, speeding construction significantly. They are also perceived as much more sustainable than traditional materials.

 

Symons inspected several projects using this new approach during a 2013 study tour of New Zealand funded by the Institution of Structural Engineers Educational Trust. She says the developments are an excellent example of productive collaboration between the local timber supply chain and the design and academic community, backed by government funding.

The building is designed to resist a one in 2,500 year seismic event, well above current code requirements

One of the key technologies is Expan, developed by the Sustainable Timber Innovation Company, a joint academic/industry research consortium. On the Merritt building, designed by architect Sheppard & Rout and engineer Kirk Roberts, steel tendons pass through the centre of the primary laminated veneer lumber (LVL) box section beams and columns that make up the seven portal frames that form the main structure. Energy dissipaters - dubbed “plug and play” connectors or “structural fuses” - containing necked steel bars are fitted at the moment connectors (see diagram).

In a major earthquake the building will rock, opening up gaps between the horizontal and vertical sections of the portal frame. Energy will be dissipated by the ductile yielding of the necked bars. Once the earthquake is over, the elasticity of the tendons will close the gaps and the structure will self centre. The necked steel bars are claimed to be simple and economic to replace.

Dissipaters on show

The design of the building, which opened in March, was developed very soon after the 2011 earthquake. Symons says there was a natural desire to have the dissipaters “on show” to emphasise the building’s seismic preparedness. She adds: “The building is designed to resist a one in 2,500 year seismic event, well above current code requirements.

“Timber is often thought to be the expensive option, but the cost of the 1,800m2 Merritt building is comparable with traditional construction.”

The construction manager on site told me that a key benefit of the timber approach was the simple site management, as only a single gang of workers were involved

The Trimble building, which opened a month later, also uses the Expan system, but with Macalloy bars rather than tendons. Designed by Opus, it replaces an earlier building that had to be demolished after the earthquake, and consists of two, two storey blocks with a total floor area of 6,000m2.

Again, post-tensioned LVL is used for the main structural frame, with box section columns and beams fitted with structural fuses similar to those on the Merritt building. LVL shear walls contain vertical voids housing the Macalloy bars, and are fitted with structural fuses at their bases.

An Australian-developed timber/concrete composite system forms the upper floor. “The construction manager on site told me that a key benefit of the timber approach was the simple site management, as only a single gang of workers were involved,” says Symons.

Timber joint

Timber joint

She explains that on these projects, the post-tensioning had a different purpose than traditional post-tensioned prestressed concrete: “The real point is to solve the problem of conventional massive moment connections between the structural elements. This approach is potentially much simpler and cheaper. Research into the Expan connection design continues, and the system could have applications in seismic zones elsewhere in the world.”

Long span LVL portal frames are now making significant inroads into the industrial/agricultural sector in New Zealand, Symons adds. These are based on another variation of the Expan system, involving “quick connect” connections developed at the University of Auckland. These not only speed erection on site, but also act as easily replaced structural fuses in the event of an earthquake.

“Initial cost is around 15% higher than traditional steel, but these are more than offset by the savings in programme,” Symons reports.

She adds: “It’s interesting to note that many of the planned post-earthquake buildings in the city are to be timber structures. New Zealand has a history of developing successful home grown industries, and locally produced LVL and now cross laminated timber are available on a commercial scale.”

Norwegian heights

Timber tower

Timber tower

Construction of the world’s tallest timber tower has begun in Bergen, Norway. The 14-storey Treet apartment building will be 49m high, taking the record from Melbourne’s 10-storey, 32m tall Forte building, and will feature a glue laminated (glulam) timber frame and a central cross laminated timber (CLT) core.

Client for the project is Norwegian housing association BOB, and the structural designer is Sweco, whose project structural engineer Rune Abrahamsen says his practice has done feasibility studies on timber buildings of up to 20 storeys.

“We found they were structurally feasible, but somewhat more expensive than steel or concrete,” says Abrahamsen.

“But erection time is significantly shorter, and the carbon footprint is much smaller.”

The Treet building’s main structural frame is made up of glulam members up to 1m square. This would easily achieve a 90 minute fire resistance without additional protection, Abrahamsen says.

The central service core, which contains the staircases and elevator shaft, is not connected to the main structural frame.

“Connecting CLT and glulam would have led to design complications we are not yet comfortable with,” Abrahamsen says.

Construction will be speeded by the use of prefabricated timber frame pods for all 62 apartments.

These are assembled inside the main frame in stacks four high, and comply with the onerous Passivhaus standard for sustainability.

With such a lightweight frame, wind-induced sway could have been a problem, but three concrete floor levels have added enough mass to bring sway into acceptable limits, Abrahamsen reports.

Glass and metal cladding will protect the structural frame from Bergen’s notorious weather.

BOB reports that it has already sold more than half the apartments off plan.

 

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