Graz University's Professor CLT – Gerhard Schickhofer

The story of how CLT moved from timber engineering theory to mass building material, all within a matter of years, centred on Graz University's Institute of Timber Engineering and Wood Technology, and particularly, the research work of Gerhard Schickhofer.

The Institute of Timber Engineering and Wood Technology's Building Technology Centre on a quiet day
Photo Philip Ott/Nussmueller Arkitekten
CLT is a revolution in the architectural and construction world. Stories of new buildings, taller and taller, larger and larger, ever more ambitious, outstripping all that came before are appearing in the architectural media at an almost industrial rate, monthly if not weekly. The large-scale panels were once called massive wood or massive timber. These days the gathering transformation brings new meaning to massive change.

CLT has also created a revolution in Styria, the South Eastern region of Austria where the panels were first developed, where the necessary research was first conducted, and where over 60% of CLT production still originates. This is about to change. Styria has been producing most of the thick timber boards before dispatching it to exclusively European sites, (excepting small consignments to Australia and a couple of other far off parts of the world) but factories are presently opening across Europe, and will inevitably compete with the Austrians. Secondly, factories are opening in different parts of the planet, as Canada, Japan and Australia lead the accelerating catch up effort, shifting the focus from Europe alone to CLT buildings going up on a pan-Global level.

One Styrian hub of this industrial timber take-off is Graz Technical University and specifically, its Institute of Timber Engineering and Wood Technology. It is here that all CLT’s early technical engineering research towards making wood into a usable material in the wider world was conducted, and today, by the leading British timber engineer, Richard Harris, noted, “continues to lead the world in research in the field.” The person who got the Institute up and running, is professor Gerhard Schickhofer, whose early research work was instrumental in getting cross laminated timber off the ground, and led directly to the panels certification and the founding of KLH, the first dedicated CLT producing factory.

Schickhofer continues to oversee the Institute, regularly presents on the timber engineering conference circuit and has become an influential voice in the broadening debates regarding the material’s future, not least that of the timber towers race. Early on, it must have felt like something of a roller coaster for Schickhofer himself, to observe the results of his, and then the Institute’s work, within a chapter – less than twenty years - of his professional lifetime, turned into this snowballing phenomenon.

Gerhard Schickhofer, surrounded by
his research archive
We have the technology - This photo and all others below, and also illustrations are courtesy
of Gerhard Schickhofer/Timber Engineering and Wood Technology

Today, he and his small team track CLT’s growth, through indicators like the number of heavy CLT production machinery sold and delivered in a year. He is confident that the growth is exponentially going to increase, pretty much all over the tree-lined world.

“Everywhere we have trees or woods there’s going to be CLT, they’ll be CLT type panels produced,” Schickhofer stated in a conversation when I visited the Institute in January 2018. As if to underline this, a few days later, one of the main Austrian timber industry website’s, Timber On-Line, ran a feature highlighting how only Africa remains as a CLT factory free continent.

“There is now a really incredible developing process, it is really international.” Schickhofer was speaking in his small office, off to the side of a larger research room on the Institute’s first floor. I had arrived on the overnight train from Zurich, and as I made my way to the meeting in cold early morning sunlight, I was struck by the surroundings, a leafy street in the city’s suburbs, which, while passing a few other TU buildings, seemed an unlikely setting for this revolution. At the end of this road, the Institute came into view, a largish shed devoted to pressuring, pulling, cracking, crushing, and more refined forms of testing the properties of chunks of Austrian mountainside Spruce – and other woods – and the story line returned to expectations. At the scrubby end of the road sat a walled timber building, almost completely without windows, a few cars parked outside, and, it seemed, just a sole entrance door.

The Institute''s Building Technology Centre

Schickhofer’s office bore the hallmarks of a particularly tidy and ordered academic, stacked to brimming over with books and research documents, including floor to ceiling shelves of reports, each one carefully archived. We began talking, and quite soon into the conversation, he recalled a moment, which left a deep impression. By the early 2000’s the KLH factory had already started producing CLT, albeit comparatively small amounts and other large scale saw milling companies were taking an interest. “They were thinking, ‘This could be an interesting way to go’. Mayr-Melnhof, Binderholz, Stora Enso and Hasslacher - they were the first to begin looking at it. By the mid-2000’s, all these main sawmill owners had started their CLT production lines.

“In a really strong way, this was a really important turnaround. I can remember talking with one of the saw milling company people, and them saying, that it was only really economically viable if they were be able to produce 100, 000 m³ of the cross laminated panels a year. That compared to KLH’s first year of 5000.” The scaling up that the big companies envisaged, and their conviction that they’d indeed be producing these sorts of levels in just a few years, had a big impact on him, contributing to the sense that the growth of CLT was just beginning, and was now only the tip of an iceberg.

By 2005 the Institute was well advanced, the new building opened in 2001 and the first handful of PhDs were either completed or in progress. Schickhofer had completed his own PhD ten years earlier, under the then current head of timber engineering, Professor Richard Pischl. Reading its title Elastic Analysis of Flexibly Jointed Laminated Timber Plates will likely leave the uninitiated none the wiser (in its original German doesn’t come across as any pithier, “Starrer und nachgiebiger Verbund bei geschichteten flächenhaften Holzstrukturen). But the subject was critical to the emergence of CLT.

His work on the potential of laminated timber panels, placed crosswise one on top of the other, started in 1990. Beginning with comparing the behaviour of layered plastic composites with wooden panels, he extended the focus into cross-layered wooden panels, which hadn’t received much engineering attention. Depending on the research results, there was potential for commercial timber materials. “It was the missing link,” he says, “in the portfolio of engineered timber.” Up to then, the grain or timber strands in glulam and laminated veneer lumber, only ran in one direction.

Merk’s 1995 three storey Dickholz housing in Aichach

Schickhofer’s research didn’t come out of nowhere. Originally focused on shell structures, once he gravitated to panel systems, he joined a small number of middle European timber specialists, who’d been looking at the same idea. The best known was development work conducted by Merk, the Bavarian timber company, which had already developed a cross laminated product called Dickholz, or ‘Thick Wood.’ The stories run parallel. In 1995, Merk’s founder, architect Karl Moser built the first multi-storey housing in Merk’s hometown Aichach. Another early researcher had been a young German, Pius Schuler, whose PhD work was published a year before Schickhofer’s, in 1993. Schuler went on to set up a Swiss company which has produced BlockHolz, a similar three layered panel system, and is focused primarily on domestic housing.

There was also the work of Julius Natterer, at the time the best-known timber engineer in the German-speaking world. The Munich born Natterer was director of IBOIS, a timber research department deep in the bowels of Lausanne’s scientific university, EPFL. One strand of research, one Natterer claimed to have invented, was Brettstapel, a glue free massive wood panel system.  Rather Brettstapel was an all wood system, connected with dowels. Schickhofer acknowledges Natterer’s influence, but points out that Brettstapel – which has since developed into another massive timber material, see this edition’s feature on Brettstapel here - isn’t an industrial system. “If you’re looking for an industrial product, Brettstapel doesn’t conform. And it doesn’t use the inherent possibilities of industrial processes.”

Inside the Building Technology Centre

Schickhofer’s work was unambiguously industrial. The research’s long-term goal was to contribute to developing panel systems, which met industrial standards of consistency and regularity. Completed in 1994, around the time, he points out, of Moser’s three-storey Dickholz housing, further research continued out and on from the PhD. The panels, originally vertical and horizontal BrettsperrHolz, began to be described as CLT, Schickhofer is keen to point out, after he used the term in a presentation at a COST Seismic Behaviour workshop in Venice, Italy in 2000.

“Wolfgang Weirer, and the other two, were actually very small saw millers, with only the production line, and no actual sawmills.” Schickhofer, at the time a young post doctoral assistant within the structural engineering department became a key member of a working group around professor Pischl, which gradually crystallised into the Institute. A new research programme using Schickhofer’s PhD as its theoretical basis was developed with the three sawmillers and the Sawmillers Association. Schickhofer set to co-ordinating this R&D proposal for Government funding. Submitted in mid 1996 and with the Styrian Sawmill Association as the industrial partner covering half the funding, the application was successful, bringing in the remaining 50% of the research budget. The research work began in January 1997, and propelled the next chapter, including the first CLT production.

An early issue was on the quality of the original wood - primarily waste wood, and how to ensure the resulting panels met various technical standards. 90% of the boards’ wood which went to make up the panels, needed to be S10, the European strength grade. Much of the wood was coming from the skinny outer edge of trees, but there wasn’t enough technical data on its strength or stiffness, and only a general consensus that the fibrous skin wasn’t strong enough to use.  Through 1997 and the first half of 1998, testing was conducted, and it was discovered that the outer wood was much stronger than had been thought; the waste wood was, after all, usable. Other issues became clear. One of which were adhesives. “No-one had thought about the different adhesives.”

Details from Schickhofer presentations, showing the original objective of using the edge's or sideboards of wastewood, and below, the actual way logs are exploited

There were other technical issues; optimising pressure levels to produce the required strength of panels. Waste-wood, by definition low in quality, meant glues needed to be as effective as possible. “You had to consider the quality of the material. And the adhesives, what kind of adhesives, these had to also be optimised.”

Details from Schickhofer presentations showing the different phases of the bonding process

By then, the first panels, 3 metres long, had already rolled off the first test CLT press, which had been hand built at one of the sawmills. It was, Schickhofer emphasises, an entirely grassroots, bottom up effort. All through the first five or so years, the sawmills were, according to Schickhofer very close to going broke, at the edge of the limit of their budgets. “It was always a risk.” The university, now with the central funding, began to help with R&D.

An early connector-tie system
between CLT plate and concrete
Presenting the connector system further into development
Photo Proholz Austria

Production tests began, at first on “a very simple, single production line.” Roughly the same time, in a parallel development, a new machine came on the market, the first press system, using water-based pressure, adding as many questions as it answered. “In the beginning it was not a big product or project, trying to build prototypes with architects.” The thickness of the panels was off-putting and confusing, “even though Austrians like massive buildings.”

Another focus was on what the panels could do. As Schickhofer remarks, “a lot depends on the size of the panel.” Different configurations of panel forms, for different building uses, such as deck slabs for floors, and wall panels, began to build up a matrix of types, all of which needed to be tested, first in the Institutes labs, and then within live projects. The different types and sizes of panels also needed connector tie’s to a floor, or ceiling, so connectors too became an area of research and testing. Those were one set of variables. Another was establishing optimum pressure levels for the glue bonding processes. If too high, there was a problem, and similarly if it was too low. “It was always difficult to find the optimal sections. The quality can be different, which is not a good thing for an industrially produced product. It isn’t meant to be.” Further tests were conducted, working on a model, analysis of tests for characteristic values.

In April 1998, this intensive period of research was finally completed. The results of the testing was submitted to the Austrian and EU bodies responsible for approving materials which formed the basis of commercial products Approval by the Austrian Government was received at the end of 1998, in December.

Bridge over Murau's waters - Wandritsch bridge, with CLT slab under construction

Some of the earliest structures panels were produced for bridge projects, many in the Murau valley over the river Mur, home of the saw millers, who were beginning to produce the now approved panels. Bridges provided useful contexts for the first tests of differently sized pre-fabricated panels; “they helped in the search for refining what the new product might finally be.”

Next bridge Raabsteg Bridge, Feldbach, again with CLT slab

One of the earliest was Kohlbacher bridge, where the production line limitations, connected to the hardening of adhesives, became an issue. “They tried to find out the limit of different individual elements, how strong they were and how far would they’d go.” For the bridge, the prefabricated panel deck plate, comprising 21 panel layers were the same thickness as they would have been in concrete. Kohlbacher bridge was followed by Wandritsch road bridge, near St. Ruprecht, again testing a different deck plate, this time consisting of nine 19 mm layers of CLT, in all 171 mm. This was followed in turn by the Raabsteg - Feldbach footbridge, a 35 metre bridge, with a 5 layer CLT panel, in 1998.

By the turn of the millennium the saw millers’ plans were beginning to be realised. The first production line was properly operational, and already in 2000 manufactured the majority of central Europe’s 50, 000 m³ CLT like massive wood panels. There wasn’t really any other plan other than to start production. “No plan B” says Schickhofer, “they had to go.” The immediate pressure was that the material produced needed to go somewhere.

The Frohnleiten CLT r&d housing, on site and completed

The answer came, says Schickhofer when the saw miller’s realised they needed to become builders, “and they started turning into builders. They had engineers in their companies, calculating the buildings they were working on.” Soon the first CLT buildings began to be tested. Pilot projects, both regional, then national, were initiated. Panel systems were being modelled and tested for full buildings of increasing complexity, with the objective of uncovering answers to research questions. The first wave of these included a set of terrace houses in Frohnleiten completed in 2000, multi-storey housing in Leoben, the Austrian Federal Forestry HQ in Vienna; as well as two buildings that had come out of the research work themselves; Schickhofer’s Building Technology Centre in Graz, designed by one of Graz’s Nussmueller Arkitekten and the first production shed at KLH factory in Katsch, in the Murau valley. By late 2000, a larger research related project, Block A, four storey residential housing in another Murau valley town, Judenberg West, was underway.

One of the Judenberg West four storey housing block under construction, and complete

“It was all so different, and not so easy to understand, why should we use such thick wall systems.” For the Building Technology Centre, research focused on an innovative connector technique, under-stressed panel framework, and further testing of the CLT elements. Through this turn of the millennium period, the engineering faculty was also building its research capacity. The research capacity crystallised fully in 2004 when the Institute for Timber Engineering and Wood Technology was launched, led by Schickhofer.

Parallel, though separately, new press systems were coming on the market, which spurred on the mills. “We needed presses tor these big companies like BinderHolz, StoraEnso and Mayr-Melnhof. They had to have advanced presses and Minda and Ledinek were developing these just at the time. There was a lot of discussion regarding presses, how to optimise the different press systems,” hydraulic, vacuum and frame system presses. Other CLT press system manufactures joined in the growth; Austria’s Springer, Leiße Weinig from Germany, and Fankhauser from Switzerland who produced vacuum presses.

By the early to mid-2000’s, with the likes of StoraEnso and Binderholz committing to major factory production CLT appeared to be on cusp of taking off. Research had broadened, with PhD’s and other work starting up across central Europe and further afield. KLH-UK opened its doors in 2005, while the other company pioneering CLT in Britain, Eurban, looked at 25 projects. In Germany, Norway and Sweden early examples of massive wood buildings began to appear. In 2006 production nearly doubled from 2000, approaching 100, 000 m³. By 2008, the figure had more than doubled again to 215, 000 m³, the graph becoming steepening dramatically so that within three years, 2011, production had reached 450, 000 m³, all from central Europe, and 300 000 m³ from Austria. Outside of central Europe, small presses were starting up, adding a further 25, 000 m³ to the 2011 total.

The confidence about the rate of growth was at times overplayed. During a 2012 presentation in Milan, Schickhofer, predicted reaching the first million tonnes m³ in 2015. Currently, circa early 2018, CLT is anticipated to reach this million figure in 2020. But the main point, CLT’s take-off, was difficult to argue with. In 2010 Eurban’s project enquiry log reached 1300, the next year KLH-UK imported 123, 000 m2 to Britain alone, according to figures provided by the British subsidiary recently. This figure declined the next year, due to the knock-on effects, of economic recession that had overwhelmed many European economies from 2009 onwards, though counter-balanced by the take up of CLT across Europe. That same year Murray Grove, London’s ‘original timber tower’ was completed, and the tall towers race began to take shape. In Vorarlberg, at the other Western end of Austria, Hermann Kaufmann, the country’s best-known timber architect was already in discussions about the Life Cycle Tower, a timber-concrete hybrid tower, which was completed in 2012. Arguably, its most significant impact has been on subsequent towers, including the world’s  current tallest, ActonOstry’s Commons Tallwood House, Vancouver, where Kaufmann acted as a consultant .

Room with a view – Inside Brock Commons Tallwood House
Photo Michael Elkan/Hermann Kaufmann Architects
Our time is nearly up. I need to leave the Centre, to head for the Styrian forests, and a meeting at one of the big incomers, the StoraEnso factory. Schickhofer is talking about the scale of CLT’s growth, and recalls speaking at a conference on the Canadian Pacific West Coast, the first part of the New World where the excitement about CLT’s prospects has been taking off. During the visit he was shown the decimation of part’s of BC’s forests by the Mountain Pine Beetle; 18 million hectares of forested land in Western Canada, twice the size of Austria have been lost, all in effect waste wood, but potentially adaptable for CLT.  

In Asia, Japan is too has been moving fast. In 2013 a decision was made at ministerial level to launch CLT in the country. Five years later, the necessary research has been completed for approval, and there are, Schickhofer tells me, already eight CLT companies. “They are very organised, it is top down, while here it was bottom up. Every year” he adds, “I see this developing process” as he describes how CLT is advancing in other parts of the world.

Both he and the Institute’s expertise are in demand. The research agenda continues. There is a current focus on ‘complete construction kits’, combined timber to timber, timber-steel and timber-concrete hybrid materials, and panel production optimisation. Though he has been outspoken and critical of tall timber, some among the research team are involved in these headline-grabbing projects, including discussions with Arup on the engineering of Haut, the Amsterdam tower. “Timber towers are a not a good development, I’m afraid,” he declares of timber towers. “It’s natural that human to want to be the best. But it makes no sense.”

Onward and upward
Kaufmann Bausysteme's Modular box factory in Leoban
Photo Kaufmann Bausysteme

“CLT is a building product, not a building system. There needs to be much more work on a building systems based on CLT.” As an example he points to the risks of moisture. Moisture ingress can find its way into the floor structure, a leak in under floor services piping can stay undetected until it could be too late. The larger the building, the greater the risk is the implication, leaving the image of the consequences of moisture failure in a thirty-storey tower to the imagination. “You can’t make a copy and paste, from concrete into wood building. You have to think about building systems from the beginning to the end.”  As an example, of both a building system and a safer methodology are the 3D prefabricated box units, currently being made by Kaufmann Bausysteme’s factory in Leoben, the industrial town near Graz, where the fusion of prefabrication in factory conditions removes much of the risk. “All the components are built within factories, which you can be sure are safer than building on site.” Schickhofer is enthusiastic, not only related to tall towers, but for other reasons; “it’s an attractive method for city re-densification,” both in new build, though also roof stacking, and adding additional storeys to buildings.”

As one of the early mission-driven community at the heart of the rise and rise of CLT, his is a rational vision; an ardent advocate of post-industrial modularity and prefabrication. Like KLH, and others in the Austrian engineered timber community, the commitment is to ever-greater systematization and standardisation. The Kaufmann factory’s modules are a recent example of where the prefabrication revolution has reached, demonstrating greater modular complexity. What happens next, as the growth in factories and production migrates out of Austria and Central Europe, indeed out of Europe, will be a major talking point in engineered timber circles through the next decade. Schickhofer, through the last two and half decades, is one in a small number of central European building luminaries, there since before the beginning. His role in bringing CLT to its current phase transition, has been likewise, completely critical.

Finnish Architect Marco Casagranda's Tikku, in Helsinki city centre, used the Kaufmann Bausysteme's modular boxes to poetic effect – Photo Nikita Wu