HASSLACHER Green Tower FAQs

Depending on the configuration, the wood portion of the timber hybrid tower comes in at approx. 90 volume percent of all materials used in the tower. The reason for that is that the large-dimension timber elements in the lower part of the whole structure make up the majority in terms of volume.

Contrary to many other wind power systems, we use four individual foundations instead of a conventional base plate. These individual foundations are designed to be relatively small, meaning that they save approx. 50% of concrete compared to the usual 1,000 m₃ of concrete used. The timber supports are connected to the four individual foundations via adapters that are pre-mounted in the factory. These four individual foundations consist of one base plate each and a plinth in the shape of a truncated pyramid. The anchor rods used to connect to the timber supports protrude out of those. The foundations are connected to an external earthing ring.

We are currently working on implementing deep foundations (anchoring technology using pilots). This will further reduce the volume of concrete used significantly. Thus, the foundations will be even easier and faster to remove.

Timber boasts excellent properties when it comes to loads in grain. Thus, a timber structure is very good at transferring tensile and compressive loads. Loads perpendicular to the grain as caused by torsional forces, however, present a significant challenge to timber as a construction material. Since these forces are often exceedingly large due to the wind power system’s rotor blades, a hybrid tower was chosen as the first development stage. The reason for this is that steel is very well suited to transferring high torsional forces in the tower’s head area thanks to its shear modulus. We are able to transfer the high torsional forces at the tower’s foot through spreading out the structure.

Our industrially bonded weathering layer made from pine heartwood is at least 4 cm thick and lasts 20 years, but can also be made thicker depending on the wind power system’s life cycle. The fact that this weathering layer protects the load-bearing timer structure well, was proven in series of tests.

When it comes to material fatigue, we are able to reference many tests done on the timber elements by material testing institutes. Comprehensive material fatigue tests have shown that timber initially shows fatigue tendencies similar to those of steel or concrete. Interestingly, however, timber virtually stops fatiguing after a certain period of time. This proven effect is shown in fatigue curves or so-called Wöhler curves.

This effect can be explained through the natural adhesives in the wood as well as the dynamic type of load transfer of the timber materials. This is why we assume that the timber structure will last significantly longer than 20 years. Furthermore, our timber structures are always designed in such a way that all “predetermined breaking points” are located within the steel elements, meaning the timber structure’s life cycle always exceeds that of the mechanical parts (both in terms of connecting means and the wind power system itself).

Bonding processes for load-bearing timber elements must be performed under controlled conditions (meaning in a factory hall) and may only be done by certified companies. Critical weather conditions such as exceedingly low temperatures or high humidity, but also dust particles, can compromise the quality of the bonded joints to the point where their structural integrity can no longer be guaranteed. Thanks to the high rental costs for tall cranes, wind power towers must be erected in a minimum number of days. This does not leave time for the two-component adhesive to cure in a closely timed assembly process. Furthermore, possible bonding defects can no longer be corrected once the elements are installed at the construction site. This is why we only use mechanical connections at the construction site. This has the further benefit that assembly crews that specialise in timber construction are not required.

The fact that timber towers are better suited for forest locations than steel or reinforced steel structures is obvious. Why, then, are we mentioning that our timber hybrid tower is particularly suited for forest locations? It’s all in our “wood protects wood” weather protection system. This leaves the wooden structure exposed instead of covering it up with a diaphragm or coating. Additionally, this integrated weathering layer results in extremely low maintenance costs that only extend to an annual visual inspection. Not using lacquer gives our tower a much lower environmental footprint. To make our package complete, land owners can deliver the trees felled to create the wind farm areas directly to the HASSLACHER group.

Clear-cutting forest areas for wind power systems is a sensitive issue. Both land owners and local inhabitants are highly critical of large-scale clear cutting. However, the size of the systems currently set up requires adequately sized crawler cranes whose cantilevers require areas up to 200 m long in a single direction. To significantly reduce clear cutting, we are developing various assembly concepts. Since timber elements weigh much less than comparable steel elements, we are able to use much smaller mobile cranes all the way to the steel adapter.  But we also develop assembly concepts involving rotating tower cranes. These flexible-height cranes, combined with our segmentable timber elements then allow for minimal clear cutting compared to current concepts. By the way, our timber frame structures are perfectly suited for connecting rotating tower cranes. Additionally, our timber hybrid tower system allows for the entire wind power system to be built gradually. This eliminates all waiting periods following the timber structure’s erection. This also contributes reducing the number of cranes required for erection and their various spatial requirements to one crane and allows us to select the most space-saving crane depending on location and availability.

The material that is wood is a regionally available, regrowing raw material. Thus, land owners can deliver the trees that accrue during clear cutting to the HASSLACHER group. The towers are made in Germany and Austria, the markets in which most of the timber hybrid towers are built. Due to an elaborate quality assurance system, regional production of the timber hybrid tower can also take place in other countries on the basis of a licensing model. The relatively simple individual foundations can furthermore be established by regional construction companies. The timber elements can also be structurally reused regionally.

This option definitely exists. But in this context, we need to mention two things. First of all, the supplied wood’s utilisation in the wind farms to be erected can only be guaranteed in financial terms (no direct utilisation). The reason for this is that every log has a different density, meaning that not every log is suited for the dynamic loads created by a wind power system. Secondly, delivery only makes sense if the location is within a 200-km radius of a HASSLACHER group sawmill. Due to the number and distribution of mills, Austria and Germany are both well covered.

Our first timber hybrid tower pilot system will be built in Rhineland-Palatinate in 2025 (according to notice by the German Federal Ministry for Economy and Energy) for certified configuration. Further cooperation agreements for systems in Austria and Germany have been concluded with various project development companies. The first German and Austrian wind farms are being prepared for 2026 (logistics concept, assembly areas, legal permits etc.)

Currently, we possess type certification for a timber hybrid tower with a hub height of 137 m. In the framework of further development, we focus on hybrid towers with a hub height between 160 and 170 m. From a technical perspective, greater hub heights up to 200 m are possible provided appropriate resources are used.

Our tower concept also offers an interesting alternative for NH 130-150, which are currently implemented with closed, round all-steel towers most of the time. Our tower, together with the other components of the wind power system, can also be erected economically and without interruption using a crane and our stiffer tower is less susceptible to wind-induced cross-vibration during assembly, which makes its erection easier to plan and reduces downtimes. Our tower can be erected by wind power system manufacturers and offers them all possibilities of optimising their building procedures for their projects. Furthermore, we would like to stress that a lot of steel could be substituted with our timber structure in the lower area of NH 130-150m systems.

The HASSLACHER Green Tower timber hybrid tower is currently built from laminated spruce beams. Compared to other wood species, spruce has an average bending strength and boasts average values regarding weathering effects. Also, spruce is currently easier to source and more affordable than other wood species. Tests with other wood species (birch, pine, larch etc.) are currently underway and could be used for connecting elements in the future. In the future, wood species such as pine and larch will assume important roles regarding weathering protection, especially in specific climate zones.

The HASSLACHER group is among the largest wood processing firms in Europe. The timber hybrid towers can be produced at several sites in Austria and Germany. Accordingly, the HASSLACHER group, as the supplier of the timber elements for the HASSLACHER Green Towers, is capable of establishing an annual production of 100 towers within relatively short time (one year). In the mid- and long-term, further production capacities can be unlocked. Regarding wood availability, we are able to reference the wide-ranging purchasing channels of the group, ensuring the required availability.

The lower the resource requirement, the lower the structure’s environmental footprint. Our timber hybrid tower was designed to be extremely resource efficient. If one converted the used wood into a closed wall structure, the load-bearing structure would only have a wall thickness of 7 cm! This low resource requirement is the result of its open timber frame design, years of accompanying research and development work with our partners and 125 years of experience with such timber structures within the group.

The investment required for the timber hybrid tower are competitive through

1. minimal material use due to the use of an open timber frame construction at a converted load-bearing wall thickness of approx.. 7 cm
2. Resource savings through the use of small-volume individual foundations
3. low-costs logistics through segmentable elements.
4. simple and quick assembly thanks to high degree of pre-fabrication and exclusively mechanical joints at the construction site
5. most efficient overall structure due to 12 years of development work

• and therefore, attractive purchasing price for the erected overall structure.
• Low maintenance costs thanks to intelligent wood protection measures
• material value of the wood at the end of the useful life higher than removal costs (positive removal earnings)
• lower provisions for removal for the counties and thus, lower capital commitment
• stable raw material price of the wood when compared with steel
• no carbon pricing to the timber construction
• lower removal costs through simply removing the connecting material in the timber structure and the time-saving cutting of the relatively small individual foundations.

For this purpose, a round wire leading to an earthing ring that runs around the foundations is installed. Additionally, there are various concepts on how to better distribute lighting. To protect the timber structure from direct hits, lighting spikes are installed on it and connected to the conductor wire in a lightning-proof way. The spikes of the outer lightning protection system in this area are positioned in such a way that here, no lightning hit can be expected when observing the specific lightning current parameters.

Wood only burns when the flames are supplied with oxygen from all sides. The massive diameter of the supports and diagonals prevents this oxygen delivery from three sides, meaning that only the side exposed to the fire would slightly char. Thanks to the density of the solid wood construction (glued laminated timber) and the associated protective carbon layer, the comparatively low heat conductivity of wood (cool core) and its limited oxygen supply, only a thin layer (several mm) of the surface would char before the fire would be go out automatically. The timber structure’s structural integrity is therefore not substantially threatened by a fire.
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The timber hybrid tower’s development is not only based on 125 years of group experience, but also on many years of development. Over the years, many material tests, fatigue verifications, climate studies and various scientific studies were conducted by renowned institutes. In the course of this, the Technical University of Graz, the University of Stuttgart, the Technical University of Munich, the Technical University of Braunschweig and the RheinMain University were involved. Additionally, tests were undertaken at the material testing facility in Leipzig. Moreover, a number of specialised design offices were consulted for development work. TÜV Nord was responsible for the system’s type certification. From this perspective, we have combined the current scientific insights with more than 125 years of experience of a production company to develop a safe and economical tower concept for our customers.

The HASSLACHER group is a family-run company with more than 120 years of history in timber processing and engineered timber construction. The group operates a dozen production facilities all over Europe and employs 2,000 staff. The timber hybrid tower structure consists of glued-laminated beams. The HASSLACHER group is now the world leader for glued laminated timber. Its experience with production and quality assurance is accordingly vast.

Cutting, planing and glued laminated timber production now exhibit an extremely high degree of precision when it comes to construction element production. Apart from a number of quality-assurance measures (see FAQs quality assurance) cutting, planing and joinery processes take place with a precision of +/- a tenth of a mm (1/10 mm). Moreover, all construction elements are once again 3D-measured by a laser unit and compared to the blueprints. Laser measurement is used across the entire assembly process, too, by the way. In the framework of the certification process, load cases must be calculated assuming a possible tilt of the wind power system. Such possible tilts exceed the internal tolerances in production by many factors.

Unlike synthetic materials like steel and concrete, wood, as a natural product, has inherent variations in density and strength. To ensure structural integrity, a high degree of material homogeneity must be achieved through careful selection and processing. In addition to the quality assurance measures in the cutting area, a range of quality-assuring measures follow in the glued laminated timber production facilities. For example, every single lamella that is used on a glued laminated beam to be used in a glued laminated timber structure is checked several times. Each lamella is exposed to defined tensile loads, x-rayed in the framework of an imaging process (e.g. to expose deep defects), electronically measured for their moisture content, and subsequently acoustically struck to determine the lamella’s strength from the returning sound. For all analysed individual lamellas, a stiffness spread is created and only those lamellas that lie within a defined range are approved for production. Furthermore, the position of every single lamella inside the beam is defined on the basis of its strength and documented. Thanks to these comprehensive quality-assurance measures, an extremely high degree of homogeneity and thus, material safety, can be reached for this natural material.

The success of structural mass timber construction in the last 20 to 30 years is based in large parts on the safe adherence to the above mentioned quality processes. The HASSLACHER group is a pioneer in this area and regularly invests in state-of-the-art production technology to further advance this commitment to quality. The HASSLACHER group’s many references involving demanding construction schemes that were and are realised all over the world are the best proof.

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Wind turbines are highly stressed structures and experience shows that such systems experience frequent issues. This was clear to us from the very beginning of development. This is why we only use technology that has been tested and proven for decades for our tower concept. We possess decades of experience with tower and bridge structures from glued laminated timber that are exposed to the weather and dynamic loads.

Even in the use of rod dowel connections for dynamically stressed structures, we can draw on decades of experience. Additionally, we avoid all bonding processes at the construction site, that – according to our experience – can hardly be performed in a quality-assured way under critical weather conditions and associated time and cost pressure. 125 years of experience on how to develop and build timber structures, combined with state-of-the-art quality assurance methods and concepts flow into our pilot system projects. This will confirm the tower’s structural stability that was proven in the test report.