Quality assurance in the timber hybrid tower’s production process
The supports and diagonals of HASSLACHER Green Tower’s timber hybrid tower are made from glued laminated timber (glulam). This is timber glued together from several board layers (so-called lamellas) in the same direction of growth. In order to achieve the dimensions needed for the transfer of loads from the wind power system, several glulam beams are bonded to form even greater composite cross-sections. This block bonding of several individual beams to large-dimension supports allows for very large cross-sections.
Since wood is a natural material, it is subject to natural deviations in terms of its density and thus, its stability. That’s why mass timber engineering seeks to reach an extremely high degree of homogeneity for the material in the framework of its material selection process. A high degree of material safety can only be guaranteed if the individual lamellas exhibit a very low spread in their mechanical properties.
This homogenisation process takes place at three different levels:
1. Mechanical grading and classification of the used lamellas and subsequent selection of lamellas with defined properties from the available graded product
Among others, 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, their dynamic e-modulus is determined by means of laser vibrometer and their stiffness by means of impulse hammers to trigger their structure-borne 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. These automated lamella grading methods are based on EN 14081 and EN 13182-1, the determination of their strength classes on EN 338 and DIN 4074.
2. Lamination effect through bonding, leading to a homogenisation of their properties and limiting the size of individual wood features
Every bonded joint, both between the lamellas and at the end of the lamellas (finger joints) as well as between the glued elements in block bonding, is tested several times. For instance, a glue application reader determines if enough glue was used or if the finger joints are adequately covered with glue. Additionally, glue application is fully tested for the right epoxy resin/hardener ratio. Other significant quality assurance processes include the checking of the glue/joint integrity via delamination tests and shear tests to determine the material’s shear resistance.
3. The use of a large number of lamellas and simultaneous stress, a so-called system effect is achieved
The higher the number of lamellas being situated close together in the cover layers, the more likely the individual lamellas’ deviating properties will balance each other out statistically. The system coefficient allows for the multiplication of the bending and compressive strength in grain according to EN 1995-1-1 and DIN 1052. In this, the position of each lamella inside a (laminated) beam is optimally defined and documented depending on its strength.
These internal measures to produce the glulam beams culminate in a CE-certified building product for which the HASSLACHER group’s plants are each certified. This includes additional external monitoring of the production methods incl. assessment and inspection of their performance consistency acc. to EN 1408:2013, speaking for the high quality standards applied to the product’s production.
As evident through the previous description, this is a highly industrial production process. The HASSLACHER group’s plants produce some 450,000 m³ of glued laminated timber annually.
Let us close by mentioning that the European standards for timber construction, despite the described homogenisation measures for the structural design of structures using the natural material that is wood, stipulate a material safety coefficient of 30%. This safety coefficient is only 10% for prepared materials such as steel and concrete.
