GSA technology

With profiled steel rods glued into glulam parallel or at right angles to the grain direction, large forces can be introduced into timber components over a small area.

In most cases, steel rods in the form of threaded rods with metric threads are used. These are glued into the wood with an adhesive specially developed for this application (mainly 2-component epoxy resins and PUR adhesives).

Together with Professor Ernst Gehri, the glulam manufacturer neue Holzbau AG, CH-Lungern, has been researching and developing glued-in threaded rods since the end of the 1990s. Under the brand name "GSA technology", a wide variety of connecting parts have been developed that are successfully used in various timber structures. In order to facilitate the use of GSA® technology (GSA = threaded rod anchor) outside of Switzerland, neue Holzbau AG, together with the adhesive manufacturer Astorit AG, CH-Einsiedeln, applied for a general building authority approval for the 2K-EP adhesive GSA-Harz and GSA-Härter for gluing steel rods into timber construction materials and received it at the end of 2012 (Z-9.1-778). Compared to DIN 1052-2008-12 or the national annex DIN EN 1995-1-1/NA, significantly higher performance can be achieved with glued-in anchors with this approval.

Ductile bonded into the wood
Joints with bonded-in threaded rods are among the most efficient fasteners in modern timber engineering. As with all other fasteners, all components such as steel rod, glulam, adhesive and geometric parameters (edge and center distances, etc.) must be optimally matched. In order to achieve optimum connections, certain rules and requirements must be met:
The connection must be designed in such a way that the steel bars fail (ductile) at break and brittle failure of the timber (shear failure in the bonded joint) or the adhesive (cohesive and adhesive failure) are excluded. For tensile connections with several bars acting simultaneously, the necessary even distribution of force on the individual bars can only be achieved if the individual bar connection is sufficiently ductile.
For efficient connections, the (too) large edge and center distances of DIN 1052 of DIN EN 1995-1-1/NA must be significantly reduced. If these edge and center distances are too large, only a few bars can be placed on the timber cross-section. The efficiency of the connection therefore remains low. A denser arrangement increases the risk of splitting. The effective anchoring of the rod must be placed inside the wood. With GSA® technology, this is done by milling off the thread in the front area. Further information is also available under GSA basics.

Failure at the edge of the drill hole
In addition to the shear strength of the timber construction material, the elongation at break of the timber parallel to the grain must also be taken into account. The elongation of the steel rod must not exceed 2.40 ‰ in the case of spruce. This approach limits both the choice of steel strength and the gluing length. Failure does not normally occur in the adhesive, but at the edge of the drill hole. The calculated strength value therefore refers to the shear strength of the timber construction material. This can be altered by the choice of building material (e.g. hardwood). As this is a failure of the timber, the pull-out strength values must be adapted to the respective service class. Systems with glued-in anchors are only suitable for damp components to a limited extent. GSA® technology is limited to service classes 1 - 2, whereby the pull-out values in FK 2 are reduced by a factor of 0.8.

Quality assurance
Once installed, the fasteners are almost invisible and subsequent visual inspection is practically impossible. Reliable quality control is therefore essential. In addition to the adhesive (mixing ratio / curing process), the threaded rods and the wood (tensile strength of the board, tensile strength of the finger joint) must also be comprehensively tested. The quality of the joint also depends to a large extent on the quality of the workmanship. The quality of the drill hole, for example, has a decisive influence on the strength. High-performance connections should be carried out by specialist companies with sufficient expertise and experience.

More anchors for high loads
The standard and also the approvals for bonded-in threaded rods deal exclusively with the single anchor. However, the technology of bonded-in anchors only becomes interesting when many anchors work together, as the group behavior is decisive for the connections. In addition to low costs, good connections are also characterized by their simplicity. Over the years, we have succeeded in developing various fasteners for a wide range of applications. These are parts that are inexpensive to produce in series and can be connected on site with just a few bolts.

GSA® technology is also suitable for complex components. From truss girders and tension-reinforced components (timber reinforcement) to timber-concrete composites and transverse tension, transverse compression and shear reinforcements, high-performance solutions can be found everywhere. In the interaction between wood, steel and adhesive, wood is the component that requires special care. In comparison with building materials such as concrete or steel, too little attention is often paid to the quality testing of timber. However, this is urgently required, especially for sophisticated engineering structures. Although all processing steps (such as finger-jointing and surface bonding) are rigorously checked during production, the basic material wood is not. Without knowledge of the base material, neither reliable design values nor, even less, high-performance components and high-performance connections are possible.