Glued, ductile wood-steel connections

Glue-laminated timber with glued-in steel parts is pretty handy. All you need to do is tighten a few bolts at the building site and the construction is finished. However, the critical question is how well wood, steel and adhesives go together. All theoretical strength is futile if the elasticity of the connecting elements is not coherent. Mistakes made at this point may cause bolts to tear off on one corner of the girder connection, while they are not even under load on the other side.
"Ductility is getting far too little attention in timber construction", says Bruno Abplanalp. His company Neue Holzbau, Lungern/CH, has developed a system for which threaded rods are mounted with a two-component epoxy resin. This ensures that all connections are actually doing what they should: making their static contribution. In late 2012, the GSA system received the Technical Approval of the German Institute for Building Technology (DIBT).

Designed for heavy-duty ports

GSA stands for "Gewinde System Anker" (thread system anchor). It is a positive and non-positive connection system with glued-in steel rods for timber engineering. The reason it was developed is that Bruno Abplanalp, expert in timber construction, missed a solution that is fully suitable for large, heavy-duty ports. "Most research and all approvals merely focus on the structural behavior of a single anchor. What they usually calculate is the pullout resistance at the point of shear failure at the hole wall. But that's not enough", says the Swiss. For in a connection several individual anchors are responsible for the structural behavior. What is therefore asked of a connection – in addition to sufficient strength and rigidity – is high ductility.

Break at deformation of half a millimeter

"Conventional glued-in anchors react to tensile loading with extreme brittleness. The breaks occur at deformations of 0.5 to 1 mm", Abplanalp explains. "If only one anchor out of a group of anchor fails, you can expect a failure of the whole port", he warns. "If the anchor is tailored to pullout-resistence, no plastic deformation capacity exists, no ductility." Although a high strength steel can increase the pullout resistence, it does not make a connection ductile. This will not occur with GSA technology, because all components are coherent. That is what DIBT has confirmed under the approval number Z-9.1-778.

According to Neue Holzbau, all components must be subjected to meticulous quality assurance:Glulam: The standard tensile strength of the lamellae and finger-joints must be assured.Threaded rods: Since the GS-anchor is sized to the steel, accurate knowledge of the steel strength is needed. In addition, the strength capacities of the anchors may vary from one another only marginally.Mixing ratio of resin: To ensure a perfect bond, the mixing ratio must be reviewed. The GSA-resin is a two-component adhesive based on epoxy resin. Apart from a thorough mixing process (by a static mixer), mixing ratios are also crucial.Tested quality: Of all components control samples are drawn and tested for their material properties.

Delicate, noble – also for hardwood

In modern timber construction, the GSA technology is suitable for both softwood and hardwood. This is not surprising. After all, with ash glulam up to strength class GL 60 the Swiss have timber products in their range which can take full advantage of this potential.
Standardization and static optimization of the GSA technology enable the design of filigree architecture with concealed steel parts. The supporting elements are fitted on CNC joining units with high precision and – if possible – delivered ready for installation. "At the building site, the GSA connecting elements can be assembled to build large structures in no time", emphasizes Bruno Abplanalp. <

FACTS & FIGURES - NEUE HOLZBAU

Established: 1983
Location: Lungern/CH
CEO: Bruno Abplanalp
Business Line: Timber Engineering, glulam made of spruce, larch and hardwood, CNC joinery, GSA Technology, Engineering
Employees: 45
Production: 6,500 m³ of glulam a year
Markets: 90% Switzerland, 10% export