Copyright: © Chair of Structures and Structural Design Test setup for tension shear tests

In engineered timber construction the connections usually are the weakest parts of the structure, thus defining the dimensions of the structural elements. Therefore, the development of timber connections aims at increasing both strength and stiffness of the joints. Compared to mechanical fasteners glued connections yield significantly higher strength and stiffness without reduction of the load-bearing cross section or secondary failure modes as splitting in nail or bolt connections.

Apart from glued finger-joints and scarf joints for timber to timber connections various methods of gluing steel elements into the timber have been developed over the years. Glued-in steel rods are commonly used for local reinforcements and high strength connections of timber elements. Glued-in perforated steel sheets serve as high-strength shear connectors in timber-concrete-composite structures and have been applied for timber-timber connections as well. The preferred adhesives to bond steel elements to wood are 2K epoxy or polyurethan glues, due to their high strength and gap filling capability. All these reinforcing and joining methods add the bonding of steel elements as an additional work step in the finishing of the glulam members.

The Chair of Structures and Structural Design of RWTH Aachen University is therefore researching, in cooperation with the Institute of Building Materials Research (ibac) and three industrial partners, a new way to embed steel sheets ab initio in the glulam production process. This method can generate synergies within the production process and enables the use of adhesives that require pressure during curing such as 1K-PUR glues. The embedded thin steel sheets serve as local reinforcements to boost the strength of bolt and steel plate connections and create high-performance glulam joints.

Copyright: © Chair of Structures and Structural Design Failure modes and mean shear strength of different steel-wood combinations in the evaluation tests.

To select suitable steel-adhesive combinations for the ab initio installation of the steel sheets in the glulam, different types of glue and steel lamellas have been evaluated regarding the parameters bonding strength, curing process of the glue, surface treatment of the steel, resistance to corrosion and costs.
For the tests Jowat SE provided three adhesive types: a 2K-Epoxy (EP), a 1K-Polyurethan (PUR) and a 2K Emulsion Polymer Isocyanate (EPI), while a common Melamin Urea Formaldehyde adhesive (MUF) for glulam production was tested for comparison. The steel sheets were made of standard cold-rolled S235 grade with non-treated, sand-blasted, electro-galvanised and strip galvanised surface and an A2 stainless steel with non-treated and sand-blasted surface. All combinations of adhesives and steel sheets were tested on specimens of C24 grade defect-free spruce wood. The experimental setup is shown in Figure 1.
After cleaning the steel sheets with ethyl acetate, the steel-wood specimen were joined and pressed with clamps according to the reaction time of the employed adhesive. Before testing they were kept in normal climate for one week. The results of the tension shear tests are depicted in Figure 2.

The PUR adhesive reached the highest shear strength values except in combinations with raw and sendzimir-galvanized S 235. The EP adhesive showed a good bond to all metal surfaces and high shear strength while the EPI adhesive was prone to adhesive metal failure and therefore had the lowest mean shear strength. In further tests both adhesion and mean shear strength could be significantly increased by pretreating the steel surfaces with a primer. The shear strength of the MUF adhesive was below the values of the EPI with a high percentage of adhesive metal failure.
For the subsequent test programme it was agreed among the project partners to use electro-galvanised steel as a first choice as it combined good adhesion in the shear tests with protection against corrosion, easy machining and cost efficiency. Sandblasted and sendzimir-galvanised steel sheets were chosen as alternatives. Stainless steel was not taken into further account due to the high costs and the more difficult machining. It remains an option in highly corrosive environments though.

Currently a number of test series is being conducted with embedded steel sheets and larger size specimens to examine the influence of various parameters on the load bearing behaviour of the steel-wood-bonding. For an easy integration into the glulam production process one side of the steel sheet would be glued with a high-performance adhesive while the other side would be glued with the MUF adhesive. In the pull-out test an adhesive metal failure on the MUF-bonded side followed by a wood shear failure on the EP/PUR/EPI-bonded side was observed. The bond of the MUF adhesive could be improved by applying a thin layer of the EP adhesive on the steel surfaces. Comparative tests with specimen where the steel sheets were glued on both sides with one of the high-performance adhesives resulted in higher pull-out strengths.
The thin glue-line of the ab initio bonding method, compared to bonding steel elements into saw-cut kerfs, results in a bigger influence of the bonding length, as peak stresses at the end of the shear joints increase with length and the glue-line has virtually no plastic deformation. To reduce the stress peaks an optimisation of the steel sheet geometry will be approached through experimental tests and numerical simulations.

To examine the effects of climatic exposure and aging of the bond the specimen will undergo treatments according to EN 302-1. For further study of the duration of load (DOL) and creep effects on the bond, long-term tests in alternate climate conditions with a constant load will be executed. The ab initio reinforcement will then be tested in the production process with the fabrication of real-size connections of glulam elements.

The ab initio bonding of steel sheets into glulam, which is examined in an ongoing research project at the Chair of Structures and Structural Design, aims to develope a process-integrated reinforcement method for glulam elements to enable high-performance joints. Various combinations of adhesives and steel sheets have been evaluated for the performance of the bond as well as cost efficiency and handling. In further series of tests and numerical simulations, influencing factors on the performance of the adhesive bond such as geometry, climatic impact, long-term load and production process are being determined to specify the load bearing properties and dimensioning for practical application. The new joining method with ab initio bonded steel sheets enables on-site connections of large timber elements with high load bearing capacity and easy assembly.
The project is funded by the Research Initiative "Zukunft Bau" of the Federal Institute "Bundesinstitut für Bau-, Stadt- und Raumforschung".