Glued-In Rods for Timber Structures - Development of a Calculation Model

This report relates to GIROD WP1 – “Development of a calculation model”. WP1 consists of four sub-WPs: 1.1 “Theoretical work”, 1.2 “Bond line tests”, 1.3 “Tests for calibration” and 1.4 “Calibration of model”.

In WP1.1 theoretical models for rational prediction of pull out strength have been developed. The models include a very simple ideal plastic model, a linear elastic fracture mechanics model, a bar shear lag fracture model, a Timoshenko beam shear lag fracture model and a 3D non-linear finite element fracture model. Several simulations have been made by the finite element model in order to investigate the effect of various geometry and material parameters on the pull out strength. The theoretical formats developed in WP1.1 has been further studied and evaluated in WP1.4 by means of test results.

In WP1.2, the bond line properties for three different adhesives, a fibre reinforced phenolresorcinol, PRF, a 2-component polyurethane, PUR, and an epoxy, EPX, have been determined by tests of small specimens in pure shear. Specimens with a very small bond area were tested in order to ensure as uniform stress as possible. The load versus deformation response of the bond line is recorded. An initially chosen test set-up was used for a large amount (approximately 30) of pre-tests. The mean strength of the adhesives was lower than expected (less than 6 MPa) and the test response was often unstable for the PUR and EPX. The PRF adhesive failed due to crushing of the threads in the adhesive, while the PUR and EPX adhesives failed in the wood/adhesive interface region with a large amount of wood fibres left on the adhesive. Due to the unstable response, these results were not suitable for evaluation, and a second test set-up was designed. A few (12) pre-test with the PRF-adhesive were performed with this second test set-up, producing useful results. Using a slightly modified version of the second test set-up, the main test series was
performed with a total of 61 successful tests. The mean strength was found to be 7.1 MPa for the PRF, 10.5 MPa for the PUR and 13.1 MPa for the EPX adhesive at 0º load to grain angle. The density of the wood was measured and it was found that it had no significant influence on the strength for the PUR adhesive. A small effect was found for the EPX and PRF bonded specimens. The mean work to failure was found to be 11.8, 9.6 and 22.0 kJ/m2 for the PRF, PUR and EPX
adhesives respectively. A method for evaluation of the effective fracture energy from the tests has been proposed. The method is based on evaluating the initial slope of the descending stress– displacement curve, rather than the conventional calculation of the area below the curve. This initial, negative, slope of the descending part of the stress-displacement curve, which can be used as a measure of the brittleness of the bondline, was evaluated for the three adhesives. It was found that the EPX and the PUR were the more brittle ones and that the PRF was more ductile. The load to grain angle was found to have a major influence on both the strength and the ductility. At 0º the average shear strength was 13.1 MPa and the alternate load to grain angles resulted in shear strengths of 12.8, 10.7 and 7.1 MPa for 22.5º, 45º and 90º respectively. The more ductile
behaviour of the cross grain specimens is explained in part by a propagating (in the circumferential direction) failure mode.

In WP1.3 a large number of full-scale short term ramp load tests of glued-in rods have been made. Three glues have been tested (PRF, PUR and EPX), various joint geometries (rod length, rod diameter, wood cross section dimensions and angle between rod and grain direction of the wood) and densities of the wood. The specimens were conditioned at 65% relative humidity before testing. Each test series comprised 7 nominally equal tests. The failure mode observed in
these tests was pull out of the rod, i.e. not splitting of the wood. The testing work is finished. A more detailed presentation of the test results than given in this report is compiled by project partner FMPA.

In WP1.4 strength design methods for the basic short term constant climate pull-out strength of glued-in rods are proposed. A basic proposal discussed in greater detail has been used in WP8. For adhesives that don’t shrink significantly and have some bond to the rod (epoxy and PUR) a design equation that is simple and based on rational mechanics has been developed. For other adhesives empirical strength design by tests is proposed. The basic design equation has been verified by short time ramp load test results obtained within WP1.3 and WP7. In WP1.4 there are moreover FE-results, showing the non-linear fracture mechanics prediction of the performance of full scale joints tested in WP1.3, as obtained using basic material property data from WP 1.2. The FE-results also comprise verification analysis of the small specimen test method.