Bond behaviour of self-tapping screws with continuous threads and long embedment length being used as reinforcement in glue-laminated timber elements - long-term behaviour
Final Report Abstract
The research project is the continuation of the research on the bond behaviour for the force transmission between the fully threaded self-tapping screws with long bonding lengths and the glued laminated timber matrix (Gl24h). While the first project part was oriented on short-term behaviour, the continuation includes experimental and theoretical investigations of the creep behaviour, of long bonding lengths under constant tension load in cyclic controlled climate. After creep tests (in duration of 6 months per series), the evaluation of the residual withdrawal capacity of the specimens is done and compared with the short-term test results. The specimens consisted of different screw diameters (8 and 12 mm), bonding lengths depending on screw diameter (20d, 25d and 30d) and different angles between the screw axis and wooden grains (0°, 30° and 90°). Long-term tests under constant load and cyclic climate underlined a strong influence of timber matrix structure on the bond creep behaviour. A clear dependency of the creep relative displacements and the angle between the screw axis and wooden grain direction is detected. The creep displacements are highest under the angle of 90° between the screw and timber matrix, caused by ductile bending of wooden grains interlocked between the screw’s threads. The stiffness of the bond for 0° angle is the highest and thus, the smallest creep displacements were obtained. Furthermore, the creep model is developed, based on the five parameter Burger model with the addition of shrinkage and swelling component, for the prediction of the relative creep factors. The calculated values exceeded the values for timber structures given in EC5 and are, as expected because of the cyclic climate, higher as those achieved in previous long-term tests by TRAKO in sheltered natural outdoor climate. The accumulation of creep damage and corresponding reduction of the withdrawal capacity is shown through residual tests. Here, also the results depend on the orientation how the screws are inserted with respect to the grain direction. Specimens with smaller angles had higher reduction of withdrawal capacity (up to 15%). Also, by some specimen’s configuration a change in failure mode is recorded. The climate and constant load induced creep displacements which reduced deformation capacities in loaddisplacements curves (the bond was partly delaminated) and caused earlier reaching of maximum bond stresses. In contrary to the short-term bond behaviour, there is a lack of research of the long-term response for the self-tapping screws in glue laminated timber. The results presented here are the basis for the further investigations of time-dependent strength values for these fasteners, which will allow wider application of the long continually threaded screws as a reinforcement and in connections for the conventional lifetime of several decades (ca 50 years) of a timber structure. For the usage of the self-tapping screws as practical, easy to install connector and reinforcement measure in glue laminated timber the familiarity of development of additional deformations through time (i.e., creep) and capacity reduction (i.e., load duration effects) is mandatory. The reinforcement configurations, like one developed in Chair of Structures and Structural Design and Institute for Building Materials Research, through projects “Reinforcing with screws” and by many other authors will benefit from the knowledge of creep response. The developed creep modelling approach can be used for the estimation of the relative creep factor for the fully threaded screws in glue laminated timber and necessary refinement of ones given by standards. Furthermore, the knowledge of residual loads and their influencing parameters, like dependency of the bond residual capacity on the screw orientation regarding the timber matrix, can be used for the improvement of the reinforcing screw’s configuration and orientation in timber elements. Among others, the residual bonding stresses are able to calculate using the climate dependable relative creep factors and initial deformation. The presented mechanical description of the short-term load-deformation characteristics and its phases serves depiction how bonding wave propagates along the screw length and causes debonding like a “zipper” effect. Until now, by many researches the high priority was given in addressing the short-term behaviour issues. Further and more extensive research of the bond´s long-term response is necessary. The orientation should be put in investigations of load-term effects under different constant load factors (i.e., 0.4, 0.5, 0.6) with the smaller bonding lengths (10d or 15d) that will provoke bond failure, which will reduce costs and necessary personal involved in tests. The experimental tests duration should be longer (minimum 1 year) and mechano-sorptive effects under constant and change in climate should be observed, for a more detailed and accurate description of moisture induced stresses. The residual load tests should be made not only at the end of the long-term tests, but multiple times during the test durations, which will give more data for analytical damage model development.
Publications
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„Das Dehnungs‐ und Tragverhalten von Brettschichtholz beim Lasteintrag durch Vollgewindeschrauben,“ Bautechnik, Bd. 94, Nr. 11, pp. 776-789, 2017
Trautz
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„Residual load-bearing capacity of self-tapping screws after long-term load,“ in Proceedings of the IASS Symposium 2018 (July 16-20), Creativity in structural design IASS2018, MIT, Boston, USA, 2018
Koj, Trautz, Pranjic und Grizmann
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„Wenn man mit Schrauben bewehren will: Erfahrungen und Forschungen zum Einsatz von Vollgewindeschrauben als Verstärkungs- und Fügungsmittel im Ingenieurholzbau,“ in Aachener Holzbautagung 2019, Aachen, 2019
Trautz
