Cross laminated timber diaphragms under shear

Significance 

Cross-laminated timber is an engineered wood product that is typically made of an uneven number of rigidly and orthogonally connected layers. This material possesses out-of-plane and in-plane features making it interesting for many applications such as shear walls, beams, folded panels, and floor elements. In view of the properties and resistances as a structural element, cross-laminated timber is differentiated between in-plane and out-of-plane loading.

For the case of cross-laminated timber under out-of-plane loading, test arrangements, characteristic values and design procedures are currently agreed. However, for cross-laminated timber diaphragms under in-plane loading, some attributes are still not agreed on, therefore leading to conservative regulations such as compression and tension in direction of the top layers.

In order to benefit from high capacities of cross-laminated timber in-plane, an in-depth understanding of all the required mechanical features that are dictated by the geometrical layup of these elements, and the development and verification of practical test arrangements to establish these features are necessary.

Consolidated understanding of the cross-laminated timber features under in-plane shear is important for conventional structural applications including floor diaphragms, cantilevered walls, and deeps beams, in all these cases potentially featuring notches and holes. The existing technical approvals as well as assessment documents for cross laminated timber products have differing regulations to establish their load-carrying capacities in-plane.

The orthogonal structure of cross-laminated timber diaphragms under shear can lead to three possible shear failure mechanisms; gross shear, torsion, and net-shear. Torsion resistance has been investigated comprehensively, but establishing in-plane gross- and net-shear strength is still challenging.

Philipp Dietsch, Michael Schulte-Wrede, Heinrich Kreuzinger and Mike Sieder at Technical University of Munich in collaboration with Reinhard Brandner and Julia Dröscher at Graz University of Technology reported a test arrangement and an evaluation procedure for cross-laminated timber diaphragms under shear stress. The proposed method is based on an elementary compression test; the test results that assumed gross-shear failures, were investigated implementing theoretical methods from plate theory. Their research work is published in Construction and Building Materials.

The proposed shear test configuration was successfully applied to the full spectrum of potential arrangements. The results demonstrate its functional and operational efficiency as well as reliable shear failures of all tested cross-laminated timber diaphragms.

The authors observed that all the samples with layers of boards bonded at their narrow faces failed in gross-shear, which was then followed by a net shear failure. All samples without narrow face bonding were observed to fail in net-shear. Therefore, the authors proposed this robust and reliable test arrangement for adoption in EN 16351 or in a suitable test and/or product standard.

The design method, which was originally developed for establishing shear properties from cross-laminated timber elements in gross-shear failure, was amended slightly and adopted for computing in-plane shear attributes also for cross-laminated timber elements in net-shear. The authors propose to incorporate the presented design approach, by taking into account all three potential failure mechanisms of cross-laminated timber diaphragms under shear stress, in Eurocode 5 or in any other timber design standard.

Cross laminated timber diaphragms under shear: Test configuration, properties and design. Advances in Engineering

About the author

Reinhard Brandner studied Forest Products Technology and Management at the University of Applied Sciences in Salzburg / Kuchl. He earned a diploma degree in 2006. Between 2005 and 2014 he was a researcher at the Competence Centre holz.bau forschungs gmbh, also active as Area-Leader and Scientific Assistant. Since 2009 he is a Univ.-Assistant at the Institute of Timber Engineering and Wood Technology at Graz University of Technology and since 2011 Deputy Head of the Institute. He obtained his PhD degree in Civil Engineering Sciences addressing the fields of Timber Engineering, Wood Technology and Stochastics, at Graz University of Technology in 2012 which was awarded 2013 with the Leo-Schörghuber Prize of the Holzforschung / Technical University of Munich (TUM). Since 2013 he is in a tenure track position and since 2014 Assistant Professor.

His research topics are timber engineering, wood technology and stochastic modeling of materials, connections and structures. He has published more than 270 publications of which more than 30 are published peer-reviewed in international scientific journals and conference proceedings.

About the author

Philipp Dietsch studied Civil Engineering at TUM from 1999 – 2005. His studies were accompanied with an academic year at McGill University, Montreal/Canada during which he was awarded the Antje Graupe Pryor award for exceptional academic performance and extracurricular commitment. He carried out his Diploma Thesis at the Center for advanced Wood Processing at UBC, Vancouver/Canada. For this work, he received the price for outstanding theses, awarded by the Association of German Engineers (VDI).

After his studies, Philipp Dietsch became research associate at the Chair for Timber Structures and Building Construction, TUM, Munich. Apart from his teaching assignments in timber engineering, his focus lies on the structural research on wide-span timber structures, e.g. robustness of systems. He contributed to the expertise about the Bad Reichenhall ice-arena collapse and was part of most activities at the chair related to the assessment and evaluation of failure mechanisms in existing timber structures. Thereupon he focused on monitoring processes and repair/ reinforcement strategies such as the reinforcement of beams under shear and tension perpendicular to the grain. In 2012 he was awarded the doctoral degree (summa cum laude) for his thesis “Design and application of shear reinforcements for glulam timber beams”.

In 2012, Philipp Dietsch became Team Leader “Timber Structures” at the Chair for Timber Structures and Building Construction, currently supervising a team of 10 scientists. The numerous courses taught are focused timber construction and timber engineering. For his commitment to teaching, Mr. Dietsch was repeatedly awarded the “Doce et Delecta”, a prize awarded by students, in 2016 he was awarded the Teaching Excellence Prize awarded by the Bavarian Minister of Science.

Philipp Dietsch is Chairman of COST Action FP1402 “Basis of Structural Timber Design – from research to standards” and delegate in European and German standardization committees (CEN/TC 250/SC 5 „Eurocode 5“ and Mirror Committees “EN 1990” and “Timber Structures”). In addition, Philipp Dietsch is the Convenor of CEN/TC 250/SC 5/Working Group 7 “Reinforcement” and member of the Project Team PTSC5.T1 “CLT and Reinforcement”. In 2019, Philipp Dietsch will take over Chairmanship of INTER, the International Network on Timber Engineering Research.

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Reference

Reinhard Brandner, Philipp Dietsch, Julia Dröscher, Michael Schulte-Wrede, Heinrich Kreuzinger, Mike Sieder. Cross laminated timber (CLT) diaphragms under shear: Test configuration, properties and design. Construction and Building Materials, volume 147 (2017), pages 312–327.

 

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