Citation: Dziurka, D.; Derkowski, A.; Wieruszewski, M.; Kuli ´ nski, M.; Mirski, R. GL Beams Reinforced with Plywood in the Outer Layer. Materials 2022, 15, 3976. https://doi.org/ 10.3390/ma15113976 Academic Editor: Tomasz Sadowski Received: 12 April 2022 Accepted: 31 May 2022 Published: 2 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article GL Beams Reinforced with Plywood in the Outer Layer Dorota Dziurka * , Adam Derkowski , Marek Wieruszewski , Marcin Kuli ´ nski and Radoslaw Mirski Department of Mechanical Wood Technology, Faculty of Forestry and Wood Technology, Pozna´ n University of Life Sciences, Wojska Polskiego 28, 60-637 Pozna´ n, Poland; adam.derkowski@up.poznan.pl (A.D.); marek.wieruszewski@up.poznan.pl (M.W.); marcin.kulinski@up.poznan.pl (M.K.); radoslaw.mirski@up.poznan.pl (R.M.) * Correspondence: dorota.dziurka@up.poznan.pl; Tel.: +48-61-84-87-619 Abstract: Glulam beams are increasingly used in the construction industry because of their high strength and the possibility of using round timber with smaller cross-sections. The load-bearing capacity of beams is strongly related to the quality of the outer layers and, in the case of wood, especially the tension zones. For these reasons, this study decided to replace the outer lamella with tensile plywood. The produced beams were subjected to static bending strength and modulus of elasticity evaluation. It was shown that the best static bending strength values were obtained for beams containing plywood in the tension layer. However, the change in structure in the tension zone of beams made of glued laminated timber results not only in an increase in the load capacity of elements produced in this way but also in a decrease in the range/range of the obtained results of bending strength. This way of modifying the construction of glued laminated beams allows a more rational use of available pine timber. Keywords: beams; glued elements; solid wood; plywood; mechanical properties 1. Introduction The growth of the construction industry is causing this industry to look not only for alternative construction materials but also for the development of current ones. Engineering Wood Products (EWP) are construction-related materials. EWPs involve, among other things, obtaining a full-value product from a material that was initially unsuitable for specific applications due to its size or insufficient quality. Nowadays, there has been a development in Europe and worldwide in the manufacture and use of glulam, mainly GLT (Glued Laminated Timber). GLT has the typical characteristics of solid wood: low weight, good strength, resilience, durability, ease of processing, and the unique feature of being easy to shape cross-sections. Its cross-section has a layered structure, which allows the production of elements with variable section height, depending on the needs. The timber elements are glued with adhesives that guarantee high static and dynamic loads [1–3]. The high properties of GLT do not mean that the development of this product has been abandoned. It seems that one of the oldest ways of strengthening wood, or improving its performance characteristics, is to combine it with steel. Initially, steel was used to join individual elements, or as anchors to connect wood to other materials. However, quite a long time ago, there were already solutions where steel was used to reinforce timber in the sense that is now understood and accepted [4–6]. One of the first precursors of the use of steel as a wood reinforcing element was Dziuba at Pozna ´ n University [7], and the work is continued by Mirski [8]. Steel tendons can be used to strengthen newly designed beams and renovate existing ones [9–11]. An attractive solution, in which wood is instead a cladding for H-profile steel, was proposed by Duan and the team [12]. The idea of steel reinforcement is to introduce a stiffer material in the section, which increases the stiffness of the beam and thus reduces its deflection. Another way to increase the load-bearing capacity of glued laminated beams and timber is to use all kinds of Materials 2022, 15, 3976. https://doi.org/10.3390/ma15113976 https://www.mdpi.com/journal/materials