*Article* **Estimating Mechanical Properties of Wood in Existing Structures—Selected Aspects**

**Tomasz Nowak \*, Filip Patalas and Anna Karolak**

Faculty of Civil Engineering, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland; filippatalas@wp.pl (F.P.); anna.karolak@pwr.edu.pl (A.K.) **\*** Correspondence: tomasz.nowak@pwr.edu.pl; Tel.: +48-71-320-22-63

**Abstract:** The paper presents and discusses selected methods of wood classification and the evaluation of its mechanical properties. Attention was mainly paid to methods that may be particularly useful for examining existing elements and structures. The possibility of estimating the modulus of rupture—MOR and modulus of elasticity—MOE based on the non- destructive (NDT), semidestructive (SDT), and destructive tests (DT) were considered. Known international, European, and American standards and research approaches were indicated. The selected testing methods and their interpretation were presented. These were, among others, the method of visual assessment, the resistance drilling method, methods of determining the dynamic modulus of elasticity, and procedures for testing small clear specimens. Moreover, some of our own research results from the conducted experimental tests were presented and discussed. In the destructive tests, both large elements and small clear specimens were examined. The results obtained from individual methods were compared and some conclusions were presented. The summary discusses the fundamental difficulties and limitations in applying the presented procedures and interpretations.

**Keywords:** timber structures; estimating mechanical parameters; small clear specimens; non-destructive tests; semi-destructive tests; resistance drilling; ultrasonic wave; stress wave; visual grading
