Experimental Evaluation of Shear and Flexure Behavior of Loblolly Pine Cross Laminated Timber Beams

Abstract

Cross-laminated timber (CLT) is a commonly used mass timber product that utilizes the strength, sustainability, and constructability advantages of timber while reducing many of the limitations associated with traditional timber construction. Recently, CLT has gained traction as a building material for protective structures due to its sustainability, improved resistance under short-duration loading, and ability to be rapidly deployed and assembled. However, design methods for CLT subjected to blast loading and other dynamic load applications are generally lacking. Such methods need robust understanding of the performance of CLT under quasi-static loading as a baseline for characterizing stiffness, strength, failure mechanisms, and post-peak responses that can then be used to quantify dynamic increase factors (DIF). This study experimentally evaluated the shear and flexure behavior of Loblolly Pine CLT beams subjected to quasi-static out-of-plane loading, with the primary objective of advancing and validating analytical methods for predicting the stiffness and expected strength. A total of 65 one-way bending beam tests were executed using a combination of contact-based sensors and Digital Image Correlation (DIC) techniques to capture the response of the CLT beams. The test matrix included varying the ply count, shear span-to-depth ratio, loading configuration, major/minor strength direction, and member width. The measured and calculated mechanical properties included the effective bending stiffness, apparent bending stiffness, effective shear rigidity, flatwise bending capacity, and flatwise rolling shear capacity. These values were compared to the predictions obtained using the shear analogy method as detailed in PRG-320 and PDC-TR 18-02. The study discovered that the ultimate strength predictions were generally conservative, but the degree of conservatism was dependent upon specimen geometry, loading configuration, and failure mode. The shear and flexural failures were highly dependent upon the shear span-to-depth ratio with shorter ratios resulting in shear failures and larger ratios resulting in flexural failures. The stiffness results follow similar trends to previous tests on comparable specimens with the apparent bending stiffness and the effective shear rigidity measuring higher than predicted and the effective bending stiffness measuring lower than predicted. The global response and post-peak behavior of the specimens was highly dependent upon the failure mode. The flexural failures were very brittle with sudden drops in the load carrying capacity of the beams. Conversely, the rolling shear failures developed more progressively and often displayed a gradual “stair-stepping” reduction in the load carrying capacity. Altogether, this study contributes toward a better understanding of the strength, stiffness, failure mechanisms, and residual response of Loblolly Pine CLT beams.