Effective Measuring Area and Test Protocol in Picture Frame Method to Measure In-plane Shear Modulus of Cross-laminated Timber

This study established a standardized test method for the in-plane shear modulus of cross-laminated timber (CLT) KS F 2082. Among the test methods used to evaluate the in-plane shear modulus of CLT, the picture frame method was considered appropriate for achieving a pure shear state. However, the test procedures for the picture frame method have varied among studies owing to the lack of a standardized test method. Therefore, the deformation measurement area and loading protocol are discussed herein. The measurement area was determined based on the shear behavior of the test specimens using digital image correlation. The tensile and compressive deformations were similar when the side measurement area length was less than 70% of the side length of the test specimen. In addition, a three-stage loading protocol, including a two-step preloading, was proposed to settle the specimen during loading. Testing of a plywood panel showed that the difference between tensile and compressive deformations decreased as the stages progressed. Considering the larger difference in the first phase compared to the subsequent phases, it was concluded that at least one preloading step was required to achieve a pure shear state. A CLT specimen was tested following these procedures and exhibited linear load-deformation curves with a small difference (2.0%) in the tensile and compressive directions. Therefore, the test procedure proposed in KS F 2082 appears to be suitable for achieving a pure shear state and is an effective testing method for CLT.