CYCLIC ELASTO-PLASTIC DEFORMATION CAPACITY OF DOG BONE TIMBER-STEEL HYBRID CONNECTIONS

Abstract

 In recent years, utilization of timber-based materials for low-rise non-residential buildings has been actively promoted for environmental considerations, advances in timber design, and the development and support of new laws. However, compared with other structural materials, it is difficult to secure the rigidity and strength of joints in timber-based structures, so additional seismic elements such as shear walls and braces are typically required. Therefore, in this study, a moment frame without the need for such seismic elements was developed. The proposed frame is a hybrid structure that uses steel for the columns and wood materials for the beams, whilst ensuring good overall toughness. A timber-steel hybrid joint (where the timber and H-shaped steel are connected by lag screws and a steel dog bone) is used. The dog bone at the ends of the beam bends at an early stage to avoid damage to the timber. This is expected to improve seismic energy absorption performance comparable to that of a steel rigid joint at the beam ends.

 The fracture behavior and mechanical properties were verified through cyclic bending tests of 1:2 scale partial specimens of the proposed joint. Subsequently, equations for evaluating the rotational stiffness and strength of the joint were proposed. The results of the experiment were reproduced using the obtained values from the equations. Finally, detailed example of a timber-steel frame model and a steel frame model were designed by using the joint rotational stiffness obtained from the experiments. Nonlinear response history analysis was undertaken based on the Ai distribution and artificially generated seismic waves (BCJ-L1 and L2). The seismic performance comparison of the two models was performed using time-history analysis, and the feasibility of the timber-steel hybrid structure building was verified.

 The following shows the findings and results obtained in this study:

 1) In the testing of the proposed timber-steel hybrid beam-to-column joint specimens, the dog bones yielded before the laminated timber joint in all tests, and a stable elasto-plastic bending moment-rotation angle relationship was obtained. While, it was confirmed that slip failure may occur depending on the lap length and the presence or absence of wedges.

 2) The proposed equations for evaluating the rotational stiffness of bonded timber joints and dog bones gives an error of 20% compared to the experimental values. The behavior of the non-linear rotating spring-loaded wire rod model with the estimated value of rotational stiffness showed reasonable agreement with the experimental results.

 3) In the test specimen with a wedge and a wrap length, the theoretical maximum strength of the steel lap was higher than the experimental maximum strength of the dog bone. The experimental results showed that the laminated timber was almost intact, and the validity of the yield strength evaluation formula was confirmed.

 4) It was confirmed that the timber-steel frame model can be designed with a smaller floor weight and stiffness in earthquake design than the steel frame model, and the timber-steel frame model could suppress the maximum response inter-story drift angle in the time history response analysis than the steel frame model.