DEVELOPMENT OF SEISMIC REPAIR FOR THE DAMAGED STEEL MEMBERS (PART 2): SEISMIC REPAIR BY RETIGHTENING TURNBUCKLE BRACES

Abstract

 Most of school gymnasiums which are composed of braced steel frame are expected to play a role as a disaster prevention base and public shelter in severe earthquakes. Therefore, it is necessary to establish seismic repairs that can be used continuously after earthquakes, although a damage reduction is also important. In the previous study, the damage evaluation based on the residual out-of-plane deformation has been established by an experimental study. This paper focuses on the seismic repair by retightening turnbuckle brace, and examined (1) performance recovery, (2) internal forces, and (3) workability.

 Firstly, performance recovery by retightening turnbuckle brace was investigated by a cyclic loading experiment. In this experimental program, after "damage loading" to damage to the braces, seismic repairs were performed. After that, "repair loading" was conducted to confirm the performance recovery. It was found that the turnbuckle can be retightened even if the maximum story drift ratio (R_max) reached to 4.0%. In addition, from the comparison of hysteresis curves, it became clear that the performance of brace was recovered by retightening the turnbuckle. However, even though multiple seismic repairs restored the maximum strength and yield strength, the elastic stiffness was reduced.

 Secondly, the out-of-plane bending moment as the internal force was discussed. It was indicated by the experiment that the out-of-plane bending moment in tension depends on the eccentric distance. The distance can be obtained by the structural model proposed in this paper. Moreover, the out-of-plane bending moment in tension is restrained by making the thickness of the gusset plate and the diameter of brace equal in size. On the other hand, in compression, the out-of-plane bending moment needs to be considered in the case of a brace with a large diameter.

 Lastly, the torque in retightening the damaged turnbuckle was investigated from the viewpoint of workability. It became clear that repairing out-of-plane deformation is easier than introducing axial force. Considering retightening the damaged turnbuckle with a 1m rod, the nominal diameter is M24 or smaller when the damaged brace is repaired and M20 or smaller when an axial force is introduced. In addition, after the multiple repairs, the torque coefficient is almost constant that the screw part of the turnbuckle was not damaged and the workability was not affected even after multiple retightening.