エネルギーの釣合に基づく1層NCブレース架構の地震応答予測

抄録

 The low cost and stable performance of steel dampers are frequently employed to reduce displacement and acceleration responses in an earthquake. Dampers cannot dissipate elastic energy but may increase a structure's absolute acceleration. To overcome these problems, the dampers can be designed for early yielding, so they may be damaged by a small earthquake.
 The authors have proposed Z-type and anti-Z-type NC braced frames that reduce elastic acceleration response by elastic energy accumulation due to incremental deformation. Residual deformation after an earthquake can be reduced by release of NC brace residual tension, so this system is superior to reuse performance. Qualitative performance of these frames is obtained from shaking table tests and numerical analyses, but quantitative estimation of the seismic response values is not clarified and differences among the seismic performances of these braced frames is not investigated.
 Seismic response estimation based on energy balance is often applied in the seismic design field. The concept of the method was proposed by Housner, and its usefulness for seismic response prediction was verified by Akiyama. The energy balance method is an efficient tool for evaluating accumulated plastic deformation and a structure's maximum deformation in an earthquake.
 In this paper, numerical analyses of these NC braced frames as well as an X-type brace are carried out to examine their seismic responses. A seismic response prediction method based on the energy balance principle is formulated and the method's accuracy is evaluated from numerical results. Conclusions are summarized as follows.
 1) Z-type, anti-Z-type and X-type NC braced frames can reduce elastic acceleration response by elastic strain energy accumulation. From numerical results, Z-type and anti-Z-type frames can reduce by up to 40% maximum acceleration, and X-type frames can reduce by up to 22.8%.
 2) Maximum responses of all braced frames under elastic vibrations have the following tendencies. Maximum acceleration is reduced in the order, X-type with initial tension, normal X-type and Z-type, and anti-Z-type. Maximum story-drift deformation of X-type is the same irrespective of initial tension. Maximum story-drift deformation and residual story-drift deformation of both Z-type and anti-Z-type frames are equivalent.
 3) Maximum responses of each braced frame under elasto-plastic vibrations have the following tendencies. Both X-type and anti-Z-type frames show bi-linear cyclic curves around the origin and Z-type around δ_c^* calculated by Eq.(1), and maximum acceleration and maximum deformation from the center of a cyclic curve. Maximum story-drift deformation and residual story-drift deformation of both Z-type and anti-Z-type frames are equivalent.
 4) A seismic response prediction method based on the energy balance principle is formulated, and the maximum acceleration and maximum story-drift deformation of NC braced frames under elastic vibrations and residual story-drift deformation of Z-type and anti-Z-type frames can be predicted. All predictions of seismic responses are in good agreement with the corresponding numerical result. Reduction ratio of maximum acceleration by elastic strain energy accumulation can be calculated by the prediction method without numerical analyses.
 5) Damage to the NC braced frames can be evaluated safely by the prediction method. The residual story-drift deformation of both Z-type and anti-Z-type frames under elasto-plastic vibrations can be accurately predicted.