Masaaki Tsuji

Non-monotonic optimal damper placement via steepest direction search

An efficient and systematic procedure is proposed for finding the optimal damper positioning to minimize the dynamic compliance of a 3-D shear building model. The dynamic compliance is expressed in terms of the transfer function amplitudes of the local interstorey drifts evaluated at the undamped fundamental natural frequency. The dynamic compliance is minimized subject to a constraint on the sum of the damping coefficients of added dampers. Optimality criteria are derived and the optimal damper positioning is determined via an original steepest direction search algorithm. This algorithm enables one to find an optimal damper positioning sequentially for gradually increasing damper capacity levels. A non-monotonic design path with respect to the total damper capacity level often appears in the application of this algorithm. A new augmented algorithm via parameter switching is devised to find this non-monotonic design path. Copyright

Optimum viscous dampers for stiffness design of shear buildings

The problem in this paper is to find the optimum sets of story stiffnesses and of damping coefficients of the dampers of an elastic planar shear building with viscous dampers (SBVD model) subject to constraints on maximum interstory drifts due to a set of spectrum-compatible earthquake motions, on upper bounds for each damping coefficient of dampers, and on the sum of damping coefficients of dampers. Two basic characteristics of an ordered set of optimum SBVD designs have been disclosed: (1) greater damping coefficients are distributed within the specified upper bounds among the dampers placed on stories with greater interstory drifts; (2) the effect on stiffness reduction of the optimization of the damping coefficients of dampers from an initial design of an SBVD model with uniform dampers is greater when non-uniform maximum interstory drifts with larger differences between the minimum and maximum values are specified. A design guideline for the effective configuration of viscous dampers for stiffness design of an SBVD model is proposed.

Application of an optimum design method to practical building frames with viscous dampers and hysteretic dampers

An innovative optimum design system for structures with passive-type dampers is proposed. The design system depends on the type of dampers. A realistic application example is presented first of the optimum structural design method to practical building frames with hysteretic dampers. A computer program based on the gradient projection algorithm is used for initial design of a 100 m-high building frame located at Osaka, Japan. The effect of hysteretic dampers is incorporated in the calculation of design horizontal loads. The present method has the flexibility that manual modification by structural designers can be added to the initial design in order to satisfy multiple design conditions specified in the Japanese building structural design codes. Subsequently another practical method is presented for optimum structural design of building frames with viscous dampers. This method is a two-step design procedure. The first step consists of the stiffness design of a reduced shear-building model with viscous dampers. The second step is the optimum design for building frames subjected to static design loads. The design horizontal static loads are determined in the first step. Several design examples are presented to demonstrate the usefulness of the proposed design method.

Automatic generation of smart earthquake-resistant building system: Hybrid system of base-isolation and building-connection

A base-isolated building may sometimes exhibit an undesirable large response to a long-duration, long-period earthquake ground motion and a connected building system without base-isolation may show a large response to a near-fault (rather high-frequency) earthquake ground motion. To overcome both deficiencies, a new hybrid control system of base-isolation and building-connection is proposed and investigated. In this new hybrid building system, a base-isolated building is connected to a stiffer free wall with oil dampers. It has been demonstrated in a preliminary research that the proposed hybrid system is effective both for near-fault (rather high-frequency) and long-duration, long-period earthquake ground motions and has sufficient redundancy and robustness for a broad range of earthquake ground motions.An automatic generation algorithm of this kind of smart structures of base-isolation and building-connection hybrid systems is presented in this paper. It is shown that, while the proposed algorithm does not work well in a building without the connecting-damper system, it works well in the proposed smart hybrid system with the connecting damper system.

Effect of Non-linearity of Connecting Dampers on Vibration Control of Connected Building Structures

The connection of two building structures with dampers is one of effective vibration control systems. In this vibration control system, both buildings have to possess different vibration properties in order to provide a higher vibration reduction performance. In addition to such condition of different vibration properties of both buildings, the connecting dampers also play an important role in the vibration control mechanism. In this paper, the effect of nonlinearity of connecting dampers on the vibration control of connected building structures is investigated in detail. A high-damping rubber damper and an oil damper with and without relief mechanism are treated. It is shown that, while the high-damping rubber damper is effective in a rather small deformation level, the linear oil damper is effective in a relatively large deformation level. It is further shown that, while the oil dampers reduce the response in the same phase as the case without dampers, the high-damping rubber dampers change the phase. The merit is that the high-damping rubber can reduce the damper deformation and keep the sufficient space between both buildings. This can mitigate the risk of building pounding.

Design of steel frames for specified seismic member ductility via inverse eigenmode formulation

Abstract A new direct method of ductility design is presented for planar moment-resisting steel frames such that a set of member stiffnesses and the corresponding member strengths of a frame with realistic cross-sectional proportions are found for a specified distribution of member ductility factors under design major earthquakes. An inverse method of stiffness design is developed such that every predominant member-end strain in a frame under design earthquakes estimated on the basis of lowest eigenvibration would coincide with the specified value. A concept called ‘a set of similar cross-sections’ is introduced to make it possible to obtain a set of members with realistic cross-sectional proportions. It is then shown that this inverse formulation for specified predominant member-end strains is quite useful for developing a direct stiffness design method for specified mean maximum member-end strains under design earthquakes. It is further demonstrated that, if a frame is designed elastically for specified ductility factors based on a new concept of a ‘virtually elastic frame’ even under design major earthquakes, then the so-designed frame exhibits the desired response ductility characteristics specified for inelastic frames. Several design examples are presented to illustrate the usefulness of this design method and time history response analyses are performed to demonstrate the validity and accuracy of this design method.

Experimental Study on Influence of Hardening of Isolator in Multiple Isolation Building

An innovative multiple isolation building system is proposed and the influence of hardening in seismic isolators on the response of a multiple isolation building is investigated by shaking table vibration tests for a scaled structural model. From the observation in recent earthquake disasters in far-fault ground motions, e.g. the 2011 off the Pacific coast of Tohoku earthquake, a significant concern is reminded that the long-period and long-duration ground excitation may cause severe damages to the existing base-isolated buildings. In order to enhance the seismic vibration suppression performance of those buildings, the multiple isolation structure has been developed recently as one of the innovative solutions. The multiple isolation structure is defined as a seismic isolated building which has multiple isolation stories by inserting supplemental isolators in the middle story in addition to the base. In this paper, the advantages of the proposed multiple isolation system subjected to an extremely strong far-field earthquake ground motion as the worst scenario are studied by the vibration test for a scaled model. In the scaled model, a non-linear restoring-force characteristic (hardening) is provided by the geometrical non-linearity which can be realized by inserting linear springs in the direction perpendicular to the vibration direction. The influence of this hardening property on the structural responses is studied by comparing with the responses of the same model without hardening effect. In the comparison with the base-isolated building, the fundamental seismic vibration suppression performances are evaluated in terms of the transfer functions of both a multiple isolation system and a base isolation system to the base input by sweeping frequencies of stationary sine waves using a controllable compact shaking table. In the numerical simulations, the effect of the friction in the isolation layers is also investigated.

Hybrid Control System for Greater Resilience Using Multiple Isolation and Building Connection

An innovative hybrid control building system of multiple isolation and connection is proposed and investigated using both time-history and input energy responses for various types of ground motions together with transfer functions. It is concerned that the seismic displacement response at the base-isolation layer of the existing base-isolated buildings may extremely increase under long-period and long-duration ground motions which are getting great attention recently. In order to enhance the seismic performance of those base-isolated buildings, a novel hybrid system of multiple isolation and building-connection is proposed and compared with other structural systems such as an independent multiple isolation system, a hybrid system of base-isolation and building-connection. Furthermore, the robustness of seismic responses of the proposed hybrid system for various types of ground motion is discussed through the comparison of various structural systems including non-hybrid systems. Finally the optimal connection damper location is investigated using a sensitivity-type optimization approach.

A Simple Response Evaluation Method for Base-Isolation Building-Connection Hybrid Structural System under Long-Period and Long-Duration Ground Motion

An innovative hybrid control building system of base-isolation and building-connection has been proposed in the previous study. This system has two advantages, (i) to resist an impulsive earthquake input through the base-isolation system and (ii) to withstand a long-duration earthquake input through the building-connection system. A simple response evaluation method without the need of nonlinear time-history response analysis is proposed here for this hybrid building system under a long-period and long-duration ground motion. An analytical expression is derived in the plastic deformation of an elastic-perfectly plastic single-degree-of-freedom (SDOF) model with viscous damping under the multi impulse which is the representative of long-period and long-duration ground motions. A transformation procedure of a base-isolation building-connection hybrid structural system into an SDOF model is proposed by introducing two steps, one is the reduction of the main base-isolated building to an SDOF system and the other is the reduction of the connecting oil dampers supported on a free wall to an oil damper with a newly introduced compensation factor on a rigid wall. Application of the analytical expression of the plastic deformation to the reduced SDOF model including the compensation factor on the connecting oil dampers enables the development of a simplified, but rather accurate response evaluation method. The time-history response analysis of the multi-degree-of-freedom (MDOF) model and the comparison with the proposed simplified formula make clear the accuracy and reliability of the proposed simplified response evaluation method.

Optimal Placement of Hysteretic Dampers via Adaptive Sensitivity-Smoothing Algorithm

Since hysteretic dampers have nonlinear restoring-force characteristics with sensitive plastic flow and input earthquake ground motions propagating random media are extremely random in time and frequency domains, the seismic response of a building structure with hysteretic dampers deviates greatly depending on the installed quantity and location of dampers. This characteristic could become a barrier and difficulty to the reliable formulation of optimal placement problems of such dampers. In order to overcome such difficulty, a new optimization method including a variable adaptive step length is proposed. The proposed method to solve the optimum design problem is a successive procedure which consists of two steps. The first step is a sensitivity analysis by using nonlinear time-history response analyses, and the second step is a modification of the set of damper quantities based upon the sensitivity analysis. Numerical examples are presented to demonstrate the effectiveness and validity of the proposed design method.