Narutoshi Nakata

A FRAMEWORK FOR MULTI-SITE DISTRIBUTED SIMULATION AND APPLICATION TO COMPLEX STRUCTURAL SYSTEMS

In this technical note, the development of a framework for multi-site distributed simulations is presented. The algorithm is suitable for any combination of physical (laboratory) and analytical (computer) distributed simulations of structures, their foundations and the underlying sub-strata subjected to static and dynamic loading. Two examples of multi-site testing and multi-platform simulation are given. The main contribution in this note is the separation between time-step integration and stiffness formulation, which enables the use of static analysis and testing as modules of the main control module referred to as the simulation coordinator. The approach proposed is intuitive, simple and efficient. It is therefore recommended for use in distributed analysis using different programs, distributed testing facilities (e.g. the NEES equipment sites) or a combination of analysis and testing.

TECHNICAL NOTE A FRAMEWORK FOR MULTI-SITE DISTRIBUTED SIMULATION AND APPLICATION TO COMPLEX STRUCTURAL SYSTEMS

In this technical note, the development of a framework for multi-site distributed simulations is presented. The algorithm is suitable for any combination of physical (laboratory) and analytical (computer) distributed simulations of structures, their foundations and the underlying sub-strata subjected to static and dynamic loading. Two examples of multi-site testing and multi-platform simulation are given. The main contribution in this note is the separation between time-step integration and stiffness formulation, which enables the use of static analysis and testing as modules of the main control module referred to as the simulation coordinator. The approach proposed is intuitive, simple and efficient. It is therefore recommended for use in distributed analysis using different programs, distributed testing facilities (e.g. the NEES equipment sites) or a combination of analysis and testing.

Direct Acceleration Feedback Control of Shake Tables with Force Stabilization

This study presents a new strategy for shake table control that uses direct acceleration feedback without need for displacement feedback. To ensure stability against table drift, force feedback is incorporated. The proposed control strategy was experimentally validated using the shake table at the Johns Hopkins University. Experimental results showed that the proposed control strategy produced more accurate acceleration tracking than conventional displacement-controlled strategies. This article provides the control architecture, details of the controller design, and experimental results. Furthermore, the impact of input errors in shake table testing on the structural response is also discussed.

A multi-purpose earthquake simulator and a flexible development platform for actuator controller design

This paper presents a multi-purpose uniaxial earthquake simulator that has been designed and developed at the Johns Hopkins University. The earthquake simulator is coupled with a flexible platform that facilitates the development of actuator control strategies to enhance accuracy in shake table tests. The development platform provides a full access to processes in control including an actuator servo control loop, enabling high-fidelity controller designs to be implemented and experimentally investigated for earthquake simulators. This paper presents construction details, hydraulic components, dynamic specifications, an integrated control and data acquisition system, and example applications of the earthquake simulator. The paper demonstrates the versatility of the earthquake simulator and the development platform that can be adopted elsewhere.

Experimental Seismic Response of a Full-Scale Cold-Formed Steel-Framed Building. II: Subsystem-Level Response

AbstractThe objective of this paper is to employ the results from the extensive instrumentation installed on recently tested full-scale cold-formed steel (CFS)-framed buildings to reveal a deeper understanding of the behavior of the building under seismic excitations. In particular, this paper complements a companion paper that focuses on system-level design and response. Here, utilizing strategically located string potentiometers, strain gauges, and accelerometers, the responses of the walls and diaphragms are isolated from the overall building response and studied. The interaction of shear walls along a wall line, as well as across stories is studied through measured data on strains in hold-down anchors, strains on floor-to-floor strap connecting shear-wall chord studs, and displacements across shear-wall sheathing and openings. The behavior of the floor diaphragm is studied through displacements measured perpendicular to the plane of one wall of the building and accelerometers throughout the floor of t...

Seismic Design of Multi-Story Cold-Formed Steel Buildings: The CFS-NEES Archetype Building

Lightweight cold-formed steel (CFS) framing is an effective building solution for low and mid-rise structures. However, systems level response and component contributions as well as their interactions such as those from lateral-load resisting systems, floor diaphragms, studs to track connections, etc., are not fully understood. Existing building codes for the CFS frame buildings are based solely on the stiffness of the lateral-load resisting frames and do not explicitly incorporate systems response. This paper presents the first-phase of a multi-year project aimed at generating knowledge and tools needed to increase the seismic safety of CFS frame buildings. The first phase of the study focuses on the design, instrumentation plan, and preliminary analysis of full-scale two-story CFS frame buildings that are tested on shake tables at University at Buffalo NEES Facility in the second phase. Design of the two-story CFS buildings incorporates a “state of the practice” ledger framing system that attaches floor and roof joists to the inside flanges of the load-bearing studs via a combination of track and clip angles. The instrumentation plan for the shake table tests is developed to capture both systems and component level response of the buildings. The preliminary analysis includes development of new modeling capabilities that incorporate cross-section limit states (local and distortional buckling) into frame analysis engines such as OpenSees to enable more accurate incremental dynamic analysis. This paper provides detailed design of a prototype CFS frame building and instrumentation plan for the shake table tests at Buffalo.

RATIONAL POLYNOMIAL APPROXIMATION MODELLING FOR ANALYSIS OF STRUCTURES WITH VE DAMPERS

The objective of this paper is to introduce a Rational Polynomial Approximation (RPA) method for modelling the response of structures that contain discrete elements with linear frequency-dependent stiffness and damping characteristics. The RPA method consists of two steps: First, system identification is performed to obtain a rational polynomial approximation for the systems transfer functions. Then, a time-domain model for the system is realised. The main advantage of the RPA method is that the resulting model is a system of ordinary differential equations, facilitating time history analysis of both linear and nonlinear structures using standard time-step integration algorithms and procedures. Viscoelastic (VE) dampers comprise one of the primary classes of frequency-dependent dampers with both frequency-dependent stiffness and damping. VE dampers are used for mitigation of seismic- and wind-induced structural vibration. When using VE dampers in analysis, effective modelling of the frequency-dependent characteristics of the VE damper plays a key role in accurate simulation of structural responses. Following a description of the theory behind the RPA method, the efficacy of the method is verified through several numerical examples employing VE damped structures. The results are compared with Kasais fractional derivative model for VE dampers. Application of the RPA method to nonlinear structures is also given. The RPA method is shewn to be effective and efficient for modelling VE damped structure.

Error Analysis of Digitally Controlled Servo Hydraulic Actuators for Structural Testing

Advanced control strategies, such as a model-based control, are essential to improve accuracy of servo hydraulic actuators in structural testing. Such control strategies require a servo-loop, often referred to as an inner-loop, of the actuator to be digitally controlled. To make the best use of digitally controlled inner-loops that have numerous advantages over conventional analog counterparts, a thorough understanding of the actuator inner-loop is needed. This study presents an error analysis of digitally controlled servo hydraulic actuators with a focus on sources and growth of errors in the actuator inner-loop. First, effects of errors such as measurement errors and electrical noises on the performance of servo hydraulic actuators are analytically investigated. Then, using detailed models of those errors and an open-loop servo hydraulic actuator, an extensive numerical simulation is performed to evaluate the effect of errors on the actuator performance. Numerical results show that the displacement measurement error has an undesirable influence on the actuator responses in the high-frequency range. Such responses make it difficult to accurately assess structural and nonstructural systems in the high-frequency range using the hydraulic actuators. In particular, velocity and acceleration responses of the hydraulic actuator are highly sensitive to the displacement measurement error. This investigation provides fundamental knowledge that is required for an effective use of digitally controlled servo hydraulic actuators.

IIR Compensation in Real-Time Hybrid Simulation using Shake Tables with Complex Control-Structure-Interaction

This article considers the use of actuator compensation in real-time hybrid simulation (RTHS) containing experimental substructures with complex control-structure-interaction (CSI). The existence of CSI in shake table testing is derived using theoretical relations. An infinite-impulse-response (IIR) compensator is developed to compensate for the shake table time delay as well as the effects of CSI. The efficacy of the IIR compensator is verified through numerical and experimental investigations of substructure shake table testing completed at Johns Hopkins University. IIR compensation is not limited to substructure shake table testing, and the concept is applicable to any RTHS that suffers from complex CSI.

Mixed Force and Displacement Control for Testing Base-Isolated Bearings in Real-Time Hybrid Simulation

ABSTRACT This paper presents a robust mixed force and displacement control strategy for testing of base isolation bearings in real-time hybrid simulation. The mixed-mode control is a critical experimental technique to impose accurate loading conditions on the base isolation bearings. The proposed mixed-mode control strategy consists of loop-shaping and proportional-integral-differential controllers. Following experimental validation, the mixed-mode control was demonstrated through a series of real-time hybrid simulation. The experimental results showed that the developed mixed-mode control enables accurate control of dynamic vertical force on the base isolation bearings during real-time hybrid simulation.