Anthony Blakeborough

The development of real-time substructure testing

Full–scale dynamic testing of civil engineering structures is extremely costly and difficult to perform. Most test methods therefore involve either a reduction in the physical scale or an extension of the time–scale. Both of these approaches can cause significant difficulties in extrapolating to the full–scale dynamic behaviour, particularly when the structure responds nonlinearly or includes highly rate–dependent components such as dampers. Real–time substructure testing is a relatively new method which seeks to avoid these problems by performing tests on key elements of the structure at full or large scale, with the physical test coupled in real time to a numerical model of the surrounding structure. The method requires a high performance of both the physical test equipment and the numerical algorithms. This paper first reviews the development of structural test methods and the emergence of real–time substructure testing. This is followed by a brief description of the equipment that is needed to implement a substructure test. Several novel developments in the numerical algorithms used in real–time substructure testing are presented, including a new, fast algorithm which allows nonlinear response of the surrounding structure to be computed in real time. Results are presented from a variety of tests which demonstrate the performance of the system at small and large scale, with either linear or nonlinear test specimens, and with varying numbers of degrees of freedom passed between the physical and numerical substructures. Finally, the usefulness and possible applications of the test method are discussed.

Laboratory testing of structures under dynamic loads: an introductory review

This paper introduces and reviews the theme of laboratory testing of structures under dynamic loads. The emphasis is on the simulation of earthquake effects, for which three principle methods are discussed: shaking tables, pseudo–dynamic testing and real–time testing. The latest developments in these areas are discussed in depth in the subsequent papers in this issue. While shaking tables and pseudo–dynamic methods are quite well established, both techniques have undergone significant advances in recent years, including improvements in control to ensure accurate reproduction of dynamic loads, and the construction of very large facilities aimed at eliminating the significant scaling problems. Development of the substructuring method has enabled large–scale pseudo–dynamic tests of parts of structures, coupled to numerical models of the remainder. Attempts are now being made to extend this approach to shaking tables. Recently, considerable efforts have been devoted to methods of testing both at large scale and in real time. Two approaches are discussed: the real–time substructure method, in which a physical test and a numerical model interact in real time; and effective force testing, in which equivalent seismic forces are applied by actuators operating under force control. Both methods have been shown to be feasible, but require further development. Although the techniques described have been developed primarily for seismic testing of structures, there is considerable potential for their application to other load types in the fields of civil and mechanical engineering.

A comparison of viscous damper placement methods for improving seismic building design

This article compares the effectiveness of five viscous damper placement techniques, two standard and three advanced, for reducing seismic performance objectives, including peak interstory drifts, absolute accelerations, and residual drifts. The techniques are evaluated statistically for two steel moment-resisting frames under varying seismic hazard levels, employing linear viscous dampers and nonlinear time history analyses. Usability of the methods is also assessed. All the placement methods meet the desired drift limit but advanced techniques achieve additional improvement in drift reduction and distribution. Performance differences between the advanced techniques are minor, making usability a significant selection factor amongst the methods.

Compensation of actuator dynamics in real-time hybrid tests:

Abstract Real-time hybrid testing is a novel approach to the dynamic testing of structures, in which the system under test is split into physical and numerical substructures which are tested and analysed in parallel, with data passed between them in real time. The success of a test is highly dependent on the performance of the actuators which provide the interface forces and displacements between the two substructures. This paper presents several numerical schemes to compensate for the non-ideal dynamics of typical servohydraulic actuators and evaluates them through a series of real-time hybrid tests on simple mass-spring systems. It is shown that effective schemes can be developed on the basis of a simple representation of the actuator response as combination of a delay and an amplitude error, both of which can vary during a test. The use of a modified online delay estimator, together with one of three simple forward extrapolation schemes, is found to be highly effective in minimizing experimental errors related to the actuator dynamics.

Novel load cell for measuring axial force, shear force, and bending movement in large-scale structural experiments

This paper describes the design, construction and testing of a load cell to measure the axial force, shear force, and bending moment at the end of a structural beam. The capacities of the load cell are 780 kN in axial load, 350 kN in shear, and 200 kNm in bending. These magnitudes, together with the requirement that the load cell should be kept as slim as possible, necessitated a novel design comprising three steel double-spring elements machined with semicircular channels to provide localized strain amplification. The load cell was designed with the aid of detailed finite element analysis and was machined from grade 55 steel. After strain gaging, it was subjected to an extensive series of calibration tests. Results from these tests are reported, together with those from some early experiments in which two load cells were used to measure the behavior of structural steel knee elements.

Reconfiguring practice: the interdependence of experimental procedure and computing infrastructure in distributed earthquake engineering.

When transitioning local laboratory practices into distributed environments, the interdependent relationship between experimental procedure and the technologies used to execute experiments becomes highly visible and a focal point for system requirements. We present an analysis of ways in which this reciprocal relationship is reconfiguring laboratory practices in earthquake engineering as a new computing infrastructure is embedded within three laboratories in order to facilitate the execution of shared experiments across geographically distributed sites. The system has been developed as part of the UK Network for Earthquake Engineering Simulation e-Research project, which links together three earthquake engineering laboratories at the universities of Bristol, Cambridge and Oxford. We consider the ways in which researchers have successfully adapted their local laboratory practices through the modification of experimental procedure so that they may meet the challenges of coordinating distributed earthquake experiments.

Modelling of joint crowd-structure system using equivalent reduced-DOF system

For human assembly structures in which the mass of the crowd is significant compared to that of the structure, it is necessary to model the passive crowd as a dynamic system added to the main structural system. Earlier work by the authors has analysed the frequency response of a joint crowd-structure system in which the structure is treated as a single degree-of-freedom (SDOF) system and the seated and standing crowds are each modelled as a two degree-of-freedom (2DOF) system. It was found that the occupied structure has dynamic properties different to the empty structure. This paper investigates representing the joint crowd-structure system as an equivalent reduced-DOF system that would have the advantage of simplifying the analysis. The modal properties of the equivalent reduced-DOF system, if known, can give a useful indication of how the passive crowd affects the modal properties of the occupied structure. Two equivalent reduced-DOF systems are investigated - SDOF and 3DOF systems. The errors between the responses of the equivalent systems and the full model are calculated and presented in the paper. The results show that the full model exhibits the behaviour of a SDOF system for structures with natural frequencies less than 4 Hz (when empty), whereas for structures with natural frequencies above 4 Hz the equivalent 3DOF system gives a better fit to the full model.

Process and Future of Data Integration within the European Earthquake Engineering Laboratories

AbstractIn common with many scientific disciplines, earthquake engineering research is increasingly focusing on large international collaborations to address complex problems. Data integration is a key requirement to facilitate joint research efforts and improve experimental outcomes. Development and implementation of a novel virtual database is presented, linking 22 leading European earthquake engineering research institutions, making data integration possible at a European level without the need for a large, centralized repository. The importance of establishing appropriate work methodologies to succeed in a distributed and highly heterogeneous work environment involving many dispersed institutions is described. The future of earthquake engineering data integration is considered, highlighting Semantic Web technologies as the main technological foundation to lead this integration.

An analytical response of church bells to earthquake excitation

Occasionally there is an earthquake in the UK during which church bells are reported to have been set ringing. The motion of a medium sized church bell has been simulated and the response to earthquake records from the Parkfield earthquake of 1966 has been calculated. The response of the bells is found to depend on the mechanical properties of the bell and the tower in which it is hung. The analyses do show that for a bell to ring in an earthquake the peak ground acceleration is in the range 0.97 to 29.4 m s−2 considerably in excess of the range indicated by the MMI VI “church and school bells ring” of 0.4 to 1.5 m s−2. The best correlation between recognised earthquake parameters and the ringing of the bell was obtained by combining the spectral values for clapper-bell angle, obtained from the linearised set of equations, at the bells natural frequencies using the SRSS method. The values of this parameter to set the bell ringing was in the range 0.25 and 0.38 rad, compared with the actual striking angle 0.54 rad.

Seismic Performance Assessment of a Conventional Multi-storey Building

Recent earthquakes have revealed that conventional seismic design philosophy allows for large levels of nonstructural damage. Nonstructural earthquake damage results in extensive repair costs and lengthy functional disruptions, as nonstructural systems comprise the majority of building investment and are essential to building operations. A better understanding of the expected overall seismic performance of code-compliant buildings is needed. This study investigates the seismic performance of a conventional building. A 16-storey steel office building was designed using a modern seismic structural code (Eurocode 8). This study is the first to assess in detail the substantial earthquake repair costs expected in a modern Eurocode concentric braced frame structure, considering nonstructural systems with the FEMA P-58 procedure. The breakdown of total repair costs by engineering demand parameter and by fragility group is novel. The seismic performance assessment indicated that substantial earthquake repair costs are expected. Limitations of the Eurocode nonstructural damage methodology were revealed in a novel manner using FEMA P-58, as the prescribed drift limits did not minimize nonstructural repair costs. These findings demonstrate the need for design procedures that improve nonstructural seismic performance. The study results provide a benchmark on which to evaluate retrofit alternatives for existing buildings and design options for new structures.