SECED 2015 Conference: Earthquake Risk and Engineering towards a Resilient World 9-10 July 2015, Cambridge UK OVERVIEW OF SEISMIC HYBRID TESTING Daniel McCRUM1 Abstract: Even though computational power used for structural analysis is ever increasing, there is still a fundamental need for testing in structural engineering, either for validation of complex numerical models or material behaviour. Many structural engineers/researchers are aware of cyclic and shake table test methods, but less so hybrid testing. Over the past 40 years, hybrid testing of engineering structures has developed from concept through to maturity to become a reliable and accurate dynamic testing technique. In particular, the application of hybrid testing as a seismic testing technique in recent years has increased notably. The hybrid test method provides users with some additional benefits that standard dynamic testing methods do not, and the method is much more cost effective in comparison to shake table testing. This paper aims to provide the reader with a basic understanding of the hybrid test method and its potential as a dynamic testing technique. Introduction Physical testing in a structural engineering context can be; monotonic, cyclic or dynamic. A monotonic test is essentially a tension or compression test under increasing load. For example, the steel coupon test consists of a steel bar being pulled under tension and results provide the Young’s Modulus, yield stress and ultimate stress of the steel. A cyclic test applies repeated tension and compression cycles to a specimen. Cyclic tests are quasi-static in that they are performed at a slow loading rate, and provide vital information about the fatigue performance and energy dissipation capacity of the material/structural component under investigation. Figure 1 provides typical load cycles for example in a moment resisting connection according an American Institute for Steel Construction code (ANSI/AISC (2005)). Importantly, cyclic tests are easily repeatable. Figure 1. Typical load cycles used in cyclic testing of moment resisting connections Dynamic tests replicate dynamic loading and can vary from free-vibration, impact, earthquake to wind loading. In earthquake engineering, shake table test facilities have been developed to provide fully dynamic testing capabilities and give the most realistic laboratory replication of dynamic effects. A shake table is a rigid platform that is moved by dynamic actuators to mimic ground motion during an earthquake event. The first ever shake table was wheel operated and used to characterise construction types, taking place at the University of Tokyo, Japan in 1893 (Reitherman (2012)). The first ever hydraulically powered shake table was conceived by Professors Clough and Penzien at UC Berkely, California, United States in 1969 (PEER (2014)). The rigid table was first used in 1972 and measuring 6m sq. is still the largest multi-directional shake table in the United States (PEER (2014)). Shake table test 1 Lecturer in Structural Engineering, Queen’s University Belfast, UK, d.mccrum@qub.ac.uk D.P. McCrum facilities are expensive to construct and maintain whilst the limitations of payload capacity of the table often result in experimental structures having to be scaled. Subsequently, the length, force and time need to be scaled appropriately to ensure similitude between scaled and unscaled results. In dynamic time history analysis, building structures are often idealised as frame elements with lumped masses at the nodes. Numerical integration is performed on the equation of motion to solve for the acceleration, velocity and then displacement of the structure at each incremental time step. However, what if the stiffness of the structure is provided by a physical test rather than numerically calculated? This would mean that the displacement being solved for would be more accurate as the stiffness is the real stiffness of the structure. This is the fundamental principle behind the hybrid test method. The hybrid test method involves the physical testing of a critical part of a structure whilst simultaneously numerically modelling the remainder of the structure. The critical part of the structure’s response may be difficult to numerically model e.g. highly nonlinear material response or else rate dependent behaviour, and the numerically modelled part of the structure has a more predictable response e.g. linear or only slightly nonlinear. Therefore, rather than testing the entire structure, it may only be necessary to test the critical part of the structure. The results from the physical test feed into the numerical model of the remainder of the structure to provide a more accurate overall prediction of the displacement structural response during dynamic excitation. Essentially, a hybrid test replaces numerically calculated structural response with physically tested structural response. In most cases, only part of the structure is physically tested, however in some experiments, the entire structure has been tested. This will be discussed in more detail later on in this article. Importantly, tests can be run at full-scale thus removing all potential issues associated with scaling dynamically tested structures. Shake table tests often need to be performed on scaled specimens as fully scaled specimens would exceed the capacity of most shake table facilities. The development of different forms of hybrid testing such as; pseudo dynamic testing, substructured testing, real-time hybrid testing and geographically distributed hybrid testing will be discussed in this paper. The content of this paper is based on McCrum (2014). Pseudo Dynamic Testing Dynamic test methods have developed over the past 40 years to make use of computational power and advances in hardware to provide more cost effective method of full-scale dynamic testing as compared to shake table testing. Rather than using multiple hydraulic actuators to push a shake table the actuators have been more efficiently employed in hybrid testing, see Figure 2. The method combines physical testing with simultaneous computational modelling. The method has been widely implemented in earthquake structural engineering testing and is discussed in general terms in this paper. Physical testing remains an important part of the progression of engineering science and the hybrid test method has advanced with developing computational power to achieve a cost effective full-scale dynamic testing method. 2 D.P. McCrum Figure 2. Schematic of the hybrid test method (Carrion & Spencer (2007)) The pseudo dynamic test method is a computational dynamic analysis procedure in which the stiffness term is physically measured. The assumption of lumped masses at the nodes is necessary as dynamic actuation can only be applied at lumped mass nodal locations (refer to Figure 1). If we consider the equation of motion below; (1) in which M is the mass matrix, is the nodal acceleration vector, C is the damping matrix, is the nodal velocity vector, is the restoring force vector (which is equal to , where K is the stiffness matrix and is the nodal displacement vector) and Fi+1 is the external excitation force applied to the system (i is the current time step and i+1 is the next time step). Time steps are usually 0.01s as this is the time increments at which typical earthquake time histories are recorded at. As per Figure 1, if the excitation is an earthquake ground acceleration of , then . Figure 1 indicates how the displacement at the next time step, is calculated using the equation of motion by the numerical component. The displacement of the structure at the next time step (i+1) is then sent to the actuator. The actuators then apply the displacement, xi+1 to the structure. The restoring forces are measured by the actuators, R1 and R2. These are then sent back to the numerical component. The restoring forces are then applied to the equation of motion and used to calculate the acceleration, velocity and subsequently the displacement for the next time step. This process is referred to as closed looped. In Figure 2, the structural masses (M) and damping (C) are numerically modelled, whilst the entire two-storey structural frame is experimentally tested. Concept of Substructuring The concept of substructuring is critical to the overall cost effectiveness of the hybrid test method. It was first introduced in the United States in 1985 (Dermitzakis & Mahin (1985)) and subsequently implemented by many others. An example of substructuring can be seen in Figure 3. Figure 3 shows a steel concentrically braced frame structure in which the response of the ground floor braced frame on one side of the building is critical to the overall dynamic response of the structure. Rather than testing the entire structure (Figure 3(a)), the critical ground floor braced frame that has been identified through numerical simulation is then physically tested (Figure 3(b)). The physically tested part of the structure is referred to as the physical substructure. Figure 3(c) shows the numerical part of the structure, referred to as the numerical substructure. Results from the physical test are sent back to the numerical substructure to solve for the overall structural response. Substructuring prevents the need for scaling test structures or the testing of entire structures. 3 D.P. McCrum Figure 3. Concept of substructuring applied to concentrically braced frame structure; (a) entire structure; (b) critical elements tested; and (c) numerical model Real-time Hybrid Testing The challenge to overcome when a pseudo dynamic test that is run close to or at real-time is that the computation of the displacement for the next time step needs to be performed very quickly. Considerable research has been undertaken to improve the hydraulic actuator systems and numerical methods associated with accurate and timely actuation. The first system capable of performing a real-time hybrid test was performed in Japan in 1992 (Nakashima & Kato (1992)). In a real-time hybrid test the structure is tested at or close to real-time therefore velocity dependent behaviour is physically measured. The damping and inertia response of the tested structure are measured in the force feedback and not defined computationally as in the pseudo dynamic test method. Figure 4. Schematic of real-time hybrid test system Figure 4 presents a schematic of a real-time hybrid test system. The analysis is initiated by the Computational Structural Model in which the command displacement (referred to as command displacement as the command is being sent to the actuator) for the first time step is calculated through numerical integration of the equations of motion. The structural model is separated into numerical and physical substructures by the user. The digital command displacement signal is converted to an analogue signal for use by the actuator. The analogue command displacement is sent to the Actuator Controller. This controller ensures accurate application of the command displacement to the test specimen. An actuator command displacement is sent by the Actuator Controller to the actuator and applied to the Test System. Data acquisition devices provide the measured displacement that is sent back to the 4 D.P. McCrum Actuator Controller to check that the displacement has been accurately applied to the structure (referred to as an inner control loop). With the displacement error tolerance of the inner control loop satisfied, the measured restoring force and measured displacement from the actuator are converted back from an analogue to digital signals for use by the Computational Structural Model. The measured displacement and restoring force of the experimental substructure is combined with the numerical substructure and the next time step command displacement is calculated. This procedure is repeated until the end of the test which is typically the end of the input time history (e.g. recorded earthquake). Distributed Hybrid Testing Hybrid testing is ideally suited to performing tests in geographically separate locations as communication between the physical and numerical substructures can take place over the internet rather than through a cable in a laboratory. Researchers can benefit greatly from the combination of one or more experimental sites. The first implementation of a geographically distributed hybrid test was performed between Kyoto University and Osaka City University, Japan (Watanabe et al. (2001)). Since then, geographically distributed hybrid tests have been conducted internally within the United States (Spencer et al. (2004)), United Kingdom (Saleem et al. (2008)) and Taiwan (Yang et al. (2003)) and between a number of countries, notably Korea/Japan (Watanabe et al. (2001)), the United States/Japan (Park et al. (2005)), the United States/Taiwan Wang et al. (2007)) and New Zealand/United States (Ma et al. (2007)). Figure 5. Schematic of client/server configuration and data communication for a distributed hybrid test (adapted from Park et al. (2005)) A number of distributed hybrid test frameworks have been developed over the past 15 years to improve the communication between the physical and numerical substructure locations. Figure 5 presents a standard client/server framework that forms the foundation of many of the frameworks developed to date. The framework shown consists of two experimental sites (Institution I & II) and one main computational site. The command displacements calculated by the Analysis Engine (typically by a finite element package) on the Main Computer are transferred across the internet and controlled locally by each Local Server as shown in Figure 5. The command displacement is then applied to the local test structure. The restoring force and measured displacements are then sent back to the Client System. The data communication between the sites is co-ordinated over the internet using the standard transmission control protocol/internet protocol. The protocols control how two computers talk to each other over the internet. The protocols are responsible for sending, receiving and checking the contents of the data packets. The way in which data is sent and received varies between the different distributed hybrid test frameworks that have been developed, but typically a protocol is set-up to ensure application of the command displacement has been achieved at each experimental site before sending back to the Main Computer site. The data 5 D.P. McCrum communication can be paused to allow all sites to complete the test time step and is particularly important if there are network interruptions i.e. communication delays over the internet. Improvements on the transmission and storage of data over the internet have been a large focus of research in distributed hybrid testing, in particular different approaches to dealing with random delays in internet communication. More recently, Ojaghi et al. (2014) demonstrated the feasibility of performing distributed hybrid tests in real time over the internet, through a series of experiments conducted between Oxford and Bristol universities in the UK. The tests used existing hardware and control systems at both sites, with modifications designed to minimise local delays and to prioritise real-time communications over other processes. Real-time hybrid testing was achieved across a variety of relatively simple test set-ups. However, the method was tailored to the particular laboratories involved and was not readily transferable to other sites. Recent Applications of the Hybrid Test Method In earthquake engineering research, the pseudo dynamic test method has been applied to structures that do not have any rate dependent behaviour, whilst real-time hybrid tests have been applied to rate dependent devices such as viscous dampers. In order to provide readers of this article with an understanding of the capability of hybrid testing, Figures 6-9 present a small selection of some of the hybrid experiments undertaken to date. Figure 6. Pseudo dynamic test of a full-scale three-storey reinforced concrete plan irregular structure (Negro et al. (2004)) Figure 7. Full-scale substructure pseudo dynamic test of a full-scale three-storey plan irregular steel concentrically braced frame structure (McCrum & Broderick (2013)) 6 D.P. McCrum The experiment shown in Figure 6 was a pseudo dynamic test performed on a full-scale three-storey moment resisting reinforced concrete framed structure at the European Laboratory for Structural Assessment in Ispra, Italy. The structure was constructed as per 1970’s design typical of southern Europe and had plan irregularity. The entire structural frame was tested including barrels containing water to provide live loading. The white frame is the test structure and the orange frame is the reference frame for measuring displacements. Figure 7 presents a substructured pseudo dynamic test performed on a three-storey two-by-one bay steel braced plan irregular structure at the Structures Laboratory in Trinity College Dublin, Ireland. The ground floor braced bay frame of the steel concentrically braced frame structure was tested at full-scale as shown in Figure 7. The remainder of the structure was numerically modelled. Figure 8. Schematic of full-scale substructured real-time hybrid test of a full-scale semi-active control device (Carrion & Spencer (2007)) The experiment shown in Figure 8 presents a schematic of a real-time hybrid test performed on a full-scale magnetorheological damper within a three-storey steel moment resisting frame undertaken at University of Illinois at Urbana-Champaign, United States. Figure 9 shows the physical substructure of a real-time hybrid test of a three-storey, one-bay steel braced frame structure with viscous dampers (referred to as DBF in Figure 9). Tests were undertaken at the Advanced Technology for Large Structural Systems Research Center at Lehigh University, United States. Performance based design of the steel framed structure with viscous dampers was assessed. Figure 9. Substructured real-time hybrid test of large-scale steel frame with nonlinear viscous dampers (a) photo of test set-up; and (b) schematic of damper (Dong et al. (2014)) 7 D.P. McCrum Within the discipline of structural engineering in the United Kingdom & Ireland, hybrid test facilities exist at University of Oxford, University of Bristol, University of Cambridge and Trinity College Dublin. Within the United States the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES (2014)) has brought together 14 experimental sites across the United States. Other major facilities are located in Italy at the Joint Research Centre (JRC (2015)), South Korea at the Korea Advanced Institute of Science & Technology (KAIST (2015)), Taiwan at the National Centre for Research on Earthquake Engineering (NCREE (2015)) and the Disaster Prevention Research Institute in Japan (DPRI (2015)). Conclusions The need for physical dynamic testing still exists in the progression of structural engineering science. In structural engineering, cyclic tests were developed to investigate the effects of low cycle fatigue with loading applied at a very slow rate. To overcome issues related to slow loading rates, shake table and hybrid test methods were developed towards the later end of the 20th century to investigate true dynamic behaviour. The hybrid test method is a dynamic testing technique that replaces numerically calculated structural response with physically tested structural response. Hybrid tests can be performed at an extended timescale (pseudo dynamic test) for structures with a non-velocity dependent response e.g. steel framed structures, or else at real-time (real-time hybrid test) for structures with velocity dependent response e.g. damping device. The concept of substructuring within hybrid testing greatly reduces the cost of full-scale testing as only the critical part of the structure and not the entire structure needs to be physically tested. An interesting development is geographically distributed hybrid testing which involves testing the physical substructure(s) in one or more locations and solving the numerical substructure in another separate geographical location. Data from the physical test is sent over the internet to solve the equation of motion of the combined physical and numerical substructures. This offers great opportunities in the future to share testing facilities and expertise. This test method still requires some further research and development before it becomes reliable and robust. The hybrid test method has reached a level of maturity over the past 40 years to become a reliable, robust and accurate dynamic test method that can be used to perform full-scale seismic tests of engineering structures in a cost effective manner. REFERENCES ANSI/AISC 341-05 (2005) Seismic Provisions for Structural Steel Buildings. American Institute of Steel Construction (AISC), Illinois, United States Carrion JE and Spencer FB Jr. (2007) Model-based Strategies for Real-time Hybrid Testing, NSEL Report Series. Report No. NSEL-006 ed., University of Illinois at Urbana-Champaign Dermitzakis SN and Mahin SA (1985) Development of substructuring techniques for on-line computer controlled seismic performance testing. 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