Development of Online Hybrid Testing: Theory and Applications to Structural Engineering provides comprehensive treatments of several topics pertinent to substructure online hybrid tests. Emphasis has been placed on explaining the three frameworks:
- the host-station framework,
- separated model framework and
- peer to peer framework
These have been developed within the Internet environment and are particularly suitable for distributed hybrid testing. In order to help readers to understand the essence of online hybrid testing and further to build up their own systems, an engineering practice has been introduced at the end of this book with the source code appended. Development of Online Hybrid Testing: Theory and Applications to Structural Engineering is primarily written for readers with some background in structural dynamics, finite elements, and computer science. Material that has previously only appeared in journal articles has been consolidated and simplified which provides the reader with a perspective of the state-of-the-art.
- Presents basics and implementations of time integration algorithms for online hybrid tests, along with the applications for real engineering projects
- Includes current progress on the development of substructure online hybrid tests as a means of investigating the seismic behaviour of large-scale structures
- Provides source code for the example tests
Chapter 2
Basics of the Online Hybrid Test
Abstract
In this chapter, the basics of the online hybrid test, which mainly include the components of the test system, the capacities of the online hybrid test, and the procedures commonly adopted for online hybrid test, are introduced. This chapter is to provide users who are new to the online hybrid test with a general introduction to this technique. After reading this chapter, the readers are expected to have the fundamentals to plan a simple online hybrid test and understand how physical and computational components work together during the execution of an online hybrid test.
Keywords
Test method
Test procedure
Test application
Development history
Advantages
Constraints
Major components
Chapter Outline
2.1 Introduction 11
2.2 Basic Concepts and Applications 12
2.2.1 Test Methodology 12
2.2.2 Research Applications 13
2.2.3 Advantages and Constraints 15
2.3 Implementation and Major Components 17
2.3.1 Implementation 17
2.3.2 Major Components 18
2.4 Single DOF Structure with Explicit Scheme 19
2.4.1 Test Structure 19
2.4.2 Test Method 20
2.4.3 Test Results 22
2.5 Substructure Test with OS Scheme 23
2.5.1 Test Structure 23
2.5.2 Test Method 23
2.5.3 Test Results 24
2.6 Conclusions 25
References 25
2.1 Introduction
The seismic performance of structural systems under earthquake loading is no doubt an area requiring extensive research. In fact, many research bodies all over the world have been engaged in investigating the seismic response of various types of structural systems. In general, research on the seismic performance of structural systems can be classified into two groups: analytical research and experimental research. Because of innovation in the fields of electronics and the mechanics, the progress of those two groups of research has been remarkable. It is now by no means difficult to simulate the static and dynamic earthquake response of complex structural systems using numerical techniques, such as the finite element method. The development of experimental hardware has also made it feasible to conduct large-scale static and dynamic tests with careful test control.
About 40 years ago, a new type of research techniques to study earthquake response behavior of structural systems was developed. This technique is unique because it combines the experiment and numerical analysis, and utilizes the benefits of both experimental and analytical research. With this technique, one can directly simulate the earthquake response of structural systems with respect to the time domain, but without using a shake table device. This technique has been designated with various names: computer actuator online test, hybrid experiment, pseudodynamic test, or online computer test control method. Recently it is more often designated as hybrid simulation. In this book, this technique is designated as the online hybrid test control method, and simply referred to as the online hybrid test.
2.2 Basic Concepts and Applications
2.2.1 Test Methodology
The basic idea of the online hybrid test method is quite simple if one is familiar with the procedures involved in conventional quasi-static testing and numerical time integration techniques used in dynamic analysis of structures. First, the structure to be tested is idealized as a discrete-parameter system that has a limited number of degrees of freedom (DOF), each of which is controlled by an actuator in a quasi-static manner. This is why the online hybrid test is also referred as the pseudodynamic test. Consider, for example, the four-story frame shown in Fig. 2.1a. Since the axial stiffness of the floor beams is usually much higher than the flexural stiffness of the columns and the response of the frame under a horizontal base motion is expected to be dominated by the inertia forces developed at the floor levels, one can idealize the structure as a 4-DOF system, as shown in Fig. 2.1b. The equations of motion for this structure can thus be expressed as:
+Cv+r=f
(2.1)
in which M and C are the mass and damping matrices of the structure, v and a are the vectors of nodal velocities and accelerations, r is the nodal restoring force vector, and f is the external excitation. For a linearly elastic system, =Kd, where K is the stiffness matrix of the structure and d is the vector of nodal displacements. If the structure is subjected to a horizontal ground acceleration ag, =-M1ag, where {1} is a unit vector.
Figure 2.1 Online hybrid testing of a two-bay frame. (a) Two-bay frame, (b) discretized model.
Once the structural model is discretized, its equations of motion are solved by means of a direct step-by-step integration scheme in an online hybrid test, with the mass and viscous damping properties of the structure modeled analytically. In every time step of a test, the displacement response computed for each DOF is imposed on the structure in a quasi-static fashion by means of actuators, and the restoring forces, r, developed by the structure are measured with load transducers and are used to compute the response in the next time step. Hence, one can see that the online hybrid test method is essentially similar in concept to dynamic structural analysis, except that the stiffness properties of the structure are directly measured from the structural specimen during a test. Since the inertia effects are modeled analytically, such a test can be conducted in a quasi-static fashion with conventional testing equipment. The details of this method have been summarized by Mahin and Shing [1].
2.2.2 Research Applications
A most desirable feature of the online hybrid test method is its versatility. It can be used to evaluate the earthquake performance of large full-scale structures [2-5], small-scale structures [6], and structural subassemblies and components [7-10]. Both the planar and three-dimensional response of a structure can be investigated with this method [11,12]. Even though the shaking table test method has been known as the most direct experimental technique to simulate the earthquake response of structures, it has some limitations, such as the size and weight of a structure that can be tested. Furthermore, the cost of a table goes up rapidly with its size, capacity, and number of DOF. On the other hand, an online hybrid test can be carried out with a large-scale structure because of the relatively slow rate of load application. For a given hydraulic power capacity, a slow-moving actuator can have a larger-diameter piston and, thereby, produce a larger force than a fast-moving actuator.
The physical limitations of an online hybrid test system are, however, the number and capacities of actuators that are available, and the dimensions and load capacities of reaction systems that are used to support the structural specimen and the actuators. However, the capacity of such a system can be expanded gradually to meet any changing needs. When compared with conventional quasi-static tests, the online hybrid test method can be considered as a major enhancement that requires only a small incremental investment. The online hybrid test method is especially attractive for evaluating the earthquake performance of multi-DOF structures and structural subassemblies. While conventional quasi-static testing is useful for comparing the performance of different structural designs under a standardized load history, it does not account for the ductility demand of an earthquake ground motion on a structural specimen or the proper distribution of earthquake-induced forces. Such problems can be resolved with the online hybrid test method, in which the displacement history and pattern applied to a structure are determined from the equations of motion.
With an analytical substructuring procedure, the online hybrid test method can be applied to structural subassemblies. Very often, the damage inflicted upon a structure by seismic excitation is localized in a few critical regions or subassemblies. In such a case, there is no compelling reason to test the entire structural system. One...
