Introduction to Structural Motion Control
Author(s): Jerome J. Connor
Size: 4.6 Mb, 752 Pages.
Description
Conventional structural design procedures are generally based on two requirements, namely safety and serviceability. Safety relates to extreme loadings, which have a very low probability of occurring, on the order of 2%, during a structure’s life, and is concerned with the collapse of the structure, major damage to the structure and its contents, and loss of life. Serviceability pertains to medium to large loadings, which may occur during the structure’s lifetime. For service loadings, the structure should remain operational (i.e., the structure should suffer minimal damage, and furthermore, the motion experienced by the structure should not exceed specified comfort limits for humans and motion-sensitive equipment mounted on the structure). Typical occurrence probabilities for service loads range from 10% to 50%.
Safety concerns are satisfied by requiring the resistance (i.e., strength) of the individual structural elements to be greater than the demand associated with the extreme loading. Once the structure is proportioned, the stiffness properties are derived and used to check the various serviceability constraints such as elastic behavior. Iteration is usually necessary for convergence to an acceptable structural design. This approach is referred to as strength-based design since the elements are proportioned initially according to strength requirements.
Applying a strength-based approach for preliminary design is appropriate when strength is the dominant design requirement. In the past, most structural design problems have fallen in this category. However, the following developments have occurred recently that have limited the effectiveness of the strength-based approach. First, the trend toward more flexible structures such as tall buildings and longer-span horizontal structures has resulted in more structural motion under service loading, thus shifting the emphasis from safety toward serviceability. Second, some of the new types of facilities such as space platforms and semiconductor manufacturing centers have more severe design constraints on motion than the typical civil structure. For example, in the case of micro-device manufacturing, the environment has to be essentially motion free. Third, recent advances-in material science and engineering have resulted in significant increases in the strength of tradition civil engineering materials. However, the material stiffness has not increased at the same rate. The lag in material stiffness versus material strength has led to a problem with satisfying the requirements on the various motion parameters. Indeed, for very high-strength materials, the motion requirements control the design. Fourth, experience with recent earthquakes has shown that the cost of repairing structural damage due to inelastic deformation was considerably greater than anticipated. This finding has resulted in a trend toward decreasing the reliance on inelastic deformation to dissipate energy and controlling the structural response with other types of energy dissipation and absorption mechanisms.
Motion-based structural design is an alternate design paradigm that address these issues. The approach takes as its primary objective the satisfaction of motion related design requirements such as restrictions on displacement and acceleration and seeks the optimal deployment of material stiffness and motion control device to achieve these design targets as well as satisfy the constraints on strength. Structural motion control is the enabling technology for motion-based design.
This book provides a systematic treatment of the basic concepts and computational procedures for structural motion control. Examples illustrating the application of motion control to a wide spectrum of buildings are also presented. Topic covered include optimal stiffness distributions for building-type structures, the role of damping in controlling motion, tuned mass dampers, base isolation systems, quasi-static active control, and dynamic feedback control. The targeted audience is practicing engineers and graduate students.
This work was motivated by the authors’ interest in the design of structure for dynamic excitation and by members of the Structural Engineering Community who have been enthusiastic supporters of this design paradigm. The author is particularly indebted to Professor Akira Wada (Tokyo Institute of Technology) and Dr. Mamoru Iwata (Nippon Steel Corporation) for their intellectual support. Their contributions have significantly strengthened the discussion of the practical aspects of passive control. Without the financial support provided by the School of Engineering, Massachusetts Institute of Technology, this work would not have been possible. The author is most appreciative. Special thanks are also due to Petros Komodromos, George Kokosalakis, Victor Pellon, and Paul Kassabian for their painstaking effort in producing the text files.
