Mechanics of Rubber Bearings for Seismic and Vibration Isolation

James M. Kelly, Dimitrios Konstantinidis

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781119972808
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Widely used in civil, mechanical and automotive engineer­ing since the early 1980s, multilayer rubber bearings have been used as seismic isolation devices for buildings in highly seismic areas in many countries. Their appeal in these applications comes from their ability to provide a component with high stiffness in one direction with high flexibility in one or more orthogonal directions. This combination of vertical stiffness with horizontal flexibility, achieved by reinforcing the rubber by thin steel shims perpendicular to the vertical load, enables them to be used as seismic and vibra­tion isolators for machinery, buildings and bridges.

Mechanics of Rubber Bearings for Seismic and Vibration Isolation collates the most important information on the mechanics of multilayer rubber bearings. It explores a unique and comprehensive combination of relevant topics, covering all prerequisite fundamental theory and providing a number of closed-form solutions to various boundary value problems as well as a comprehensive historical overview on the use of isolation.

Many of the results presented in the book are new and are essential for a proper understanding of the behavior of these bearings and for the design and analysis of vibration or seismic isolation systems. The advantages afforded by adopting these natural rubber systems is clearly explained to designers and users of this technology, bringing into focus the design and specification of bearings for buildings, bridges and industrial structures.

This comprehensive book:

• includes state of the art, as yet unpublished research along with all required fundamental concepts;
• is authored by world-leading experts with over 40 years of combined experience on seismic isolation and the behavior of multilayer rubber bearings;
• is accompanied by a website at www.wiley.com/go/kelly

The concise approach of Mechanics of Rubber Bearings for Seismic and Vibration Isolation forms an invaluable resource for graduate students and researchers/practitioners in structural and mechanical engineering departments, in particular those working in seismic and vibration isolation.

1

History of Multilayer Rubber Bearings

Multilayer rubber bearings are widely used in civil, mechanical and automotive engineering. They have been used since the 1950s as thermal expansion bearings for highway bridges and as vibration isolation bearings for buildings in severe acoustic environments. Since the early 1980s, they have been used as seismic isolation devices for buildings in highly seismic areas in many countries. Their appeal in these applications is the ability to provide a component with high stiffness in one direction and high flexibility in one or more orthogonal directions. The idea of using thin steel plates as reinforcement in rubber blocks was apparently suggested by the famous French engineer Eugène Freyssinet (1879–1962). He recognized that the vertical capacity of a rubber pad was inversely proportional to its thickness, while its horizontal flexibility was directly proportional to the thickness. He is of course best known for the development of prestressed concrete, but also for the discovery of creep in concrete. It is possible that his invention of the reinforced rubber pad was driven by the need to accommodate the shrinkage of the deck due to creep and the prestress load while sustaining the weight of a prestressed bridge deck. In any case, he obtained a French patent in 1954 for “Dispositif de liaison élastique à un ou plusieurs degrés de liberté” (translated as “Elastic device of connection to one or more degrees of freedom”; Freyssinet 1954; the patent, with an English translation, is given in the Appendix). It seems from his patent that he envisaged that the constraint on the rubber sheets by the reinforcing steel plates be maintained by friction. However, in practical use a more positive connection was desired, and by 1956 bonding of thin steel plates to rubber sheets during vulcanization was adopted worldwide and led to the extraordinary variety of applications in which rubber pads are used today.

This combination of horizontal flexibility and vertical stiffness, achieved by reinforcing the rubber by thin steel shims perpendicular to the vertical load, enables them to be used in many applications, including seismic protection of buildings and bridges and vibration isolation of machinery and buildings.

The isolation of equipment from vibration via anti-vibration mounts is a well-established technology, and the theory and practice are covered in several books, papers, and reviews; the survey by Snowden (1979) is an example. Although the isolated machine is usually the source of the unwanted vibrations, the procedure can also be used to protect either a sensitive piece of equipment or an entire building from external sources of vibration. The use of vibration isolation for entire buildings originated in the United Kingdom and is now well accepted throughout Europe and is beginning to be used in the United States. Details of this method of building construction can be found in Grootenhuis (1983) and Crockett (1983).

The predominant disturbance to a building by rail traffic is a vertical ground motion with frequencies ranging from 25 to 50 Hz, depending on the local soil conditions and the source. To achieve a degree of attenuation that takes the disturbance below the threshold of perception or below the level that interferes with the operation of delicate equipment (e.g., an electron microscope), rubber bearings are designed to provide a vertical natural frequency for the structure about one-third of the lowest frequency of the disturbance.

The first building to be isolated from low-frequency ground-borne vibration using natural rubber was an apartment block built in London in 1966. Known as Albany Court, this building is located directly above the St James’ Park Station of the London Underground. This project was experimental to a certain extent, and the performance and durability of the isolation system in the years since its construction was monitored for several years by the Malaysian Rubber Producers Research Association (MRPRA, now the Tun Abdul Razak Research Centre) in conjunction with Aktins Research and Development (Derham and Waller 1975).

Since then, many projects have been completed in the United Kingdom using natural rubber isolators. These have included Grafton 16, a low-cost public housing complex that was built on a site adjacent to two eight-track railway lines that carry 24-hour traffic. In this project the isolators produced a vertical frequency of 6.5 Hz to isolate against ground motion in the 20 Hz range. Several hotels have been completed using this technology, for example, the Holiday Inn in Swiss Cottage in London. In addition, a number of hospitals have been built with this approach, which is particularly advantageous when precision diagnostic equipment is present.

More recently, vibration isolation has been applied for use in concert halls. In 1990, the Glasgow Royal Concert Hall, which is sited directly above two underground railway lines, was completed in Glasgow, Scotland. The building has a reinforced concrete structural frame that is supported on 450 natural rubber bearings. In addition to housing the 2850-seat concert hall, it also contains a conference hall and a number of restaurants.

Another concert hall is the International Convention Centre in Birmingham, England, which was completed in 1991. Home of the City of Birmingham Symphony Orchestra, the building comprises ten conference halls and a 2211-seat concert hall. The entire complex was built at a cost of £121 million and is supported on 2000 natural rubber bearings to isolate it from noise from a main line railway running in a tunnel near the site.

The International Congress Center (ICC) in Berlin (Figure 1.1), Germany, constructed between 1970 and 1979, was Berlin's largest post-war project. It is 320 m (1050 ft) long, 80 m (260 ft) across and 40 m (130 ft) high. It has a cubic content of 800 000 m3 (1 000 000 yd3), and the total weight of steel in the roof is 8500 tons (18700 kips). A “box-in-box” construction, developed specially for this center, permits several functions to be held simultaneously under one roof. The building is supported on neoprene bearings (Figure 1.2) which range in size up to 2.5 m in diameter that can carry loads of 8000 tons (17600 kips; Freyssinet International 1977). They were constructed in segments which were placed in position with space between the segments to allow for bulging of the neoprene layers – described in the literature on the center as a kind of architectural shock absorber – and were intended to exclude outside noise and absorb vibrations from an adjacent highway and railway. ICC Berlin has over 80 halls and conference rooms, with seating capacities ranging from 20 to 5000, with a sophisticated information and direction system. The largest hall (Hall 1) can seat up to 5000 and has the second-largest stage in Europe.

Figure 1.1 The International Congress Center (ICC) in Berlin, Germany. Reproduced from Hans-Georg Weimar, Wikimedia

Figure 1.2 2.5-m diameter bearing for the ICC Berlin. Reproduced by permission of Freyssinet, Inc.

Two recent applications of vibration isolation to concert halls in the United States are the Benaroya Concert Hall in Seattle, Washington, completed in 1999 and the Walt Disney Concert Hall in Los Angeles, California, completed in 2003. The first uses rubber bearings to mitigate ground-borne noise from trains in a tunnel below the hall. The Walt Disney Concert Hall is built directly above a loading dock for an immediately adjacent building. The interesting thing about these two buildings is that they are located in highly seismic areas, yet there was no attempt on the part of the structural engineers for either project to combine both vibration isolation and seismic isolation in the same system. Experimental results of tests done at the shake table at the Earthquake Engineering Research Center of the University of California, Berkeley, many years before the construction of these two concert halls, demonstrated that it was possible to design a rubber bearing system that would provide both vibration isolation and seismic protection. In the concert hall projects, lateral movement of the bearings that support the buildings is prevented by a system of many vertically located bearings, the additional cost of which is substantial and could have been avoided by appropriate design.

Seismic isolation can also be provided by multilayer rubber bearings that, in this case, decouple the building or structure from the horizontal components of the ground motion through the low horizontal stiffness of the bearings, which give the structure a fundamental frequency that is much lower than both its fixed-base frequency and the predominant frequencies of the ground motion. The first dynamic mode of the isolated structure involves deformation only in the isolation system, the structure above being to all intents and purposes rigid. The higher modes that produce deformation in the structure are orthogonal to the first mode and, consequently, to the ground motion (Kelly 1997). These higher modes do not participate, so that if there is high energy in the ground motion at these higher frequencies, this energy cannot be transmitted into the structure. The isolation system does not absorb the earthquake energy, but rather deflects it through the dynamics of the system. This type of isolation system works when the system is linear, and even when undamped; however, a certain level of damping is beneficial to suppress any possible resonance at the isolation frequency. This damping can be provided by the rubber compound itself through appropriate compounding. The rubber compounds in common engineering use have an intrinsic energy dissipation equivalent to 2–3% of linear viscous damping, but in compounds referred to as high-damping rubber this can be increased to 10–20% (Naeim and Kelly 1999).

The first use of rubber for the earthquake protection of a structure was in an elementary school, completed in 1969 in Skopje, in the Former Yugoslav Republic of Macedonia (see Figure 1.3). The building is a three-story concrete structure that rests on large blocks of natural rubber (Garevski et al. 1998). Unlike more recently developed rubber bearings, these blocks are completely unreinforced so that the weight of the building causes them to bulge sideways (see Figure 1.4). Because the vertical and horizontal stiffnesses of the system are about the same, the building will bounce and rock backwards and forwards in an earthquake. These bearings were designed when the technology for reinforcing rubber blocks with steel plates – as in bridge bearings – was neither highly developed nor widely known, and this approach has not been used again. More recent examples of isolated buildings use multilayered laminated rubber bearings with steel reinforcing layers as the load-carrying component of the system. These are easy to manufacture, have no moving parts and are extremely durable. Many manufacturers guarantee lifetimes of around 50 or 60 years.

Figure 1.3 The first rubber isolated building: the Pestalozzi elementary school completed in 1969 in Skopje. Courtesy of James M. Kelly. NISEE Online Archive, University of California, Berkeley

Figure 1.4 Unreinforced bearing in the Pestalozzi school building in Skopje. Courtesy of James M. Kelly. NISEE Online Archive, University of California, Berkeley

The first base-isolated building to be built in the United States was the Foothill Communities Law and Justice Center (FCLJC), a legal services center for the County of San Bernardino that is located in the city of Rancho Cucamonga, California, about 97 km (60 miles) east of downtown Los Angeles (see Figure 1.5). In addition to being the first base-isolated building in the United States, it is also the first building in the world to use isolation bearings made from high-damping natural rubber (Derham and Kelly 1985) (Figure 1.6). The FCLJC was designed with rubber isolators at the request of the County of San Bernardino. The building is only 20 km (12 miles) from the San Andreas fault, which is capable of generating very large earthquakes on its southern branch. This fault runs through the county, and, as a result, the county has had for many years one of the most thorough earthquake-preparedness programs in the United States. Approximately 15 794 m2 (170 000 ft2), the building is four stories high with a full basement and was designed to withstand an earthquake with a Richter magnitude 8.3 on the San Andreas fault. A total of 98 isolators were used to isolate the building, and these are located in a special sub-basement. The construction of the building began in early 1984 and was completed in mid-1985 at a cost of $38 million (Tarics et al. 1984). Since then, many new buildings have been built in the United States on seismic isolation systems.

Figure 1.5 Foothill Communities Law and Justice Center, Rancho Cucamonga, California. Courtesy of James M. Kelly. NISEE Online Archive, University of California, Berkeley

Figure 1.6 Natural rubber isolator for the Foothill Communities Law and Justice Center showing laminated construction. Courtesy of James M. Kelly. NISEE Online Archive, University of California, Berkeley

The same high-da...

Table of contents

• Cover
• Title Page
• Copyright
• About the Authors
• Preface
• 1: History of Multilayer Rubber Bearings
• 2: Behavior of Multilayer Rubber Bearings under Compression
• 3: Behavior of Multilayer Rubber Bearings under Bending
• 4: Steel Stress in Multilayer Rubber Bearings under Compression and Bending
• 5: Buckling Behavior of Multilayer Rubber Isolators
• 6: Buckling of Multilayer Rubber Isolators in Tension
• 7: Influence of Plate Flexibility on the Buckling Load of Multilayer Rubber Isolators
• 8: Frictional Restraint on Unbonded Rubber Pads
• 9: Effect of Friction on Unbonded Rubber Bearings
• Appendix: Elastic Connection Device for One or More Degrees of Freedom
• References
• Photograph Credits
• Author Index
• Subject Index