Manish Kumar

Manish Kumar, Ph.D., P.E.

State University of New York at Buffalo
Indian Institute of Technology Kanpur

Try this web application for seismic isolation of bridge using elastomeric bearings
Seismic Isolation Web App

Send comments and suggestions regarding article to mkumar2@buffalo.edu


Seismic Isolation

History of Seismic Isolation

History of Seismic Isolation goes long back. People have been using seismic isolation in one form or another for long time. Evidence found proves that Greeks have been implementing this concept in their structures to make flexible rocking columns which used to isolate the upper structure from ground motions.
Seismic isolation in modern times may be claimed to be started by John Milne, a British Scientist, who was professor of mining engineering at the Imperial College of Engineering in Tokyo from 1875-1895.In 1885, he built a base isolated wood house which was founded on piles. The heads of the piles incorporated balls made of cast-iron plates with saucer like edges. Structure was modified many times to finally perform adequately under real earthquakes. Japan and New Zealand was ahead in implementing this concept. In 1909, J. A. Calantarients filed a patent for lubricated free joints on a layer of fine material. System behaved like an isolation system. In 1969, first application of rubber isolated bearings was implemented in 3-story reinforced concrete elementary school building. It was a simple rubber bearing without any reinforcing steel shims. School building was later demolished due to high bulging of rubber.
1980s can be considered the decade when seismic gain popularity all around world and many structures around the world were built using seismic isolation. Japan, by that time, had implemented many building projects with isolation concepts. Significant research was devoted to this topic and many initiatives around the world were taken by government agencies and private sector to prepare the framework for development of regulations and guidelines for seismic isolation of structures. No of buildings in Japan constructed using seismic isolation increased exponentially after 1995 Kobe earthquake.
France and South Africa had already successfully implemented seismic isolation concept to their Pressurized Water Reactors (PWRs) during 1978-1984. Each of the four units at Cruas in France was isolated using 1,800 neoprene pads(500x500x65 mm). A modified isolator design was used for regions with higher seismicity; where flat sliders are installed in between top of pads and upper mat. Flat sliders used a lead-bronze alloy lower plate and a polished stainless steel upper plate. Two unit plants in Koeberg, South Africa, with a SSE of 0.3g, were isolated using this design, where total of 2000 neoprene pads (700x700x100 mm) were used. A similar design was proposed to be implemented for Karun River plant in Iran whose construction was stopped in 1978
First application of seismic isolation of in USA was seismic isolation of the Foothills Community Law and Justice Center(1982-1985). Acceptance of seismic isolation in US was rather slow; however, it gained popularity after 1989 Loma-prieta earthquake and many new and existing structures were isolated in 1990s.
Isolators used for first few applications were high damping rubber bearings which was followed by elastomeric bearing and lead-rubber bearing. Sliding bearings gain popularity in 1990s and structures now a days are isolated using primarily Elastomeric, Lead-rubber, and Friction pendulum family of bearings(sliding).

NPP_France
Four seismically isolation nuclear power plant units in Cruas, France (Forni et. al., 2011)

Seismic Isolation is a concept in which an isolation bearing is used to isolate the superstructure from ground movement transferring reduced inertia forces by means of very low lateral stiffness isolators compared to main structure. It is governed by a fundamental period shift to higher periods which brings a reduction in forces attracted by the structure but at the cost of increased isolator displacements. However, relative displacement of superstructure, which is primarily responsible for damage in the superstructure, is decreased achieving intended response. If there's a limit on isolator displacement because of neighboring structure of buildings, limited seat width of bridges or due to utilities, increased isolators displacement response can be controlled with the help of added dampers.

BaseIsolatedBuilding
Effect of seismic (base) isolation on the response of a structure

 

Location of Bearings
(1) Between superstructure and foundation for the buildings
(2) Between deck and piers for bridges

Here is a video demonstrating the effect of base isolation.

https://www.youtube.com/watch?v=ZqlXp3czrrM

Linear theory of seismic isolation approximate the behavior of seismically isolated structure with 2 degree of freedom system. First degree of freedom is horizontal displacement of isolator and second is the horizontal relative displacement of superstructure. Some important conclusions that can be drawn from linear theory of base isolation are as follows:
As the time period of isolator and superstructure grows apart, that is, superstructure becomes more and more stiffer than isolator, first mode is primarily relative displacement of isolator and second mode for which the participation becomes very less, is the relative deformation of superstructure.
 
There are different kinds of systems used for seismic isolation:
1. Elastomeric Bearings
2. Sliding Bearings
3. Metallic yielding devices

1. Elastomeric Bearings

Elastomeric bearings are made up of alternate layers of rubber and steel. Steel plates used are called shims, and are used as reinforcements to increase the strength and reduce the lateral bulging. Damping is provided by energy dissipation capabilities of rubber. Initially high damping rubber was used to provide the required damping, however high damping rubber had inherent scragging effects. Low damping rubber, made of natural rubber, saw an increased demand in application on account of its availability and ability to predict the behavior more accurately. Development of lead rubber bearings provided an efficient method of achieving damping requirements by energy dissipation in lead. Lead has an excellent energy dissipation quality and it recover its properties even after many cycles of yielding.

  • High damping elastomeric bearings
  • Low damping elastomeric bearings
  • Lead Rubber Bearings

This video shows a good presentation of an elastomeric bearings.

 https://www.youtube.com/watch?=LKDu6CCedaA

Rubber has very low stiffness in horizontal direction but is almost incompressible in the vertical compression. So it is good for vertical load transfer from structure to the ground and at the same time provides isolation from horizontal ground motion by having very low shear stiffness. However, rubber does not perform adequately in tension. Modern day rubber bearings, with good quality control, have been able to provide some tension capacity up to a pressure of 3G, where G is the shear modulus of the rubber. After vertical tension reaches 3G, rubber cavitates and loses its capacity. Tension is not a major issue except in near fault areas, where vertical ground motion needs to be considered.

LeadRubberLeadRuuber1
Internal construction of a lead-rubber (LR) bearing (Constantinou, et al., 2007)

2. Sliding Bearings

Sliding bearings provide isolation by two or more metallic hardware sliding on each other. Damping and energy dissipation is provided by the friction on the sliding surfaces.

  • Flat sliding bearings
  • Friction pendulum bearings

Flat sliding bearings were mainly developed for non-seismic bridge applications to accommodate thermal and vehicle loading displacements. They provide none or very low horizontal stiffness and hence no restoring force, hence their applications for seismic applications have been very limited.

Friction pendulum bearings, on the other the hand, can provide desired stiffness and restoring force by providing the curvature to the sliding surfaces. They can accommodate very high displacement demands and can work efficiently even in very cold regions as opposed to rubber bearings.

SlidingBearing
Sectional view of a single friction pendulum bearing (Constantinou et al, 2007)

Relevant references

1. Kelly, J. M. (1993). Earthquake-resistant design with rubber. London, Springer-Verlag.
2. Naeim, F. and J. M. Kelly (1999). Design of seismic isolated structures: From theory to practice. New York, John Wiley & Sons.
3. Constantinou, M. C., Whittaker, A. S., Kalpakidis, I., Fenz, D. M., and Warn, G. P. (2007). "Performance of seismic isolation hardware under service and seismic loading." Technical Report MCEER-07-0012, University at Buffalo, State University of New York, Buffalo, NY.
4. Constantinou, M., Kalpakidis, I., Filiatrault, A., and Lay, R. A. E. (2011). "LRFD-based analysis and design procedures for bridge bearings and seismic isolators." Technical Report MCEER-11-0004, University at Buffalo, State University of New York, Buffalo, NY.

Seismic isolation and energy dissipation devices companies

Listed below are the companies providing seismic isolation and energy dissipation devices*
Some international companies providing services in this field are
  • Alga
  • Andre Rubber
  • Bridgestone
  • FIP Industriale
  • Nippon Steel
  • Oiles Corporation
  • Skellerup

*Information taken from MCEER Technical Report-07-0012 (Constantinou et al)

 

Important links

Youtube channel of Dynamic Isolation Systems, Inc.

 

Projects and presentations on seimsic isolation