Technical earthquake consideration becomes an important aspect of design for critical infrastructure , and base isolation is one of the available and well-proven methods for making structures more resilient if they are susceptible to earthquakes .
The technical benefits of a base isolation system to earthquakeprone civil engineering structures are undeniable , including :
• Significant reduction in relative displacement / distortion within the structure . This can be especially beneficial to the operation of mechanical equipment with a low tolerance to distortion , such as cranes , or hydromechanical equipment , such as gates , which may become inoperable due to the distortion in the structure .
• Significant reduction in overall shaking of the structure during an earthquake , which reduces damage to the structural elements due to cracks , particularly in concrete elements induced by shaking effects . Such cracks are often detrimental to the longevity of the structure , as reinforcement corrodes due to increased water penetration .
• Significant reduction in earthquake forces on the structure , which could potentially result in smaller structural elements , as earthquake loading requirements are lower .
While beneficial in many ways , base isolation systems are not without their challenges , and designers need to weigh these up against other alternatives . Challenges include :
• Significant maintenance requirements and costs , which include the need for periodic access to perform a condition assessment of the isolation system and ultimately replacement of isolators that have failed or reached their service life end .
• Large lateral displacement at the base of the structure due to differential movement of the structure ’ s base and its foundation . This requires special consideration for connections of services such as water pipes , power cables , communication networks and other structures which need to be linked with junctions that can allow such large movements .
• Unlike traditional fixed-base systems , there is a need for additional knowledge by designers , contractors and maintenance personnel on all relevant aspects of the base isolation system , its operation parameters and maintenance requirements .
Examples of civil engineering structures where base isolation has proved to be beneficial , include :
• Buildings – it ’ s often applied to high-rise buildings . Due to their high centre of gravity , such buildings are prone to earthquake risk . This is not however restricted to high-rise buildings ; it also includes buildings that host sensitive equipment or which are structurally vulnerable to earthquakes , such as those constructed from large shell elements . Examples of iconic buildings with base isolation include the Apple Park building in the United States of America and Shinagawa Season Terrace building in Japan .
• Nuclear power plants – due to the sensitivity and high-risk nature of nuclear power plants , extra protection is often provided to a nuclear facility by using base isolation on nuclear reactor structures so that they are sufficiently protected from potential earthquake damage . Built in the 1970s , Koeberg Nuclear Power Plant ( NPP ), located near Cape Town , is one of the first NPPs in the world constructed with a base isolation system .
• Bridges – often a bridge ’ s super-structure is isolated from its sub-structures by using isolators at the interface between the two bridge elements . However , some creative designs have been implemented on the South Rangitikei Rail Bridge in New Zealand in the 1970s , where base isolation devices were installed at the base of the bridge ’ s 75m tall piers .
• Liquid storage tanks – ground motion induces hydrodynamic forces on the stored fluid , which results in increased fluid pressures on the tank ’ s walls . This could result in potential environmental contamination or safety risks if such tanks should fail . To keep economical wall thicknesses of lateral walls , storage tanks , particularly where safety and environmental risks are high , such as with liquified natural gas , are increasingly stored in base isolated tanks to limit potential risks associated with ground movement .
• Railway lines – buildings constructed near railway lines can be protected from vibrations induced by trains through the use of base isolation , which can be applied to either the railway structure or the buildings ’ foundations .
Base isolation is an important technical solution for dealing with the effects of ground vibrations / movements on infrastructure . Designers are encouraged to explore base isolation as a potential technical solution wherever there are risks to the structure from ground vibrations .
The application of base isolation does not need to be restricted to new infrastructure ; retrofitting existing buildings has proved to be highly beneficial for older structures designed using older seismic design methods which were not as stringent as the current design standards . RACA
“ South Africa is a particularly stable region in terms of seismic risk ; however , for critical structures with a long service life , designers often have to consider longer earthquake return periods during design .”
www . refrigerationandaircon . co . za RACA Journal I May 2023 49