Construction World November 2022

• 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. • 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 center 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 75 m 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 civil 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. 

7 CONSTRUCTION WORLD NOVEMBER 2022