Civil Engineering December 2022 | Vol 30 No 11

T he base isolation system is essentially a method of decou- pling or isolating the structure’s supporting base from its foundations. This reduces the transfer of vibrations between the structure and its foundation, which significantly reduces forces transferred to the structure from its foundations. The benefit of this method becomes apparent when con- sidering earthquake-induced ground movement without base isolation, where vibrations from the earthquake are transferred directly to the base of the structure without any attenuation. For structures with a base isolation mechanism, earthquake forces transferred to the structure are significantly reduced, which results in a more earthquake-resilient structure. Traditionally, civil structures have a concrete base rigidly fixed to the ground. This direct coupling between the ground and the structure’s base means that all vibrations experienced from the ground are directly transferred to the base of the structure. Base isolated structures, however, are constructed with base iso- lation devices between the base of the structure and the ground. TYPES OF BASE ISOLATION There are two main types of base isolation: elastomeric/rubber isolators and sliding isolators. The primary function of both types is to reduce vibrations induced on the structure, provide adequate stiff- ness to restrain the movement of the structure due to vibrations, and to dissipate energy induced by ground motion on the structure. Base isolation systems are often coupled with energy dissipators to reduce the energy that the structure is subjected to. These dissipa- tors can take the form of various mechanisms, but commonly a lead/ metallic core at the center of the bearing is used, which dissipates energy through shear plastic deformations or through dedicated damping devices such as viscous fluid dampers or friction dampers. 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 de- sign. For example, in the design of dams, earthquake return periods for as long as 1 000 to 10 000 years are often considered. Longer earthquake return periods often result in a require- ment for such structures to endure significantly higher magni- tude earthquakes compared to ordinary structures where earth- quake return periods of up to 100 years are typically considered. As such, despite South Africa being a stable region, 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. BENEFITS AND CHALLENGES The technical benefits of a base isolation system to earthquake- prone civil engineering structures are undeniable and include: Q Q 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. Q Q 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. Q Q 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: Q Q 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. Q Q Large lateral displacement at the base of the structure due to differential movement of the structure’s base and its Kabelo Seosenyeng Design Engineer GIBB Engineering Base isolation benefits and applications in the South African context South Africa may not be a country that experiences frequent or sizeable earthquakes, but the possibility still exists, and our infrastructure needs to be protected. This is where the method of base isolation demonstrates its value. 24 December 2022 Civil Engineering Koeberg is one of the first nuclear power plants in the world constructed with a base isolation system Struc tural eng i neer i ng