By Kabelo Seosenyeng, design engineer at GIBB Engineering
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.
The base isolation system is essentially a method of decoupling 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 and associated internal services/plant rooms.
The benefit of this method becomes apparent when considering earthquake-induced ground movement without base isolation, where vibrations would be 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 resilient structure.
Traditionally, civil structures have a concrete base rigidly fixed [in]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 structure. Base isolated structures, however, are constructed with base isolation devices between the base of the structure and the ground.
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 stiffness 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 dissipators can take the form of various mechanisms, but commonly a lead/metallic core at the centre 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 design. 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 requirement for such structures to endure significantly higher magnitude earthquakes compared to ordinary structures where earthquake 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.
The technical benefits of a base isolation system to earthquake-prone 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.