Technical
BASE ISOLATION BENEFITS AND APPLICATIONS IN THE SOUTH AFRICAN CONTEXT
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 .
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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 ,
RACA Journal I May 2023
© RACA Journal 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 ,
Base isolation mitigates risks for sensitive systems such as wet services and HVAC equipment / plant room , and particularly in the application of liquid storage tanks , where ground movement is possible or likely . www . refrigerationandaircon . co . za