Bearing the brunt20 April 2020
Seismic isolation bearings decouple structures, such as buildings and bridges, from the ground. This allows such structures to behave more flexibly during seismic activity
Earthquake engineering provides several ways of protecting a building from the forces exerted by an earthquake. These can be broadly categorised into four areas – reinforcement, ground stabilisation, vibration control, and base isolation.
Reinforcement can give a building’s superstructure sufficient strength and flexibility to withstand the forces of an earthquake, while ground stabilisation involves augmenting the structure’s foundations to prevent the ground from breaking up and causing it to collapse.
Vibration control, meanwhile, allows a building to dissipate vibration energy through damping, absorption of resonant frequencies with mass dampers, and dispersal of wave energy using passive or active systems. If an earthquake has high energy at a frequency matching a resonant frequency for the building, this will lead to violent oscillations (back and forth movement). Vibration control seeks to minimise this effect.
And finally, base isolation prevents earthquake forces reaching the building’s superstructure by allowing primarily horizontal movement of the building relative to its foundations. As well as reducing peak forces, this can also help to prevent resonant oscillation by reducing the buildings’ stiffness and, therefore, lowering its natural frequency. Base isolation is also the only solution that reduces floor accelerations experienced by people inside the building.
Large earthquake-proof structures are likely to combine these approaches to first limit the forces reaching the structure and then ensure the structure is able to withstand any residual forces. Older buildings and non-isolated buildings have fundamental vibration periods of 0.2 to 0.5 seconds, which corresponds to the high energy frequencies of earthquakes. By extending the fundamental periods (reducing the natural frequency), resonance is avoided, and buildings move smoothly without significant structural deformations.
SEISMIC BASE ISOLATION
Seismic isolation bearings are used to provide seismic base isolation. They allow the base of a building to move relative to the ground, decoupling the building from the movement of the ground. To do this effectively, base isolators must perform four functions:
|- Transmit vertical loads, supporting the building and preventing it from leaning|
- Allow horizontal displacements, reducing the building’s stiffness and the peak forces experienced. It is this function that decouples the building from the ground and provides the core function of isolation
- Dissipate energy, further reducing resonance and preventing excessive horizontal displacements
- Self-centre, so that the building returns to the neutral mid-position after a seismic event and, more importantly, ensure it does not cumulatively progress towards the end of the range of horizontal movement for the isolators during a seismic event. This is most important for buildings located very close to a fault, where highly asymmetric accelerations typically occur.
One simple form of base isolation bearing is a metallic roller bearing. This is quite literally a series of metal cylinders on which a building stands, allowing the ground to move from side to side, without exerting significant lateral forces on the building above it. Roller bearings do not have inherent damping and they require some form of additional constraint to prevent the building rolling off the rollers.
A friction pendulum bearing has a plain sliding bearing that sits on a concave horizontal surface. Because the surface is concave, gravity provides a restoring force to return the support to its central position. A cylindrical perimeter wall also prevents the slider from moving past the edges of the concave surface. The HQ of technology giant Apple, which is based in California, United States, and referred to as the ‘spaceship’ for its circular design, is reportedly isolated using a slightly more sophisticated version of friction pendulum bearings. Each of the 692 units has two stages so that friction is increased as the isolator approaches the end of its 1.2m range of motion.
Springs with dampers, elastomeric, and lead-rubber bearings, also provide restoring forces, as well as inherent limits on their range of motion. These types of isolation device include some damping to dissipate energy. Elastomeric rubber bearings are the most commonly used form of base isolation. They consist of alternating horizontal layers of neoprene or natural rubber and mild steel plates, produced as a single moulding. The steel plates prevent the rubber layers from bulging. There is very little compliance in the vertical direction.
Elastomeric rubber bearings are commonly used for large span bridges, as well as for buildings. Last year, OE reported how rubber bearings produced by Bridgestone Corporation had been supplied to the Tokyo Aquatics Centre and the Ariake Arena for the rescheduled 2021 Olympics. The two venues feature a roof seismic isolation structure.
Tokyo 2021 aquatics centre
Bridgestone said that installing the bearings beneath the roof instead of under the foundation of the venue would help to lower the burden placed on the roof’s structural support elements. This type of installation, it added, is often used in hall- or dome-shaped facilities with large, open spaces such as sporting arenas.
Pendulum rubber bearings, meanwhile, combine properties of friction pendulum and rubber bearings. They use a series of short reinforced concrete columns that are mounted between bearing cups on the foundation and the building. Rubber between the columns and the bearing cups provides a restorative force as well as damping.
Energy may be dissipated in a number of ways within rubber isolators. Damping properties may be added to the rubber itself through the use of additives to create high damping rubber bearings (HDRBs) with damping coefficients of typically between 10% and 15%, although some formulations can achieve as much as 20%. Alternatively, where higher levels of damping are required, lead plugs or silicone fluids may be inserted, giving damping values of over 15%. Dedicated damping devices may also be installed in parallel with low damping isolation devices.
BASE ISOLATION PROS & CONS
A major advantage of base isolation is that, by reducing the transmission of forces, the extent and complexity of structural reinforcement is reduced. This saves cost during analysis, materials and construction. Damage also tends to be concentrated within the isolation units, enabling rapid and low-cost repair of earthquake damaged buildings.
As well as providing protection against seismic activity, base isolation also provides a degree of protection against blast damage, by providing a small amount of flex, and therefore reducing peak forces. It is sometimes possible to retrofit base isolation units into existing buildings.
However, buildings constructed on soft ground may not be suited to base isolation, and the method also becomes less effective for very tall buildings. Furthermore, installation may also require more highly-skilled construction personnel than for other methods.
In conclusion, seismic base isolation has been used in earthquake zones around the world since the 1970’s and is growing in popularity. It has proven to be reliable and has also shown to be an effective and economical way to protect buildings and occupants. In addition, it may reduce the overall cost of a construction project by reducing the requirement for structural reinforcements and maintenance.
|BOX OUT: Earthquakes in the UK?|
Many parts of the world are associated with seismic activity, including Japan and the west coast of America. While the UK is not seen as a quake hot spot, the British Geological Survey (BGS) reports that between 20 to 30 earthquakes are felt by people each year, and a few hundred smaller ones are recorded by sensitive instruments.
Most of these are very small and cause no damage, but some British earthquakes have caused damage, if not devastation. The largest known British earthquake occurred in 1931 near the Dogger Bank, a large sandbank in a shallow area of the North Sea about 60 miles (97km) off the Yorkshire coast. With a magnitude of 6.1, it caused minor damage to buildings on the east coast of England. The most damaging UK earthquake, meanwhile, was in the Colchester area in 1884. Around 1,200 buildings needed repairs, chimneys collapsed, and walls were cracked.
According to BGS, a magnitude ‘4’ earthquake happens in Britain roughly every two years, and we experience a magnitude ‘5’ roughly every 10 to 20 years. The driving forces for earthquake activity in the UK are unclear, it adds, but likely causes include regional compression from motion of the Earth’s tectonic plates, and uplift resulting from the melting of the ice sheets that covered many parts of Britain thousands of years ago. BGS updates its website regularly with earthquakes around the British Isles in the last 50 days.
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