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What every structural and civil engineer should know about "seismic isolation solutions"
Your bridge, building or other structure is to be protected against earthquakes by means of seismic isolation – but what solutions are available?
Key requirements
Seismic isolators
Seismic isolators are specially designed structural bearings which, like most bearings, form the required connections between a structure’s superstructure and its substructures – thereby typically transmitting weight forces while also resisting other forces and accommodating movements and rotations as required by the structure’s design.
However, seismic isolation bearings are additionally designed to perform in a very specific way during an earthquake, protecting the main structure and its users. For optimal performance, a seismic isolator should offer the following benefits:
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Isolation of the superstructure from violent, potentially damaging ground oscillations – thus drastically reducing shear and bending stresses in the structural components.
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Enabling the superstructure to return to its former position after the dynamic event.
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So that the seismic energy is released in a safe, controlled manner, thus also protecting secondary structural elements against unacceptable deformations.
Key types of seismic isolator
Earthquake protection devices
High-Damping Rubber Bearings
High-Damping Rubber Bearings, such as LASTO®HDRB, which are similar to anchored elastomeric bearings with no sliding surfaces but fabricated using special high-damping elastomeric compounds which provide high resistance to horizontal (shear) deformation and result in high energy dissipation during seismic movements.
A round High-Damping Rubber Bearing (HDRB) of the LASTO®HDRB type as installed in a concrete structure
1. | Mounting plate |
2. | Connectors to transfer horizontal loads |
3. | Elastomeric block with horizontal steel reinforcing plates |
4. | Mounting plate |
Lead Rubber Bearings
Lead Rubber Bearings, such as LASTO®LRB, which are also similar to anchored elastomeric bearings with no sliding surfaces but feature a vertical lead core which increases stiffness and damping (energy dissipation) during seismic events – thus optimising the size of the device and enabling a threshold to be defined between the behaviours expected during events of low and high intensity.
A Lead Rubber Bearing (LRB) of the LASTO®LRB type as fabricated, with anchor plates top and bottom
1. | Mounting plate |
2. | Connectors to transfer horizontal loads |
3. | Elastomeric block with horizontal steel reinforcing plates |
4. | Lead core |
5. | Mounting plate |
Seismic isolation – decoupling of structures
from ground motions
Curved Surface Sliders
Curved Surface Sliders, such as RESTON®PENDULUM, which have the effect of decoupling the supported structure from ground motions during a seismic event through harmonic pendulum-like relative displacements. As the supported structure becomes horizontally displaced, seismic energy is dissipated thanks to the friction at the isolator’s sliding interface(s) with their special high-performance sliding material. During the oscillations, the curvature of the sliding interface results in a slight (almost imperceptible) lifting of the superstructure. After the earthquake, the same curvature causes the supported structure to return by gravity to its original position.
Cross-section illustration of a Curved Surface Slider seismic isolator of the RESTON®PENDULUM MONO type – with ONE curved sliding surface (on top) to accommodate displacements
Cross-section view of a Curved Surface Slider seismic isolator of the RESTON®PENDULUM DUPLO type – with TWO curved sliding surfaces (top and bottom) to accommodate displacements
Seismic protection strategy
Identifying and implementing the optimal seismic protection strategy
The elastomeric isolators (High-Damping Rubber Bearings and Lead Rubber Bearings) are similar in many respects. For example, LASTO®LRB and LASTO®HDRB seismic isolators are typically suitable for applications involving vertical loads of up to 30,000 kN and horizontal seismic displacements of up to 150% – 200% of the total rubber height of the device. However, the damping performance of up to 30% offered by LASTO®LRB isolators is significantly higher than that offered by LASTO®HDRB isolators (up to approximately 20%).
Curved Surface Sliders, on the other hand, are generally suitable for applications involving substantially higher loads and displacements. RESTON®PENDULUM bearings, for example, can typically be designed to carry loads of up to 150,000 kN and to accommodate horizontal seismic displacements of up to +/- 1000 mm, while also achieving damping factors above 30%.
Where Curved Surface Sliders are to be used to support and protect a structure, it must be noted that, if different sizes/designs are used for the same superstructure, then all of them must be designed for the same oscillation period. It should also be noted that Curved Surface Sliders should not be combined with elastomeric isolators due to the slight lifting experienced by the superstructure during the oscillations at the locations of the Curved Surface Sliders – but not at the locations of the elastomeric isolators.
Whatever type of seismic isolator is selected to support a structure, it is generally recommended to verify the performance of the devices as designed and manufactured – by means of seismic testing of randomly-selected specimens at an accredited test facility which can simulate the seismic conditions for which the device was designed.
Such testing is generally conducted regardless of the isolator’s size or specification and of any previous testing of comparable devices – the uniqueness of each application warrants the carrying out of testing for every application.
Identifying and implementing the optimal seismic protection strategy for a particular application – considering the structure’s design, use and location, for example – requires specialist expertise and experience.
Please feel free to contact us when you require a seismic isolation (or any other type of seismic protection) solution for your structure – we will be happy to assist you.
Any questions?
Expert Knowledge
To learn more about earthquake protection, please read our “Expert Knowledge”.
Earthquake protection is a commitment – we design solutions that make infrastructures and buildings safer.
