Thechnical Information

Seismic isolation is the most effective earthquake mitigation technique for medium to low-rise building structures constructed on dense to medium soils. The seismic isolation system employs isolator units that are rigid in the vertical direction to resist the gravity loads with minimal deflections, and flexible in the lateral directions to isolate the superstructure from the ground motions during an earthquake. Isolators are typically located between the superstructure building and its foundation or at the basement ceiling level (i.e., between columns and ceiling of the basement story). In seismic isolated bridges, the seismic isolators are often installed between bridge piers and bridge deck.

The natural period and effective damping of a base isolated structure is significantly increased with respect to its fixed-base (non-isolated) counterpart. This results in significant decrease in the peak acceleration that the superstructure would experience during an earthquake (known as spectral acceleration). The decreased induced acceleration is translated to the reduced inertia force and mitigated seismic demand in the superstructure. The internal forces and deflections can be mitigated such that the superstructure remain elastic with minimal nonstructural damages during a severe earthquake. Seismic isolation is the only practical technology that can lead to an immediate occupancy (operational) performance objective.

Elastomeric Isolators

LBK, Ltd. has developed various types of elastomeric isolators for seismic isolation of structures and industrial equipment units. These isolators consist of alternating layers of rubber and reinforcement materials that provide lateral flexibility and high vertical stiffness. The most commonly used type is the steel plate reinforced elastomeric isolator, also known as laminated rubber bearings. However, there are also novel elastomeric isolators that use other reinforcing materials like steel mesh or fiber fabric to confine the lateral bulging of the rubber layers and provide even greater lateral flexibility. Here, we will review the different categories of elastomeric isolators manufactured by LBK, Ltd.

Types of rubber materials

Low damped rubber:

Unfilled natural or neoprene rubber are the two most commonly employed rubber materials in our elastomeric bearings. They exhibit great ultimate tensile strength and elongation. The inherent damping of these unfilled rubber materials is typically low, of the order of %2 to %5. Therefore, an isolator device fabricated using these rubber materials must be supplemented with added dampers to mitigate the resonance vibration of the base isolation system.

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High damped rubber:

Special fillers and other additives are included in the rubber compound to improve its damping properties.  The equivalent viscous damping ratio of our high-damped rubber material increases up to 10% to 15%. As such, unless otherwise dictated by the design calculations, no supplemental dampers are typically required.

Types of internal reinforcement

• Steel plate
• Steel mesh
• Fiber fabric

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Types of boundary conditions

Fixed connection:

The top and bottom layers of elastomer have thick steel end plates bonded to their entire plan area. These end plates are bolted to flanges fixed to the foundation and superstructure.

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Partially bonded connection:

The bearing isolator is designed with two thick steel end plates that are partially bonded to the central core of the outer elastomer layers. The top and bottom end plates are attached to the foundation and superstructure through two bolted steel flanges. This partial bonding together with the negligible flexural rigidity of the internal reinforcement layers allows for rollover deformation during shear loads. The rollover deformation provides added lateral flexibility and improves the seismic isolation performance of the isolator. The isolator is capable of resisting uplift forces and preventing relative slip due to the partial bonding at its contact surfaces.

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Unbonded connection:

The bearing isolator is placed between the foundation and the superstructure without any mechanical or chemical connection at the contact surfaces. Friction transfers shear forces to the bearing isolator. The bearing can undergo rollover deformation under shear loads, which improves its lateral flexibility and seismic isolation performance. The steel end plates and steel flanges are eliminated, unlike other elastomeric isolator types, resulting in significant cost savings in the seismic isolation system. The unbonded application of isolators is best suited for situations where no uplift forces are applied to the isolator. LBK, Ltd. also offers special mechanical uplift restraining devices that are supplemented with unbonded bearings to resist uplift forces and provide stability against overturning. This system is especially suited for seismic isolation of large and heavy industrial equipment units. Please contact us for more information.