Types of Base Isolation for Seismic Engineering

Seismic design is critical for buildings and structures in areas prone to earthquakes. One of the most effective strategies to mitigate seismic forces is through the implementation of base isolation systems. There are six major types of base isolation devices that are widely adopted in seismic design:

Elastomeric Bearings

Elastomeric bearings are flat rubber pads composed of elastomeric materials. They are designed to deform elastically under load, absorbing seismic energy. These bearings are highly effective for transferring vertical and horizontal forces from the structure to the foundation.

High Damping Bearings

High damping bearings are designed to absorb and dissipate seismic energy through their high damping characteristics. These bearings use materials with a significant proportion of elastic and viscous behavior, which helps in reducing the amplitude of seismic motions.

Lead Rubber Bearings

Lead rubber bearings consist of a rubber core with a layer of lead wrapped around it. The rubber core provides flexibility, while the lead layer significantly increases the bearing’s capacity to absorb and dissipate energy. This design makes them particularly useful in high-rise buildings.

Flat Slider Bearings

Flat slider bearings are composed of a flat plate with a thin layer of lubricant sandwiched between it and the foundation or structure. These bearings allow for horizontal movement, preventing the transmission of seismic forces to the structure. They are effective in reducing soil-structure interaction.

Curved Slider Bearings or Pendulum Bearings

Curved slider bearings, also known as pendulum bearings, are designed to provide a combination of sliding and rotational movement. These bearings are more complex and are used in systems where both translational and rotational seismic motions are significant.

Ball Roller Bearings

Ball roller bearings are cylindrical components that can rotate and roll, providing a mechanism for horizontal movement. These bearings are highly efficient in transmitting vertical loads while allowing for lateral movement. They are ideal for use in high-mass structures.

Types of Isolated Footings in Seismic Engineering

Isolated footings are a critical component in seismic design, providing a localized base for columns to support the structure without transferring excessive loads to the foundation. There are several types of isolated footings, each designed to suit specific conditions:

1. Flat Pad Plain or Reinforced Isolated Footing

A flat pad isolated footing is constructed under each column independently, typically in square, rectangular, or circular shapes. The thickness of the footing is uniform and is designed to reduce bending moments and shearing forces. Reinforcement is often added to increase the ultimate load-carrying capacity.

2. Sloped Isolated Footing

Sloped or trapezoidal footings are designed with a top slope of 45 degrees from all sides. This design reduces the amount of concrete and reinforcement needed, as the slope itself provides resistance. The reduced use of materials makes this type of footing economically viable for projects with limited concrete supply.

3. Stepped Isolated Footing

Stepped footings consist of multiple stacked concrete sections, resembling steps. This design was previously popular in residential buildings, but its use has declined in recent times. Stepped footings are stacked upon each other, typically with three concrete cross-sections creating steps. This design is used for uneven ground conditions.

Isolated Footing Design Guidelines and Specifications n IS 456: 2000

The design of isolated footings must adhere to strict guidelines to ensure structural integrity and seismic safety. These guidelines are specified in IS 456: 2000, which covers the detailing of isolated footings, including:

Reinforcement Detailing

Reinforcement is a critical component of isolated footings, and its detailing must be carefully considered. IS 456: 2000 provides guidelines on the amount and type of reinforcement required to safely distribute loads to the soil without causing excessive settlement or movement.

Load Distribution and Settlement

Proper load distribution is essential to ensure that the isolated footing can distribute column loads without causing significant settlement or movement. The design must account for the soil conditions and the uniformity of load distribution to prevent differential settlement.

Bearing Capacity and Safety

The bearing capacity of the isolated footing must be sufficient to support the column loads without failing. Safety factors are also considered to ensure that the structure can withstand accidental overweighting or other unexpected loads. This ensures that the isolated footing provides adequate safety against sliding and overturning.

Conclusion

Base isolation and isolated footings play a crucial role in seismic engineering by protecting structures from the damaging effects of earthquakes. By understanding the different types of base isolation devices and isolated footings, engineers can design structures that are safer and more resilient in seismically active regions.