Control Systems for Seismic Protection of Structures – Seminar
Submitted By: Mahesh Chand Sharma
Civil engineering structures located in environments where earthquakes or large wind forces are common will be subjected to serious vibrations during their lifetime. These vibrations can range from harmless to severe with the later resulting in serious structural damage and potential structural failure.
Seismic Protection of Structures
The Traditional Technique of a seismic Design – ( increase the stiffness of structures by enlarging the section of columns, beams, shear walls, or other elements)
Modern Approach through Structural Controls – (by installing some devices, mechanisms, substructures in the structure to change or adjust the dynamic performance of the structure)
Basic Principles of Seismic Response Control
Control systems add damping to the structure and/or alter the structure’s dynamic properties. Adding damping increases the structural energy-dissipating capacity, and altering structural stiffness can avoid resonance to external excitation, thus reducing structural seismic response.
- Passive control systems
- Active Control systems
- Semi-active control systems
- Hybrid control systems
Passive control systems
The passive control system does not require an external power source and being utilizes the structural motion to dissipate seismic energy or isolates the vibrations so that response of structure can be controlled. The passive control devices includes
1. Base Isolation
2. Passive Energy Dissipating (PED) Devices
A building mounted on a material with low lateral stiffness, such as rubber, achieves a flexible base.
During the earthquake, the flexible base is able to filter out high frequencies from the ground motion and to prevent the building from being damaged or collapsing
– deflecting the seismic energy and
– absorbing the seismic energy
Various Type of Base Isolation
- -Low-Damping Natural or Synthetic Rubber Bearing
- – High-Damping Natural Rubber Bearing
- – Lead-Rubber Bearing (Low damping natural rubber with lead core)
- – Flat Sliding Bearing
- – Spherical Sliding Bearing
– Rubber Layers: Provide lateral flexibility
– Steel Shims: Provide vertical stiffness to support building weight while limiting lateral bulging of rubber
– Lead plug: Provides source of energy dissipation
Low Damping Natural or Synthetic Rubber Bearings
Linear behavior in shear for shear strains up to and exceeding 100%.
Damping ratio = 2 to 3%
- Simple to manufacture
- Easy to model
- Response not strongly sensitive to rate of loading, history of loading, temperature, and aging.
-Need supplemental damping system
High-Damping Natural Rubber Bearings
- Damping increased by adding extra-fine carbon black, oils or resins, and other proprietary fillers
- Maximum shear strain = 200 to 350%
- Damping ratio = 10 to 20% at shear strains of 100%
- Effective Stiffness and Damping depend on:
– Elastomer and fillers
– Contact pressure
– Velocity of loading
– Load history (scragging)
- damping properties can be improve by plugging a lead core into the bearing
- damping of the lead-plug bearing varies from 15% to 35%.
- The Performance depends on the imposed lateral force
- The hysteretic damping is developed with energy absorbed by the lead core.
- Maximum shear strain = 125 to 200%
The imposed lateral force is resisted by the product of the friction coefficient and the vertical load applied on the bearing
Passive Energy Dissipating Devices (PED)
Mechanical devices to dissipate or absorb a portion of structural input energy, thus reducing structural response and possible structural damage.
•Visco-elastic Dampers – Visco-elastic (VE) dampers utilize high damping from VE materials to dissipate energy through shear deformation. Such materials include rubber, polymers, and glassy substances.
Active Control systems
In the active control, an external source of energy is used to activate the control system by providing an analog signal to it. This signal is generated by the computer following a control algorithm that uses measured responses of the structure
Types of Active Control systems
- Active Mass Damper Systems -It evolved from TMDs with the introduction of an active control mechanism.
- Active Tendon Systems – Active tendon control systems consist of a set of pre-stressed tendons whose tension is controlled by electro-hydraulic servomechanisms
- Active Brace Systems
Semi-active Control Systems
It compromise between the passive and active control devices. The structural motion is utilized to develop the control actions or forces through the adjustment of its mechanical properties. The action of control forces can maintained by using small external power supply or even with battery.
- Stiffness control devices
- Electro-rheological dampers
- Magnetorhelogical dampers
- Friction control devices
- Fluid viscous dampers
- Tuned mass dampers
- Tuned liquid dampers
ER fluids that contain dielectric particles suspended within non-conducting viscous fluids. When the ER fluid is subjected to an electric field, the dielectric particles polarize and become aligned, thus offering resistance to the flow.
Stiffness control devices
– the stiffness
-the natural vibration characteristics
So create a non-resonant condition during earthquake
MR fluid contains micron-size, magnetically polarizable particles dispersed in a viscous fluid. When the MR fluid is exposed to a magnetic field, the particles in the fluid polarize, and the fluid exhibits visco-plastic behavior, thus offering resistance to the fluid flow.
Hybrid control systems
Combine controls system together
- Passive + Active
- Passive + Semi-Active
- Reduce external power requirement
- Improve reliability
When loss of electric during earthquake, hybrid control can act as a passive control
- Reduce construction and maintenance costs due to active or semi-active