Dampers and their Seismic Performance
In seismic structures upgrading, one of the lateral force reduction caused by the earthquake is use of dampers. During an earthquake, high energy is applied to the structure. This energy is applied in two types of kinetic and potential (strain) to structure and it is absorbed or amortized. If structure is free of damping, its vibration will be continuously, but due to the material damping, vibration is reduced. Input energy caused by earthquake to structure is presented in the following equation:
E = EK + ES + Eh + Ed
In this equation, E is earthquake input energy, Ek is kinetic energy, Es is reversible strain energy in the elastic range and Eh is the amount of wasted energy due to inelastic deformation and Ed is the amount of amortized energy by additional damper. In seismic isolation systems, use of energy dissipation systems, allocated a special place to their selves. Damping increasing is possible by using various methods such as the flow of a soft metal, two metal friction on each other and a piston motion within a slimy substance or viscoelastic behavior in materials such rubber-like substances.
Damping effect on structural response
Damping increasing reduces structural response ( acceleration and displacement) damping effect at low frequency (close to zero) have no effect on spectrum amount and at high frequency, it has low effect on response acceleration. Figures 1 and 2 show the most effect of damping increasing in the frequency of 0.3 to 2.5 seconds.
Types of dampers
Dampers are classified based on their performance of friction, metal (flowing), viscous, viscoelastic; shape memory alloys (SMA) and mass dampers. Among the advantages of using dampers we can infer to high energy absorbance, easy to install and replace them as well as coordination to other structure members.
In this type of damper, seismic energy is spent in overcoming friction in the contact surfaces. Among other features of these dampers can be classified as avoiding fatigue in served loads (due to the non-active dampers under load) and their performance independent to loading velocity and ambient temperature. These dampers are installed in parallel to bracing.
In this damper, transferred energy to the structure is spent to submission and non-linear behavior in used element in damper. In these dampers, metal inelastic deformation is used such as for formability metals such as steel and lead for energy dissipation. In all conventional structures, energy dissipation is based on deformation of steel members after the submission.
In this damper, by using viscous fluid inside a cylinder, energy is dissipated. Due to ease of installation, adaptability and coordination with other members also diversity in their sizes, viscous dampers have many applications in designing and retrofitting. This type of dampers are connected to the structure in three ways:
- damper installation in the floor or foundation ( in the method of seismic isolation)
- connecting dampers in stern pericardial braces
- damper installation in diagonal braces.
In connecting dampers on the floor or foundation of structures, we can use a combination of dampers with isolators.
Mass is placed on a fulcrum which acts as a roller. And it allows to mass with move as a transfer-lateral movement to the floor. Springs and dampers are placed between mass and anchor members to the floor and frame and they are placed relative in “opposite phase” and sometimes are adjacent vertical. And these anchor members transmits structural lateral force. Bidirectional transfer dampers are made as a spring-damper in two vertical directions. And they provide controlling the structure movement in two vertical structures.
Shape Memory Alloy (SMA)
Shape Memory alloy (SMA) are created from metals which have the following properties:
- Their flexibility is very similar to the flexibility of the rubber piece.
- After applying many deformation, they can back to their original state, by heating.