Seismic Dampers Q & A

DEFINITIONS

 

Fluid Viscoelastic Dampers

A viscoelastic damper which is adapted to control the vibration of a structural member, a tank, a pipe, etc. which may be caused by an earthquake or wind while producing minimum reaction force when the structural member and so on is slowly displaced as a result of the thermal deformation of the member itself or another member connected thereto. Such a property of a viscoelastic damper can be obtained by using dilatant liquid which presents relatively small resistance when the speed of the motion is small and produces progressively great resistance as the speed of the motion increases. To the end of enhancing this favorable property of the viscoelastic damper, the shoulder surfaces that are exposed to the dilatant liquid are rounded. By providing a spiral groove or projection in the member exposed to the dilatant liquid to produce rotational motion between the two ends of the damper, this property is enhanced even more.

Shock Transmission Units

A shock transmission unit (STU) is a simple device which provides the engineer a method of temporarily creating a fixed connection, when desirable, which would during normal operations remain as a moveable connection. The device is sometimes referred to as a Lock-Up Device. The unit is connected between adjoining separate structures or between elements of structures and has a benign effect on the bridge during normal periods of time. Upon receipt of a sudden short duration shock (dynamic) load the device locks up and transmits the load through the structure. In effect the device creates a rigid link within a fraction of a second when the sudden load is applied, affording the possibility of sharing the load throughout the structure. However, once the shock load is removed the device again reverts to its benign influence and the structure behaves in a normal  manner.

Shock absorbers

Taylor Devices designed such concepts as fluidic orifice control, dynamic fluid compression, self-adjusting shock absorbers and the liquid spring.

Tuned Mass Damping Systems

The optimum parameters of tuned mass dampers TMD for suppressing the dynamic response of a base-excited structure in a specific mode is investigated. The base excitation is modelled as a stationary white noise random process. The stationary response of the structure with TMD is analyzed for the optimum parameters of the TMD system. The criterion selected for optimality is the minimization of the root mean square (r.m.s.) displacement of the main structure. The parameters of TMD that are optimized include the damping ratio, the tuning frequency ratio and the frequency bandwidth of the TMD system. The optimum parameters of the TMD system and corresponding effectiveness are obtained for different damping ratios of the main structure and mass ratios of the TMD system. In addition, the effectiveness of an optimally designed TMD system is compared with that of an optimum single tuned mass damper. It is shown that the optimally designed TMD system is more effective for vibration control than the single tuned mass damper.

 


 

Q & A

 

Q    Do Taylor Dampers provide any special benefits to structures with high value contents, such as hospitals, government buildings and computer data centers, compared to alternative products like friction dampers, sliding joints or plastic hinges formed by yielding the structure?

A    In addition to the greatly enhanced seismic performance provided by Taylor fluid viscous dampers, building floor accelerations are also significantly reduced during an event, in many cases by as much as 50-60%. Other damping products such as friction dampers, sliding joints or plastic hinges can only create hysteretic damping (displacement-dependent) and result in higher floor accelerations with the inherent risk of equipment damage and human injury. A major benefit of using Taylor Viscous Dampers is that sensitive equipment is less likely to be destroyed or damaged during the event, allowing the building to provide undisrupted service for the affected community during and after the event.

Q    Is there any financial benefit on the overall construction costs when Taylor Devices’ fluid viscous dampers are used as opposed to friction dampers?

A    Fluid viscous dampers have velocity dependent damping properties; their damping forces are naturally out-of-phase with the stresses in the building. This benefit can allow the reduction of shear walls, use of smaller columns and beams, use of smaller and less complicated foundations and overall reductions of concrete/steel mass, generally offsetting the cost of the fluid viscous dampers. Friction damping forces are in-phase with structural response. As a result, they constantly add forces into the structure, which necessitates even stronger foundations and beam to column joints, thus adding additional costs to the building.

Q    Taylor Fluid Viscous Dampers are very compact, how can such a physically small product help a large building or bridge to be more survivable during an earthquake?

A    With most structures, a relatively small amount of damping provides a large reduction in stress and deflection by dissipating energy from the structure.  For example, with an automobile suspension, the damper, or shock absorber, is used to control the motion of the springs.  The damping forces required are quite small compared to the springs, which must support the vehicle and deflect under bump loadings.  A similar situation exists with a building where the spring forces are supplied by the building columns or base isolators which both support the building and deflect under load.  It requires only a small amount of viscous damping force to reduce building deflection by a factor of two or three while simultaneously reducing overall column stresses.

Q    How can Taylor Fluid Viscous Dampers reduce building deflection and stress at the same time?  If we use dampers to limit the deflection, won’t this increase the load into the building columns?

A    Fluid Viscous damping reduces stress and deflection because the force from the dampers is completely out of phase with stresses due to flexing of the columns.  This is only true with fluid viscous damping, where damping force varies with stroking velocity.  Other types of damping products such as yielding elements, friction devices, plastic hinges, and visco-elastic elastomers do not vary their output with velocity; hence they can, and usually do, increase column stress while reducing deflection.  Consider a building shaking laterally back and forth during a seismic event.  Column stress is at a maximum when the building has flexed a maximum amount from its normal position.  This is also the point at which the flexed columns reverse direction to move back in the opposite direction.  If we add a Fluid Viscous Damper to the building, damping force will drop to zero at this point of maximum deflection.  This is because the damper stroking velocity goes to zero as the columns reverse direction.  As the building flexes back in the opposite direction, maximum damper force occurs at maximum velocity, which occurs when the column flexes through its normal, upright position.  This is also the point where column stresses are at a minimum.  It is this out of phase response that is the most desirable design aspect of fluid viscous damping.

Q    How much damping is needed?

A    A typical building normally has internal structural damping of 1 to 3 percent of critical.  Optimal performance of a building with fluid viscous damping is achieved with added damping in the range of 20 to 25 percent of critical.  Again, using the comparison with an automobile, most conventional autos use dampers with 20 to 30 percent of critical damping.  Experiments with building models have indicated additional improvements with damping increased to as much as 50 percent of critical, but eventually the gain goes past the point of diminishing returns from the point of damper cost.

Q    We are not located in an area of seismic activity, why should we be interested in dampers?

A    Fluid Viscous Dampers are also very effective in reducing building deflections under wind loadings without changing the stiffness of the building!  In the case of tall buildings, wind motion can also cause complaints of motion sickness and general discomfort from the occupants on higher floors.  The motion is similar to an automobile with worn out shock absorbers.  Fluid Viscous Dampers can reduce wind deflection by a factor of 2 or 3, greatly reducing occupant discomfort without creating localized stiff sections.  New buildings designed with Fluid Viscous Dampers for mitigation of wind motion can be built with reduced lateral stiffness detailing, resulting in a less costly overall structure.

Q    Why are Taylor Fluid Viscous Dampers better than friction dampers such as sliding joints and plastic hinges?

A    There are three major differences between our Fluid Viscous Dampers and friction devices.  The primary difference is that the constant force output of a friction damper increases maximum column or pier stress under any deflection of the structure.  Fluid Viscous Dampers do not increase column stresses due to their inherent out of phase response output.

The second difference is that friction dampers put out an essentially constant force when deflected, independent of velocity.  This response causes continual stress in the structure during all thermal expansion and contraction of the structure.  Fluid Viscous Dampers put out virtually zero force at the low velocities associated with thermal motion.

The third difference is that friction dampers restrict a structure from restoring itself to its original position after seismic events.  Fluid Viscous Dampers allow the structure to re‑center itself perfectly at all times.

   How do Taylor Fluid Viscous Dampers compare to visco-elastic devices?

A    Visco-elastic devices have an output that is somewhere between that of a damper and a spring.  Under high level seismic inputs, the spring response dominates, producing a response that increases column stresses at any given deflection.  This does not happen with Fluid Viscous Dampers.

One of the most serious problems with visco-elastic devices is an unacceptable increase in force at low temperatures coupled with an accompanying overloading of the bonding agent used to “glue” the visco-elastic material to its steel attachments.  At high temperatures, unacceptable softening or reduction of output occurs.  This thermal variance from high to low temperature can be in the range of fifty to one.

In comparison, Taylor Fluid Viscous Dampers include a bi-metallic orifice which acts like a thermostat to provide uniform performance over a temperature range of -40 degrees F to +160 degrees F. This excellent thermal stability is combined with all steel construction, having internally threaded joints and no welded or bonded parts.

   Do you have any life test data on your dampers, particularly the seals?

A    Taylor Devices has been building Fluid Viscous Dampers continuously since 1955.  Taylor products do not use commercially available seals, but instead rely on our own proprietary machined seal design using high strength structural polymers rather than soft elastomers.  This seal design does not degrade with age, and we have test units that date back to 1955 that operate perfectly today with no leakage and no refilling or seal changes of any type needed.  Equally important to our seal design is our piston rod construction.  All Taylor Devices= piston rods are made from solid stainless steel using aircraft quality material only.  Each rod is hand finished to a mirror-like finish of less than 2 micro-inch surface roughness, then microscopically impregnated with Teflon7 by a proprietary process.  The long term corrosion resistance of this design has been proven in literally thousands of severe applications in steel mills, smelters, and chemical plants.  In addition, our products have been applied to literally hundreds of military applications on ships, aircraft and missiles.  Our total production of fluid viscous energy absorbers exceeds two million units.

Q   How do we go about sizing Taylor Fluid Viscous Dampers for an application?

A    Most structural engineering software allows for the use of viscous equivalent damping to simulate structural damping.  All Taylor Fluid Viscous Dampers have an output identical to this model. Instead of running your simulation with the normal 1 to 3 percent structural damping, you can elevate these values to 20 to 50 percent of critical.  This will give a tremendous improvement in seismic behavior, greatly reducing both stress and deflection.

All we need to select the damper that satisfies your requirements is to be given the value of the required damping constant, the velocity exponent, and the maximum translational velocity of the damper.  In a viscous damping model, the output of the damper is:

Fdamper   =    C*V%
Where C   =    damping constant (lb*sec/in)
V    =    velocity (in/sec)
% =    velocity exponent  (0.3 # % # 1.0)
Once performance requirements have been satisfied using linear damping (% = 1.0), further refinement can be evaluated with lower velocity exponents.

Q    What type of mountings should be used?

A    Taylor dampers are available with either threaded stud mounting, clevis type mounting, and/or base plate mounting.  The clevis mounts include a spherical insert bearing.  The clevis mounts are normally used on bridges, base isolated structures, in chevron bracing, or on any application with more than plus or minus 2 inches of stroke.  Base plate or threaded stud mounting is generally used with diagonal bracing.

Q    What are Taylor’s materials of construction?

A    All Taylor Fluid Viscous Dampers utilize solid stainless steel piston rods, hand polished to a mirror-like finish, and Teflon7 impregnated.  Our seal has a history of over 40 years of use and is patented.  For long stroke applications, the piston rod is protected against bending by a heavy walled external guide sleeve.  The cylinder, end cap and sleeve are constructed of alloy steel and corrosion protected by painting, cadmium plating, or chrome plating.  Stainless steel construction is available for all external parts as an option, and is recommended for bridge use or outdoor service.

   What is Taylor’s operating fluid?

A    The operating fluid used in a Taylor Fluid Viscous Damper is a silicone fluid, manufactured in accord with U.S. Federal standards, and is cosmetically inert per U.S. FDA standards.  Flashpoint of the silicone oil is in excess of 600 degrees F, thus classified as nonflammable and noncombustible under U.S. OSHA standards.  This silicone fluid is a pure fluid polymer that cannot settle-out or break down into components.  Potential oxidation is prevented by permanently sealing the silicone fluid volume inside the damper.

 

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