796 Wind and Earthquake Resistant Buildings

8.2. DAMPING DEVICES FOR REDUCING MOTION PERCEPTION

Engineers have learned from building occupants and owners, and from wind tunnel studies,that designing a tall building to meet a given drift limit under code-specied equivalentstatic loads is not enough to make occupants comfortable during windstorms. However, theyhave only limited control over three intrinsic factors, namely, the height, the shape, and themass, that inuence the dynamic response of buildings. Additionally, the behavior of a tallbuilding subjected to dynamic loads such as wind or seismic activity is difcult to predict

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Figure 8.29. Structural steel unit quantities.

798 Wind and Earthquake Resistant Buildings

with any accuracy because of the uncertainty associated with the evaluation of a buildingsdamping and stiffness, as well as the complicated nature of loading.

The present state of the art is such that an estimate of structural damping can bemade with a plus or minus accuracy of only 30% until the building is constructed and thenonstructural elements are fully installed. It is well-known that wind-induced buildingresponse is inversely proportional to the square root of total damping, consisting ofaerodynamic plus structural damping. So, if damping is quadrupled (increased by fourtimes), a 50% response reduction is achieved, and if damping is doubled, the dynamicresponse is reduced by 29%. Because of the inherent damping of a building respondingelastically to wind loads in the range of 0.5 to 1.5% of the critical response, it is impracticalto increase the damping to, say, four times as much by use of modied structural materials.

Suppression of excessive vibrations can be dealt with limited success in a variety ofways. Additional stiffness can be provided to reduce the vibration period of a building to aless sensitive range. Changes in mass of a building can be effective in reducing excessivewind-induced excitation. Aerodynamic modications to the buildings shape, if agreeable tothe buildings owner and architect, can result in reduced vibrations caused by wind. However,these traditional methods can be implemented only up to a point beyond which the solutionsmay become unworkable because of other design constraints such as cost, space, or aesthetics.Therefore, to achieve reduction in response, a practical solution is to supplement the dampingof the structure with a mechanical damping system external to the buildings structure.

8.2.1. Passive Viscoelastic DampersFigure 8.30a shows schematics of a viscoelastic polymer damper. An early example ofapplication of this type of damper is the World Trade Center Towers, conceived in the1960s, constructed in the early seventies, and destroyed by terrorists on September 11,2001. These buildings were designed with viscoelastic dampers distributed at approxi-mately 10,000 locations in each building. The dampers extended between the lower chordsof the oor joists and gusset plates mounted on the exterior columns beneath the stiffenedseats (Fig. 8.28).

Viscoelastic dampers dissipate energy through deformation of polymers sandwichedbetween relatively stationary steel plates. Their energy dissipation depends on both relativeshear deformation of the polymer and relative velocity within the device. The device istypically used to reduce occupants perception of wind-induced motions. It does not requireconstant operational monitoring and is not dependent on electric power.

The Columbia Searst Center, a 76-story building in Seattle built in 1984, is anotherexample of using this technology to reduce occupant perception of wind-induced buildingmotion. The dampers used in this building consist of steel plates coated with a polymercompound. The plates are sandwiched between a system of relatively stationary plates.As the building sways under the action of wind loads, the steel plates which are attachedto structural members are subjected alternately to compression and tension. In turn, theviscoelastic polymer subjected to shearing deformations absorbs and dissipates much ofthe strain energy into heat, thus reducing wind-induced motions.

8.2.2. Tuned Mass DamperA typical application of a tuned mass damper (TMD) consists of a heavy mass installednear a buildings top in such a way that it tends to remain still while the building movesbeneath it. This strategy allows the mass at top to transmit its inertial force to the building

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in a direction opposite to the motions of the building itself, thereby reducing the buildingsoscillations.

The mass itself need weigh only a small fraction0.25 to 0.70%of the buildingstotal weight, which corresponds to about 1 to 2% of rst modal mass. Tuned simplymeans the mass can be adjusted to move in a fundamental period equal to the buildingsnatural period so that it will be more effective in counteracting the building oscillations.In addition to the initial tuning when it is rst installed, the TMD may be ne-tuned asthe building period changes with time. The period may increase as the building occupancychanges, as nonstructural partitions are added, or as elements contributing nonstructuralstiffness loosen-up after initial wind storms.

Thus a TMD may be considered as a small damped mass of single-degree-of-systemriding piggy-back atop a building. Although its mass is a small fraction of the buildingsmass, its vibration characteristics are adjusted to mimic those of the buildings. Forexample, if a tall building sways, say, 24 in. to the right at a fundamental frequency of0.16 Hz, the TMD is designed to move to the left at the same frequency.

The idea of using the inertia of a oating mass to tame the sway of a tall buildingis not entirely new. In fact, the invention of the TMD as an energy-dissipative vibrationabsorber is credited to Frahm, who developed the concept in 1909. The theory was laterdescribed by Den Hertog in his classic textbook in 1956, and since then has been appliedin automotive and aircraft engines to reduce vibrations. Since the wind forcetime rela-tionship is not harmonic (sinusoidal), the basic ideas developed by Den Hartog have beenmodied in building applications to account for the random nature of wind.

When activated during windstorms, the TMD becomes free-oating by rising on anearly frictionless lm of oil. To dissipate energy, the TMD must be allowed to move withrespect to the building. In the earlier TMDs installed in tall buildings, spring-like devicesconnecting the mass to the building pull the building back to center, as the building swaysaway from its equilibrium position. The mass is also connected to the building with adamping device, in the form of a hydraulic actuator, which is controlled to provide apredetermined percentage of critical damping. This limits the lateral displacements of themass relative to the building.

The TMDs advantages become academic in a power failure. It needs electricity towork and if thats lost in a heavy windstorm, when the TMD would most be needed, itwouldnt work. So it is advisable to have the TMD wired to an emergency power system.

During a major wind storm, the mass will move in relation to the building some 2to 5 ft. The system is controlled to activate when a predetermined building lateral accel-eration occurs. This motion is registered on an accelerometer and, if the allowable limitis reached, the mass is activated automatically.

Figure 8.30a. Viscoelastic polymer damper. A building damping of about 4% can be attainedusing these dampers. Buildings equipped with viscoelastic dampers include the World Trade Center,New York, destroyed on Sept 11, 2001, and the Columbia Searst Center, Seattle.

800 Wind and Earthquake Resistant Buildings

8.2.2.1. Citicorp Tower, New YorkThe Citicorp Tower (shown in schematic view in Fig. 8.30b) consists of a unique structuralsystem of perimeter-braced tubes elevated on four 112-ft-high columns and a central core.It rises approximately 914 ft above grade. The tower is square in cross section with plandimensions of approximately 157 by 157 ft. The top 140-ft portion of the tower slopesdownward from north to south.

The TMD designed for the building consists of a concrete block 29 29 9 ft thatweighs 410 tons (820 kips). It is attached to the buiding with two nitrogen-chargedpneumatic spring devices and two hydraulic actuators that are controlled to providedamping to the TMD and linearize the springs. One set counters northsouth buildingdynamic motion and the other set counters eastwest motion. The spring stiffness, andthereby the TMD frequency, is adjusted (tuned) by changing the pneumatic pressure. Italso has an antiyaw device to prevent twisting of the block, and snubbers to preventexcessive motion of the block.

The TMD is capable of a 45-in. operating stroke in each orthogonal direction. Theoperating period is adjustable independently in each axis. The mass block is supportedwith twelve 22-in.-diameter pressure-balanced bearings connected to a hydraulic pump.

The block positioned at the buildings 63rd oor (780 ft high) represents approxi-mately 2% of rst-period modal mass of the building. The motions of the block arecontrolled by pneumatic devices and servohydraulics resulting in a system that has thecharacteristics of a spring-mass-damper system, as shown schematically in Fig. 8.30c.

To dissipate energy, the TMD is allowed to move with respect to the building. It iscontinuously on standby, and is designed to start up automatically whenever the acceler-ations exceeds a predetermined value. The TMD kicks in whenever the accelerations fortwo successive cycles of building motion exceed 3 milli-g (1 milli-g = 1/1000 of acceler-ation due to gravity. Therefore, 3 milli-g corresponds to an acceleration of approximately1.16 in./sec2).

The system continues to operate as long as building motions continue and stopsonly a half-hour after the last pair of building cycles for which maximum acceleration is

Figure 8.30b. Tuned mass damper for Citicorp Tower, New York: (1) building elevation; (2)plan; (3) rst-mode response; (4) TMD atop the building.

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greater than 0.75 milli-g. The TMD provides the building with an effective structuraldamping of about 4% of critical. This is a signicant increase above the inherent dampingestimated to be just under 1% of critical. Since wind-induced accelerations of a buildingare approximately proportional to the inverse of the square root of the damping, when inoperation the TMD reduces the building sway oscillations by over 40%.

The Citicorp TMD is installed on the 63rd oor. At this elevation, the building maybe represented by a single-degree-of-freedom system with a modal mass of 40,000 kipsresonating biaxially at a 6.8-sec period with a critical damping factor of 1%. The TMDis designed with a moving mass of 820 kips, biaxially resonant with a period of 6.7 secondsplus or minus 20%, and an adjustable damping of 8 to 14% of critical. Observe that themoving mass represents approximately 2% of the rst-period modal mass, which typicallycorresponds to about 0.6 to 0.7% of the total mass.

8.2.2.2. John Hancock Tower, Boston, MAThe TMD for the John Hancock Mutual Life Insurance Co.s glass-clad landmark in Bostonis somewhat different from that for Citicorp Tower. It was added as an afterthought toprevent occupant discomfort. Second, Hancock Tower is rectangular in plan and consistsof moment frames unlike Citicorps diagonally braced frame (Fig. 8.30d). Because of thebuildings shape, location, and vibration properties, its dynamic wind response is mainlyin the eastwest direction and in torsion about its vertical axis. There is a TMD near eachend of an upper oor. They are tuned to a vibration period of approximately 7.5 sec. Thetotal eastwest moving mass represents about 1.4% of the building rst-mode generalizedmass, while in the twist direction the moving masses represents about 2.1% of the buildings

Figure 8.30c. Schematic view of a TMD operating on top of the Citicorp Center. The TMDconsists of a 400-ton concrete block bearing on a thin lm of oil. The structural stiffness of theTMD is aided by pneumatic springs tuned to the frequency of the building. The TMD dampingsystem is aided by shock absorbers.

802 Wind and Earthquake Resistant Buildings

generalized torsional inertia. The dampers, then, move only in an east-west direction andwork together to resist sway motions in the short direction, or in opposition to stabilizetorsional rotations of the building. They are located 220 ft apart, and when moving inopposition act in effect as a 220-ft lever arm to resist twisting. Hancocks dampers eachhave a 300-ton mass consisting of lead blocks contained in a steel coffer box. They alsoactivate at 3 milli-g of acceleration. In operation the masses may move up to six feet withan operating cycle of about 7.5 sec. Each mass block is supported on sixteen 22-in.-diameter pressure-balanced bearings connected to a hydraulic pump.

The TMDs in both of these towers are used only to assure occupants comfort. Theirbenecial effects in reducing wind-induced dynamic forces are not relied upon for struc-tural integrity under extreme wind loads.

Both the John Hancock Tower and Citicorp Tower TMDs are called passive-poweredbecause, although the reduction in the buildings sway response comes from the inertial forceof the dampers, initially power is required to activate the masses. The sliding masses installedin these towers cannot move until their oil bearings are pressurized to levitate the masses.

8.2.2.3. Design Considerations for the TMDThere are a number of practical considerations in the design of the TMD. One of these isthe need to limit the motions of the TMD mass under very high wind loading such as willoccur in the design storm or under ultimate load conditions. One way of doing this is touse a nonlinear hydraulic damper in the TMD. By employing such a damper, the motionsof the TMD mass can be greatly reduced under very high wind loading conditions orunder strong seismic excitation. A further safeguard against excessive TMD motion is toinstall hydraulic buffers around the mass. When the mass comes into contact with thebuffers, high velocities are quickly reduced.

Both the Citicorp and John Hancock TMD systems have sensors and feedback andelectronic control systems, but these were designed to make the TMD operate like a passivetuned mass damper. Tuned mass dampers can in principle be readily converted to be anactive system by incorporating sensors and feedback systems that can drive the TMD massto produce more effective damping than is possible in a purely passive mode. As a result,a larger effective damping can be obtained from a given mass. This approach has beenused in several commercially available ready-to-install systems. The TMD is thus mademore efcient, a benet to be weighed against the increased cost, complexity, and main-tenance requirements that are entailed with an active system.

Figure 8.30d. Dual TMD system: John Hancock Tower Boston, MA. Two 60,000-pound massesat each end of the building reduce expected motion by 50%. Effective damping is increased fromabout 1% to 4%.

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8.2.3. Sloshing Water DamperA simple sloshing type of damper consists of a tuned rectangular tank lled to a certainlevel with water. The tuning of the system consists of matching the tanks natural periodof wave oscillation to the buildings period by appropriate geometric design of the tank.If obstacles such as screens and bafes are placed in the tank, dissipation of the wavestakes place when water sloshes across these obstructions resulting in a behavior similarto that of a TMD, and the result is again that the tank behaves as a TMD. However, analysisindicates that a sloshing water tank does not make as efcient use of the water mass as atuned liquid column damper.

8.2.4. Tuned Liquid Column DamperA tuned liquid column damper (TLCD) is in many ways similar to a TMD that uses aheavy concrete block or steel as the tuned mass. The difference is that the mass is nowwater or some other liquid. The damper is essentially a tank in the shape of a U. It hastwo vertical columns connected by a horizontal passage and lled up to a certain levelwith water or other liquid. Within the horizontal passage, screens or a partially closedsluice gate are installed to obstruct ow of water, thus dissipating energy due to motionof water. The TLCD is mounted near the top of a building, and when the building moves,the inertia of the water causes the water to oscillate into and out of the columns, travellingin the passage between them. The columns of water have their own natural period ofoscillation which is determined purely by the geometry of the tank. If this natural periodis close to that of the buildings period then the water motions become substantial. Thusthe buildings kinetic energy is transferred to the water. However, as the water moves pastthe screens or partially open sluice gate in the horizontal portion of the tank, the drag ofthese obstacles to the ow dissipates the energy of the motion. The end result is addeddamping to reduce building ocillations.

8.2.4.1. Wall Center, Vancouver, British ColumbiaShown in Fig. 8.30e is the plan for the mechanical penthouse of a building called WallCenter, a 48-story residential tower in Vancouver, British Columbia. From wind-tunneltests, predicted 10-year accelerations were in the range of 40-milli-g, depending on thestructural systems considered in the preliminary design. To minimize occupants percep-tion of motion due to wind excitations, a limit of 15 milli-g was chosen as the designcriterion for a 10-year acceleration. A damper using water serves a dual purpose by alsoproviding a large supply of water high up in the tower for re suppression. Initially, asloshing water damper was considered but the TLCD was found preferable due to itsgreater efciency in using the available water mass. The design turned out to be aremarkably economical solution considering the saved cost of having to install a high-capacity water pump and emergency generator in the base of the building as initiallyrequired by re ofcials. The total mass required was on the order of 600 tons whichcorresponds to a large volume of water. However, sufcient space was available. Also ahelpful factor was that the motions of the tower were primarily in one direction only.Therefore only motions in one direction needed to be damped, which simplied the design.Figure 8.30f illustrates the TLCD design consisting of two identical U-shaped concretetanks. Since the building was concrete, it was relatively easy to incorporate the tanks intothe design and to construct them as a simple addition to the main structure. The structuraldesign is by Glotman Simpson Engineers, Vancouver, British Columbia, Canada. The

804 Wind and Earthquake Resistant Buildings

design of the TCLD is by Rowan, Williams, Davis, and Irwin, Inc., Guelph, Ontario,Canada.

8.2.4.2. Highcliff Apartment Building, Hong KongAnother example of a tall building that uses TLMD to control accelerations and provideenhanced structural performance during typhoon conditions, is the 73-story Highcliffapartment building in Hong Kong, one of the windiest places on earth. The building soarsto a height of 705 ft (215 m) with an astonishing slenderness ratio of 20:1. A unique

Figure 8.30e. Mechanical penthouse cross section of the Wall Center, a 48-story building inVancouver, British Columbia. Two specially shaped tanks containing 50,000 gallons of water providethe mass for the buildings TLCD. Structural engineering by Glotman Simpson Consulting Engineers,Vancouver, British Columbia, Canada; TLCD by Rowan, Williams, Davies, & Irwin, Inc., and MotionEngineering, Inc., Guelph, Ontario, Canada.

Figure 8.30f. TLCD for Wall Center, Vancouver, British Columbia. The motions of the towerwere primarily in one direction only. Therefore only one direction needed to be damped. Two TLCDsextend nearly the full width of the tower. Within each tank is a long horizontal chamber at thebottom and two columns of water at each end. The dampers work by allowing the water to moveback and forth along the bottom chamber of the tank and up into the columns of water.

450 (Typ.) 4000 (Typ.)

Solid fill.Dimensions tied afterbuilding frequency is known.Both sides.

Removable sections.Dimensions tied afterbuilding frequencyis known.Both sides.

450

127826391

3000

Water level (nom.) Water level (nom.)

3885

3500

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structural system that incorporates all vertical elements as part of the lateral system, incombination with a series of tuned liquid mass dampers, ensures the safety and comfortof the buildings occupants.

Photographs of the building are shown in Fig. 8.30g. The structural engineering isby the Seattle rm of Magnusson Klemencie Associates.

8.2.5. Simple Pendulum DamperThe principle feature of the system shown in Fig. 8.30h is a mass block slung from cableswith adjustable lengths. The mass typically represents approximately 1.5 to 2% of thebuildings generalized mass in the rst mode of vibration. The mass is connected to hydraulicdampers that dissipate energy while reducing the swinging motions of the pendulum.

The adjustable frame is used as a tuning device to tailor the natural period of vibrationof the pendulum. The frame can be moved up and down and clamped on the cables toallow the natural period of the pendulum to be adjusted. The mass is connected to anantiyaw device to prevent rotations about a vertical axis. Below the mass there is a bumperring connected to hydraulic buffers to prevent travel beyond the hydraulic cylinders strokelength.

Figure 8.30g. Highcliff apartment building, Hong Kong.

806 Wind and Earthquake Resistant Buildings

8.2.5.1. Taipei Financial CenterAn example of a tuned mass pendulum damper (TMPD) architecturally expressed as abuilding feature is shown in Fig. 8.30i. At a height of 1667 feet (508 m), consisting of 101stories, the building, called Taipei Financial Center, is poised to steal the crown from thetwin Malaysian Petronas Towers as the tallest building in the world. A special space has

Figure 8.30h. (1) Simple pendulum damper; (2) Hydraulic dampers attached to mass block. (Pho-tograph courtesy of Dr. Peter Irwin of Rowan, Williams, Davis, & Irwin, Inc., Guelph, Ontario, Canada.)

Figure 8.30i. Spherical damper, Taipei Financial Center, Taiwan. A 20-ft (6-m)-diameter steelball assembled on site in layers of 5-in. (12-cm)-thick steel plate is suspended from level 92 byfour sets of cables. Eight hydraulic pistons, each 6.5 ft (2 m) long, attached to the ball, dissipatedynamic energy as heat.

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been allocated for the TMPD near the top of the building and people will be able to walkaround it and view it from a variety of angles. The TMPD, consisting of a 730-ton steel ball,will be brightly colored, and special lighting effects are planned. The architecture of thebuilding is by C.Y. Lee and Partners, Taiwan; structural engineering is by Evergreen Con-sulting Engineering, Inc., Taipei, Taiwan, and Thornton-Tomasetti Engineers, New York; andthe design of the TMPD is by Motioneering, Inc., a company in Ontario, Canada, thatspecializes in designing and supplying damping systems for dynamically sensitive structures.

8.2.6. Nested Pendulum DamperIn situations where the height available in a building is insufcient to allow installation ofa simple pendulum system, a nested TMD may be designed as illustrated in Fig. 8.30j. Thedesign shown is for a North American residential tower. The total vertical space occupiedby the damper, which has a natural period of about 6 sec and a mass of 600 tons, is lessthan 25 ft (7.62 m), as compared to 30 ft (9.14 m) required for a simple pendulum. Thedesign of the damper is by Rowan, Williams, Davis, and Irwin, Inc., Guelph, Ontario, Canada.

A nested pendulum damper is installed at the top of the 70-story, 971-ft-tall Land-mark Tower, Yokohama, Japan. The damper requires only a one-story-high space, and issemi-actively controlled. Wind-induced lateral accelerations are expected to be reduced atleast 60%. The damper design is by Mitsubishi Heavy Industries, Ltd., Tokyo, Japan.

Figure 8.30j. (1) Simple pendulum damper; (2) nested pendulum damper.