Fluid Viscous Dampers to reduce Wind-induced Vibrations in Tall Buildings

Transcription

1 Fluid Viscous Dampers to reduce Wind-induced Vibrations in Tall Buildings Abstract R.J. McNamara, SE McNamara / Salvia Inc. 160 Federal Street, 16 th Floor Boston, MA Douglas P. Taylor, President Taylor Devices, Inc. 90 Taylor Drive North Tonawanda, NY Philippe Duflot, Eur. Rep. Taylor Devices Europe 18 rue J-B Vannypen B-1160 Brussels Using supplemental fluid viscous dampers to dissipate energy and reduce building response to dynamic inputs is gaining worldwide acceptance. The concept of supplemental dampers added to a structure is that they absorb much of the energy input to the structure from a transient, not by the structure itself, but rather by supplemental damping elements. This paper presents an application of fluid viscous dampers in a high-rise structure to suppress the anticipated wind-induced accelerations. The description of the damping system, the design criteria and cost data are discussed. The viscous damper system proves to be a very cost-effective method to reduce wind motions. Introduction Damping is one of many different methods for allowing a structure to achieve optimal performance when it is subjected to seismic, wind, blast or other types of transient shock and vibration disturbances. A conventional approach would dictate that the structure must inherently attenuate or dissipate the effects of transient inputs through a combination of strength, flexibility and deformability. By adding fluid viscous dampers, the energy input from a transient is absorbed, not by the structure itself, but rather by the supplemental dampers. Fluid damping technology was validated for seismic use by extensive testing in the period The long history of military applications proved the reliability of the system. Additional testing revealed that fluid dampers were also well suited to improve the performance of structures subjected to wind. The tested performance of structures with fluid dampers showed that tremendous gains in performance could be realized at relatively low cost. The implementation of fluid damping technology began swiftly and today more than one hundred major structures are using fluid

2 dampers to obtain enhanced performance during seismic or wind excitation. This paper presents an application of fluid viscous dampers in a high rise building to effectively increase the level of damping within the structure itself and thereby reduce undesirable oscillations. The structure, a 39-story office tower, was designed using conventional wind engineering methods of code loadings and deflection limitations and was model tested in a wind tunnel in Canada. Wind tunnel tests indicated that the structure would experience unacceptably high acceleration levels. The wind tunnel predicted acceleration levels were almost double the industry standard for office towers. The high response level was produced by the proximity of an existing 52 story building. Since the main intent of the damper installation is to reduce wind motions, the fluid viscous dampers need to provide a large force output at very low displacement levels. To minimize the cost and number of fluid viscous dampers, a motion amplification device was included in the design in one direction of the structure; that being the stiffest with the lowest predicted movements. Acceleration criteria As a structure becomes taller, lighter or more slender, the possibility of excessive accelerations of the upper floors during relatively common wind events become more likely. The building occupants are usually sensitive to acceleration and its change rather than displacement and velocity. Acceptability criteria for accelerations are not currently codified. General guidelines do exist that are generally accepted by the wind engineering community. The International Organization for Standardization (ISO) has recommended, for commercial or office occupancies, 1 and 5 year return periods that depend on the natural period of the building. Past experiments showed that the acceleration perception threshold under cyclic motion was about 5 mg. The accelerations became annoying when they reached 20 mg. However, the perception and annoyance are dependent on ambient motions and activities. Occupants in apartments or hotels are more sensitive than in offices. Acceptability of motion perception varies widely. In common practice, the suggested values range from mg for 10 years return with 10 mg being used for apartments and 30 mg being used for offices. Damping system concept In order to reduce the projected motion levels, tuned mass damper and sloshing damper systems were investigated and evaluated for cost and project impact. Tuned mass dampers and sloshing dampers required valuable office space at the top of the building and proved to be very expensive, although very effective. Fluid viscous dampers proved to be the most costeffective and least space-intensive on the office tower. An extensive design program was undertaken with various damper configurations vertically located in the tower and with many variations on fluid viscous damper specifications. Since the main intent of the damper installation is to reduce wind motions, the fluid viscous dampers need to provide a large force output at very low displacement levels (± 3 mm). In order to insure reliability at this low movement and to keep the number and cost of the dampers to a minimum, a motion amplification device called a Toggle Brace Damper system (TBD) was included in the design. The motion amplification device was used in one direction

3 of the structure, that being the stiffest with the lowest predicted movements. The TBD system is a lever style mechanism that multiplies the deflection of the building while simultaneously reducing the required amount of applied force at the damper mounting points. The system combines a substantially braced column with a driving arm connected to the column and upper floor with hinge pins. The hinge connection incorporates flexural constraint elements to prevent out of plane buckling of the pinned connection. The end result is that a simple mechanical lever is used to increase the effective damper stroke. The fluid viscous dampers are then designed for both 100-year return wind and moderate earthquake excitations (seismic zone 2, Av=0,12g). Description of the structure The 39-story office tower consists of three lateral systems at different levels as shown in the structural elevation diagram. From the 1 st to 7 th floors and above the 34 th floor are diagonal braced lateral systems at the inner-core. The remaining floors are a moment frame along the perimeter of the building. The basic structural lateral system of building is classified as a moment-frame-tube system. The typical floor system is a composite metal deck with joist girder system. Typical floor area is m². Total weight of the building is about tonnes and supported by 3,6 m-1,8 m diameter drill shafts tied with concrete beams at innercore and another 22-drilled shafts at perimeter columns. Fluid viscous dampers on E-W direction are diagonally placed in two bays with 5,8m length at the inner-core on every other floor between the 7 th floor and the 34 th floor while TBD systems are assigned to two 9,5 m length bays along N-S direction at the same level. The damper's location is shown in Fig. 1. Fig. 1 Fluid viscous damper location The static lateral analysis and design was conducted by ETAB 6.2. The dynamic response and

4 fluid viscous damper design of the TBD system was analyzed by SAP2000. The floor masses are lumped at the center of the mass. The building dynamic properties are tabulated in table 1. Table 1: Dynamic properties of building for first six modes Mode Shape Period (sec) 5,26 5,00 3,65 1,92 1,82 1,71 Effective Mass (%) 66,1 62,6 81,2 15,3 12,8 8,5 Direction X (E-W) Y (N-S) Rotation X (E-W) Y (N-S) Rotation Wind tunnel results indicated that average story drifts from 7 th floor to 34 th floor on E-W (X) direction are larger than the (Y) direction. The bay length (5,8 m) on X-direction at inner-core where the damper is placed is shorter than that of Y-direction (9,5 m). The overall building stiffness in the X-direction is less than that in the Y-direction. For cost effective dollar design, the TBD system in the Y-direction was used to magnify the story drift. The damping values C are kn.s/m from 6 th floor to 25 th floor and kn.s/m from 26 th floor to 35 th floor. In the X-direction, damping values C are kn.s/m from 6 th floor to 25 th floor and kn.s/m from 26 th floor to 35 th floor. The damper layout is shown Fig. 1. The configuration of TBD devices on the building are described both in table 2 and in Fig. 2 and Fig. 3. In order to activate fluid viscous damper elements, a total of 361 full modes are considered in building dynamic analysis. The damper output force is directly proportional to velocity, the exponent constant was set as a unit, which produced perfect linear viscous behaviour. Fig. 2 Diagonal fluid viscous damper on E-W(X) direction Fig. 3 Toggle brace damper system on N-S(Y) direction Table 2 : Toggle Brace configuration with story height 4m Bay Length (m) Low brace angle Upper brace angle Low brace length (m) Upper brace length (m) Motion amplification factor Force amplification factor 9, ,5 7,2 2,8 2,9 6,1

5 Design Criteria The design criteria for office building are compliant to BOCA 96 and Massachusetts State Building Code. The lateral structural systems are designed to meet AISC strength requirements and seismic provisions for zone 2B. No reductions due to the damping increase by fluid viscous dampers are taken into account at the static load design stage. The design coefficients for the equivalent lateral load of BOCA 96 are tabulated in table 3. Wind design criteria are for 100-year return for strength and 10-year return for serviceability. Table 3 : Equivalent lateral load design parameters for BOCA 96 Design wind load Design earthquake load Wind speed 144 km/h Seismic zone 2A Design category B Peak acceleration (Av) 0,12 g Importance factor 3 Reduction factor (R) 4,5 Aspect ratio of depth to width 1 Soil factor (S3) 1,5 Aspect ratio of depth to width 1 Building period (Ta) 3,65 s The TBD system and fluid viscous dampers are introduced to improve the serviceability of the building by raising story drift index and reducing the acceleration at the top floor of building. Wind Tunnel Test Results and Wind Time History Generation To determine structural wind loads and wind-induced accelerations, a 1:400 scale model of the building was placed in a wind tunnel with all surrounding buildings within a full-scale radius of 500 m. The wind tunnel test was carried by RWDI, Ontario. The magnitude of simulated wind speed for a 100 year return period was scaled to correspond to a highest velocity speed of 145 km/h at 10m above ground in open terrain, which is consistent with the Massachusetts Building Code and ASCE-93 Standard. A high frequency force-balance wind tunnel test was performed where the bending moments and shears are measured directly from the model to give generalized wind forces. These were then combined with structural characteristics, such as building mass, mode shapes and an estimate of the structural damping to determine the full scale dynamic behaviour of the building. Fig. 4 and 5 show the build roof maximum acceleration response which occurred at 36 th floor. For this office building, the acceleration at the highest occupied (36 th floor) level is predicted as high as 41 mg for 10 years return which was higher than the desired criterion of 30 mg. Fig. 4 The build roof acceleration response on (E-W) from wind tunnel test

6 Fig. 5 The build roof acceleration response on (N-S) from wind tunnel test Building Story Drift High-rise building design is usually governed by stiffness rather than member strength due to its inherent flexibility. This is especially true in moderate seismic zones. Under normal wind conditions, large deflections or story drifts of a building may result in damage of the nonstructural partitions and cladding, overall building stability and comfort of tenants. As stated earlier, the major building lateral structural system is a moment frame tube system that is designed to meet strength requirement of current specifications. After sixty fluid viscous dampers are introduced, the deflection and minimum story drift index are much improved as shown in Fig. 6 and Fig. 7. Fig. 6 Story drift on E-W and N-S (wind load) Fig. 7 Story drift on E-W and N-S (seismic load)

7 Results The wind and seismic effect comparison of the Massachusetts State Building Code and National Building Code (BOCA 93) on the office building are plotted on Fig. 8 and Fig. 9. Fig. 8 Comparison of building behaviour for equivalent static lateral load Fig. 9 Building base shear and overturning moment comparison for different load condition Wind tunnel tests revealed that the wind exerts more pressure at 100 m and above on E-W direction, but diminished quickly on lower floors. In general, wind load shows more severe influence on the office building than that of an earthquake condition. The effectiveness of fluid viscous dampers on the office building are summarized in table 4. This table shows that the fluid viscous dampers will improve the building dynamic behaviours from 20% to 30%. These dampers gave the building additional inherent damping, which is equivalent to an entire building structural damping ratio around 3% of critical. Table 4: Results summary table for time history analysis

8 Wind load condition Seismic load condition E-W (X) dir N-S (Y) dir E-W (X) dir N-S (Y) dir Response Accel. at 37 th floor (m/s²) 0,696 0,455 2,415 2,845 Without Displ. at 37 th floor (m/s) 0,528 0,284 0,597 0,665 Damper Base shear (kn) Response Accel. at 37 th floor (m/s²) 0,523 0,305 1,938 1,976 With Displ. at 37 th floor (m/s) 0,417 0,185 0,556 0,584 Damper Base shear (kn) th -15 th Max. stroke (mm) Max. damper force (kn) th -25th Max. stroke (mm) Max. damper force (kn) th -35th Max. stroke (mm) Max. damper force (kn) Overall Evaluated by energy 1,89% 2,0% 3,56% 3,8% Damping Evaluated by acceleration 1,94% 3,08% 3,56% 4,58% Conclusion A non-linear time history analysis coupled with wind tunnel tests was used to predict the peak acceleration of a high rise building subject to wind induced oscillations. The wind tunnel predicted acceleration levels were much higher than the industry standard for office towers. The increased level of damping has been shown to reduce undesirable accelerations in the building in order to guarantee occupant comfort. The addition of a fluid viscous damping system resulted in a significant reduction in acceleration; the building deflection and floor accelerations were reduced by approximately 35%. Cost of the installation, including the motion amplification device, was less than one million dollars. The fluid viscous damping system proved to be a very cost effective method to effectively reduce wind-induced vibrations. For large force output at very low displacement, a motion amplification device has been included in the design in order to reduce the quantity and cost of the dampers. References 1. McNamara R.J., Huang C.D., Wan V. (2000), "An Efficient Damper System for High Rise Buildings", Fifth Conference on Tall Buildings in Seismic Regions, University of Southern California, LA, CA. 2. Constantinou M.C., Tsopelas P., Hammel W. (1998), "Testing and Modeling of an Improved Damper Configuration for Stiff Structural Systems", Technical Report to the Center for Industrial Effectiveness and Taylor Devices, Inc. 3. Taylor D.P. (1999), "Fluid Viscous Dampers in Seismic Isolation."