VIBRATION CONTROL AND FULL-SCALE MEASUREMENT OF A STEEL TV TOWER WITH A DAMPER DEVICE OF PTTMD

Transcription

1 13 th Worl Conference on Earthquake Engineering Vancouver, B.C., Canaa August 1-6, 24 Paper No VIBRATION CONTROL AND FULL-SCALE MEASUREMENT OF A STEEL TV TOWER WITH A DAMPER DEVICE OF PTTMD Renle MA 1 an Minjuan HE 2 SUMMARY A kin of amping evice name Penulous Tank as Tune Mass Damper (PTTMD) was esigne an installe on the Heilongjiang steel TV tower of 336 m in height to control the vibration of the tower. Fullscale measurement in site has been unertaken to investigate the ynamic property of the tower an the efficiency of the amping evice. In this paper, the esign of the amper evice is introuce, an the full-scale measurement methos of the tower with an without amper evice are presente. By analyzing of the self-power spectrum an mutual power spectrum of the acceleration response of the tower uner artificial excitation, the ynamic moes an the amping ratio of the tower are ientifie. The result shows that the amping ratio are increase eviently after the tower is installe the PTTMD. This amping evice is effective to control the vibration of the tower uner win loa or earthquake action. INTRODUCTION For a structure, the esign of win-proof an seismic-resistance is quite important. The traitional metho is to esign the structure to be strong enough so that it can resist the loas. Recent ecaes, a more active metho is to install amping evices. As to each kin of amping evice, enough space is neee to install the system. Besies, a large mass is require as a part of this system. For a high an slener steel tower, the floor space on the tower is very limite. Therefore, It is quite ifficult to fin a large enough room to install this large an heavy evice. Authors have ha an opportunity to esign the Heilongjiang Steel TV Tower that is 336m in height an is the highest one among those steel TV towers in China so far. In this structure, a kin of amping evice calle Penulous Tank as Tune Mass Damper (PTTMD) is esigne an installe to reuce vibration of the tower. PTTMD means that the water storage tank, which supplies life water an services the fire sprinkler system neee in the tower, is suspene as the mass of the tune mass amper system. Several amper bars use to provie amp to the PTTMD. As a structural control system, PTTMD system is less room occupie, low cost, convenient maintenance an higher efficiency. It is especially suitable to tall towers because of the limite space on the tower. 1 Professor, School of Civil Engineering, Tongji University, Shanghai, P.R. China. 2 Professor, School of Civil Engineering, Tongji University, Shanghai, P.R. China. hemj@mail.tongji.eu.cn

2 OUTLINES OF HEILONGJIANG STEEL TV TOWER AND ITS PTTMD The Steel TV Tower The tower is locate in Heilongjiang province, the northeast of China. It is a lattice steel structure mae of circular section steel pipes an connecte with flanges. The total height of the tower above the groun level is 336 m. The tower consists of several parts incluing main boy truss, an observation plate, an observation ball, an antennae truss an a well truss in the center as shown in Fig1. Uner the tower there is a four floors builing. The main boy truss of the tower is octagon in plan with a base raius of 28 m. From 18 m to 214m at level of the tower there is a plate an a ball calle observation builing. The area 2 of the observation builing is 36 m. A lot of evices neee in tower are installe an tourists can sightsee there. The upper truss calle antennae truss is square in plan an is use to install antennae of television, broacast an communication. The well truss is esigne as circle in plan in orer to ecrease the win loa transmitting on the tower. Three elevators an a stair are arrange within the well truss. The 6 weight of the steel tower is kg. If the weight of ornament on tower is inclue the total weight of 6 the tower is up to. 1 kg Antennae truss Observation ball Observation plate Main boy truss Well truss Fig. 1. Heilongjiang Tower PTTMD The system of PTTMD is shown in Fig. 2. It is mae up of a water tank of circular cyliner, 4 amper bars with hyraulic pressure, 8 pieces of flexible cable an some flexible water pipe. The water tank is 2 m in raius an 3. m in height. It weighs 3. 1 kg incluing 3. 1 kg of water an 1 kg of tank.

3 The working concept of the amper bar is shown in Fig. 3. It comprises oilcan, valve an accumulator etc. The amping ratio of the amper system can be ajuste by changing the oil pressure in the amping bars. The tank is suspene so that there is less power consumption of the tank uring the motion process. The suspene point of water tank is esigne as a hinge so that the vibration of the tower in ranom irection can be controlle. The water pipe is flexible to meet the nees of the tank function without limiting the motion of the tank Water tank 2 Damper bar 3 Flexible suspension cable 4 Flexible water pipe (infall) Flexible water pipe (outfall) Fig.2. System of PTTMD 1 Oil tank 2 Commuting valve 3 Oilcan 4 Energy accumulator Safe accumulator 6 Flexible cable 7 Water tank Fig.3. Working concept of the amper bar CALCULATION METHOD OF THE TOWER WITH VIBRATION CONTROL DEVICE OF PTTMD AND PARAMETERS DESIGN OF THE DEVICE The movement of the Tower with PTTMD Tower is iealize a multi egrees of freeom (MDOF) system subjecte to seismic action F (t) an control force F PTTMD (t). Every mass of the system is concentrate on the intersection of horizontal members an vertical members. The control evice of PTTMD is locate in j th mass of the tower. The moel of tower, the simplifie iagram of the tower an the amper system are shown in Fig4a, Fig4b an Fig4c respectively. The equations of movement for the tower an the PTTMD are given by MX & + CX& + KX = F( t) FPTTMD ( t) ( ) ( ) (1a) m & x + c x& x& j + k x x j = (1b) where M, C an K are mass, amping an stiffness matrices of the tower respectively, x is the isplacement, x j an x are the isplacement of the j th mass an the isplacement of the PTTMD, m, c an k are the mass, amping coefficient an stiffness of the PTTMD respectively. The control force is expresse by

4 F. t) = f... ( t) = c.. PTTMD ( j ( x& j x& ) + k ( x j x ) (2) Fig.4c. Simplifie iagram of amper Fig.4a. Moe of tower Fig.4b. Simplifie iagram of tower The efficiency of PTTMD affecte by the waving of the water in the tank Movement of the water tank is translational an not rotational as show in Fig. It means that the movement of any point in the tank is the same as that of the noe between steel cable an the tank. The movement of water an tank is analyze by the theory of flui mechanics. It is suppose that the shape of the tank is a cube with imensions shown in Fig6. The tank vibrates along x irection. The movement of the water in the tank meets Laplacian equation as Φ Φ Φ = = 2 2 x + z (3) Where Φ is potential energy of the water incluing two parts of rigi boy movement an wave movement. Accoring to the solution of the equation, bounary conition an initial conition, the force on the tank wall cause by the wave movement of water can be got. If the tank is circular, similar conclusion can be got by transform of coorinates. From the result we realize that the force on the tank wall cause by the wave motion of water varies with the imension of the tank. The larger the ratio epth of water to raius of the tank is the less the force is. From what was iscusse above it is explaine that not all the mass of water in the tank can be as the vali mass of PTTMD. The vali mass of PTTMD can be expresse as m = m tan + δm (4) k water

5 Where m tan k an m water are mass of tank an water respectively. δ is a reuction coefficient of the mass of water. It has relation with the force on the tank wall cause by the wave motion of water an can be simplifie as R h δ = 1 th π () 2 πh( π 1) R where R an h are raius of tank an epth of water in the tank respectively. For Heilongjiang steel tower the reuction coefficient δ is.97. In the esign of PTTMD, if the ratio epth of water to raius of the tank is large enough an the movement of the tank is not so large, the reuction coefficient δ is equal to 1. approximately. l z y x φ a l h K a b W Fig.. Movement of PTTMD Fig. 6. Coorinate of water tank Fig.7. Mechanic moel Vibration property of PTTMD system Mechanic moel of penulous tank uner vibrating is shown in Fig 7. Frequency of the tank is analyze in energy metho. Kinetic energy is maximal when ϕ = an potential energy is maximal when ϕ = ϕ max. Accoring to conversation law of energy, equation of the penulous tank is written as 2 2 Wl ω φ ( ) 2 max = Ka + Wl φmax (6) 2g 2 The meaning of φ, a, l, K an W in equation (6) is shown in Fig 7. From (6) the frequency of PTTMD is Particularly, when a = l, Equivalent stiffness of PTTMD is 2 g Ka ω = + 1 (7) l Wl g Kl ω = + 1 (8) l W W K = K + (9) l Parameters calculation of the PTTMD an ajusting uring installation Several parameters such as mass, amping ratio an frequency of the amper system will influence the efficiency of vibration control.

6 Mass Mass inclues two parts of water an tank. Mass of water mainly meets the nees of function of reserving enough living water an fire protection water. Mass of the tank itself is ominate by the structural strength. The esign iea of PTTMD is not to increase too much aitional weight, which is no goo to 4 structure. Total weight of PTTMD on Heilongjiang tower is 3. 1 kg as mentione above. Frequency Frequency of the amper system can be calculate by equation (8). From (8) we know that the weaker the stiffness is the lower the frequency is an the longer the penulous length is the lower the frequency is too. Accoring to the TMD theory optimum frequency of amper system is nearly equal to the funamental natural frequency, which is erive base on the structure of tower. Funamental natural frequency of Heilongjiang tower is about from theoretical calculation. It is s very low because the tower is extreme tall. We ha to ecrease the stiffness an increase the penulous length. For the space limite the penulous length coul not be too long. Zero stiffness of the amper system was aopte. From equation (8) l =. 2m is got. The tuning of the frequency was mae by ajustment of penulous length of the tank. At first the tank was suspene accoring to the calculate length. Then the frequency was measure an the length was ajuste again. At last the length of l =. 24m was aopte an the measure frequency is f = s Damping ratio Accoring to theoretical analyses the optimum-amping ratio is about ξ =. 9. Finally the tuning of the amping ratio was mae base on the measure value of ξ =. 89 after ajusting pressure of hyronitrogen in the accumulator etc. FULL-SCALE FIELD MEASUREMENT OF THE HEILONGJIANG STEEL TV TOWER The measurement system Process of the measurement system The structure vibrate by transient manpower excitation. Accelerometers being put on the proper position of tower putout the perceptive signals. Electric charge amplifiers woul amplify the signals. Computer woul analyze the signals after A/D converting. Vibration excitation The metho of vibration excitation was performe by cutting a stresse cable connecte the tower an the groun. The upper en of the cable was tie to the tower of m at level an the other en was tie to the anchor that is fixe uner groun shown in Fig 8. The cable comprises three parts A, B an C. Part A is a long cable an B an C are short ones. Part B an part C all connect with A an anchor. Besies, part C links with an ergometer an an exerting force evice. It parallels to B. When the force in C shown on the ergometer reaches to a specifie value, part B was stresse by ajusting the bolt till the force in C is equal to zero an then C is taken off. Force vibration was performe through cutting B. Arrangement of accelerometers Ten accelerometers were place in same irection an at ifferent level. Nine of them were on the tower an one was on the tank. The arrangement of accelerometers is liste in Table 1.

7 Table 1. Arrangement of accelerometers on the tower No At level(m) On tank Steel cable Part A Exerting force Ajusting bolt Part B Cutting point Part C Ergometer Anchor Steel cable Part A Part C Part B Fig. 8. Excitation of the tower Full-scale measurement uner ifferent exciting force Force vibration measurements were performe uring still atmospheric conitions to minimize aeroynamic an other artificial isturbance effects. To contrast the response of the tower with or without PTTMD an know the efficiency of PTTMD uner ifferent exciting force, the tower is measurement uner four specifie ifferent exciting force tabulate in Table 2. Table 2. Specifie ifferent exciting force of measurement No. Specifie exciting force 1 PTTMD is fixe on the floor (without PTTMD), exciting force is equal to 2kN 2 The tank was suspene (with PTTMD), exciting force is equal to 2kN 3 The tank was suspene (with PTTMD), exciting force is equal to 3kN 4 The tank was suspene (with PTTMD), exciting force is equal to 4kN

8 Analysis of the measure ata When the tower was vibrate by excitation, acceleration history responses were recore. A few response curves are shown in Fig Acc i Acc i t i t i 4 No. 6 No Acc i Acc i t i t i 4 No. 8 No. 9 Fig.9 Acceleration history responses of No. 6, 7, 8 an 9 on the tower By the ranom vibration analysis metho, self-power spectrum an mutual power spectrum of acceleration responses of the tower can be got. In Fig. 1, the self-power spectrum of No. 8 an mutual power spectrum between No. 8 an No. 6, 7, 9 are shown Pow k Pow k Freq k Freq k (a) Mutual power spectrum between No. 8 an No. 6 (b) Mutual power spectrum between No. 8 an No Pow k 3 Pow k Freq k Freq k (c) Self-power spectrum of No. 8 () Mutual power spectrum between No. 8 an No. 9 Fig.1 Self power spectrum an mutual power spectrum of some point on the tower

9 The natural frequencies of every moe evaluate from the peak of the power spectra. The amping ratio evaluate by half-power banwith metho. The frequencies an amping ratio of the tower are shown in Table 3. Table 3. The natural frequencies an amping ratio of the tower Natural frequencies f( i 1 / s) Moe Damping ratioξ i Measurement value Theoretical value Deference 1th moe %.28 2th moe %.2 3th moe %.7 Accoring to the phase angle an the ratios of peak values between one measurement point an a benchmark point on power spectra, the moe shapes of tower can be got. The former three moe shapes of the tower are shown in Fig Measurement Calculation (a)1th moe shape (b)2th moe shape (c)3th moe shape Fig.11. The former three moe shapes of Heilongjiang steel TV tower The amping ratios uner ifferent specifie exciting force from 2kN to 4kN are shown in Table 4. Table 4. Damping ratios of the tower with or without PTTMD Without PTTMD With PTTMD Exciting force of 2kN Exciting force of 3kN Exciting force of 4kN ξ With the increasing of the exciting force, the vibration of the tower increases an the movement of the tank become great, so that the effectiveness of the PTTMD increases. CONCLUSIONS From the calculation of win loa an earthquake action we know that their values vary with the amping ratio. The higher the amping ratio is, the less the win loa or earthquake action is. After the PTTMD was installe on Heilongjiang steel tower, the amping ratio at its funamental frequency reache to.33 which was gotten when the tower was excite by a force of 4 kn in the cable.

10 The amping ratio increase 17.9% compare with the amping ratio of the tower without PTTMD. Accoring to esign characteristic perio of groun motion of the tower an the site characteristic in Heilongjiang, the seismic action ecrease about 4%. An the vibration acte by win ecrease about 7% calculate with Chinese coe for loa of builing. Because the experiment was carrie in site, the excite force couln t be very large. When the tower was excite by the force in the cable from 2kN to 4kN, the amping ratio change from.29 to.33. It shows that the greater the structure vibrates, the larger the structural amping ratio is. This suggests that the efficiency of PTTMD becomes greater with the increasing of vibration amplitue of tower. When the tower is acte by a greater win or a stronger earthquake, the effectiveness of the PTTMD to control vibration will be evient. The groun motion with more high frequency vibration ivision almost oesn t bring to motion of the penulous tank with longer vibration perio. This almost still an great mass will give a great inertial force, which acts on the structure in the opposite irection of the structure movement. If the water tank neee by the water system on the tower is put on the floor an not fitte as a amper evice, it will vibrate with the vibration of the tower uner earthquake action an bring a greater aitional earthquake action. It will bring a negative action to the structure. REFERENCES 1. Glanville MJ, Kwok KCS, Denoon RO. Full-scale amping measurement of structures in Australia. Journal of Win Engineering an Inustrial Aeroynamics 9 (1996): Zhuang BZ, Liang YD, Zhang YQ. Structural Stochastic Vibration. Beijing: National Defence Inustry Press, Li GQ, Chen SW, Li J. The ynamic characteristic measurement of Shanghai Jinmao Builing. Journal of Civil Engineering 2; 33(2): 3-39