Recent Advances of Structural Vibration Control in Mainland China

Jingping OU and Hui LI

ABSTRACT

Structural vibration control has become a workable technology to protect infrastructure against earthquake and wind loads. Serious efforts have been undertaken in past two decades in mainland China and fruit achievements have been made in this area. In this paper, recent advances of vibration control, including experimental and theoretical studies on performance of control devices, controlled structural analysis methods, issues of full-scale implementation and design codes in mainland China, are reviewed. The trend of structural vibration control is finally discussed. Key words: structural control, smart dampers, passive structural control, active structural control 1. INTRODUCTION

The research and application of base isolated buildings were dated back to mid 70’s of last century, two buildings have been constructed by using base isolation technology and the base isolation layer consisted of sand. However, considerable attention was paid on structural control as a new research direction and discipline after the concept of structural control was firstly introduced in mainland China by Wang (Ou, 2003).

Before 1996, a viscous fluid damper, namely hydraulic mass damper, was studied by Li (Ou et al, 2004), and several of metallic and friction dampers were developed by Zhou (Ou et al, 2004). Ou et al (1996) reviewed the state-of-the-art of passive supplemental damping strategies including viscoelastic (VE) dampers, viscous dampers, metal dampers and friction dampers, tuned mass dampers and tuned liquid dampers. Later, Ou and his group developed several passive energy dissipation devices and systematically studied the passive structural control technologies in experimental, theoretical, design and implementation aspects (Ou et al, 2004). In 2001, Building Seismic Design Code published by the agency of Ministry of Construction of China was published, in which design methods for both passively damped buildings and base-isolated buildings were included. The Design Specification of Base Isolated Buildings was also published at the same time. Tan and Qian (1998) studied the behaviour of viscous fluid wall. More than 50 buildings have been implemented with passive energy dissipation for the purpose of retrofitting or strengthening the structures.

Because of their mechanical simplicity, low power requirements, and large, controllable force capacity, semi-active systems provide an attractive alternative to active and hybrid control systems for structural vibration reduction. Semi-active control systems including variable orifice fluid damping devices, variable stiffness systems and MR dampers, were also experimentally and ____________

Jingping OU, School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, P.R. of China Hui LI, School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, P.R. of China

analytically studied. MR dampers have been implemented at cables of two cable-stayed bridges to reduce wind-rain induced vibration of stay cables.

Ou (2003), Ou and Li (2004) proposed a homogeneous design method of all control systems based on linear optimal control theory. Ou (2003) and Ou et al (2004) published two books titled, respectively, “Structural Vibration Control-active, semi-active and smart control systems” and “Structural Vibration Control-passive energy dissipation systems”, which systematically introduced and summarized the fruit achievements made in structural vibration control field throughout the whole world.

In this paper, the achievements of structural vibration control made in the past two decades in mainland China are introduced, especially made in Harbin Institute of Technology (HIT). 2. BASE ISOLATED BUILDINGS AND BRIDGES

A seismic base isolation system is typically placed at the foundation of a structure. By means of its flexibility and energy absorption capability, the isolation system partially reflects and partially absorbs some of the earthquake input energy before this energy transmitted to the structure, and thus resulting in a reduction of energy dissipation demand on the structural system and an increase in its survivability.

Many researches on base isolated buildings were carried out in the 80’s-90’s. The structural analytical approaches of base-isolated buildings, the behavior of isolators and full implementations have been developed.

A lot of base isolated buildings were built and the statistic column diagram was shown in Fig.1. Fig. 2 shows an example of full implementation of a hybrid base isolated building (Liu et al, 2002). Besides seismic design code and design specification for base isolated buildings published by the agency of Ministry of Construction in 2001, the design codes for base isolated highway bridges and base isolated railway bridges were also, respectively, published by the agencies of Ministry of Transportation and Ministry of Railway.

Figure 1. Statistic diagram of base isolated buildings

Figure 2. Suqian City Gymnasium with base isolation

Frame

Masonry

F+M

45 40 35 30 25 20 15 10 5 0

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Before1994 1995 1996 1997 1998 1999 2000 year

More than 500 full-scale implementations of base isolated buildings and bridges have been accomplished to alleviate their earthquake and traffic-induced response so far.

With the construction of subway facilities, the demand of the isolated buildings over subway facilities increases. 3. PASSIVE ENERGY DISSIPATION SYSTEMS

The base isolation technology cannot be used to reduce the response of tall buildings, structures located in soft sites and the occasion of structures subjected to wind load. The passive energy dissipation technologies, i.e. passive energy dissipation devices incorporated into a structure, is to absorb or consume a portion of the input energy, thereby reducing energy dissipation demand on primary structural members and minimizing possible structural damage when the structure subjected to earthquake as well as wind load.

A lot of research on passively damped structures has been conducted by many Chinese scientists and engineers. Ou et al (1996) comprehensively summarized the achievements of passively damped structures made throughout the whole world. During the ensuing years, Ou and his research group developed several passive energy dissipating devices, and systematically studied the behavior of passive energy dissipation devices, structural analysis approaches, design consideration and implementation issues of passively damped buildings and offshore platforms (Ou et al, 2004). The studies in details are omitted here and only following main conclusions and remarks are given: 1) Both metallic dampers and friction dampers are only adopted for the purpose of against earthquake hazards in consideration of fatigue degeneration of metallic dampers and performance deterioration of friction dampers; 2) The Young’s modulus of VE materials made in China is large, however, the loss factor is low, thus the energy dissipation capacity is not enough to effectively suppress vibration of passively damped buildings; 3) Viscous fluid dampers are most effective against wind induced motions as well as those due to earthquakes. The behavior models of the four kinds of passive dampers were respectively obtained by experimental-based modeling along with mechanics-based modeling approaches.

The relationship between the model parameters, and the configuration and material properties of passive dampers was experimentally, theoretically and numerically established and the results were then used to develop the design criteria and methodology for the dampers.

The response time history of the passively damped structures can be obtained by resolving equations of motion using numerical methods. However, it is a little bit too complicated for all engineers to calculate the response time history of passively damped structures. The proportional damping of the structures with added passive energy dissipation devices is assumed. The equations of motion for the modal coordinates are decoupled in this case, i.e. neglecting the non-diagonal elements of the modal damping matrix. Further, the earthquake response spectrum with large damping ratio can be used to evaluate the response and examine the improved seismic performance of the controlled structures. The simplified design method has been adopted in the Building Seismic Design Code of China. The Specification for Passively Controlled Structures is to be published in the end of this year. Additional, He (2001) proposed the performance-based design approach for passively damped buildings and compared the damage index of the passively damped structures with that of uncontrolled buildings showing the high performance of the damped structures.

134 passive friction dampers with 100kN to 200kN sliding force capacity have been implemented in Shenyang Government Office Buildings in 1997 to retrofit this building, as shown in Fig. 3 (Wu and Li, 1998). The pseudo-dynamic tests of a 1/3 scale model of the building with and without friction dampers were carried out to examine the seismic capacity of the retrofitted buildings. Some of critical existing buildings located around Beijing had been retrofitted by using passive energy dissipation technology since 1998, such as Beijing Hotel, as shown in Fig. 4 (Wang et al, 2001), Beijing Railway Station (Ou et al, 2004), and so on. Beijing Hotel is an 8-story RC frame structure, and 7 JARRET dampers along X direction and 8 JARRET dampers along Y direction were installed at each story. The performance of the dampers incorporated into Beijing Hotel was tested in HIT and the damping force versus the displacement of a damper is also shown in Fig. 4. 14 metallic dampers developed by Wu and Ou (1997) have been attached into two structures (Ou et al, 2004), which were located over the fault and the acceleration magnitude of the input earthquake motions is larger than 400gal. More than 50 buildings have been constructed or retrofitted by implementing passive energy dissipation devices so far.

Figure 3. Shenyang Government Office Building retrofitted by friction dampers

Figure 4. Beijing Hotel retrofitted by viscous dampers

4. ACTIVE STRUCTURAL CONTROL

The active structural control was firstly experimentally studied in 1996 by Soog (Ou, 2003). A 1/4

scaled five story steel frame attached with an active mass damper was employed to perform the active control test. The practice of active structural control is still in doubt, therefore, resulting in the slow shift to application from experimental and theoretical study stage.

Active mass dampers were implemented on Nanjing TV tower to reduce wind-induced vibration with effort of international cooperation between China and USA (Cao, et al, 1998), as shown in Fig.5. Ou et al (2002) developed an innovative active mass damper with magnetic driver instead of the

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hydraulic servo-system of conventional AMD, as shown in Fig.6. Recently, the performance of this system is being investigated.

Figure 5. Nanjing TV Tower with AMDs Figure 6. AMD with magnetic driver 5. SEMI-ACTIVE AND SMART DAMPING CONTROL 5.1 Semi-active control

Semi-active control strategies are particularly promising in addressing many of the challenges to

this technology, offering the reliability of passive devices, yet maintaining the versatility and adaptability of fully active systems, without requiring the associated large power sources and can operate on battery power. Studies have shown that appropriately implemented semi-active damping systems perform significantly better than passive devices and have the potential to achieve, or even surpass, the performance of fully active systems. Semi-active control systems including variable-orifice fluid dampers, controllable friction devices, variable-stiffness devices, and controllable fluid dampers are reviewed in this paper.

Li and Liu (Ou, 2003) developed an AVS system, which has similar configuration and working mechanism with that proposed by Kobori et al (1993). Experiments were carried out to investigate the behavior of active variable stiffness system. Both of the structural analysis and a small-scaled model shaking table test indicated that variable stiffness system not only changed frequencies of the controlled structure, but also increased damping ratio.

Sun (Ou, 2003) developed a hydraulic actuator with a controllable orifice. Later, Li et al (2002) experimentally investigated the electromechanical behavior, which is dependent on exciting frequency and applied voltage. The benefits of this semi-active damper was proved through a shaking table test of a 1:4 scaled five-story steel frame attached this damper at the first story.

5.2 Smart damping control

The fabrication technology of MR fluids and the behavior of viscousity, sedimentation, rheology, magnetization and yield strength of MR fluids were tested (Ou, 2003). Ou (2003) developed and tested series of MR dampers with force capacity of 5-400kN, as shown in Fig.7, which are suitable for

full-scale applications in civil engineering. Full-scale implementation issues of MR dampers have been accomplished in two bridges. Ko et al (2003) implemented 256 MR dampers (made in Lord Corporation, USA) with 2.26kN capacity at the cables of Dongting Lake Cable-stayed Bridge of China to suppress the wind-rain induced dramatic vibration, as shown in Fig. 8(a). Ou (2003) also implemented 40 MR dampers with 8kN capacity at the cables of Shandong Binzhou Yellow River Highway Bridge, as shown in Fig. 8(b). The MR dampers were manufactured by Harbin Taider Ltd, China and the semi-active control algorithm was employed to determine control force.

Figure 7. Behavior of MR dampers

(a) Dongting Lake Bridge (b) Binzhou Yellow River Highway Bridge

Figure 8. Cables attached with MR dampers

The variable friction damper with a PZT actuator was proposed by Ou (2003), respectively, in mainland China. The mechanism to adjust sliding force is that the nominal pressure force can be adjusted through applying voltage to PZT layer due to its reverse piezo-electric property. The behavior of PZT variable friction damper was experimentally studied, shown in Fig. 9.

Figure 9. PZT variable friction damper Shape memory alloys (SMAs) are multi-functional materials due to they have self-sensing,

self-actuating and energy dissipation properties. Many researches focus on developing SMA-based

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isolators with self-centering performance. Recently, Li et al (2003) developed two self-sensing SMA dampers, as shown in Fig. 10. The self-sensing SMA dampers have following characteristics: 1) The SMA wires are always elongated during vibration no matter direction of the controlled structure move in; 2) The SMA dampers can self-sense deformation itself, and thus resulting in a potential way to quantificationally assess the safety and damage of the controlled structures post-earthquake hazards; 3) One of SMA dampers can dissipate much more energy because the configuration of the SMA damper with amplifying function deform the SMA wires in the damper larger than the corresponding drift of the controlled structure.

The self-sensing properties as well as energy dissipation capacity of SMA damper were tested and the results were shown in Fig. 11. A shaking table test of a 1:4 scaled model attached with SMA dampers at the first story was carried out to examine effectiveness in response reduction by SMA dampers.

Figure 10. SMA damper and its mechanism

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(a) Energy dissipation capacity (b) Self-sensing property

Figure 11. Self-sensing property and energy dissipation capacity of the SMA damper

6. CONTROLLED OFFSHORE PLATFORMS Offshore platforms located in Bohai oil field exploitation of China suffer from ice attack in

service, which causes human uncomfortable feeling and accumulative fatigue damage. In contrast with buildings, the space available to install dampers is very limited for offshore platform. Ou et al (2004) firstly proposed the damped components, i.e. a viscous fluid damper or VE damper was incorporated into the element of jacket. However, the experimental results of a 1:10 scaled offshore platform model incorporated with the damped components indicated the deformation of the

components is too small to efficiently utilize their capacity, resulting in poor control effectiveness. Ou (2003) further proposed the smart isolation layer that is essentially concentrated energy dissipation technology. The deck was separated from jacket and the smart isolation layer was installed between the deck and jacket, as shown in Fig. 12. The smart isolation layer consists of conventional bear isolators and actuators, the former can be rubber type of isolators or self-centering shape memory alloy-based isolators, the later can be passive viscous fluid dampers, MR dampers or active actuators. The experiment on a 1:10 scale offshore platform model with and without the smart layer was carried out, and the response of a full-scale offshore platform subjected to ice and earthquake ground motions with and without various smart isolation layers was numerically simulated, as shown in Fig.13. The results showed that the smart isolation layer can achieve large reduction of response. Actively controlled platform attached active mass damper on the deck, as shown in Fig. 14 was also tested and the fuzzy logic control algorithm was employed to determine the active control force in the test.

Figure 12. Smart isolated offshore platform

Figure 13 Responses of the platform with different isolated layers

Figure 14. Platform with active mass damper

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7. UNIFIED DESIGN APPROACHES FOR ALL CONTROL SYSTEMS

Simplified design approaches of passive energy devices have been developed by researchers around the world. However, the design methods of semi-active dampers and smart dampers are very inefficient at present stage.

Although the benefits of semi-active dampers and smart dampers have been demonstrated by many tests and numerical studies, the mechanism of semi-active dampers and smart dampers achieving the almost same performance as the full active control systems is not well understood yet.

Figure 15. Proportion of control force in time domain Ou (2003), and Ou and Li (2004) analyzed the characteristics of active optimal control force and

revealed that the proportion of damping force to elastic restoring force in entire active control force is very large, as shown in Fig. 15. This conclusion means that active control force behaves essentially as a damping force and dissipates energy to reduce controlled structural response. Analysis of the frequencies and damping ratio of actively controlled structures also supported the above point, i.e. the frequency of the actively controlled structure changed very little, however, damping ratio of the controlled structure increased dramatically, in comparison with the uncontrolled structure. Ou (2003) defined following index to quantify the damping force characteristic of active control force:

)()sgn()( tuuytu iiiyi −= (1a)

∫∫ −

= Ti

Tyiii

udttu

dttuuyHyi

0

0

)(

)()(γ (1b)

⎩⎨⎧

<≥−

=0101

)sgn(xx

x (1c)

⎩⎨⎧

<≥

=0001

)(xx

xH (1d)

where uj(t) and )(tyi are the control force and velocity between the ith inter-story. An unit value of

yiuγ means that the active control force behaves exactly as a damping force, in contrast, zeroyiuγ means

that the active control force does not behave as a damping force at all during the entire excitation. The semi-active devices are essentially passive dampers with tunable parameters, and thus they

can only provide the damping force being opposite to the velocity in the direction. Because of the

damping characteristic of active control force, the semi-active damper can achieve the almost same performance as the full active control system.

Based on the above knowledge, a design approach based on LQR control algorithm for all control devices including passive energy dissipation devices, semi-active control devices, smart damping devices and active control devices was proposed by Ou and Li (Ou,2003; Ou and Li, 2004). Assume the semi-active control system achieves the same performance of the active control system such that the maximum semi-active control force is equal to the maximum active control force, i.e. the response of the semi-active control system is the same as the active control system, the two equivalent principles can be described

maxmax iis uu = (2a)

maxmaxmaxmax , iisiis yyyy == (2b)

where uimax, yimax and maxiy (i=1,2,…,p）are, respectively, the maximum active control force of the ith actuator, and the corresponding drift and velocity of the story attached with the ith actuator; correspondingly, uismax, yismax and maxisy are, respectively, the maximum semi-active control force of the ith semi-active damper, and corresponding drift and velocity.

The semi-active control force of variable damping control system is described by

)()()( tytctf isidid = (3)

where cid is the variable viscous damping coefficient and can be adjusted over the range of [cidmin, cidmax]; fid(t) is the damping force provided by the variable damping devices and fid(t)=-usi.

The maximum damping coefficient cidmax can be determined according to equation (2)

maxmaxmaxmax maxmax iuiiduisidis uycycuiis

=== (4a)

max/maxmax iuiiid yuc = (4b)

wheremaxisuisy and

maxiuiy (i=1,2,…,p) are the corresponding velocities of controlled structure

with semi-active control systems and active control systems, respectively, at the time of maximum semi-active control force and maximum active control force.

Suppose that maximum damping coefficient is s times of the minimum damping coefficient, the minimum damping coefficient is

scc idid /maxmin = (5)

The feasibility of the design approaches is proved through two benchmark problems. Here, only one of them is introduced, which is wind-resistant 76-story RC building benchmark control problem taken from the website of www-ce.engr.ccny.cuny.edu/people/faculty/agrawal/benchmark.html.

Numerical calculations are carried out to investigate the feasibility of the proposed design method. The entire active control force, the damping force and elastic restoring force in time domain under wind load are shown in Fig. 15. The active control force versus the corresponding drift is shown in Fig.16.

The results in Fig. 15 indicated that the active control force mainly behaves as a damping force and the elastic restoring force is very little. The approximately elliptical curve in Fig.16 is observed that is similar with the behavior of a passive linear viscous damper.

The value of yiuγ is about 0.99 that means ninety nine percent of the active control force behaves

as a damping force in this numerical study. 20 variable orifice dampers are designed by using Eqs.(3) and (4). The response and the

semi-active control force are also shown in Fig. 16. The results in Fig.16 indicated that the designed semi-active control forces can trace well the active control forces and achieve the almost same performance.

Figure 16. Control forces versus the corresponding drift

5. CONCLUSIONS

Structural control technologies offer many new ways to protect structures from natural and other types of hazards. Semi-active control systems exhibit bright prospects in civil engineering because of their mechanical simplicity, low power requirements, and high force capacity. Studies on control theories for nonlinear systems are still very limited so far. The performance-based design method of controlled structures is an ongoing topic. The development of prototype design standards or specifications for active, semi-active and smart control systems should be paid much more attention in the near future.

ACKNOWLEDGEMENTS

This research is financially supported by the National Natural Science Foundation of China under the grant number 50238040 and the National Hi-Tech Research and Development of China under the grant numbers 2001AA602023, 2002AA3131110, 2002AA335010.

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