4.3.1 Active Control of Structures
Th e concept of adaptive behavior has been an underlying theme of active control of structures, which are subjected to earthquake and other environmental type of loads. Th e structure adapts its dynamic characteristics to meet the performance objectives at any instant. A futuristic smart bridge system (an artist’s rendi- tion) is shown in Figure 4.1 [5].
A thermomechanical approach to develop a constitutive rela- tion for bending of a composite beam with continuous SMA fi bers embedded eccentric to neutral axis was used by Sun and Sun [6]. Th e authors concluded that SMAs can be successfully used for the active structural vibration control. Th ompson et al.
[7] also conducted an analytical investigation on the use of SMA wires to dampen the dynamic response of a cantilever beam constrained by SMA wires.
To date, active structural control has been successfully applied to over 20 commercial buildings and more than 10 bridges. One example of active control is the Kurusima bridge in Shikoku area
in Japan. Th e bridge was designed with the application of active vibration control as integrated structural components. Several modes of the bridge tower, which were anticipated to be excited by wind vortex, were carefully protected by appropriate controllers during the construction phase. It therefore made it possible for the tower of this bridge to be built much lighter and more slender than one following traditional design. Active tuned mass dampers have been installed in the 11-story build- ing, the Kyobashi Seiwa building in Tokyo (the fi rst full-scale implementation of active control technology) and the Nanjing Communication Tower in Nanjing, China.
Th ere are two serious challenges that remain before active control can gain general acceptance by engineering and construction professionals at large. Th ey are (1) Reduction of capital cost and maintenance (2) Increasing system reliability and robustnessActive control systems consist a set of sensors, a controller, an active control system (actuators), and an external power supply. A schematic sketch of active control system is shown in Figure 4.2.
Nowadays, much work on structural control is focused on intelligent structures, developments of actuating materials and piezoceramics. Due to its limited frequency bandwidth, SMA has traditionally been used for passive strategies such as damp- ers or other types of energy dissipation devices instead of being used in active control strategies as actuators. On the other hand, it is well known that a SMA actuator is capable of producing relatively large control forces despite its slow response time. Th is unique characteristic of SMA makes it very attractive for civil
engineering control applications where large forces and low- frequency band width are mostly encountered. Th ey conducted experimental studies on active control of a fi ve-story building model with SMA actuators [8]. It was proven that despite its slow response, it is feasible to use SMA for active control of civil engi- neering structures. But while selecting the alloy type, utmost care should be taken in specifying the temperature range or the transition temperature or the process in alloy-making so as to suit our requirements. Dynamics of SMA should be considered in control design.
4.3.2 Passive Control of Structures
Two families of passive seismic control devices exploiting the peculiar properties of SMA kernel components have been Actuators
Feedback control
Pier sensors Wireless
sensors
Sensors for intelligent vehicles
Composite pipes for de-icing bridge deck (with geothermal energy)
Structural controls Stream
sensors
Optical fiber sensors
Advanced composite materials Protective coating
Data acquistion (from sensors) and processing Multidirectional
carbon or other fibers
Epoxy binders (matrices)
Cable with sensors Dampers
FIGURE 4.1 Futuristic smart bridge system (an artist’s rendition). (Courtesy: USA Today, 3 March 1997.) J. Holnicki-szulc and J. Rodellar (eds.), Smart Structures, Requirements and potential applications in mechanical and civil engineering, NATO Science Series 3. High Technology, Vol. 65, Springer 1999.
Active control
Structure Response Excitation
Controller
Sensors Power
Sensors
FIGURE 4.2 Schematic sketch of active control system.
implemented and tested within the Memory Alloys for New Seismic Isolation and Energy Dissipation Devices (MANSIDE) project. Th ey are special braces for framed structures and isola- tion devices for buildings and bridges.
4.3.3 Hybrid Control
Th e term hybrid control generally refers to a combined passive and active control system. Since a portion of the control objec- tive is accomplished by the passive system, less active control eff ort, implying less power resource, is required. Similar control resource savings can be achieved using the semiactive control scheme where the control actuators do not add mechan- ical energy directly to the structure, hence bounded-input/
bounded-output stability is guaranteed. Semiactive control devices are oft en viewed as controllable passive devices. A side benefi t of hybrid and semiactive control systems is that, in the case of a power failure, the passive components of the control still off er some degree of protection unlike a fully active con- trol system. Th e materials described above can be used to make the hybrid control scheme workable. In the case of ER/MR dampers, hybrid control scheme is a viable option for realistic structural control.
4.3.4 Smart Material Tag
Th ese smart material tag can be used in composite structures.
Th ese tags can be monitored externally throughout the life of the structure to relate the internal material condition. Such mea- surements as stress, moisture, voids, cracks, and discontinuities may be interpreted via a remote sensor [6].
4.3.5 Retrofi tting
SMAs can used as self-stressing fi bers and thus they can be applied for retrofi tting. Self-stressing fi bers are the ones in which reinforcement is placed into the composite in a nonstressed state.
A prestressing force is introduced into the system without the use of large mechanical actuators by providing SMAs. Th ese materials do not need specialized electric equipments nor do they create safety problems in the fi eld. Treatment can be applied at any time aft er hardening of the matrix instead of during its curing and hardening. Long- or short-term prestressing is intro- duced by triggering the change in SMA shape using temperature or electricity. Th ey make active lateral confi nement of beams and columns a more practical solution. Self-stressing jackets can be manufactured for rehabilitation of existing infrastructure or for new construction.
4.3.5.1 Restoration of Cultural Heritage Structures Using SMA Devices
An innovative technique using superelastic SMA devices for the restoration of a cultural heritage structure especially masonry buildings were implemented under the framework of the European Commission-funded ISTECH Project. Masonry buildings are
largely vulnerable to earthquakes because of their low resistance and ductility during earthquake ground motion. To enhance the seismic behavior of cultural heritage structures, the most common method traditionally used has been the introduction of localized reinforcements. Usually steel bars or cables served this purpose by increasing stability and ductility. But in many cases, these reinforcement techniques prove inadequate to prevent collapse.
Th e development of the connection technique was based on the idea of using the unique properties of Ni–Ti alloys espe- cially its super elasticity and high resistance to corrosion [9].
Th e idea was to connect the external walls to the fl oors, the per- pendicular walls or the roof with an SMA Device that should behave as follows:
1. Under low-intensity horizontal actions (wind, small intensity earthquakes), the device remains stiff , as tradi- tional steel connections do, not allowing signifi cant displacements.
2. Under higher intensity horizontal actions (i.e., strong earthquakes), the stiff ness of the device decreases, allow- ing “controlled displacements,” which should reduce amplifi cation of accelerations (as compared to stiff con- nections) and permit the masonry to dissipate part of the transmitted energy, mainly owing to elasticity exploitation and microcracks formation in the brick walls; consequently, the structure should be able to sustain a high-intensity earthquake without collapse, though undergoing some minor damage.
3. Under extraordinary horizontal actions, the stiff ness of the device increases and thus prevents instability.
4.3.5.2 SMA for Seismic Retrofi t of Bridges
Unseating of supports was the major cause of bridge failures during earthquakes. Retrofi t measures to reduce the likelihood of collapse due to unseating at the supports have been in place for many years. Th e damage to bridges in the recent Chi-Chi, Kobe, and Northridge earthquakes indicate the need to provide better methods of reducing the damaging eff ects of earthquakes in bridges. Th e use of restrainer cables and restrainer bars to limit the relative hinge displacement became popular in the United States following the collapse of several bridges due to loss of support during the 1971 San Fernando earthquake. Recent earthquakes have demonstrated that restrainers were eff ective in some cases. However, many bridges with restrainers sustained serious damage or collapse. Bridges that had been retrofi tted with restrainer cables failed in both the 1989 Loma Prieta and 1994 Northridge earthquakes. Failure of Japanese restraining devices also occurred during the 1995 Kobe earthquake.
Experimental tests of restrainer cables have shown that failure occurs in the connection elements or the through-punching shear in the concrete diaphragm. In addition, restrainers do not dissipate any signifi cant amount of energy, since they are gener- ally designed to remain elastic. Analytical studies of bridge and restrainer systems have demonstrated that a very large number
of restrainers are oft en required to limit joint movement to acceptable levels, particularly for high seismic loads. In those cases, the excessive number of restrainers would induce large forces in other components of the bridge, such as bearings and columns. Th e shortcomings of traditional restrainers can poten- tially be addressed with the use of SMA restrainers. Th e SMA restrainers, in the superelastic phase, act as both restrainers and dampers (Figure 4.3).
Energy dissipation and base isolation are found to be optimal candidates for structural control of structures. As far as bracing systems are concerned, until now all the applications and the research studies on this technique were focused on the energy dissipation capability. For seismic retrofi tting purposes, the supplemental recentering devices (SRCD) are found to be useful as they provide forces to recover the undeformed shape of the structure at the end of the action. In existing structures, in fact, particularly when they were designed without any seismic pro- vision, the energy dissipation can turn out to be insuffi cient to limit damage to structural elements. It would then be necessary to strengthen some elements to fully achieve the design objec- tives. Local strengthening would imply expensive work, also involving nonstructural parts. Retrofi tting could turn out to be economically in convenient, and, yet some residual displace- ment could occur in case elastoplastic devices are used. An alternative strategy can be pursued by using SMA devices having supplemental force to recover the undeformed struc- tural confi guration, resulting in the elimination of any residual displacement, while accepting yielding in structural elements.
A comparison of properties of Ni–Ti with steel is given in the Table 4.1.
4.3.6 Self-Healing
Experimentally proved self-healing behavior [10], which can be applied at microlevel of a material, widens their spectrum of use.
Here signifi cant deformation beyond the fi rst crack can be fully recovered and cracks can be fully closed.
4.3.7 Self-Stressing for Active Control
Self-stressing for active control can be used with cementitious fi ber composites with some prestress, which impart self-stressing
TABLE 4.1 Comparison of Properties of Ni–Ti SMA with Typical Structural Steel
Property Ni–Ti SMA Steel
Recoverable elongation (%) 8 2
Modulus of elasticity (MPa) 8.7 × 104 (A) 2.07 × 105 1.4 × 104 (M)
Yield strength (MPa) 200–700 (A) 248–517 70–140 (M)
Ultimate tensile strength (MPa) 900 (f.a.) 448–827 2000 (w.h.)
Elongation at failure (%) 25–50 (f.a.) 20 5–10 (w.h.)
Corrosion performance Excellent Fair Note: f.a. denotes fully annealed and w.h. denotes work hard- ened, which are two types of treatment given to the alloy. A and M denote the two phases of the alloy namely, austenite and martensite.
(b)
SMA restrainer bar/damper
Real system
Stay cable
SMA damping device (a)
FIGURE 4.3 SMA restrainer bar used in multispan simply supported bridge abutments and intermediate piers confi guration.
thus avoiding diffi culties due to the provision of large actuators in active control, which require continuous maintenance of mechanical parts and rapid movement, which in turn created additional inertia forces.
In addition to SMAs, some other materials such as polymers can also be temporarily frozen in a prestrained state that have a potential to be used for manufacturing of self-stressing cementi- tious composites [1].
4.3.8 Structural Health Monitoring
Use of piezotransducers, surface bonded to the structure or embedded in the walls of the structure can be used for structural health monitoring and local damage detection. Problems of vibration and UPV testing can be avoided here. Jones et al. [11]
applied neural networks to fi nd the magnitude and location of an impact on isotropic plates and experimented using an array of piezotransducers surface bonded to the plate. Figure 4.4 shows a typical health monitoring setup making use of optic fi bers.
4.3.9 Active Railway Track Support
Active control system for sleepers is adopted [5] to achieve speed improvements on existing bridges and to maintain the track in a straight and nondeformed confi guration as the train passes. With the help of optimal control methodology, the train will pass the bridge with reduced track defl ections and vibrations and thus velocity could be safely increased. Figure 4.5 shows various posi- tions of the train with and without active railway track support.
4.3.10 Active Structural Control against Wind Aerodynamic control devices to mitigate the bidirectional wind induced vibrations in tall buildings are energy effi cient, since the energy in the fl ow is used to produce the desired control forces. Aerodynamic fl ap system (AFS) is an active system driven by a feedback control algorithm based on information obtained from the vibration sensors [5]. Th e area of fl aps and angular amplitude of rotation are the principal design parameters.
Cracks Vibration meter
Damage detection of piles using optic fibers Optic fibers
Seismometer Vibration meter
Optic fibers
Optic fiber
Hitting
Damage detection after damage occured FIGURE 4.4 Health monitoring setup.