Civil Engineering Applications of Vibration Control Structural Control

Civil Engineering Applications of Vibration Control (Structural Control) Naresh K. Chandiramani, Associate Professor Room 141, Dept. of Civil Engineering

Structural Control • Vibration control of civil structures is more recent as compared to machines & aerospace vehicles. • Earthquakes and wind loads - main sources of structural vibrations. • Control vibrations by: changing rigidity, mass, damping, shape, or applying passive or active control forces. • > 20 full scale active control appl. in Japan • Passive base isolation used in USA. • Retrofitting reqd. if new seismic activity detected • High strength may result in high acceleration levels, so increasing strength alone wont always work.

Structural control versus Mechanical & Aerospace control • Environmental disturbances (wind, earthquake excitations) occur over wide range of frequency and amplitudes, i. e. , they are uncertain, whereas mechanical loads are usually deterministic. • Civil structures (without control) are stable and may get destabilized with active control, whereas aerospace structures require active control for stabilization. • Performance specifications for civil structures are coarse (e. g. , peak amplitude, time for motion to settle down).

Mathematical model of structure. Fig. 1: (a) Mathematical model, (b) Schematic of a building • In a simplified model, the masses correspond to slab masses and stiffnesses correspond to column stiffnesses (i. e, the force required per unit lateral displacement of column)

Passive control: Base isolation Fig. 2: (a) Schematic of base isolated building, (b) Model, (c) Rubber bearing

Passive control: Base isolation • Structure mounted on a suitably flexible base such that the high frequency component of ground motion is filtered out and the fundamental vibration period is lengthened. This results in deformation in the isolation system only, thus keeping the structure above almost rigid. However, if the earthquake excitation contains a major component of this fundamental period, there will be large sidesway (albeit almost rigid) motions. • San Fransisco city hall (retrofitted, 530 rubber bearings), International terminal at SF airport (267 Friction pendulum sliding bearings). • Not suitable for tall slender buildings (subject to high wind loads). For these auxiliary dampers (viscous, viscoleastic) are deployed (eg. WTC).

Passive control: Tuned Mass Damper (vibration absorber) Fig. 3: (a) TMD schematic, (b) Response

Passive control: Tuned Mass Damper (vibration absorber) • TMD, usually having mass about 1% that of structure, fitted to top of building. It is tuned to reduce vibration for given frequency range. • Absorber mass takes up vibratory energy, leaving the main mass (building) almost static. • Not very useful for earthquake excitations which occur over wide frequency range. • Main system properties (stiffness-k 1, mass-m 1) known, absorber system properties (stiffness-k 2, mass-m 2) to be designed such that absorber frequency equals excitation frequency (w 2=w). • Examples: John Hancock Tower (Boston), Citicorp Building (New York).

Passive control: Untuned viscous absorber Fig. 4: (a) Model of untuned viscous absorber, (b) Response

Types of passive control devices • Metallic yield damper: relies on the principle that the metallic device deforms plastically, thus dissipating vibratory energy. Used in earthquake applications. • Friction devices: here friction between sliding faces is used to dissipate energy. When used in base isolation systems, the friction coefficient has conflicting requirements. It should not be too large otherwise shear forces from ground during a strong earthquake will transmit to the structure. Also it should not be too small or the entire structure will move due to small/medium wind/earthquake loads. These devices can also be fitted between two storeys to damp their relative motion. Used in earthquake applications.

Types of passive control devices • Viscous/ Viscoelastic devices: Example is fluid in a cylinder with piston having an orifice. These can also be semi-active (eg. , variable orifice, variable viscosity). Used in earthquake and wind applications. • Tuned mass dampers: problems are size of the mass to be used and its displacement relative to the structure, in order that damping is effective. • Liquid sloshing dampers, Impact dampers.

Classification of Control Methods Active/Feedback control: • External source of power drives actuators (i. e. , provides input voltage). • Voltages required are computed by controller using certain algorithms with inputs from sensors. • Sensors measure motion (strains, displ, vel, accl. ) • Actuators apply forces to structure, thereby adding or dissipating energy. • Examples of sensors are acceleromters, strain gauges. • Examples of actuators are tendons, solenoids, piezoelectric stacks, active mass dampers (AMD). • Destabilization possible. • External power may not be available during earthquake.

Classification of Control Methods Passive control: • No external power required. • Passive control device (TMD, Base Isolator) imparts forces that are developed directly as a result of motion of structure (i. e. , no actuator involved). • Total energy (structure + passive device) cannot increase, hence inherently stable. • Relatively inexpensive. • Reliable during earthquake • Not as effective as active, hybrid, semi-active control.

Classification of Control Methods Hybrid control: • Uses active & passive devices. • Advantages of both active and passive systems are present and their limitations are reduced. • Essentially an active control system • Examples: viscous damping with AMD, base isolation with actuators, TMD+AMD).

Classification of Control Methods Semi-active control: • Uses devices where input power requirements are orders of magnitude less than fully active devices. In fact in some cases battery power is sufficient. • These devices usually don’t add energy to the system, hence stability ensured. • These devices can be viewed as controllable passive devices (eg. , Magneto-Rheological Fluid damper where voltage input applied to change viscosity depending on motion measured by sensors, variable orifice damper, controllable friction devices, variable stiffness devices).

Active Control • The goal is to design a control system to keep stresses/strains/displ. /accel. (called outputs) at certain locations below specified bounds (peak, rms) when disturbances (wind, earthquake) below specified bound are applied. • Designer decides choice of outputs based on comfort (e. g. accelerations) and safety (e. g. stresses).

Active Control Fig. 5: Schematic of an active control system

Active control Fig. 6: Implementation of control

Active control The above provides a simple comparison between active and passive control. In passive control, the additional stiffness is chosen and fixed, i. e. , like a re-designed structure. In passive control the actuator applies a force to the original structure, the force being proportional to the displacement measured by sensor (which is proportional to the sensor’s output voltage). In active control the main task of design is determining the proportionality constant

Active Control Outline of Design Process • Develop mathematical model of the structure and the chosen sensors and actuators. • Adopt a mathematical model for the disturbances (i. e. , wind, earthquake load). • Decide performance specifications (eg. , peak accleration, time to settle down after disturbance applied, etc). • Choose type of control algorithm (i. e. , how to obtain the ‘proportionality constant’ and hence actuation voltages/forces from sensor voltages). Then design controller so that performance specs are met. Examples of control algorithms are proportional, integral, derivative, PID, optimal control, robust control, etc.

Active control with TMD Fig. 7: Schematic of AMD applied to building

Active control with TMD Fig. 8: AMD on Kyobashi Seiwa building

Active control • First full scale application of active control to a building was done on Kyobashi Seiwa building (Japan) in 1989 (Fig. 7, 8). Two AMD’s were used. Primary one weighs 4 t and damps transverse motion. Secondary one weighs 1 t and damps torsional motion. • Can also use Magnetorheological fluid dampers (semi-active), active tendons, etc. (Fig. 9, 10, 11)

Semi-Active control with MRD Fig. 9: Control using MR dampers (a) two dampers (b) single damper

Actuators – MR Damper Fig. 10: Magnetorheological damper

Active control with Tendons Fig. 11: Active tendons used in control

Active Control with Tendons