How to Mitigate Human Induced Vibrations in Reinforced Concrete Structures?

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Human induced vibration may create serious serviceability problems in reinforced concrete structures. There are several modern strategic projects that have suffered from such problem for instance Millennium Bridge in London-UK. The bridge suffered from lateral synchronous excitation. The bridge had to closed and mitigated to eliminate such problem. From this example it becomes quite clear that human induced excitation need to be taken seriously and tackled properly. In this article, several human induced excitation mitigation strategies will be discussed.

Fig.1: London Millennium Bridge suffered from lateral excitation caused by human footsteps

How to Control Human Induced Vibrations in Reinforced Concrete Structures?

Mitigation strategies for Human induced vibration include:

• Passive vibration control methods
• Active vibration control methods
• Semi active vibration control methods (or controlled passive methods)

The above vibration mitigation approaches may be used separately or combined to provide the desired result. These major vibration mitigation systems will be discussed in the following sections:

Passive Vibration Control Methods

Passive methods commonly decline energy dissipation demand in the main structure though eating most of the energy imposed on the structure. Not only do isolation systems provide energy absorption capability but also introduce flexibility in the structure. Consequently, the amount of energy that may transfer to the structure would be declined considerably. Moreover, passive damping supplemental tools eat most of energy that the structure experience. So, such device protects structures from the detrimental effects that inputted energy may cause. Passive damping supplemental devices generate forces against the movement of the structure in which they are attached. The most outstanding advantage of such devices is that their dynamic features would not change with time. Furthermore, they do not need to connect to external source of energy to help them in controlling vibrations. That is why these devices cannot cope with variations in external loads such as variations in excitation frequencies. Finally, various passive damping supplemental devices are available to be used to tackle vibration serviceability problems of reinforced concrete structure due to human induced excitation. For example, friction dampers, viscous damper, viscoelastic treatments, tuned mass damper or vibration absorber, tuned liquid damper, pendulum tuned mass damper, unbonded braces, impact damper, constrained layer damping, and yield plates. Figure 2 and Figure 3 shows the application of passive methods to control the imposed vibration which the structure suffered from.

Fig.2: Stock Bridge Type Damper

Fig.3: Pier damper used in London Millennium footbridge to control lateral excitation (tuned mass damper)

Active Vibration Control Methods

The most obvious characteristic of active vibration control is that it is connected to an external source of energy. This energy is used to manipulate the power of actuators which in turn employ control force to a structure in its vibration mitigation function. Active vibration control strategies can adapt to various loading conditions and able to control different vibration modes in the structure. With regard to the design of active vibration control system, the design methods are considerably variable and the selection of each method is based on several factors for example favored performance objectives, the nature of the active control device, and features of the controlled target. Lastly, there are various active vibration control systems for instance active tendon control system, active mass driver, active gyro stabilizer, active pulse control system, active aerodynamic appendages, and active constrained layer damping. Figure 4 illustrates active vibration control system which initially detects vibration using sensors and then control the vibration properly.

Fig.4: , as it may be observed a sensor would detect the vibration and then the vibration would be suppressed and controlled.

Semi Active Vibration Control Methods (Controlled Passive Methods)

As the name of the technique may suggest, semi active vibration is the combination of active and passive vibration control system. Semi active vibration control devices have stiffness and damping characteristic which can be adjusted in real time but energy cannot be injected into the controlled system. There are several types of semi active vibration control devices for instance electrorheological damper as shown in Figure 5, netorheological damper, semi active tuned liquid damper, semi active pendulum tuned mass damper, and semi active tuned vibration absorber as it may be observed from Figure 6.

Fig.5: Typical arrangement of electrorheological fluid damper

Fig.6: Semi active liquid tuned damper configuration

Also Read: Types of Loads Causing Vibration Serviceability Issues in Buildings

References:

1. Contitech AG, Active Vibration Control, 2017
2. Donald Nyawako and Paul Reynolds, Technologies for Mitigation of Human Induced Vibrations in Civil Engineering Structures. The Shock and Vibration Digest (SAGE), Sheffield, v. 36, n. 6, p. 465-493, November 2007.
3. Syed Ahmed, Research on the Quality Management in London Millennium Bridge Project. [S.l.], p. 6. 2015.
4. Walk London, London Millennium Footbridge, 2009.