**6. Attenuation of adverse aerodynamic phenomena**

To begin with, it should be noted that the aerodynamic study of a deck can only be initiated after the overall geometry and main dimensions of its section have been established. Thus, at that time, considerable effort has already been put in the bridge structural design. Therefore, to avoid delays and costs, the corrective modifications to be proposed in order to diminish the possibility of wind exciting a vibration mode, as inferred from wind tunnel tests, should not lead to considerable reformulations of the initial design of the bridge. As has already been mentioned, small geometrical modifications to the bridge can lead to substantial changes of the airflow pattern around the structure and thereby modify its aerodynamic behaviour. This effect can be exploited to overcome any undesirable aspect of the bridge's aerodynamic response. Accordingly, modifications to bridge decks often consist in the addition of aerodynamic appendages, judiciously shaped, dimensioned, and positioned, or in minor changes to the original geometry.

and its construction and maintenance costs. If despite this effort, it is not possible to guarantee

Wind Action Phenomena Associated with Large-Span Bridges

http://dx.doi.org/10.5772/intechopen.73061

101

One such countermeasure is the installation of tuned mass dampers (TMD), which are a secondary vibration system designed to have the same natural frequency of the main vibration system that it intends to cancel (i.e., the bridge). When the bridge oscillates, the secondary system acts as a stabiliser because of the action of its damping force on the main vibration system. However, the installation of TMD requires the existence of considerable free space in the bridge

Over the past decades, various studies demonstrated that aerodynamic appendages, like double-flaps, flaps, or skirts, are effective countermeasures against VIV. These attachments inhibit the formation of the vortices responsible for VIV by directing high-speed flow to sweep the relevant deck's surfaces; moreover, the appendages themselves generate turbulence close to

Other countermeasures have been proposed, like guide vanes, which have been employed for example on the Great Belt Suspension Bridge, as already mentioned in the Introduction.

Since the failure of the Original Tacoma Narrows Bridge, mainly two types of deck girders have been promoted: the streamlined box-girder and the truss stiffened girder. Wind tunnel tests came to reveal that the use of either one was not an absolute assurance of absence of flut-

For the truss stiffened girder deck, one countermeasure generally used is the installation of gratings on the girder, which reduces the pressure difference between the lower and the upper surfaces of the deck and thus settles flutter. This solution was adopted in the suspension bridges over the Akashi Strait in Japan and over the Tagus River in Portugal, with central

Flutter instabilities in plate girder decks can also be controlled with gratings. The influence of

In regards to streamlined box-girders, studies have shown that the aerodynamic stability can be improved by allowing for a central slot in the box section. This increases the critical flutter

Numerous other solutions have been used to satisfy bridge construction codes' requirements for flutter. Some of them are triangular fairings [21, 26], active flaps [55], and passive flaps [56]. Galloping can particularly affect very slender box-girders. It may be difficult to control galloping through the increment of structural damping because of the small effect on the critical wind speed and given the diverging nature of the oscillation's amplitude. The countermeasure usually adopted to minimise the effects of galloping is the attachment of relatively small

the absence of VIV, then other countermeasures have to be adopted.

the surface of the bridge, which improves resistance to flow separation.

ter instabilities, and hence other countermeasures were developed.

the opening ratio and the location of the grating have been object of study.

wind speed of the bridge, and it further rises with the slot's width [24].

horizontal plates to the surface of the lower part of the girder.

deck, which is not always available.

**6.3. Aeroelastic instabilities**

spans of 1990 and 1013 m, respectively.

Fujino and Siringoringo [4] review aerodynamic countermeasures that have been used in truss and box-girder bridges built all over the world to avoid aerodynamic instabilities like galloping, torsional flutter, and vortex-induced vibration. The countermeasures are specific of the aerodynamic phenomenon to mitigate.

### **6.1. Buffeting**

The response of long-span cable-supported bridges to buffeting can be controlled through the use of auxiliary damping devices called tuned mass dampers (TMD). These are based on the secondary inertial system principle and consist of a mass attached to the structure through a spring and a dashpot. A variation of TMD is the tuned liquid damper (TLD) where the mass is replaced by a liquid (usually water).

Other types of cost-efficient dampers for mitigating buffeting response have been developed, like the tuned liquid column damper (TLCD) and its variants. This concept, first proposed by Sakai et al. [53], consists of a U-tube container in which one or more sections are partially obstructed by a plate with an orifice at the centre. The energy of structural oscillation is dissipated by the combined action of the liquid inertia, weight, and the damping effect associated with hydraulic pressure loss through the orifices within the tube. In principle, TLCD can be tuned (or retuned) to the frequency of the structure by designing it with the appropriate wetted length, or "liquid length". The advantages of TLCD systems include low cost and low maintenance, while they also represent a water reservoir that can be used for fire-fighting.

An evolution with adaptive frequency tuning capacity is the semi-active TLCD, which features an air chamber at each end of the tube where the pressures are actively controlled. Shum et al. [54] numerically investigated the performance of this system for suppressing combined lateral and torsional vibration along the construction of a real long-span cable-stayed bridge and concluded that it can effectively reduce the buffeting response of the bridge for the various construction stages. The same authors investigated a further development of the concept encompassing multiple pressurised TLCD, which provides more flexibility in the system design.

#### **6.2. Vortex-induced vibrations**

As previously mentioned, VIV is an aeroelastic phenomenon that occurs at low wind speeds and that is strongly related to the bridge section's shape. When designing the section's shape, there must be an effort towards minimising the possibility of occurrence of these phenomena [26, 40], since, generally, any ad-hoc solutions will increase the complexity of the structure and its construction and maintenance costs. If despite this effort, it is not possible to guarantee the absence of VIV, then other countermeasures have to be adopted.

One such countermeasure is the installation of tuned mass dampers (TMD), which are a secondary vibration system designed to have the same natural frequency of the main vibration system that it intends to cancel (i.e., the bridge). When the bridge oscillates, the secondary system acts as a stabiliser because of the action of its damping force on the main vibration system. However, the installation of TMD requires the existence of considerable free space in the bridge deck, which is not always available.

Over the past decades, various studies demonstrated that aerodynamic appendages, like double-flaps, flaps, or skirts, are effective countermeasures against VIV. These attachments inhibit the formation of the vortices responsible for VIV by directing high-speed flow to sweep the relevant deck's surfaces; moreover, the appendages themselves generate turbulence close to the surface of the bridge, which improves resistance to flow separation.

Other countermeasures have been proposed, like guide vanes, which have been employed for example on the Great Belt Suspension Bridge, as already mentioned in the Introduction.

#### **6.3. Aeroelastic instabilities**

not lead to considerable reformulations of the initial design of the bridge. As has already been mentioned, small geometrical modifications to the bridge can lead to substantial changes of the airflow pattern around the structure and thereby modify its aerodynamic behaviour. This effect can be exploited to overcome any undesirable aspect of the bridge's aerodynamic response. Accordingly, modifications to bridge decks often consist in the addition of aerodynamic appendages, judiciously shaped, dimensioned, and positioned, or in minor changes to

Fujino and Siringoringo [4] review aerodynamic countermeasures that have been used in truss and box-girder bridges built all over the world to avoid aerodynamic instabilities like galloping, torsional flutter, and vortex-induced vibration. The countermeasures are specific of

The response of long-span cable-supported bridges to buffeting can be controlled through the use of auxiliary damping devices called tuned mass dampers (TMD). These are based on the secondary inertial system principle and consist of a mass attached to the structure through a spring and a dashpot. A variation of TMD is the tuned liquid damper (TLD) where the mass

Other types of cost-efficient dampers for mitigating buffeting response have been developed, like the tuned liquid column damper (TLCD) and its variants. This concept, first proposed by Sakai et al. [53], consists of a U-tube container in which one or more sections are partially obstructed by a plate with an orifice at the centre. The energy of structural oscillation is dissipated by the combined action of the liquid inertia, weight, and the damping effect associated with hydraulic pressure loss through the orifices within the tube. In principle, TLCD can be tuned (or retuned) to the frequency of the structure by designing it with the appropriate wetted length, or "liquid length". The advantages of TLCD systems include low cost and low maintenance, while they also represent a water reservoir that can be used for fire-fighting.

An evolution with adaptive frequency tuning capacity is the semi-active TLCD, which features an air chamber at each end of the tube where the pressures are actively controlled. Shum et al. [54] numerically investigated the performance of this system for suppressing combined lateral and torsional vibration along the construction of a real long-span cable-stayed bridge and concluded that it can effectively reduce the buffeting response of the bridge for the various construction stages. The same authors investigated a further development of the concept encompassing

As previously mentioned, VIV is an aeroelastic phenomenon that occurs at low wind speeds and that is strongly related to the bridge section's shape. When designing the section's shape, there must be an effort towards minimising the possibility of occurrence of these phenomena [26, 40], since, generally, any ad-hoc solutions will increase the complexity of the structure

multiple pressurised TLCD, which provides more flexibility in the system design.

the original geometry.

**6.1. Buffeting**

100 Bridge Engineering

the aerodynamic phenomenon to mitigate.

is replaced by a liquid (usually water).

**6.2. Vortex-induced vibrations**

Since the failure of the Original Tacoma Narrows Bridge, mainly two types of deck girders have been promoted: the streamlined box-girder and the truss stiffened girder. Wind tunnel tests came to reveal that the use of either one was not an absolute assurance of absence of flutter instabilities, and hence other countermeasures were developed.

For the truss stiffened girder deck, one countermeasure generally used is the installation of gratings on the girder, which reduces the pressure difference between the lower and the upper surfaces of the deck and thus settles flutter. This solution was adopted in the suspension bridges over the Akashi Strait in Japan and over the Tagus River in Portugal, with central spans of 1990 and 1013 m, respectively.

Flutter instabilities in plate girder decks can also be controlled with gratings. The influence of the opening ratio and the location of the grating have been object of study.

In regards to streamlined box-girders, studies have shown that the aerodynamic stability can be improved by allowing for a central slot in the box section. This increases the critical flutter wind speed of the bridge, and it further rises with the slot's width [24].

Numerous other solutions have been used to satisfy bridge construction codes' requirements for flutter. Some of them are triangular fairings [21, 26], active flaps [55], and passive flaps [56].

Galloping can particularly affect very slender box-girders. It may be difficult to control galloping through the increment of structural damping because of the small effect on the critical wind speed and given the diverging nature of the oscillation's amplitude. The countermeasure usually adopted to minimise the effects of galloping is the attachment of relatively small horizontal plates to the surface of the lower part of the girder.
