**Wind Action Phenomena Associated with Large-Span Bridges Bridges**

**Wind Action Phenomena Associated with Large-Span** 

DOI: 10.5772/intechopen.73061

Daniel C. Vaz, Raquel A.B. Almeida and António R. Janeiro Borges António R. Janeiro Borges Additional information is available at the end of the chapter

Daniel C. Vaz, Raquel A.B. Almeida and

Additional information is available at the end of the chapter

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

#### **Abstract**

[31] Caetano E, Cunha A, Magalhães F, Moutinho C. Studies for controlling human-induced vibration of the Pedro e Inês footbridge, Portugal. Part 1: Assessment of dynamic behaviour. Engineering Structures. 2010;32:1069-1081. DOI: https://doi.org/10.1016/j.engstruct.

[32] Mottershead JE, Link M, Friswell MI. The sensitivity method in finite element model updating: a tutorial. Mechanical System and Signal Processing. 2011;25:2275-2296. DOI:

2009.12.034

82 Bridge Engineering

https://doi.org/10.1016/j.ymssp.2010.10.012

In the past, the design of bridges over increasing distances was limited by construction techniques and, as always, by economics. As technological advances have turned possible cable-supported bridges of incredible spans, a new challenge has been added to the equation: that of withstanding the action of winds without developing undesirable dynamic responses. In this chapter, the several aerodynamic phenomena of relevance to long-span bridges are classified and discussed. This will interest both experts and non-experts in the field, thanks to the overview that is given. For certain cases, codes of practice recommend wind tunnel tests. The reader is introduced to these, as well as to numerical simulations, which are currently gaining increasing importance. Next, measures for attenuating susceptibility for undesirable dynamic responses are reviewed. The chapter ends with a discussion of the Vila Real Bridge deck section, based on wind tunnel tests and numerical simulations carried out by the authors: the aerodynamics was effectively improved with geometrically subtle modifications that were proposed and adopted still in the design phase.

**Keywords:** wind actions, aeroelasticity, bridge decks, wind tunnel tests, aerodynamics, attenuation measures

#### **1. Introduction**

#### **1.1. Relevance of bridge aerodynamics**

Bridges are essential to the expansion and development of societies. Having in mind that erection sites are all different, we see a wide variety of bridges and it can even be said that no bridge is equal to the other. There are types of bridges but each case is a case; bridges are not a mass-production item. Several constraints to the design are brought into play, which

> Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

vary from place to place and over history, such as economics, construction means and time schedule, difficulties of construction, and architectural aesthetics. Technological advances have made possible the construction of bridges over ever-increasing spans using less mass of materials, which is of socioeconomic interest.

This chapter is aimed at readers without expert knowledge in the field of aerodynamics and the objective is to provide information that the reader can later complete by exploring in more

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85

While, from the functional standpoint, a bridge deck is the indispensable element of a bridge, it is also the one that is of greater concern from the point of view of the bridge's aerodynamic performance. Its shape is of paramount importance and the design has to go hand in hand with structural criteria from civil engineering and the other aforementioned criteria. Consequently, a few types of decks exist. To begin with, we can distinguish between road and railway bridges. The latter are reinforced to increase the stiffness both in torsion and in bending. Long road bridges often have wider decks in order to accommodate more lanes of traffic. In what regards wind action upon them, one can identify decks with a continuous solid surface exposed to the lateral wind, like plate-girder, box-girder, and streamlined-trapezoidal deck sections, and decks based on trusses, which are permeable to the wind (**Figure 1)**.

Each type of deck exhibits distinct susceptibility to aerodynamic phenomena. Moreover, for each one, several instances of aerodynamic studies can be found in the literature for the reason that decks of very similar geometry may nonetheless exhibit considerably distinct aerodynamic performances, be it as a result of the complexity of the aerodynamic phenomena or as a result of the dissimilarity between the dynamic properties of the decks. For example, for the same overall shape, a box-girder for a cable-stayed bridge with a central plane of cables has a

depth the available wealth of data in the published material.

**1.2. Typical bridge deck sections**

**Figure 1.** A selection of types of bridge decks.

According to the system supporting the deck, bridges can be classified into pillar-supported, arch-supported, cable-stayed, and cable-suspended. In moving from the first to the last case in this list, and generally speaking, the main span increases and, as a result, the flexibility of the bridge. Modern materials, construction techniques, and more accurate design impart to present-day bridges high flexibility and low structural damping, which are precisely the characteristics that render them more prone to aerodynamic instabilities. Indeed, the frequencies of the natural vibration modes of such bridges are sufficiently low to be within the energetic range of frequencies of the aerodynamic phenomena and thus undesirable resonance can take place.

When the structure vibrates, as a result of the fluid motion, its movement, if of sufficient amplitude, changes the flow pattern in the immediate vicinity of the structure, which in turn changes the effect of the flow on the structure. This results in a complex and dynamic interaction, called aeroelastic interaction, that does not correspond whatsoever to the situation in which the structure remains static or quasi-static.

Bridge design should bear in mind aeroelastic phenomena, in order to keep them safe for use, that is to say, under the action of wind, bridges shall not develop dangerous or inconvenient oscillations. In this regard, the deck's cross-section geometry is of paramount importance. Since it is not imposed by aerodynamics alone, such a thing as a single optimal type of deck is not available or possible, and instead there are a number of types of decks.

To show the relevance of the study of the aerodynamics of bridge decks, we give here three examples in which aerodynamic oscillations developed and had impact in serviceability or led to modification costs.

The Rio-Niterói Bridge (Rio de Janeiro, Brazil) has been the world's longest steel-box-girder pillarsupported bridge with a central span of 300 m at 72 m high. Since it opened to traffic in 1974, lowvelocity winds have often set it into oscillations [1]. When this happened, the bridge was closed to traffic, for the sake of user comfort and overall safety, though with important traffic inconveniences.

The Great Belt Suspension Bridge (Denmark) is, outside of Asia, the bridge with the longest main span: 1624 m. It opened to traffic in 1998. During the final phases of construction, low-frequency vertical oscillations of the girder were observed for lateral wind of moderate speeds (5–10 m/s) [2]. A theoretical study showed that the structural impact of the oscillations would be insignificant. However, there were concerns that the oscillations could distract drivers due to visual impact, jeopardise road safety, and convey a sense of unsafeness to the public potentially resulting in a loss in traffic volume. The oscillations were later attenuated with guide vanes mounted at the bottom edges of the girder.

More recently, in May 2010, months after being opened to traffic, the Volgograd Bridge (Russia) was closed to motor traffic for 5 days due to strong oscillations. In the autumn of 2011, this prestressed concrete girder bridge, with a main span of 160 m, was fitted with tuned mass dampers to correct the situation [3].

This chapter is aimed at readers without expert knowledge in the field of aerodynamics and the objective is to provide information that the reader can later complete by exploring in more depth the available wealth of data in the published material.

#### **1.2. Typical bridge deck sections**

vary from place to place and over history, such as economics, construction means and time schedule, difficulties of construction, and architectural aesthetics. Technological advances have made possible the construction of bridges over ever-increasing spans using less mass of

According to the system supporting the deck, bridges can be classified into pillar-supported, arch-supported, cable-stayed, and cable-suspended. In moving from the first to the last case in this list, and generally speaking, the main span increases and, as a result, the flexibility of the bridge. Modern materials, construction techniques, and more accurate design impart to present-day bridges high flexibility and low structural damping, which are precisely the characteristics that render them more prone to aerodynamic instabilities. Indeed, the frequencies of the natural vibration modes of such bridges are sufficiently low to be within the energetic range of frequencies of the aerodynamic phenomena and thus undesirable resonance can take place. When the structure vibrates, as a result of the fluid motion, its movement, if of sufficient amplitude, changes the flow pattern in the immediate vicinity of the structure, which in turn changes the effect of the flow on the structure. This results in a complex and dynamic interaction, called aeroelastic interaction, that does not correspond whatsoever to the situation in

Bridge design should bear in mind aeroelastic phenomena, in order to keep them safe for use, that is to say, under the action of wind, bridges shall not develop dangerous or inconvenient oscillations. In this regard, the deck's cross-section geometry is of paramount importance. Since it is not imposed by aerodynamics alone, such a thing as a single optimal type of deck is

To show the relevance of the study of the aerodynamics of bridge decks, we give here three examples in which aerodynamic oscillations developed and had impact in serviceability or

The Rio-Niterói Bridge (Rio de Janeiro, Brazil) has been the world's longest steel-box-girder pillarsupported bridge with a central span of 300 m at 72 m high. Since it opened to traffic in 1974, lowvelocity winds have often set it into oscillations [1]. When this happened, the bridge was closed to traffic, for the sake of user comfort and overall safety, though with important traffic inconveniences. The Great Belt Suspension Bridge (Denmark) is, outside of Asia, the bridge with the longest main span: 1624 m. It opened to traffic in 1998. During the final phases of construction, low-frequency vertical oscillations of the girder were observed for lateral wind of moderate speeds (5–10 m/s) [2]. A theoretical study showed that the structural impact of the oscillations would be insignificant. However, there were concerns that the oscillations could distract drivers due to visual impact, jeopardise road safety, and convey a sense of unsafeness to the public potentially resulting in a loss in traffic volume. The oscillations were later attenuated

More recently, in May 2010, months after being opened to traffic, the Volgograd Bridge (Russia) was closed to motor traffic for 5 days due to strong oscillations. In the autumn of 2011, this prestressed concrete girder bridge, with a main span of 160 m, was fitted with tuned mass dampers

not available or possible, and instead there are a number of types of decks.

with guide vanes mounted at the bottom edges of the girder.

materials, which is of socioeconomic interest.

84 Bridge Engineering

which the structure remains static or quasi-static.

led to modification costs.

to correct the situation [3].

While, from the functional standpoint, a bridge deck is the indispensable element of a bridge, it is also the one that is of greater concern from the point of view of the bridge's aerodynamic performance. Its shape is of paramount importance and the design has to go hand in hand with structural criteria from civil engineering and the other aforementioned criteria. Consequently, a few types of decks exist. To begin with, we can distinguish between road and railway bridges. The latter are reinforced to increase the stiffness both in torsion and in bending. Long road bridges often have wider decks in order to accommodate more lanes of traffic. In what regards wind action upon them, one can identify decks with a continuous solid surface exposed to the lateral wind, like plate-girder, box-girder, and streamlined-trapezoidal deck sections, and decks based on trusses, which are permeable to the wind (**Figure 1)**.

Each type of deck exhibits distinct susceptibility to aerodynamic phenomena. Moreover, for each one, several instances of aerodynamic studies can be found in the literature for the reason that decks of very similar geometry may nonetheless exhibit considerably distinct aerodynamic performances, be it as a result of the complexity of the aerodynamic phenomena or as a result of the dissimilarity between the dynamic properties of the decks. For example, for the same overall shape, a box-girder for a cable-stayed bridge with a central plane of cables has a

on structures. This is known as buffeting and the associated displacements are usually of small amplitude, in a way that does not change the topology of the flow around the deck. In other situations, called aeroelastic phenomena, the oscillation amplitude is large enough to interfere with the airflow. In this case, the flow pattern around a deck is affected by the structure's motion itself, giving rise to a very complex interaction between the deck's motion and aerodynamic forces.

The flexibility of long-span bridges renders them highly susceptible to aeroelastic phenomena. In this context, vortex-induced vibration (VIV), torsional flutter, coupled flutter, and galloping may arise. While in VIV, the amplitude is self-limited, in the other three phenomena, the amplitude of the deck's motion tends to increase continuously and, for this reason, they are categorised as aeroelastic instabilities. If damping is insufficient, the large amplitude of the motion may cause collapse of the structure. **Figure 3** systematises the classification of

VIV may occur at low wind speeds, and when it does occur, it is for narrow ranges of wind speed specific for a given structure (**Figure 4**). Flutter-type instabilities arise at much higher wind speeds, above a critical value. In buffeting, turbulence in the incoming flow (a large band excitation) causes a response proportional to the dynamic pressure of the wind. The next

As previously mentioned, buffeting refers to structural oscillations due to the turbulent fluctuations of oncoming wind. The structure can be considered absolutely rigid since the displacements are of very small amplitude. More specifically, the amplitude is much smaller

disrupt the boundary layer, thus not changing the topology of the flow around the deck.

Turbulence has a stochastic nature; pressure and velocity are random. The turbulence energy of natural wind concentrates in the lower frequency of the velocity spectrum. That is to say that if the characteristic lateral length of the structure is comparatively small (up to about 10<sup>2</sup>

Boundary layer is the thin region near a solid surface where viscous effects are dominant and fluid velocity evolves from zero at the surface to nearly its free-stream value away from it. The depth of this layer is known as its thickness.

**Figure 3.** A classification of flow-induced vibrations, including aeroelastic phenomena.

, and thus the vibration of the structure does not

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87

m),

sections address in more detail each type of flow-induced vibration.

flow-induced vibrations on structures.

than the thickness of the boundary layer1

*2.1.1. Buffeting*

1

**Figure 2.** From general rectangular decks to trapezoidal and streamlined decks.

higher depth-to-width ratio than one for a bridge with a double plane of cables, because the girder rigidity has to compensate the lack of torsional stiffness in the first case. This difference in aspect ratio can lead to very distinct aerodynamic responses.

In box-girder decks, the quest for high aerodynamic performance has resulted in the shift from thick rectangular decks to trapezoidal and streamlined decks (**Figure 2**).
