**7. Example of bridge design aided by wind tunnel studies: the Vila-Real Bridge**

We have seen that, in what concerns to bridges, Codes of Practice strongly encourage wind tunnel tests for cases encompassing long span or great heights. If susceptibility to adverse aerodynamic phenomena is identified in the tests, then the solutions discussed in the previous section can be considered. The economics of the project favours minor geometric changes (from the structural viewpoint) comparatively to the addition of aerodynamic appendages. Therefore, wind tunnel tests and structural design should be coordinated so that any geometrical modification possibly proposed can be reflected in the final design.

In this section, the study of the cable-stayed Vila-Real Bridge, over the Corgo River Valley, Portugal, is presented as an example of the aid wind tunnel tests can provide in the design of the deck cross-section. The tests were carried out by the authors and associated team at the Laboratory for Fluid Dynamics and Applied Thermodynamics of the Department of Mechanical and Industrial Engineering (DEMI) of Universidade Nova de Lisboa (UNL).

The deck entails a concrete single-cell box-girder with the top flange extended and supported by inclined struts, also in concrete. It is a bridge cable-stayed in the midplane, and its importance among this family of bridges is made clear in **Figure 5**. Given its main span of 300 m and deck height of 230 m above the river, wind tunnel tests were due, as pointed out in Section 3 (Codes of Practice).

When the overall geometry and main dimensions of the deck section had been established, by the engineering office in charge of the bridge structural design, it was time to address, through wind tunnel tests, the susceptibility of the bridge to aeroelastic instabilities. After a small survey of aerodynamic laboratories, the study was assigned to our laboratory. Over the years, the laboratory has acquired a reasonable experience in static tests of civil engineering structures, namely viaducts, pylons of stay-bridges, telecommunication towers, and concrete poles for overhead electrical transmission lines. The purpose of the study was to examine the

bridge's dynamic response to the action of lateral wind, for the construction and operation

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Prior to wind tunnel tests, in order to understand the way the wind approaches the Corgo Valley, numerical simulations of the wind over terrain were performed with input from sta-

For the reasons stated in Section 4.3, the tests were conducted with a sectional model of the deck supported in the wind tunnel by an elastic suspension system. The model, at a scale of 1:66.7, has a skeleton, of aluminium frames mounted on four steel bars, that confers the indispensable stiffness for it to not bend or twist in the dynamic tests, as well as brings the weight to the required range. The shape of the box-girder and top flange was then obtained with balsa wood lined with thin birch plywood, leaving exposed the portion of the aluminium

The suspension system, already referred to in Section 4.4 and described in detail in [41], was adjusted taking into consideration the suspended mass, the logarithmic decrement factor of the combined suspension assembly and model, and the ratio of torsional to heaving natural frequencies of the bridge that were made available by the design team from FE simulations. The adjustment was made for each configuration: bare deck during construction (with the underlying Eigen frequencies corresponding to the situation in which the bridge's central span is about to be closed) and fully equipped deck (with frequencies corresponding to a

The proper combination of forces inferred from the measurements of six ring strain sensors present in the assembly produces the dynamic action of the flow on the model in terms of lift, drag, and roll and pitch moments. The model was tested for various angles of attack and the RMS of the amplitude of oscillations both in torsion and vertical translation was obtained for

Since the tests were conducted in smooth flow, buffeting shows up just slightly in the results and is possibly related to the vortex field generated by flow separations along the contour of the deck. Diverging amplitude, which would correspond to flutter, has never been found. The results for the construction configuration exhibit no peaks whatsoever, while those for the operation configuration show peaks indicating possible susceptibility to VIV. **Figure 6** shows a sample of results

than the real deck since, in strict terms, the sectional model represents a deck of infinite span.

**Figure 6.** Results for fully equipped deck in the original and modified configurations at an angle of attack *α* = +2 °.

). It should be pointed out that the model is more sensitive to oscillation

configurations, and to propose attenuation measures if found necessary.

tistical wind data from a meteorological station a few kilometres far off.

continuous construction over the whole length of the bridge).

the range of reduced velocities of practical interest.

Amplitudes of: (a) vertical oscillations; (b) torsional oscillations.

(angle of attack *α* = +2°

frames that represent the inclined struts.

**Figure 5.** Deck height versus main span (adapted from [10]).

bridge's dynamic response to the action of lateral wind, for the construction and operation configurations, and to propose attenuation measures if found necessary.

**7. Example of bridge design aided by wind tunnel studies: the Vila-**

metrical modification possibly proposed can be reflected in the final design.

We have seen that, in what concerns to bridges, Codes of Practice strongly encourage wind tunnel tests for cases encompassing long span or great heights. If susceptibility to adverse aerodynamic phenomena is identified in the tests, then the solutions discussed in the previous section can be considered. The economics of the project favours minor geometric changes (from the structural viewpoint) comparatively to the addition of aerodynamic appendages. Therefore, wind tunnel tests and structural design should be coordinated so that any geo-

In this section, the study of the cable-stayed Vila-Real Bridge, over the Corgo River Valley, Portugal, is presented as an example of the aid wind tunnel tests can provide in the design of the deck cross-section. The tests were carried out by the authors and associated team at the Laboratory for Fluid Dynamics and Applied Thermodynamics of the Department of Mechanical and Industrial Engineering (DEMI) of Universidade Nova de Lisboa (UNL).

The deck entails a concrete single-cell box-girder with the top flange extended and supported by inclined struts, also in concrete. It is a bridge cable-stayed in the midplane, and its importance among this family of bridges is made clear in **Figure 5**. Given its main span of 300 m and deck height of 230 m above the river, wind tunnel tests were due, as pointed out in Section 3

When the overall geometry and main dimensions of the deck section had been established, by the engineering office in charge of the bridge structural design, it was time to address, through wind tunnel tests, the susceptibility of the bridge to aeroelastic instabilities. After a small survey of aerodynamic laboratories, the study was assigned to our laboratory. Over the years, the laboratory has acquired a reasonable experience in static tests of civil engineering structures, namely viaducts, pylons of stay-bridges, telecommunication towers, and concrete poles for overhead electrical transmission lines. The purpose of the study was to examine the

**Real Bridge**

102 Bridge Engineering

(Codes of Practice).

**Figure 5.** Deck height versus main span (adapted from [10]).

Prior to wind tunnel tests, in order to understand the way the wind approaches the Corgo Valley, numerical simulations of the wind over terrain were performed with input from statistical wind data from a meteorological station a few kilometres far off.

For the reasons stated in Section 4.3, the tests were conducted with a sectional model of the deck supported in the wind tunnel by an elastic suspension system. The model, at a scale of 1:66.7, has a skeleton, of aluminium frames mounted on four steel bars, that confers the indispensable stiffness for it to not bend or twist in the dynamic tests, as well as brings the weight to the required range. The shape of the box-girder and top flange was then obtained with balsa wood lined with thin birch plywood, leaving exposed the portion of the aluminium frames that represent the inclined struts.

The suspension system, already referred to in Section 4.4 and described in detail in [41], was adjusted taking into consideration the suspended mass, the logarithmic decrement factor of the combined suspension assembly and model, and the ratio of torsional to heaving natural frequencies of the bridge that were made available by the design team from FE simulations. The adjustment was made for each configuration: bare deck during construction (with the underlying Eigen frequencies corresponding to the situation in which the bridge's central span is about to be closed) and fully equipped deck (with frequencies corresponding to a continuous construction over the whole length of the bridge).

The proper combination of forces inferred from the measurements of six ring strain sensors present in the assembly produces the dynamic action of the flow on the model in terms of lift, drag, and roll and pitch moments. The model was tested for various angles of attack and the RMS of the amplitude of oscillations both in torsion and vertical translation was obtained for the range of reduced velocities of practical interest.

Since the tests were conducted in smooth flow, buffeting shows up just slightly in the results and is possibly related to the vortex field generated by flow separations along the contour of the deck. Diverging amplitude, which would correspond to flutter, has never been found. The results for the construction configuration exhibit no peaks whatsoever, while those for the operation configuration show peaks indicating possible susceptibility to VIV. **Figure 6** shows a sample of results (angle of attack *α* = +2° ). It should be pointed out that the model is more sensitive to oscillation than the real deck since, in strict terms, the sectional model represents a deck of infinite span.

**Figure 6.** Results for fully equipped deck in the original and modified configurations at an angle of attack *α* = +2 °. Amplitudes of: (a) vertical oscillations; (b) torsional oscillations.

To gain insight into the flow topology, two-dimensional numerical simulations of the flow were carried out, employing unsteady Reynolds-averaged Navier-Stokes (URANS) equations. By comparing comprehensively, the numerical and experimental results, it was possible to understand that, at least for this particular deck section, VIV occurs whenever the flow pattern displays simultaneously the following two features (**Figure 7**): (a) above the deck, a large coherent vortex structure on the windward part or irregular flow anywhere; (b) below the box-girder base, separated flow without reattachment. This led to the conclusion that to eliminate, or at least attenuate, the oscillatory movements of the model at certain wind speeds it would suffice to prevent vortex-shedding from at least one side of deck.

As can be seen in the results of **Figure 6**, the modifications introduced completely eliminated the prominent peak in torsional oscillations and reduced the other minor peaks (both in vertical translation and in torsion). The results in **Figure 6a** also suggest a reduction of what is one

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The execution of projects of special structures is always complex and requires contributions from different branches of engineering. It is important that the structural design team is complemented, in due time, by aerodynamic specialists with the competence and access to the means to perform the required special studies. The Vila-Real Bridge case that has been discussed here is an example of how laboratorial means were made available, of the capacity of timely response, and of how modifications to improve the aerodynamic performance of a bridge deck can turn out to be very subtle from the geometric point of view (or even favour

the aesthetics of the bridge) while being very effective with regards to aerodynamics.

parcel of the total investment for this kind of projects.

Daniel C. Vaz\*, Raquel A.B. Almeida and António R. Janeiro Borges

UNIDEMI and DEMI, Universidade Nova de Lisboa, Caparica, Portugal

\*Address all correspondence to: dv@fct.unl.pt

288. DOI: 10.1016/S0167-6105(99)00108-7

It should be pointed out that major structures that very exposed to wind action are the ones that justify wind tunnel studies, and the costs of the aerodynamic studies are always a small

This work has been funded by Fundação para a Ciência e Tecnologia (PEst-OE/EME/

[1] Battista RC. Reduction of vortex-induced oscillations of Rio–Niterói bridge by dynamic control devices. Journal of Wind Engineering and Industrial Aerodynamics. 2000;**84**(3):273-

[2] Larsen A, Esdahl S, Andersen JE, Vejrum T. Storebælt suspension bridge – Vortex shedding excitation and mitigation by guide vanes. Journal of Wind Engineering and Industrial Aerodynamics. 2000;**88**(2):283-296. DOI: 10.1016/S0167-6105(00)00054-4

of the forms of buffeting.

**Acknowledgements**

UI0667/2014).

**References**

**Author details**

**8. Conclusion**

To reduce the dimensions of the recirculation bubble below the box-girder, whenever it was formed, in order to facilitate its reattachment and thus prevent vortex-shedding, the edges at the base of the box-girder were rounded. The border beam profile was also rounded. Wind tunnel tests of the deck with these slight geometric modifications showed that they were beneficial.

The geometric modifications were then proposed to the engineering office undertaking the bridge design and a meticulous discussion followed to define the transposition of the modifications to the full-scale bridge. They are visible in **Figure 8**, and the dashed lines in the sketches therein refer to the original contour.

**Figure 7.** Streamlines from numerical simulation for fully equipped deck at *α* = +2 °, in configurations: (a) original, showing a large recirculation bubble over the deck, on the windward part, and separated flow without reattachment below the box-girder base; (b) modified, showing thinner bubbles and reattached flow below the girder (from [10]).

**Figure 8.** Photo and schemes of details of the Vila-Real Bridge deck, showing the modifications introduced by the aerodynamic study (original photo courtesy of Prof. A.P.V. Urgueira, Universidade Nova de Lisboa).

As can be seen in the results of **Figure 6**, the modifications introduced completely eliminated the prominent peak in torsional oscillations and reduced the other minor peaks (both in vertical translation and in torsion). The results in **Figure 6a** also suggest a reduction of what is one of the forms of buffeting.
