# Aerodynamic Analysis of Simple Grider Bridges under Construction Phase
## 1 Introduction  
### Background  
The developments in structural engineering allow the designers to make lighter and more efficient structures every year, translating into slender and flexible shapes [1]. These changes lead to significant savings in materials and construction, but they also make the structures more vulnerable to dynamic loads. Therefore, dynamic effects on structures now have a more significant influence on the design process, making it necessary to address the problem as soon as possible. One of the actions that may have a strong dynamic component is wind loading. The increasing lightness of exposed structures, like bridges, can render dynamic effects as decisive for the design. Furthermore, wind effects also act during construction stages, when some types of bridges show especially vulnerable situations. Steel-concrete composite bridges are an example of this. The concrete parts provide stiffness to the structure, but they are built after the steel parts. Thus, there is a certain period of time when the structure in construction is lighter and more flexible, being more vulnerable to dynamic loads. Investigations of the dynamic component of the wind load were initiated essentially due to Tacoma Narrows Bridge's collapse in 1940. However, the complexity of the phenomena involved makes it difficult to develop analytical methods to calculate the loads. In the early 1960s, [PERSON] [2, 3] set the basis to define the along-wind response of structures to wind turbulence. Later, the same author [4, 5] and other authors [6, 7] focused on simplifying wind loads to develop analytical methods that can be used in design codes. Two differentapproaches were taken regarding the cross-wind response to vortex shedding effects [8]. In Germany, [PERSON] [9] and [PERSON] [10] developed an analytical method assuming the synchronization of the phenomenon due to the amplitudes of vibration of the structure. In Canada, [PERSON] and [PERSON] [11], [PERSON] [12], and [PERSON] [13; 14; 15] developed another analytical method based on the spectral functions of the wind. For large structures, the mentioned methods are usually not precise enough, and it is necessary to combine them with wind-tunnel tests, full-scale measurements, and computational simulations to obtain accurate results [16; 17]. However, for small and common structures, they are a powerful tool for wind assessment. Nowadays, the mentioned analytical methods are included in codes like Eurocode 1 [18] or the National Building Code of Canada [19]. However, they still have a huge limitation: the lack of aerodynamic parameters for specific geometries. The main part of the research done in the field has been focused on circular cross-sections since they are common in several structures. Still, it is necessary to find the aerodynamic parameters to apply the methods with other geometries. In the past decades, several authors concentrated on analyzing the aerodynamic behavior of different simple shapes, like flat plates [20; 21; 22]. However, small geometric modifications lead to substantial variations in the parameters, being impossible to extrapolate the parameters from simple shapes. It is necessary then to analyze each specific geometry. Regarding I-beams and simple girder bridges, [PERSON] et al. [23] examined the aerodynamic behavior of simple girder bridges during the construction stages, but the study was not centered on the dynamic effects of the flow. Their research was focused on concrete beams, which are less vulnerable to dynamic wind effects than the steel beams analyzed in this paper. [PERSON] et al. [24] studied the wind flow around H-beams to understand the galloping response. During their study, only a few geometrical configurations were tested. Therefore, there is still a lack of data regarding single and double steel I-beams. In large suspension bridges, the project's budget allows making wind-tunnel tests and computational simulations to analyze the cross-section.