Alfredo Camara

Fundamental Mode Estimation for Modern Cable-Stayed Bridges Considering the Tower Flexibility

The design of cable-stayed bridges is typically governed by the dynamic response. This work provides designers with essential information about the fundamental vibration modes, proposing analytical expressions based on the mechanical and geometrical properties of the structure. Different bridge geometries are usually considered in the early design stages until the optimum solution is defined. In these design stages, the analytical formulation is advantageous, because finite-element models are not required and modifying the bridge characteristics is straightforward. The influence of the tower flexibility is included in this study, unlike in previous attempts on mode estimation. The dimensions and proportions of the canonical models proposed in the analytical study stem from the previous compilation of the dimensions of a large number of constructed cable-stayed bridges. Five tower shapes, central or lateral cable-system layouts and box- or U-shaped deck sections, have been considered. The vibration properties of more than 1,000 cable-stayed bridges with main spans ranging from 200 to 800 m long were extracted within an extensive parametric analysis. The Vaschy-Buckingham theorem of dimensional analysis was applied to the numerical results to propose the formulation for period estimation. Finally, the formulas were validated with the vibration properties of 17 real cable-stayed bridges constructed in different countries. The importance of the tower flexibility is verified, and the errors observed are typically below 15%, significantly improving the estimations obtained by previous research works.

Analysis and Control of Cable-Stayed Bridges Subject to Seismic Action

Abstract Cable-stayed bridges are key points in transport networks and at present one of the most challenging structures for the civil engineering community. The integrity of these bridges should be guaranteed even under extremely large earthquakes. This paper begins with a discussion of the advantages of a new non-linear static “Pushover” procedure that includes the three-dimensional contribution of the governing vibration modes. The efficacy and the accuracy of the proposed Pushover in the non-linear seismic analysis of bridges with significant coupling between the towers, deck and cable system is verified. In the second part of this paper, the seismic responses of several cable-stayed bridges have been studied, verifying the influence of the tower shape, cable arrangement and the main span length on the structural behaviour under strong ground motions. Severe damage is identified at critical tower sections by means of extensive non-linear dynamic analyses. Finally, retrofit solutions with viscous dampers (VDs) and yielding metallic dampers (MDs) connecting the deck and the tower in the transverse direction are explored. The proposed connection with dampers effectively prevents yielding of the reinforcement and cracking in the tower legs.

Effects of seismic devices on transverse responses of piers in the Sutong Bridge

The Sutong Bridge in China opened to traffic in 2008, and is an arterial connection between the cities of Nantong and Suzhou. It is a cable-stayed bridge with a main span of 1,088 m. Due to a tight construction schedule and lack of suitable seismic devices at the time, fixed supports were installed between the piers and the girder in the transverse direction. As a result, significant transverse seismic forces could occur in the piers and foundations, especially during a return period of a 2500-year earthquake. Therefore, the piers, foundations and fixed bearings had to be designed extraordinarily strong. However, when larger earthquakes occur, the bearings, piers and foundations are still vulnerable. The recent rapid developments in seismic technology and the performance-based design approach offer a better opportunity to optimize the transverse seismic design for the Sutong Bridge piers. The optimized design can be applied to the Sutong Bridge (as a retrofit), as well as other bridges. Seismic design alternatives utilizing viscous fluid dampers (VFD), or friction pendulum sliding bearings (FPSB), or transverse yielding metallic dampers (TYMD) are thoroughly studied in this work, and the results are compared with those from the current condition with fixed transverse supports and a hypothetical condition in which only sliding bearings are provided on top of the piers (the girder can move “freely” in the transverse direction during the earthquake, except for frictional forces of the sliding bearings). Parametric analyses were performed to optimize the design of these proposed seismic devices. From the comparison of the peak bridge responses in these configurations, it was found that both VFD and TYMD are very effective in the reduction of transverse seismic forces in piers, while at the same time keeping the relative transverse displacements between piers and the box girder within acceptable limits. However, compared to VFD, TYMD do not interact with the longitudinal displacements of the girder, and have simpler details and lower initial and maintenance costs. Although the use of FPSB can also reduce seismic forces, it generally causes the transverse relative displacements to be higher than acceptable limits.

Dynamic Effects of Turbulent Crosswind on the Serviceability State of Vibrations of a Slender Arch Bridge Including Wind–Vehicle–Bridge Interaction

The use of high-performance materials in bridges is leading to structures that are more susceptible to wind- and traffic-induced vibrations due to the reduction in the weight and the increment of the slenderness in the deck. Bridges can experience considerable vibration due to both moving vehicles and wind actions that affect the comfort of the bridge users and the driving safety. This work explored the driving safety and comfort in a very slender arch bridge under turbulent wind and vehicle actions, as well as the comfort of pedestrians. A fully coupled wind–vehicle–bridge interaction model based on the direct integration of the system of dynamics was developed. In this model, the turbulent crosswind is represented by means of aerodynamic forces acting on the vehicle and the bridge. The vehicle is modeled as a multibody system that interacts with the bridge by means of moving contacts that also simulate road-surface irregularities. A user element is presented with generality and implemented using a general-purpose finite-element software package to incorporate the aeroelastic components of the wind forces, which allows modeling and solving of the wind–vehicle–bridge interaction in the time domain without the need for using the modal superposition technique. An extensive computational analysis program is performed on the basis of a wide range of turbulent crosswind speeds. The results show that bridge vibration is significantly affected by the crosswind in terms of peak acceleration and frequency content when the crosswind intensity is significant. The crosswind has more effect on the ride comfort of the vehicle in the lateral direction and, consequently, on its safety in terms of overturning accidents.

IABSE: an opportunity for Young Engineers to develop their social skills

Universities rarely have time to develop a major tool for Civil Engineers: their social skills. As a result, recent graduates usually have troubles in presenting their work to different groups of people. During the last years, the internationalisation of Civil Engineering has increased their difficulties as they need to work in a common language that may differ with their first language. In addition, young engineers need to learn how to interact with people from other cultures around the world. These social skills demanded in the current labor market are traditionally learnt by young engineers themselves in a process that might be overwhelming. Being a member of IABSE, and specially participating in their conferences and annual meetings, offers a unique opportunity for young engineers to develop their social skills as they can get in contact and talk in front of a broad spectrum of engineers from both the industry and academia. Furthermore, young engineers can discuss topics and exchange ideas with senior and junior engineers in a friendly atmosphere.

Comfort in Slender Bridges Subjected to Traffic Loading and Hammering Effects

The verification of the Serviceability Limit State (SLS) of vibrations due to traffic live loads is typically ignored in the design of road bridges with conventional concrete decks. However, the vibrations perceived by pedestrians usually govern the design in slender and light-weight modern structures that take advantage of the improvement in the structural efficiency, material performance and constructive procedures. On the other hand, the comfort of the vehicle users is traditionally ignored in the design of the bridge because pedestrians are usually more sensitive to vibrations. However, in many highway bridges without pathways the only users of the structure are those in the vehicles (drivers and passengers). Considering all the possible bridge users and their specific sensitiveness, this paper addresses the vibration serviceability in a slender under-deck cable-stayed bridge subjected to heavy traffic loading. In this structure the prestressed concrete deck spans a distance of 80 m with a depth-to-span ratio of 1/80. The vehicle-bridge interaction accounts for aspects traditionally ignored like the wheel dimensions and the cross-slope of the bridge. A large number of time-history analyses is conducted to address the influence of road and vehicle properties on the SLS of vibrations. This work is completed with the study of the vehicle impact when it enters and leaves the bridge. The results clearly demonstrate the influence of the wheel dimensions and the road conditions, as well as the importance of high-order modes on the response.