THE STRUCTURAL AND MECHANICAL ENGINEERING OF BRIDGES
The engineering of bridges is clearly one of the fields in civil engineering that has had a greater social impact over the last 2000 years. Without bridges we would be living in a very different world; trade and travel would never have evolved the way they have. In all their brilliance, many bridges today stand majestically as symbols of the human genius. Every engineer knows that behind each greatis a very detailed study that can cross over several disciplines. The ensemble of typologies and engineering disciplines involved in bridge design makes this branch of modern civil engineering one of the most exciting.
Selecting a Structural Design
In order to determine the most appropriate, and possibly the most economical, type of structural design (such as girder, arch, cable stayed or suspended bridge) to be used for a new bridge, the following factors need to be thoroughly studied:
- geographical location
- service traffic
- construction constraints
Specific technological aspects need to be addressed for the selected construction material or materials. And for the more complex and consequently more expensive bridges other factors need to be considered such as the fluid-structure interaction phenomena for a suspended bridge with significant span or for vertical pylons subjected to river currents.
The analysis and design of construction sequences for bridges can be considered as a self standing discipline which employs engineering methods that are common to both civil and mechanical engineering. This multidisciplinary approach is most evident in the design of moveable bridges.
Numerical Simulation as a key tool in Bridge Analysis and Design
Bridges for the most part are structures that are statically indeterminate and that are often intrinsically non-linear. Even the simplest girder bridge can exhibit a statically indeterminate and intrinsically non-linear behavior. Although simplified approaches are typically available in the civil engineering practice for the most common configurations, most of the time a bridge (especially when the span increases) is a unique and complex structure. This is why numerical simulation has emerged as an indispensible tool in bridge design. For example, numerical simulation can play a vital role in:
- assessment of extreme traffic loads configurations, based on the available moving load templates and reference standard
- behavior under ultimate (ULS) and service (SLS) load carrying conditions, possibly accounting for plastic redistribution
- creep and shrinkage phenomena for concrete-based materials, relaxation of tensioned wires and cables
- linear and non linear stability analysis of slender or shallow-arch structures
- tension analysis for cable-stayed, suspended or hypostatic structures
- analysis of construction phases, accounting for both compatible and not compatible assembling conditions. The latter can be even complicated by accounting for creep during construction
- fluid structure interaction analysis for bridges with pylons subject to river currents or aeroelastic phenomena
- seismic analysis of bridges with fixed or isolated external restraints (i.e. installation of viscous dampers or energy dissipation devices)
- comfort analysis for pedestrian footbridges
- detailed analysis for the assessment of local structural behavior and connection design
- analysis of accidental conditions due to impact with vehicles or vessels
The added value of Numerical Simulation in Bridge Design
The commercial simulation software packages available today make all the above analysis so much easier. Some of them allow engineers to build custom, highly efficient routines for tackling repetitive tasks using macro languages or APIs. An example of this is the performing of code checks for various international standards. Given the increasing complexity of modern architecture, the use of engineering simulation has become an integral part of the bridge design process at the same level as the static intuition of the professional engineer.