We often take footbridges for granted without realising the design problems involved. Pedestrian movement can cause vibrations that result in the bridge feeling uncomfortable to walk over and could even result in it becoming damaged and unsafe.

Footbridges can be beautiful structures that convey pedestrian traffic. They are the result of man's everlasting need to cross natural and manmade obstacles, such as rivers, valleys and busy roads. They are as old as man himself. However simple the prehistoric structures were and however complex the new ones might look, footbridge forms have always been the subject of experimenting. The latest technology available is used to test bridge design, to ensure they will be fit for pedestrian loads as we build bigger and more varied road and rail bridges. Nowadays, apart from the main purpose of crossing obstacles, the elegance of pedestrian bridges has become equally an important factor in their design, making modern footbridges a fusion between art and engineering.

Brief History

Who made the first footbridges? Well, probably nature itself. There are numerous examples of natural made, rock arch-like bridges that were "constructed" by running water gradually destroying the soft rock layers. It is not surprising that Roman bridges were built in stone and had the arch as their basic structure. However, not only stone was used as a material in old times. Strong climbing plants, encountered in tropical rainforests, were and still are, equally good building material. Progress of technology has made use of wood and iron as well. Nowadays, it is mainly steel, reinforced and prestressed concrete that dominate footbridge design, although some new unconventional materials, such as fibre reinforced plastic, are finding their use as well.

Structural Systems

Designing bridges is all about providing an efficient way of transferring the relevant loads to the ground and making sure they are fit for purpose. The design challenge is to choose an appropriate structural system and make efficient use of materials available, depending on the expected loads the bridge has to carry, its length of span and the landscape. To satisfy both engineering and aesthetic requirements, a wide range of structural forms are available to choose from: arches, boxgirders, portal frames, trusses, suspension bridges and cable-stayed forms. Thanks to the relatively small size of footbridges (compared with road and railway bridges), designers can be innovative and experiment with new forms. This often leads to beautiful, slender, light and elegant structures. However, this also makes footbridges more prone to vibrations than ever before.

Good Or Bad Vibrations?

Some of the loads a footbridge has to carry are static (not changing in time) such as
self weight, while some are dynamic (changeable with time), such as wind or toads induced by people who are walking, jogging, jumping or even marching in step.

Marching is the strongest dynamic load which has the potential to induce bad vibrations and damage or even destroy the structure, therefore, it is considered as dangerous not only for footbridges but also for much larger road and rail bridges. This is why you can see warning to troops to break their step when crossing a bridge. Since the excessive swaying of the Millennium Bridge in London occurred, on its opening day in June 2000 when the bridge was exposed to crowds of walking pedestrians, the realisation came that civilians can subconsciously synchronise their movement with that of the structure and cause strong vibrations. Actually, every repetitive load (walking, marching, jumping) is potentially problematic if the number of repetitions in time unit (so called the excitation frequency) coincides with the frequency in which the bridge tends to oscillate when set into motion. Differently from soldiers who could be ordered to break their marching step when crossing bridges, pedestrians are not that disciplined and cannot be ordered to not synchronise between themselves and/or with the movement of the structure, especially a rope bridge). Therefore, it is important to understand the human-human and human-structure interaction to be able to design serviceable and safe footbridges and convert bad vibrations into good low-level ones.

Research Challenges

In contemporary design, an engineer is facing a complex task of estimating the pedestrian flow over footbridges, making an appropriate modelling of dynamic loads and their interactions (including possible vandal loading), making sure that the bridge will withstand expected static and dynamic loads, and that structural vibrations will never be that strong to cause pedestrians' concern or panic. Dealing with all this is far from being a daily job of footbridge designers. This is because many of these topics are still the subject of research rather than part of daily design practice. Some of skills required for making advances in the area are: vibration measurements and evaluation, computer modelling, signal processing, biodynamics of human walking, running and jumping and psychology of human reaction to vibration.

Where can you develop the required skills?

You need to choose a course in civil and structural engineering that has a strong dynamics component. This kind of courses is offered by a number of UK universities. And do not forget: "building bridges" is not only about engineering, it is also about connecting and collaborating with others. Are you ready for the challenge?