Lately, I've been taking part in 1st INFRASTAR implementation day (more info here). It was held by BASt (Federal Highway Research Institute) near Cologne, Germany.
Once there, I took a chance to walk the city and its riverbank. They have couple of road and railway bridges over there. This brought some thoughts to my mind.
According to the European codes, the design service life of a bridge is around 100 years. During that time, the traffic and the demands that we, a society, impose on a structure can change considerably. Just think of how the road traffic changed between 1918 and now!
One of the common cases, especially in case of bridges within the city limits, is need of widening of a bridge to accommodate higher number of cars, trains etc. But, this must be done not only under economical, but also under architectural constraints.
One of ways to widen the bridge is to add new, parallel structure. This strategy was used in two cases in Cologne.
In case of the railway bridge, a new structure was built next to existing one. Same material (steel) and type of structure was used, so that it is difficult to say the difference from the distance. But, closer look, reveals the progress that happened in civil engineering during the time.
Another case is a road-pedestrian bridge. Here, two different material were used (concrete and steel) but, to keep the nice look and compatibility, the box-girders were formed in similar way.
These two cases show, that an engineer needs to think of aesthetics, as well!