Footbridges are particularly susceptible to dynamic excitation from pedestrians due to their flexibility and lightweight construction. Steel footbridges are particularly susceptible; composite footbridges are less likely to be affected.
When starting the calculations in part 2C, the first thing you will need to do is estimate the weight of your structure.
There isn't enough time in the exam to go through the full process and estimate the self-weight, do your analysis, check your member sizes, adjust if necessary and then re-calculate the actual weight of the structure. You just have to make a good guess, do your analysis, and at most make a note of whether your actual weight is higher/lower than the expected weight.
So, how do you estimate the weight of a structure without knowing your member sizes?
Sizing foundations quickly is an important skill for the exam. Ideally, your first guess for foundation size in part 1a should be about right, especially if poor ground conditions are a key constraint.
The exam question typically gives the following design parameters, which can be used to size the foundations:
Railway bridges are not very common in the IStructE exam, perhaps because there are so many standard designs available. Structures carrying light railway fall into a grey area, and could be designed as highway bridges with lower span to depth ratios.
This cheat sheet sets out some common types of railway bridge that are suitable for different span ranges, along with some key considerations when laying them out. Depths are given directly instead of span to depth ratios, but you will notice that the ratios are much lower than for highway bridges.
The following cheat sheet is designed for footbridge superstructure selection.
Different superstructure options are compared in terms suitable span ranges, key constraints and span to depth ratios.
Common constraints in the exam include:
One of the most common constraints in the IStructE exam is spanning a known distance with limited construction depth (e.g. due to headroom clearance requirements underneath the structure), so the span and span to depth ratio are two of the key pieces of information you can use to identify a suitable superstructure type.