Typical Loads and Self-weights for Bridges

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?

Footbridges

For footbridges, one way is to have pre-calculated values for typical components and add them up to get a sensible total dead load. Typical weights of stiffened deck plates, plate girders and trusses are provided. These are very approximate; in reality the weight of the structure can be quite important as it affects the structure's susceptibility to dynamic analysis (more mass -> lower natural frequency). For the exam it's ok to round up and overestimate the dead load, especially if you make a note to show you know the mass is important for dynamic analysis.

Highway bridges

Graphs have been provided based on three sources:

• SCI preliminary steel composite bridge design charts. These are the best charts to use for steel-concrete composite bridges. We have done some "post processing" on the data in the preliminary design guide to calculate a steel weight per m2 of deck area.
• Bridge Engineering Handbook, 2nd Ed. This contains data from a study of bridges constructed in Japan, where the weight of steel is recorded for new bridges. The graphs plot the best fit line for each bridge type in the study. Multiple graphs are provided to present the same data with a focus on different span ranges and bridge types.
• Benaim (2008) 'The Design of Prestressed Concrete Bridges: Concepts and Principles'. This book sets out the proportions of various prestressed concrete box girder bridges for various deck widths. Excel has been used to calculate the weight of various widths of box based on these rules, with weights presented as a weight per unit deck area, and also as a weight per metre longitudinally. The former may be useful to compare different box arrangements (e.g. one 20m wide box or 2no. 10m wide boxes), whereas the latter may be useful for structural design.

Railway Bridges

At present, the cheat sheet does not have much information about sizing railway bridges, because they are uncommon in the exam. More information may be added in future.