Wind Loads

Wind loading is usually given in the exam. Depending on the question and form of structure you are considering, this may or may not be a significant load. This cheat sheet is therefore relevant in part 1a (design appraisal) and also in the calculation section.

Wind loads given in the exam are usually given as a 3-second gust speed, and/or a mean hourly wind speed, for example:

The site is located near open sea. Basic wind speed is 56m/s based on a 3-second gust; the equivalent mean hourly wind speed is 28m/s”

The cheat sheet contains the Beaufort wind scale, which is useful for putting the magnitude of the wind speed in context. In this case, the mean hourly wind speed of 28m/s is equivalent to a 10-minute mean wind speed of 29.4m/s, which is a violent storm.

If you read the examiners' reports on past papers, you will see that they expect you to understand whether the wind is significant or not. There have been some exam papers where a very high wind speed was given and a lot of candidates missed the significance. In high wind speeds the following might be relevant:

• Wind-induced oscillations of flexible bridge decks (e.g. Tacoma Narrows bridge)
• Wind-induced oscillations of individual cables
• High lateral loads on the bridge deck (minor axis bending) and substructure (overturning). The deck can be profiled to make it more aerodynamic if necessary.
• Wind loads may contribute to uplift at supports, especially for lightweight footbridges.
• Cranes cannot operate in high winds, so this may affect the method of construction.

Therefore, in part 1a it's a good idea to get a feel for the magnitude of the wind loads and use it to rule out certain structural forms (e.g. cable structures) if necessary. Note that wind induced oscillations are driven by a steady mean wind speed (not individual gusts), whereas ULS checks of structural elements will be driven by the gust speed. In some cases the wind loading will be insignificant and could be ignored (explain why).

The cheat sheet contains a method for turning the gust speed into a peak velocity pressure that can be used to calculate wind loads if necessary. The peak velocity pressure can be multiplied by the relevant force coefficient (also given in the cheat sheet) to give the pressure on the bridge deck or element under consideration.