Analysis and Solution to Human-Induced Lateral Vibrations on a Historic Footbridge

Recently, it has come to light that footbridges may have problems with human induced vibrations if their lateral period of vibration is of the order of 1 s. This has been highlighted by the much-publicized case of the Millennium Footbridge in London. This paper describes a similar problem associated with the historic Cragside Bridge built circa 1875. This bridge is a wonderful historic bridge built in the grounds of Cragside Manor in Northumberland, United Kingdom by the famous Industrialist Lord Armstrong. An investigation of the bridge revealed that it suffered vibration problems resulting from insufficient lateral stiffness. When lightly loaded (approximately six people) the fundamental lateral natural frequency of vibration was 2.6 Hz. This mode was predominantly a lateral mode of vibration. A computer model of the bridge was developed and various loading scenarios were investigated. As the bridge is very lightweight, the computer model showed that the natural frequency could reduce to 1.12 Hz if the bridge was loaded to 4 kPa. Extra stiffening elements were added to the computer and, depending on the superimposed load, the frequency could be increased from 1.12 to 1.57 Hz (for a 4 kPa uniform live load) and increased to 3.67 Hz for an imposed load equivalent to approximately nine people. It is believed that this will help to remedy the problem, but further research is being conducted to reduce the problem further.