Magazine Title: The structural engineering (IStructE)

Article Title: Applying walking and running forces

Issue: April 2010

Principal investigator: Prof. JMW Brownjohn

Of all the dynamic forces induced by humans such as walking, running, jumping, bouncing and swaying, the dynamic forces induced by walking and running are the most complex to understand and deal with, when the performance of civil structures such as footbridges, floors, grandstands and other assembly structures are considered. These forces are arbitrary in nature and at the same time difficult to forecast and even more difficult to use in practice as their action changes not only in time but also in space.

Predicting vibration performance of civil engineering structures due to pedestrian loading is an increasingly critical aspect of design.

 

There is no formal guidance on the nature of loading due to crowds of pedestrians and the result of this lack of understanding may be financially catastrophic. The example of the London Millennium Bridge is the tip of the iceberg. When the London Millennium footbridge was opened in June 2000, it swayed alarmingly due to synchronous lateral excitation from pedestrians and earned a nickname “wobbly bridge”.

 

Contrary to common belief and current codification, walking/running forces are not perfectly periodic but are narrow-band random phenomena with considerable variation between individuals as well as for the same person. Use of such forces in structural design and extension to use with more than one pedestrian really requires experimental and analytical treatment similar to that used for other types of random forces that dynamically excite civil engineering structures, such as wind, waves or earthquakes.

 

Such statistical and probabilistic treatment of moving human-induced dynamic forces currently exists only as a concept in a few research papers. Hence gathering a large number of time-varying loading records of walking/running, establishing a viable database of them and using it directly for various forms of probability-based dynamic calculations of real-life structures presents a timely opportunity to advance the whole field of vibration serviceability assessment of structures occupied and dynamically excited by moving humans, such as floors and footbridges.

 

There are two key novelties in this approach:

1.       Utilization of 'free field' measurement of three-component continuous walking/running forces by measuring movement of the human body or bodies in addition to and for comparison with standard direct measurement of forces on a treadmill.

2.       Establishing database of measured time-varying traces of walking/running forces to develop stochastic models of these forces and to then apply them to simulate dynamic response of real-life structures. This will help in applying and refining the software models to assess vibration serviceability at the stage of structural design.