EXPERIMENTAL DETERMINATION OF THE DYNAMIC CROWD EFFECT

Tianjian Ji (UMIST) and Brian Ellis (BRE)

In 1996, BS 6399 included a new section dealing with dynamic loads produced by co-ordinated jumping of crowds. However, it was recognised that one item, namely the dynamic crowd effect, required further investigation, and this was the goal for a range of tests conducted by UMIST and BRE. In response to music, a group of people can jump at the same frequency (the beat frequency) and produce a large dynamic load. This is likely to occur at some pop concerts and aerobics events. However, even if everyone in a crowd tried to jump at the same frequency, their co-ordination would not be perfect and this would effectively attenuate the resulting load. This attenuation is called the dynamic crowd effect. Although theoretical investigations have beenconducted, full-scale experimental data is desirable to calibrate the theory. To provide this data, a detailed test programme was required involving a group of people and an appropriate test structure.

A group of students and staff from UMIST volunteered to undertake an experiment to investigate the dynamic crowd effect. A floor of the steel framed test building at BRE Cardington laboratory was selected and its dynamic characteristics determined. The 60 students were equally divided into three groups and each group attended the tests in turn. Within each group, 2, 4, 8, 16 students were asked to take part in the tests which involved jumping in time to a musical beat. Finally, measurements were taken for groups of 32 and 64 people (staff included).

An electronic keyboard was used to generate selected musical beats to provide a timing signal for the jumping. Two beats were used throughout the tests. One beat was selected to generate a resonant loading of the floor and another to avoid a direct resonance. A range of measurements were taken on the day and the dynamic crowd effect has been evaluated on the basis of these measurements.

The original idea of the experiment was to answer some important questions related to structural design; however, the days events were designed to combine research and education. To provide background information for the visit, a pre-visit talk was given at the Department of Civil and Structural Engineering, UMIST, and a seminar was held at the Cardington Laboratory. A tour of the laboratory was also included. The students involvement made the large scale test possible and they provided a significant contribution to a real research programme partly financed by EPSRC. In addition, the students had the opportunity to visit the Cardington laboratory including a full size test building, to encounter actual research in structural engineering in the UK and thereby to supplement their university studies.

After completing the experiment, it was recognised that this was the largest controlled experiment yet conducted in a unique full size eight storey steel framed test building which occupies a small part of the largest enclosed research laboratory in the world.

The experimental programme will be described in the SECED meeting scheduled for the 25th November.