Over the past few years there has been increased interest in the effects of dynamic crowd loading on moderately flexible structures such as footbridges and cantilever grandstand seating decks. This has resulted from changes in the pattern of use of existing structures and a construction trend towards more slender and flexible, structures that are consequently more sensitive to dynamic loading. Some newly built structures, as well as more established structures, have needed structural modifications to provide adequate serviceability following exposure to dynamic crowd loading. These events have led to concerns over both safety and comfort and the need for additional guidance for designers.
With grandstands, the problems are most severe with crowds at lively events such as pop-concerts. Here, an excitable crowd participates in the event by handclapping, stamping, bobbing or jumping; all of which activities may be coordinated more strongly by a musical beat. The action of the crowd is then periodic, typically at frequencies in the range 1.5Hz to 3.0Hz, with the resulting loading on the structure depending on the type and intensity of activity. Typically, cantilever grandstands have natural frequencies for vertical excitation in the range 2.5Hz to 6.5Hz. It follows that there is the prospect of resonant response to one or other harmonics of the excitation with correspondingly large displacements and accelerations of the seating deck. In extreme circumstances, these could lead to panic or severe discomfort and possibly even to structural damage.
J. W. Dougill, PhD, FREng, FCGI, FIStructE, FICE, FASCE
Chairman of IStructE, DCLG, DCMS Joint Working Group
J. R. Wright, BSc,PhD, FRAeS, CEng
Emeritus Professor, School of Mechanical, Aeronautical and Civil Engineering, University of Manchester. Member of IStructE, DCLG DCMS JWG
J. G. Parkhouse, MA, CEng, MICE
Director of Parkhouse Consultants Ltd. Member of IStructE, DCLG DCMS JWG
R. E. Harrison, MEng, DMS, AMIMechE
Research Engineer, Arup North West