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Dance is movement with rhythmic steps and actions, usually to accompanying music. Similar movements include jumping, stamping and aerobics. The loading is thus related to the dance frequency or the beat frequency of the music and is periodical. These loads are here termed as dance type loads. The maximum response produced by dancing occurs when jumping is involved, and it is this situation which is of concern in this project. This type of load can be significant in structural design for three reasons:
The load produced by jumping can be several times bigger than the load resulting from standing still. This is demonstrated by the action of a child, who when trying to crush a cardboard box will jump on it.
In response to music, a large group of people could act at the same frequency (the beat frequency) and produce a large co-ordinated dynamic load, such as those seen in pop concerts and sports events.
If the fundamental frequency of a structure is close to the load frequency or an integer multiple of the load frequency, resonance will occur, which often associates with excessive vibration. The effect of resonance has been shown in a TV programme where a glass, filled with wine, is broken suddenly when a piece of classical music reaches a specific high pitch.
The safety of structures subject to dance type loads not only relates to the loads but also the structure itself. Using pre or post-stressing techniques and high strength materials, long span structures can be built to accommodate static loads and thereby provide clear space and viewing, which is important in some events. This leads to structures which have relatively low natural frequencies and which are dynamically sensitive. In the extreme this has produced structures like cantilevered grandstands which are desirable for watching sports matches and like dance floors which are ideal for accommodate a group of people, but they are particularly vulnerable to dynamic loads which leads to a structural safety problem.
Structural vibrations induced by dance type loads concern the floors used for dance, aerobics, keep-fit exercise and pop concerts, and grandstands used for sports events and pop concerts.
2. Summary of the Recent Research Work Relating to the Project
The overall purpose of the work in this area was to provide information on structural response generated by crowd loading and to improve design guidance and safety for lightweight structures, such as grandstands and floors. Over a number of years work Dr. Ji (UMIST) and Dr. Ellis (Building Research Establishment) have dealt with the loads generated by people, structural performance and structural response to the loads. The initial focus was on dancing and jumping loads and structural response to the loads which was the main safety consideration.
Theoretical studies[A1, A4], laboratory tests[A5] and site experiments[A6] had been conducted to investigate and clarify a number of problems. The study ranged from the definition of human dance type loads to the evaluation of structural response, and was related to floor design for aerobics and dance activities and grandstands for football and pop concert events. At present, the load model for dance-type loads has been established and has led to changes in the BS loading code[A2]; the analytical method which was proposed to evaluate structural vibration due to dance type loads won the Henry Adams Award, and criteria have been provided for arranging bracing systems for temporary grandstands which results in a stiffer and more elegant structure without using more materials[A7].
The research findings have been applied to solve a number of practical engineering problems. Typical examples are the dynamic assessment of two floors for holding dancing activities in a sports hall in Coventry[A8], the dynamic analysis of the Kippax Stand for the Manchester City Football Club[A9], and the bracing arrangement for a temporary grandstand at the Chelsea football ground.
3. The Objectives of the Project
The project is a continuation of the previous research and two problems are identified for further investigation. The first aims to provide a simplified method for designing structures subject to dance type loads, based on the theory proposed by Dr. Ji and Dr. Ellis, which will bridge the gap between the theory and applications. The second is to investigate the model of the dynamic crowd effect when a crowd of people is involved. This is required to deal with practical situations. A prime consideration will be calibration of the method and model against good quality full-scale test data that will be provided by BRE.
References
[A1]. Ji, T. and Ellis, B. R, (1994), Floor vibration induced by dance type loads: theory, The Structural Engineer, Vol.72, No.3, pp.37-44.
[A2]. BSI, BS 6399, Part 1: Loading for Buildings (1996).
[A3]. Ellis, B. R. and Ji, T.,(1997), Human-structure interaction in vertical vibrations, Structures of Buildings, the Proceedings of Civil Engineers, Vol. 122, No.1, pp.1-9
[A4]. Ji, T. and Ellis, B. R, (1993), Evaluation of dynamic crowd effect for dance loads, IABSE Symposium : Structural Serviceability of Buildings, Goteburg.
[A5]. Ellis, B. R and Ji, T.(1994), Floor vibration induced by dance type loads: verification, The Structural Engineer, Vol.72, No.3, pp.45-50.
[A6]. Ellis, B. R., Ji, T. and Littler, J. D., (1994), The response of grandstands to dynamic loads induced by crowds, Australasian Structural Engineering Conference, pp.457-462, Sydney,
[A7]. Ji, T. and Ellis, B. R. (1997), Effective bracing systems for temporary grandstands, The Structural Engineer, Vol.75, No.6, pp. 95-100
[A8]. Ellis, B. R., Beak, M. and Ji, T., (1992), Vibration tests and analysis at the Sports Centre, Fairfax Street, Coventry, Technical Consultancy Report: GI0813
[A9]. Ji, T. and Ellis, B. R., (1994), Dynamic analysis of the Kippax Stand, The Manchester City Football Club, Technical Consultancy Report: GI1323.
[A10] Ji, T. and Ellis, B. R.,(1992), Review of dynamic loads induced by human movements, BRE Note N98/92.