An important aspect of grandstand design is ensuring that crowd loading cannot cause excessive dynamic motion of the structure, resulting in discomfort or panic in the crowd. This is potentially a problem when crowds move rhythmically, e.g. to music, and excite the structure at one of its natural frequencies.
In 2000, a Working Group was set up by the Institution of Structural Engineers and two government departments to look at this issue. The group has recently published Recommendations1 in which two methods for assessing the ability of a structure to withstand dynamic crowd loads are presented. A grandstand may be considered to meet these Recommendations if the requirements of either one of the two methods are met. The first method, termed Route 1, is based on the previous guidance document2 and judges acceptance using only the lowest relevant natural frequency of the structure. The second method, Route 2, is a new approach which enables engineers to predict the likely vibration levels and to compare them against acceptable limits based on human comfort and safety. This more rigorous approach
provides a way of demonstrating compliance of a structure which may have otherwise failed the Route 1 test. In addition, it enables engineers to investigate the effect of management actions aimed at improving the dynamic performance of the structure e.g. by keeping rows of seating unoccupied.
The new assessment method is a significant development for engineers engaged in the assessment of grandstand dynamic behaviour. This paper discusses the basis of the new method and provides design charts to assist in its practical implementation. The design charts offer a rapid means of assessment requiring only the results from a natural frequency analysis of the empty structure. The charts also clearly illustrate how factors other than natural frequency affect the dynamic performance of grandstands subject to crowd loading.
Graham Parkhouse, MA, CEng, MICE
Ian Ward, BSc, MSc, CEng, FIMechEE