Dynamic Interaction Between Event Deck Structures and a Jumping Crowd

Abstract

Human-induced rhythmic loading is an increasingly critical aspect in the design process of civil engineering structures such as sports stadiums and floors accommodating gym and aerobic classes. There are two main reasons for this trend. First, structures are becoming more slender with improvements in materials and construction techniques and modern trends in architectural designs. And second, crowds are getting livelier than previously was the case, their activities can become better synchronized due to the presence of various auditory and visual stimuli at above mentioned events.

In recent years there is also a growing discussion on the strength and stability of event structures. A very common type of structure is an event deck structure. Visitors gather on top of these decks during festivals and in some cases festival organizers place bars underneath them to efficiently use the structure. Personnel working in these bars experience the movement of the structure due to a dancing crowd and tend to feel uncomfortable. Engineers at Tentech are aware of this phenomenon through contacts with clients.

Nowadays, in the Netherlands, event deck structures are designed to withstand a vertical static load of 5 $kN/m^2$, prescribed by Dutch design codes. The amplitude of this static load is based on a dense static crowd. But according to existing literature, a synchronically jumping crowd can cause a vertical load which far exceeds the design load prescribed by design codes. This provides a reason to further investigate the extreme load case of a synchronically jumping crowd on an event deck structure.

A missing element in the standard design of structures subjected to a synchronically jumping crowd is the consideration of dynamic interaction between human and structure. In this research the focus is on how human-structure interaction (HSI) can influence the human-induced loading and the internal stresses caused by that loading in the structure. This is done by building a 3D finite element model in Abaqus.

A 3D finite element model is used to easily vary in the position of a jumper on the structure. Modelling group effects such as the coordination factor is also easier when using a 3D model. And in the case of an event deck structure a 3D model will result in more detailed results compared to 2D models because mechanical properties of an event deck can vary over the third dimension.

A mass-spring system is suggested to represent a jumping person. To simulate a synchronically jumping crowd, multiple mass-spring systems are used. By assigning an initial velocity to the mass in the mass-spring system, it is possible to simulate a similar mechanism as a jumping person colliding with a structure. By analysing the force in the spring over time it is possible to draw conclusions on the influence of human-structure interaction on the impact peak force. And by comparing the impact forces with the reaction forces which are measured at the base of the structure, the effects of structural vibrations on the internal stresses are determined.

It is found that taking HSI into account does influence the impact peak force and the internal stresses of the structure. But for an event deck structure with a natural frequency of 20 Hz or higher, this only accounts for an individually jumping person. It is found that the more people jump on a structure, the less significant the effect of HSI will be, even when a crowd tries to jump synchronically. This is caused by the natural time lag between each jumping person.