How people interact with their environment is a topical issue and one of increasing importance. One form of physical interaction which is understood poorly, even by professionals, is concerned with human response to vibration in structures. This is important for instance when determining how dance floors, footbridges and grandstands respond to crowds of moving people and when determining how stationary people are affected by vibration in their working environment. However, human-structure interaction provides a new topic that describes the independent human system and structure system working as a whole and studies the structural vibration where people are involved and human body response to structural movements.

When a structure is built on soft soil, the interaction between soil and structure may be considered; when a structure is in water, such as an offshore platform, the interaction between the structure and fluid surrounded may be considered. Similarly when a structure is loaded with people, the interaction between people and structure may need to be considered. An obvious question is why this was not considered before? There are two reasons:

Now many structures become longer and lighter and the effect of human bodies becomes important. The newly emerged problems are the human induced vibrations of grandstands and footbridges, where crowds of people are involved. These were identified by the latest SCOSS report on structural safety [SCOSS, 2001]. The investigation of the lateral vibration of the London Millennium Bridge provided a new and interesting observation: when a crowd reached a critical number, even an increase of a few people would lead to a significantly enlarged response (Fig.2). This phenomenon of lateral vibrations had been observed from other types of footbridges when a crowd was involved [Dallard, 2000]. It might be possible for solutions if the human body were not just considered as a loading and/or an inert mass, and the knowledge of biodynamics of human body were applied into structural dynamics.

Some phenomena were observed earlier when the North Stand at Twickenham was tested when it was empty and when it was full of spectators [Ellis, 1997]:

These observations had never been reported and suggested a new concept that the human whole-body acts as a mass-spring-damper rather than an inert mass. The phenomena were reproduced in the laboratory and were explained by the new concept of human-structure interaction.

1.2 Previous Work

As human-structure interaction is a new topic and the problems in practice have recently emerged, there are not many publications available and most of them were written by Dr Ji and Dr Ellis [Ellis, 1997, 2000, Ji, 1993, 1994.2, 1995, 1997, 1999.1, 1999.2, 2000]. Research work was commissioned by the Department of Environment on the project of Loads and Vibrations Caused by People, immediately after the collapse of the rear part of a temporary grandstand in Corsica in May 1992. Work was undertaken on a range of topics from human induced loads to structural vibration subjected to human loads, from structural dynamic behaviour to structural improvement.

The fact that the human body acts as a mass-spring-damper system was also noted by investigators in Canada [Foschi, 1995] although the basic data of the body system used were incorrect due to lack of information on human body models. Our work on human structure interaction has stimulated others. The influence of human body on dynamic behaviour of beams and floors have been investigated recently [Sachse,2002, Brownjohn, 2001]. Most information on the human whole-body came from the study of biomechanics of human whole-body [Fairley, 1989, Griffin, 1990, Matsumoto, 1998, Wei, 1998]. In the experimental studies a sitting person was tested on a shaking table and measurements were taken from the interface of the test subject and the chair that was fixed on the shaking table [Fairley, 1989]]. In the theoretical studies the human body was treated as a single or multi-degree-of-freedom system (a lumped mass system). The frequencies of the body in sitting positions have been intensively investigated; however, there is limited information available on the damping but little on the modal mass of a stationary person. These items are important for the investigation of human whole-body response to structural vibration and structural vibration due to human involvement.

The recognition of the human whole-body as a mass-spring-damper model in structural vibration also led to new applications. A method for the indirect measurement of the human whole-body frequency has been developed and the human whole-body frequency in a standing position has been measured. Immediately after the indirect measurement method was reported [Ji, 1993], the Silsoe Research Institute applied the method to measure the resonant frequency of chickens which was unlikely to be obtained by using a shaking table [Randall, 1994] (Fig.3). (As the shaking table moves, the chicken is flying). Their study related to the financial aspect and the meat quality of six hundred million chickens consumed each year in the UK. Dr. Ji, as the only UK structural engineer, presented four times in the UK meeting on Human Response to Vibration [Ji, 1993, 1995, 1997, 2000]. It has been gradually realised that the theory and concept of human-structure interaction is required to explain and to deal with newly emerged engineering problems.

1.3 Structural Dynamics and Body Biodynamics

The study of human-structure interaction is concerned with both structural dynamics and body biodynamics. The former belongs to engineering while the latter is part of science. The study of structural dynamics may be described by the following diagram:

                        Fig.4: The Basic Studies in Structural Dynamics

The structure may range from a simple beam to a complex building, from a car to an aeroplane. The relationships between input, output and the model of a structure can normally be described by governing equations and the solution of the equations is the output. In the diagram the input, output and the structure are quantified, or at least quantified statistically.

For different types of problem, the governing equations may not be the same; and a variety of methods may be available for the solution of a particular equation. Alternatively, the input can be determined based on the understanding of the structure and the output, or the model can be improved based on the known output and input [Ewins, 2000]. If a similar diagram to Fig.4 is required to describe the basic studies in biodynamics of the human body, it may be represented as follows:

(a)

Two possible ways of combining the knowledge in structural dynamics and body biodynamics have been examined for the study of human-structure vibration [Ji, 2000], (1) applying structural dynamics methods into body biodynamics, and (2) applying body biodynamics methods into structural dynamics. The second is identified as the most straightforward and effective way because:

1. There are some available results in body biodynamics, which can be used directly into the study.

2. The topic of human-structure interaction aims to solve structural problems where people are involved.

3. Predictions of human response to structural vibration are required.

It is expected in the future that the spans of structures will be even longer and the human expectation of the quality of life and their working environment will be even greater. Therefore, engineers will need some improved knowledge to tackle these problems where structural safety and/or human comfort are concerned.

This project aims to develop the new topic, human-structure interaction, which links body biodynamics and structural dynamics. The new topic will be developed by applying existing knowledge in body biodynamics to structural dynamics, identifying some dynamic characteristics of human whole-body and investigating systematically several issues of structures relating to human body models as an individual and as a crowd. The theory will make it possible for predicting human body response to structural vibration and for determining structural vibration due to human involvement. This will provide useful information for designing human sensitive structures, such as grandstands and footbridges, and for understanding serviceability issues concerning human perception of vibration that relates to many office floors. Thus the quality of life and working environment can be improved.