Abstract
Horizontal ground reaction forces (GRFs) due to human walking and swaying have been investigated (respectively) through direct measurements using a treadmill and a set of force plates. These GRFs have also been measured (or estimated) indirectly using acceleration data provided by inertial measurement units (IMUs).One motivation for this research has been the lack of published data on these two forms of loading that are generated by movements of the human body in the medio-lateral plane perpendicular to the direction of walking or the direction faced during swaying. The other motivation, following from successful developments in applying IMUs to in-situ vertical GRF measurements, has been to identify best practice for estimating medio-lateral GRFs outside the constraints of a laboratory.Examination of 852 treadmill measurements shows that medio-lateral GRFs at the first sub-harmonic of pacing rate can exceed 10% of body weight. Using a smaller and more recent set of measurements including motion capture, it has been shown that IMUs can be used to reconstruct these GRFs using a linear combination of body accelerations at each of the lower back and sternum positions. There are a number of potential applications for this capability yet to be explored, in particular relating to footbridge performance.A separate set of measurements using force plates has shown that harmonic components of medio-lateral dynamic load factors due to on the spot swaying can approach 50% of body weight. Such forces provide a capability to excite horizontal vibration modes of large civil structures with frequencies below 2 Hz that are problematic for mechanical excitation. As with walking, the ability to use IMUs to estimate medio-lateral swaying GRFs outside laboratory constraints has been demonstrated. As for walking a pair of IMUs is needed, but the best linear combination varies strongly between individuals, according to swaying style. In-situ application of indirect measurement has been successfully demonstrated through a very challenging application of system identification of a multi-storey building, including estimation of modal mass.
Highlights
The effects of lateral human dynamic forces on structures became infamous due to the well-publicised behaviour of two footbridges celebrating the new millennium, Pont de Solferino [1] ( Passerelle Leopold-Sedar-Senghor) and London Millennium Bridge (LMB) [2]
It has been shown that positive work on the structure can be done by the component of the pedestrian ground reaction forces (GRFs) in the medio-lateral plane of the human body even if the pedestrian stride frequency does not coincide with the frequency of bridge lateral oscillation
The capability for forced vibration testing using human ‘shakers’ has been explored in more detail elsewhere for vertical forces and response [24], so the aim of this paper is to study the nature of human medio-lateral GRFs and their application to study of loading and response mechanisms for lateral vibrations in relevant structures
Summary
The effects of lateral human dynamic forces on structures became infamous due to the well-publicised behaviour of two footbridges celebrating the new millennium, Pont de Solferino [1] ( Passerelle Leopold-Sedar-Senghor) and London Millennium Bridge (LMB) [2]. Arup e the consulting engineers behind the LMB, describe this mechanism as 'synchronous lateral excitation' (SLE) or ‘lateral synchronous vibration’ and to this day this seems to be the most often purported cause of excessive lateral response of bridges under the action of walking pedestrians [9] According to this notion, there exists a form of positive feedback between the pedestrian and structural behaviour whereby the vibrations prompt the pedestrians to change their stride frequency, i.e. half the pacing frequency, such as to coincide with the frequency of structural motion. Based on the measurements from the LMB, Arup proposed an empirical model for the critical number of pedestrians at which the structural instability is initiated, where each pedestrian is treated as a source of negative damping to the structure This model seems to agree with some full-scale observations [4], since it is based on the measured bridge response only it lacks insight into pedestrian behaviour its general applicability can be considered uncertain. It is logical to extend the idea to using and measuring medio-lateral GRFs
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