C.S. Belsham
Professional Engineer, Southport, United Kingdom
Keywords: seismic, macroseismic intensity, multibody dynamics, simulation, testing, intensity indicator, ground motion.
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Ideally a seismic event will have been measured on a large number of uniformly distributed recording instruments; however, even in seismically active areas the distribution of instruments is sparse as a result of the high cost. Macroseismic intensity provides an alternative means of measuring ground motion of new events and importantly past seismic events.
Macroseismic intensity of ground motion is a measure of the severity of shaking at a location on a qualitative scale such as the MSK which ranges from level I, not felt, through to level XII, which corresponds to buildings destroyed. With sufficient records it can also be used to quantify the magnitude an event. However, there is a large degree of uncertainty associated with individual recordings as well as variations in consistency for intensity measures from different events. It is clearly advantageous if those uncertainties can be reduced.
Each intensity level has a set of intensity indicators which begin responding at that intensity, some of these are systems that exhibit mechanical responses and can therefore be measured against the effects of seismic input motions. This provides an opportunity to calibrate the recorded intensities against instrumented recordings and confirm that they match the intensity level they represent. The tests reported here are for MSK level V for the intensity indicator 'that a few small objects will overturn'.
Ideally the response of the indicators would be determined through physical experiments but this is an expensive and time consuming process. This paper presents an alternative approach for obtaining the response of an indicator using computer simulation of multibody dynamics and records from the European Strong Motion Database for the seismic input motion.
The use of such simulations is a new approach for confirming the association of an intensity indicator with a particular intensity level and it is also a new application in terms of the use of multibody dynamic simulations. This series of studies is carried out using a two-dimensional approximation to a three-dimensional setting by using a resultant horizontal input.
Contact between bodies is an important factor for many of the intensity indicators. This study is based on using the coefficient of restitution as it has the advantage of simplicity which is an important consideration when a large number of analyses need to be performed and it is easy to quantify in physical tests.
With the objects response being highly geometrically nonlinear and subject to random input motion, the expectation is that only general conclusions can be drawn and they are the following. The results reveal that for the events tested the intensity indicators correspond with the level criteria that a few small objects will overturn. The tests reveal that whether an object overturns is dependent on the coefficient of friction, the coefficient of restitution and the characteristics of the input motion.
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