I have still not seen in the open press what exactly was neglected in the dynamic analysis of the Millennium Bridge that caused its lowest resonant frequency of vibration to be so low. The media stated that it was a swaying effect that was not accounted for.
Was the omission of this 'swaying effect', in fact, that the mass moment of inertia of the people may have been neglected? Its effect is normally small and usually justifiably neglected. However, in the case of the Millennium Bridge, with thousands of people swaying about together, its effect may have been significant.
One of the reasons why the mass moment of inertia of additional concentrated masses is neglected, is that if reduction is used, the rotational degrees of freedom are customarily 'knocked out', as they are not usually master degrees of freedom. Depending on the computer program used, it may be difficult to add the mass moments of inertia of additional concentrated masses to 'missing' rotational degrees of freedom.
I first came across this problem in 1977 and overcame it by appropriately increasing the material density of the elements that were so affected.
Professor Carl Ross, (formerly of the University of Portsmouth), carl@tfross. freeserve.co.uk