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Bennett Associates has recently completed the seismic analysis
of 45 tonne and 10 tonne dockside cranes on behalf of the manufacturer,
Butterley Engineering Ltd. The cranes will be used at Devonport Docks for the
servicing of nuclear submarines. ANSYS (the general purpose finite element
program) was used for computer modelling.
The conventional approach for seismic analysis of most structures
is to apply the earthquake in the form of a spectrum consisting of a range of
frequencies with corresponding accelerations applied to a linear elastic model.
In the case of jib cranes this approach would have been too severe and would
have resulted in an over conservative and heavy design.
Therefore, in addition to a spectral analysis, Bennett Associates
also produced a non-linear time history analysis, using a finite element model
which incorporated non-linear features such as wheel lift-off and luff/hoist
pulley/rope sub-models. The ropes were unable to carry a compressive load and
were therefore modelled on tension only spars. This allows the jib to move in a
realistic fashion relative to the mast as the pulleys and ropes rotate and
oscillate during the seismic event. These non-linearities contribute to energy
dissipation of the earthquake and additional damping during the seismic event.
This approach resulted in reduced design wheel loads and a more economical
design. It also allowed the rocking behaviour of the crane to be analysed and to
predict if the crane would topple over in a seismic event.
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