2 DOF model for non-linear vertical seismic analysis of a loaded overhead crane

• Simplified numerical model to simulate the dynamic behaviour of an overhead nuclear polar crane loaded on his main hook. • Capacity for detection and quantification of crane-rail decoupling under seismic actions. • Time history simulations reproducing benchmark/reference experimental tests of polar cranes. • Spectrum diagrams to identify the best combinations of dynamic/mechanical parameter values to obtain the best crane performance. In nuclear power plants, polar cranes are used for fuel assembly purposes and reactor head removal and replacement. A simplified numerical model is presented to simulate the dynamic behaviour of an overhead nuclear polar crane loaded on his main hook and taking into account the possibility of the uncoupling effect of the mechanical parts of the crane. The model extends, at a relatively low computational cost, a previous model proposed in the literature, but adding the possibility of detecting crane-rail decoupling under seismic actions. Time history simulations reproducing experimental tests considered as benchmark, including experimental tests of a 1:8 scaled model of a 330 T bridge crane, have been performed aiming to analyse the reliability of the numerical model. Critical seismic acceleration threshold values, slightly higher than 0.3 g, have been calculated for which the crane-rail decoupling appears. The separation values during the uncoupling phases, calculated for higher values of seismic acceleration, adequately fit the experimental results obtained in the experimental benchmark tests. Additional simulations, including the crane-rail uncoupling effect, were undertaken in order to study the influence and the interaction of the characteristic parameters in the dynamic response of the crane. In this respect, the crane-rail separation and the dynamic amplification factor have been calculated as a function of the fundamental frequency of the crane and the rope length. Results have been arranged as spectrum diagrams to identify the best combinations of dynamic/mechanical parameter values to obtain the best crane performance, which is a relevant objective in the design phase of a polar crane. To include the possibility of sway motion, the proposed model is extended by including a degree of freedom corresponding to the angle rotated by the load. The dynamic amplification factor, applying the 2DOF and 3DOF models, generates, for different horizontal initial velocity conditions, practically identical results. Thus, the simplified numerical model (2DOF) is an appropriate and computationally low-cost option for estimating cable loading and verifying compliance with current regulations.