Study On The Application Of Base Isolation For Ultra Large Cooling Tower Structure

With the rapid development of economy in our country, the construction scale of thermal power unit in the power plant is increasing, and the body of corresponding cooling tower is larger and larger. However, the ultra large cooling tower in high intensity areas is difficult to meet the needs of seismic fortification. This article applied base isolation technique using laminated rubber bearings to the ultra large cooling tower structure. Simulation and analysis of the ultra large cooling tower with and without base isolation was carried out under the action of gravity, wind load, temperature effect and earthquake action using the finite element analysis software SAP2000, and the work below has been completed.Base isolation design using laminated rubber bearings was made for an ultra large cooling tower structure as example. The characteristic parameters and layout of isolation bearings are preliminarily determined using simplified calculation method in the code for seismic design of buildings. And appropriate numerical simulation method is chosen based on SAP2000. Comparative analysis was made between the cooling towers with and without base isolation according to the load effect combination specified in the codes, which verified the correctness of the selection and layout of isolation bearings and the feasibility of base isolation applied to ultra large cooling towers, and revealed the obvious decrease of cooling tower internal force response under the action of different kinds of loads and the combination of them when base isolation measure was applied.According to the seismic isolation analysis method in the current code, seven earthquake acceleration time history records input in three orthogonal directions were used to analyze the seismic response of the cooling tower structures with and without base isolation under consistent seismic acceleration excitation in frequent earthquakes, precautional earthquakes, and rare earthquakes corresponding to 8 degree fortification(0.2g), which demonstrates that the response of X pillar internal force, shell principal stress and base shear decreases significantly when base isolation measures are taken, and the superstructure of the ultra large cooling tower remains intact even in rare earthquakes. It is also found that the vertical earthquake action cannot be ignored.Considering the travelling wave effect, three earthquake acceleration time history records input in single direction were used to analyze the seismic response of cooling tower structures with and without base isolation under inconsistent seismic displacement excitation in frequent earthquakes, precautional earthquakes, and rare earthquakes in areas of 8 degree fortification(0.2g), under the apparent wave velocity of 200m/s, 400m/s and 600m/s according to the site of type III. It is found that the internal force of ultra large cooling towers with and without base isolation is both amplified, and the torsion moment of X pillars increases more obviously considering the traveling wave effect. The traveling wave effect has significant influence on the ultra large cooling tower without base isolation and little influence on that with base isolation.The seismic response of X pillars was studied further considering the traveling wave effect comprehensively. The comparative analysis of torsion displacement and base shear time history of cooling tower structures with and without base isolation was made, which showed the apparent torsional vibration and large base torsion moment of cooling tower under inconsistent seismic excitation. It is discovered that the relative displacement between piers of X pillars leads to torsion moment amplification of X pillars in the cooling tower without base isolation mainly, while the torsion vibration leads to the torsion moment amplification of X pillars in the cooling tower with base isolation. Base isolation measures significantly reduce the relative displacement between piers of X pillars and torsion moment of X pillars, which remarkably diminishes the adverse effect due to the torsion vibration under inconsistent seismic excitation.