Research On The Design And Calculation Method Of Reinforced Concrete Chimney And The Seismic Collapse Analysis

Chimney is an important engineering project and the reinforced concrete(RC)chimney is one of the most widely used chimney structures in China.With more and more international projects undertaken by China,and the American Codes required in the design,it is particularly necessary to interpret the difference between Chinese and American Codes and revise the Chinese Codes accordingly.In recent years,the height of chimney increases for the sake of improving the exhaust capacity and fuel utilization rate.Tall chimneys are extremely sensitive to earthquakes.In terms of the current Chinese code,it is common to guarantee the seismic fortification objectives through the conceptual designs and structural measures.However,the chimneys are still likely to be damaged and collapse.The seismic fragility analysis based on structural performance is applicable for evaluating the collapse risks of chimeys under different performance levels.In order to study and improve the design methods of RC chimney and the collapse performances under seismic excitation,the major contents and conclusions in this paper can be briefly summarized as follows:(1)According to the Chinese and American Codes(GB 50051-2013 and Design and Construction of Reinforced Concrete Chimneys and ACI 307-08),this paper presents comparative study on design and calculation methods of the RC chimney from basic wind speed,along-wind load,across-wind load,load combination,circumferential bending moment,additional bending moment,bearing capacity of normal cross section and other design details requirements.The results show that there exist some differences in the design and calculating methods of RC chimney.(2)The layered-combination model is adpoted to analyze the nonlinear relationship between bending moment and curvature of sections of the RC chimney.Then the lateral displacement and the additional bending moment due to the wind load will be calculated under the ultimate limit state and the serviceability limit state.Based on the results of additional bending moment,the stiffness reduction coefficient in the Chinese code will be determined.The influence of the reinforcement ratio on the stiffness reduction coefficient is also analyzed under the ultimate limit state.The results indicate that the value of stiffness reduction coefficient which is used to calculate the additional bending moment in the Chinese code is conservative.Note that there is no need to calculate the additional bending moment based on the American codes,the stiffness reduction coefficient could be increased.And it will decrease with the increase of reinforcement ratio within limits.(3)In terms of the incremental dynamic analysis(IDA)approach,the IDA-based seismic fragility analysis method for the RC chimney is proposed.The chimney models are developed with the help of OpenSees software and the changes of maximum value of sectional curvature ductility factors with the increase of seismic intensity are analyzed.In addition,the exceedance probabilities of different performance levels for four given ground motion intensity levels are determined and the effects of openings on the fragility of chimneys are also compared.The results illustrate that the chimney meets the requirements of seismic design under a frequent earthquake,a basic earthquake and a rare earthquake of eight degree.However,when suffering a huge earthquake,an insufficient seismic potential will occur.Setting the openings will result in the increase of curvature ductility factor at corresponding section.But the openings located at the bottom of shells will contribute to improve the ductility energy dissipation and reduce the risk of a break or dislocation.