Research On Dynamic Characteristics And Load Response Of Tower Cran

Wind load is the controlling load for tower cranes.The magnitude of wind load depends not only on the basic wind pressure,tower crane height,and tower crane shape but also on the wind vibration coefficient,which is influenced by the dynamic characteristics of the tower crane,especially its natural frequency.The current load specifications in China for estimating the natural frequency of tower cranes are very crude and require further research.Firstly,on-site tests were conducted for the empty load,half load,and full load conditions of the Zhongtian 6513 flat-top tower crane and Yongmao 5513 luffing jib tower crane.The measured values of the natural frequency of the tower crane were obtained for different lifting loads.The test results showed that the lifting load ratio(lifting load/upper mass of the tower crane)had an impact on the natural frequency,but the effect was not significant,mainly due to the relatively small lifting load ratio.Modal analysis using finite element software was then performed for these two tower cranes under the empty load,half load,and full load conditions,and the calculated natural frequencies were compared with the measured values.The maximum deviation was less than 5%,which verified the reliability and accuracy of the finite element method.By comparing the displacement contour plots obtained from the modal analysis with the finite element method,it was observed that the vibration mode of tower cranes is similar to that of a single-degree-of-freedom beam.The tower crane was simplified as a single-degree-offreedom model,where the tower body was simplified as an elastic beam with uniform crosssection,and the upper part of the tower crane was simplified as a single mass point.A simplified calculation formula for the natural frequency of the tower crane was preliminarily proposed.Due to the limited test data,which did not fully reflect the influence of parameters such as tower height and lifting load ratio on the natural frequency,a parametric analysis was conducted using finite element software.Taking into account the influence of tower crane mass distribution and height,the derived formula for the structural natural frequency was modified twice.First,modal analysis was performed based on different ratios of upper structure mass,and the natural frequency variation data with respect to the ratio of upper structure mass was obtained to modify the simplified calculation formula.Then,by adding or removing standard tower sections,tower crane models with a height spacing of 6 meters were established for luffing jib tower cranes and flat-top tower cranes,respectively,to modify the simplified calculation formula for height.Finally,the modified formula for the natural frequency was obtained.The "Design Code for Tall Structures" requires the calculation of wind load using the natural frequency of tower cranes.The proposed formula for the natural frequency can be applied to avoid the need for individual finite element simulations to obtain modal analysis data,simplifying the calculation process.By considering different tower heights,selecting various basic wind pressures,and ground roughness types,precise calculated values for wind loads under different lifting loads were obtained.The calculated values were compared with those from three different specifications to summarize the characteristics of each specification.The Davenport wind speed power spectrum was transformed into wind pressure time history curves for each stage of the tower body.These curves were input into the finite element model at different angles with respect to the tower body to analyze the wind pressure time history.Under any rotation angle of the jib where the tower crane is in working condition,the lifting moment remained constant,and the distance of the lifted load was varied to compare the wind load-induced displacement at the top of the tower body.The least favorable jib rotation angle under various wind pressures was determined to avoid working in that direction under strong wind conditions and reduce the risk of overturning.Seismic response spectrum analysis was performed on the tower crane using ABAQUS finite element software to obtain the displacement at the top of the tower body.By comparing the results with the independent height displacement values under the single-degree-of-freedom model,adjustment factors for the displacement at the top of the tower body were determined.These adjustment factors were incorporated into the displacement calculation formula for the single-degree-of-freedom model,yielding a formula for calculating the displacement at the top of the tower body considering different levels of seismic action,providing a reference for tower crane spacing.Selected seismic waves that meet the code requirements and artificial waves derived from the response spectrum were input into the ABAQUS finite element analysis software along with the wind pressure time history curve.By varying the angle between the wind load and the seismic action,the top displacement of the tower under load combinations was obtained to verify the most unfavorable action angle.