Wind Load And Wind-induced Buckling Of Large Steel Tanks

Vertical cylindrical welded steel tanks are typical thin-walled structures which are widely used for fluid and bulk storage, such as oil, natural gas, grain and cement in industrial and agricultural plants. The welded steel oil tanks are representative of these structures. With the development of economy and oil industry and the implementation of the national energy strategy, more and more oil storage tanks are put into service in recent decades, especially large steel tanks. Because of high diameter-thickness ratios and low aspect ratios, tanks are wind-sensitive and therefore very susceptible to buckling under wind loads especially when they are empty or partially filled. Buckling of tanks sometimes even occurs under moderate wind load during their construction. Over the past few decades, buckling failures of cylindrical steel tanks and silos during windstorm have occurred in many countries and regions. Because of serious economic losses and environmental problems due to the destruction of storage tanks, studies about buckling of tanks under wind load have been conducted extensively over the past few decades. However, it is found that most of the literatures focus on small and medium tanks and silos while few studies have been conducted on the buckling behavior of large practical steel tanks with stepped wall. It is thus very important to conduct research on the wind loads and wind-induced buckling behaviour of large steel tanks which will also be put into a wider application.This dissertation reports the work done to evaluate the wind loads and the wind-induced buckling strength of large steel tanks, with aim to.r.eveal the wind-induced buckling mechanism and behavior of large steel tanks and provide advices for reasonable wind-resistant design of large steel storage tank. The strategy in the research is experimental and computational, in which wind tunnel experiments are carried out on rigid models to obtain wind loads on large cylindrical tanks, and the pressures are next used in a finite element model to evaluate the buckling behaviour of large steel tanks. The layout is organized as follows:Chapter1introduces the background of this dissertation. Literature of topic on the wind loads and wind-induced buckling of cylindrical tanks as well as silos is reviewed and the suggestions of several national design codes are summarized. The argument and idea of this thesis are also given.Chapter2presents the numerical simulation of wind loads on large vertical cylindrical tanks. The basic principles of fluid dynamic and its numerical solution method, computational fluid dynamics (CFD), are fist introduced. The commercial software package for general purpose Fluent is then used to carry out the simulation of wind loads on tank models. Results of wind field around a tank and wind loads on tanks are discussed, providing a reference for the design of wind tunnel test of large storage tanks.Chapter3reports the wind tunnel test of the wind loads on an isolated tank. The test covers three types of tanks:open-top tank, flat-roof tank and the dome-roof tank. The first four moments of the measured wind pressure, including the mean and normalized deviation pressure, kurtosis and skewness of the pressure signal, are obtained to study the feature of the wind loads. The probability distribution of fluctuating wind pressure is examined and compared with the Gaussian distribution. The effect of roof on the wind loads of cylindrical wall and the correlation of the mean wind pressure and fluctuating wind pressure are discussed. Comparison of wind loads between present study and related literatures is carried out. Based on the data of wind loads obtained from wind tunnel test, a Fourier formula is fitted for the wind load distribution of the cylindrical shell for design.Chapter4contains the wind tunnel test conducted to investigate the wind loads on grouped tanks. Three types of tank groups are covered in this study:two adjacent tanks including tandem, parallel and staggered configurations, three adjacent tanks in triangular array and four adjacent tanks in square array. All together, there are519cases in the test. The effects of spacing between tanks and wind attack angle on wind pressure distributions of both external and internal wall are investigated, and the difference of wind loads on tanks in a group compared with those on an isolated tank is discussed.Chapter5describes the general behavior of wind buckling of cylindrical tanks. The basic mechanics characteristics, the basic concept of stability theory and the development history of nonlinear stability theory of thin-walled structures are fist introduced. The finite element method for buckling analysis is described. The classification of buckling of thin cylindrical shells and the concept of lateral pressure stability are put forward. Several common practical tanks with volume of2000m3～50000m3are then chosen to investigate the buckling behavior of this type of structures. Both the linear elastic bifurcation analysis and the geometrically nonlinear analysis are carried out. The sensitivities of the welded-induced and eigenvalue mode imperfections are also discussed.Chapter6examines the buckling behaviour of large steel tanks under wind loads. The wind loads applied on finite element models are obtained from the wind tunnel test and the investigation is carried out by using bifurcation analysis and non-linear analysis. The effect of initial geometric imperfections on buckling behavior is analyzed through geometrically non-linear analysis. The effects of thickness reduction of cylindrical shell and the liquid stored in tank are also included. The buckling capacity of grouped tanks is also considered and compared with that of an isolated tank.Chapter7considers the nonlinear dynamic buckling behaviour of large steel tanks under wind loads. The dynamic stability theory and the criterion for evaluate the dynamic stability of a structure are first introduced and then the dynamic buckling behaviour of large steel tanks is investigated by using dynamic time history analysis. The effect of initial geometric imperfections on dynamic buckling behavior is also included.Chapter8discusses the design method of wind girders for large steel tanks. In light of current design code and engineering method, the strengthening mechanism and damage feature of the components for wind buckling resistance, including the top angle iron, the upper wind girder and the middle wind girder of tanks, are investigated. The difference of formula for wind girder design between several standards is discussed. The buckling performance of tanks with wind girders is examined and some suggestions for design of the components for wind buckling resistance of tanks are also given.Chapter9summarizes some important conclusions and indicates some further work which may contribute to the research on this topic.