Investigation Of The Structural Behaviours And Design Optimization Of Long Span Cable-Supported Irrigation Structural System

Cables have been applied to develop various forms of Cable-supported Structures(CSS).Cables are also applied for supporting pipelines over large spaces.For example,Frank Zybach utilised the tensile strength of the cable to support a large span of pipes over an irrigation field in his invention of the centre pivot system.The concept of the Centre pivot system has been applied in irrigation structures for water application in farms.However,a novel application in this study resulted in a new concept,referred to as the Cable-supported Irrigation Structural systems(CISS).Since pipes are long and flexible,some forms of the structurally efficient cable-support system are suggested to meet their spatial requirements.The CISS is recognized to be structurally efficient and a possesses large spatial capability,thus,a frontier subject of study in civil engineering.Reviewed literature revealed several research developments of cable-supported structures,but limited studies on the concept applied in the farm structures.Based on the reviewed literature,three major structural applications of cable support including buildings,towers and bridges are recognized,in buildings,towers and bridges.CISS is proposed to offer the advantages of lightweight,spatial capability and robust structural systems by combining cable members with truss members to support pipelines to form pipe bridges.However,due to spatial requirements as elevated pipe structures and the need to satisfy structural performance,it is expediently necessary to investigate proposed structures.Furthermore,design parameters such as mid-span depth and width,cable pretension as well as static and dynamic load effect resulting in undue vibratory effects and failure of members are crucial interests of study.Thus,these resulting complications must be addressed to meet the serviceability and ultimate design requirements in the engineering application of CISS.Such requirements necessitate the study of structural optimization subject to static load,dynamic characteristics,wind-induced vibratory response and control,and collapse mitigation of the structure.Consequently,experimental and numerical approaches are deployed in this study to investigate load-displacement behaviour,vibration control and collapse mitigation of the structure.Founded on the available laboratory space,scaled-down models of the CISS were evaluated using both novel experimental setup and numerical processes with the aim to investigate the static load performance and dynamic characteristics of CISS structure.A specially developed pipe clamp loading pad(PCLP)that aided innovative loading test sets on both single and double tube pipe systems was utilized to perform the static load test.The load-displacement response and stress in members were obtained after performing a static load test.The results show that the critical design variables include the mid-span depth,number of truss modules,and CISS structure’s cable section.A structural design optimization method combining response surface analysis and Multi-Objective Genetic Algorithm(MOGA)was proposed through the acquired ideas from the parametric analysis and test results.The objective of the optimization program is to minimize the displacement and stress on members of the CISS structure.The response surface method was used to develop the performance response of the loaded CISS structure,and the outcome shows that the performance projected by the response surface model is reliable.The optimization results suggest a set of Pareto optimum solutions the span depth,number of truss modules and cable section within the constraint of limit displacement and stress of the CISS structure.The evaluation of the dynamic properties of the models was achieved using experimental modal analysis.The validity of the experimental results was tested using modal assurance criterion(MAC)with the numerical models to aid the updating of the64.8m CISS model.The resonating frequencies and damping of the structure were then compared to those obtained from the study of other CSSs and provisions in codes.The results suggest that long span CISS structures are more vulnerable to resonance considering the reduction of the modal frequencies of the 64.8m span finite element models.The low frequencies and multiple modes necessitate the application of a passive control device.The evaluation of the structure through various modification predictions of structural mass,cable pretension and tuned mass dampers(TMDs)provided useful clues to strategy for control of wind-induced vibration of the CISS structure.Due to the varying mode shape in the short frequency band,it is necessary to deploy multiple TMD along the span of the CISS structure.A significant aspect of this study involves the evaluation of the collapse resistance of the structure.Spontaneous dynamics and internal force rearrangement characterize a sudden break of a structural member.These phenomena were verified using results obtained from explicit dynamic analysis and further investigated on various design parameters of the scaled-down CISS models.Thus,critical members causing large dynamic response amplitudes were identified,and the mitigation of collapse susceptibility was evaluated.Dynamic increasing factor(DIF)methods were utilized to assess the dynamic response resulting from the pattern of responses between the cable members and other members after the sudden cable break of the CISS structure.Dynamic factor(DF)was proposed to evaluate the cable members.Comparison with provisions in other studies and post-tension institute(PTI)shows higher values DIF of the CISS cable members,which led to proposed evaluation using the dynamic factor(DF).