Faris Albermani

Numerical simulation of structural behaviour of transmission towers

Transmission towers are a vital component and management needs to assess the reliability and safety of these towers to minimise the risk of disruption to power supply that may result from in-service tower failure. Latticed transmission towers are constructed using angle section members which are eccentrically connected. Towers are widely regarded as one of the most difficult form of lattice structure to analyse. Factors such as fabrication errors, inadequate joint details and variation of material properties are difficult to quantify. Consequently, proof-loading or full-scale testing of towers has traditionally formed an integral part of the tower design. Stress calculations in the tower are normally obtained from a linear elastic analysis where members are assumed to be axially loaded and, in the majority of cases to have pinned connections. In practice, such conditions do not exist and members are detailed to minimize bending stresses. Despite this, results from full-scale tower test often indicated that bending stresses in members could be as high as axial stresses. EPRI (1986) compared data from full-scale tests with predicted results using current techniques and concluded that the behaviour of transmission towers under complex loading condition cannot be consistently predicted using the present techniques. They found that almost 25% of the towers tested failed below the design loads and often at unexpected locations. Furthermore, available test data showed considerable discrepancies between member forces computed from linear elastic truss analysis and the measured values from full-scale tests. The paper describes a nonlinear analytical technique to simulate and assess the ultimate structural response of latticed transmission towers. The technique may be used to verify new tower design and reduce or eliminate the need for full-scale tower testing. The method can also be used to assess the strength of existing towers, or to upgrade old and aging towers. The method has been calibrated with results from full-scale tower tests with good accuracy both in terms of the failure load and the failure mode. The method has been employed by electricity utilities in Australia and other countries to: (a) verify new tower design; (b) strengthen existing towers, and (c) upgrade old and aging towers.

Nonlinear analysis of transmission towers

A nonlinear analytical technique for predicting and simulating the ultimate structural behaviour of self-supporting transmission towers under static load conditions is presented. The method considers both the geometric and material nonlinear effects and treats the angle members in the tower as general asymmetrical thin-walled beam-column elements. Modelling of material nonlinearity for angle members is based on the assumption of lumped plasticity coupled with the concept of a yield surface in force space. A formex formulation is used for automatic generation of data necessary for the analysis. The developed software, AK TOWER, is used to predict the ultimate behaviour of two full-scale towers recently tested in Australia.

Elasto-plastic large deformation analysis of thin-walled structures

This paper presents an elasto-plastic, large deformation analysis of thin-walled structures. A solution procedure for treating both the geometric and material nonlinearities, based on an updated Lagrangian formulation, is proposed. The procedure is suitable for analysing large-scale space frames since the structures may be modelled using only a single beam-column element per member. It is achieved by incorporating a displacement stiffness matrix which provides the necessary coupling between the axial stretching and the flexural and torsional deformations. The geometric and the displacement stiffness matrices for a general, thin-walled, beam-column element previously derived in References 1 and 2 are used. Plastic hinge formation, the interaction of element forces at the hinges and elastic unloading are taken into account in the analysis. In particular, a single-equation, stress-resultant yield surface has been developed to model the plastification of structural steel angle sections under a combination of axial force and moments about the principal axes. Yield surfaces for tubular sections and American wide flange sections are also considered. Several numerical examples are presented to demonstrate the accuracy and efficiency of the method.

Cyclic and seismic response of flexibly jointed frames

Abstract A model simulating the hysteretic M-θ r behaviour of flexible joints under cyclic and dynamic loading conditions is presented. This model requires four parameters to describe. These parameters can be evaluated using existing test data for different types of connections. The proposed model has been cast into a two-node zero length connection element and used to model the nonlinear response of flexibly jointed frames. It is shown that the presence of flexible joints alters the vibration characteristics of the structure. Depending on the excitation frequency, the presence of flexible joints can either dampen or magnify the structural response.

Stability of thin-walled members having arbitrary flange shape and flexible web

A finite element method is presented for analysing thin-walled structural members comprising a flexible web connected to one or two rigid flanges of arbitrary shape. A general thin-walled beam-column element is used to model the flanges while a thin plate element is used to model the web. Based on the derived total potential energy functional, explicit linear and geometric stiffness matrices for the two types of element are obtained. Using static condensation and appropriate transformations, the beam-column element and the plate element are combined to yield a super element with 22 degrees of freedom capable of modelling the flexural, torsional, web distortional and coupled web and flange local buckling modes of a general thin-walled member. The technique may be used to predict the elastic buckling load of members under any loading and boundary conditions. Several numerical examples are presented to demonstrate the accuracy, efficiency and versatility of the method.

Dynamic response of flexibly jointed frames

A bounding-line model simulating the hystereticM-θr behaviour of flexible joints under dynamic loading conditions is presented. The four parameters required to describe this model are evaluated and presented using existing test data for different types of connections. The proposed model has been cast into a two-node zero length connection element and used to model the nonlinear dynamic response of flexibly jointed frames. It is shown that the presence of flexible joints alters the vibration characteristics of the structure. Depending on the excitation frequency, the presence of flexible joints can either dampen or magnify the structural response.

A COUPLED KNOWLEDGE-BASED EXPERT SYSTEM FOR DESIGN OF LIQUID RETAINING STRUCTURES

This paper describes a coupled knowledge-based system (KBS) for the design of liquid-retaining structures, which can handle both the symbolic knowledge processing based on engineering heuristics in the preliminary synthesis stage and the extensive numerical crunching involved in the detailed analysis stage. The prototype system is developed by employing blackboard architecture and a commercial shell VISUAL RULE STUDIO. Its present scope covers design of three types of liquid-retaining structures, namely, a rectangular shape with one compartment, a rectangular shape with two compartments and a circular shape. Through custom-built interactive graphical user interfaces, the user is directed throughout the design process, which includes preliminary design, load specification, model generation, finite element analysis, code compliance checking and member sizing optimization. It is also integrated with various relational databases that provide the system with sectional properties, moment and shear coefficients and final member details. This system can act as a consultant to assist novice designers in the design of liquid-retaining structures with increase in efficiency and optimization of design output and automated record keeping. The design of a typical example of the liquid-retaining structure is also illustrated

Nonlinear finite element analysis of latticed transmission towers

Current design practices for transmission tower structures are based on 3D linear elastic truss analyses and on full-scale testing experience. This paper reviews current practices and presents a nonlinear analytical technique for accurate simulation and prediction of the ultimate strength and behaviour of transmission towers under static load conditions. Both geometric and material nonlinearities are accounted for in the analysis. A formex formulation is used for the automatic generation of data that is necessary for the analysis. The behaviour of four different full-scale towers is described and the predicted results are compared with tests.

Stability of cold-formed members

A finite element method capable of predicting the buckling capacity of arbitrarily shaped thin-walled structural members under any general load and boundary conditions is presented. A rectangular thin plate element with 30 degrees of freedom is used. The linear and geometric stiffness matrices for this element are derived explicitly using symbolic manipulation, thereby eliminating the need for the expensive process of numerical integration. Further, the explicit form of the stiffness matrices makes it easier to interpret the physical significance of the various stiffness terms. For sections composed of rigid flanges and flexible web, a lower-order plate element is used in combination with a general beam-column element to form a super element for predicting distortional buckling modes. Formex formulation is used for the automatic generation of the data necessary for the analysis. Numerical examples of thin-walled structural members involving local, distortional and flexural-torsional buckling modes are presented to demonstrate the accuracy, efficiency and versatility of the method.

Nonlinear dynamic analysis of lattice structures

A computational procedure for predicting the geometric and material nonlinear dynamic response of space trusses is presented. An updated Lagrangian method, based on the incremental formulation of the equation of motion, is employed. The constitutive law is cast into the Ramberg-Osgood polynomial model. Numerical studies of two- and three-dimensional truss structures under various dynamic loading are made. The structure dynamic response including different types of nonlinearity is compared with the linear response and with the results obtained from static analysis.