Taking advantage of geometric considerations the authors give explicit expressions for the determination of axial force, bending and twisting moments in the helical wires, and for the axial force and twisting moment in the core of a 7-wire strand subjected to axial and torsional displacements. The equations given are linear, could be similarly developed for strands of any number of wires or for strands subjected to large displacements, and should be found practical by designers. Measurements on oversize epoxy models of the strand show good correlation with the theory and support the observation that axial load has no effect on the effective torsional rigidity of the strand. Results of experimental strain analyses on steel strands and on models of strands, as well as other theoretical studies on helices and strands, will be published in other papers.
Photoelasticity has been used recently by the authors to conduct many tests related to the determination of stresses in the field of tall beams, and to the optimization of those beams.Numerical methods (in particular finite -elements, and solutions of Laplace's equations) have also been used to supplement or to verify the results obtained photoelastically.The opportunity seems appropriate to weigh some of the advantages and limitations of the methods used.Photoelasticity gives readily the field solution of Tmax.An approximate method can be used to obtain normal stresses using equilibrium conditions and photoelastic data. The photoelastic test can also be used to determine the boundary conditions, or part of them when they are not known, as in the case of many hyperstatic structures. The complete and precise knowledge of the boundary conditions is not always necessary.Numerical solution of Laplace's equation which corresponds to the sum of the principal stresses can be obtained easily for the whole field with the computer. Knowledge of boundary condition is necessary.Complete solution in terms of individual principal stresses can be obtained using finiteelements.Knowledge of boundary conditions is necessary. When gradients are high, the size of the elements must be small which may require longer time in the computer.The decision on what method to use depends to a large extent on 1) The availability of a photoelastic laboratory; 2) The availability of a computer and the program; 3) The availability of personnel with the necessary experience; 4) The knowledge, or lack of knowledge of all the boundary conditions.
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