SECONDARY EFFECTS IN CABLE ROOFS

Abstract

This thesis contains a study to determine secondary effects in cable roof systems. The secondary effects which have been considered in the study are due to support flexibi lity, sequence of pretensioning, temperature and creep. A generalised algorithm for analysis of cable roof structures has been developed adopting the finite element method. The numerical schemes used for the solution of nonlinear equations are quite varied but the Newton-Raphson (instantaneous Stiffness) method has been used because of its proven efficiency for this type of problems. For the study reported, a general purpose computer program has been developed whicn caters for prescribed boundary displacements and sequential or single-step loading. The effect of support flexibility is considered in two different ways; (1 ) by adopting a shuttling technique in which the structure is divided into two substructures, the cable assembly and the supporting structure. The two sub structures are solved separately and alternately till com patibility between cable forces or boundary displacements is achievea, (2) by analysing the complete structure in one step. In this method, the stiffness terms for the support ing elements are duly incorporated in the overall stiff ness matrix of the structure. The supporting ring beam is considered as made up of 2-noded plane beam elements, a iv general algorithm is developed incorporating the cable elements and beam elements. Using the above two methods, a detailed study has been carried out on a 'bicycle wheel* type cable roof, in which radial cable trusses are supported on outer compression ring. The influence of cable roof supporting structure inter action is thus evaluated and studied. A simple flexibility coefficient approach has been used to study the effect of sequence of pret«nsioniny to determine the changes of cable tensions, and variations of thrusts and moments in the supporting ring beam structure, through various steps of pretehsioniny sequence. For this study several sequences have been assumed using bicycle wheel type cable roof as the example. The effect of rise or fall of temperature in cable systems has been incorporated by duly accounting for cable extensions or shortenings, whereas, creep strain has been calculated by using the appropriate constitutive equation and thus cable forces are modified. For the experimental study, a 3 m diameter model of bicycle wheel type roof has been used. The variations of cable tensions, beam thrusts and moment through a sequence have been determined and the behaviour of the model under symmetrical and unsymmetrical loads has been studied. Numerical stuaies have been carried out on the experi mental model used as mathematical model, as well as on a prototype.