Stability and dynamic behaviour of steel structures with non-linear restraints
This study was undertaken to investigate the effects of imperfections in the initial geometry of bracing members on the stability of the structural frameworks. The general non-linear behaviour of frameworks, consisting of single columns, or multistorey frames stiffened by curved bracings, were studied under the effects of combined vertical and horizontal load systems. The study was divided into two main parts. In part one, the study examined the structural frameworks in the following situations: i) Influence of initial bowing on the behaviour of individual members subjected to axial or eccentric forces. ii) General static behaviour of a single column restrained by curved member or members. iii) General static behaviour and instability of multistorey frameworks with non-linear cross bracings. It has been the goal of the thesis to reinforce the theory put forward to explain the particular type of instability encountered, therefore a critical state, or transient instability region, has been investigated. The characteristics of individual curved members were determined using the theory of large deformations. The general behaviour and the stability of frameworks restrained by imperfect bracing systems were studied using tangent slope and influence coefficient techniques. The results of this study have shown that the initial imperfections of bracings are very important and have major effects on the overall behaviour of the braced frame structures. The particular type of instability encountered, i. e. the critical state or the transient instability region, may be considerably influenced by the initial geometric imperfections of bracings and the relative magnitude of the ratio between vertical and horizontal applied loads on the frameworks. The critical loads have been presented in a series of curves and tables. In part two of the study, the dynamic behaviour at the critical state, i. e. in the region of transient instability, has been investigated. Numerical methods for the dynamic analysis of structural frameworks have been discussed. A new procedure of numerical differentiation has been presented and its advantage over existing procedures has been shown. The method is convenient for use with a digital computer and can also be used for solving simple problems with a calculator. In general the results of parameters studied were presented in a series of curves and tables to enable the stability and dynamic actions to be readily determined for a wide range of structural configurations. Finally, a test programme was carried out to investigate experimentally the non-linear behaviour of frameworks restrained by these imperfect bracings. Three separate models were used in the experimental programme. The experimental results were used to verify the general accuracy of the theoretical methods of analyses. In general the theoretical results and the experimental ones were in very close agreement.