The Influence of Semi-Rigid Connections on the Behaviour of Beam and Column Structural System
A storage racking system consists of three main parts: beams, uprights and the beam end connectors which provide semi-rigid connections between the beams and the uprights. The connections in storage systems are often boltless tabs. These tabs are engaged into perforations in the cold-formed upright sections at variable heights determined by the perforation pitch. The fundamental aim of this study was to investigate the behaviour of boltless semi-rigid connectors under loading and unloading conditions to determine the effects on the general behaviour of racking frames. A comprehensive literature review in semi-rigid joint behaviour was undertaken for heavy and light steel structures. This literature survey showed that most of the previous work in boltless beam end connectors and their applications in racking frames concentrated on the joint behaviour under the effect of direct monotonicaJJy increasing load. Most of the work that bad been carried out on semi-rigid frames was mainly theoretical with the assumption of a linear stiffness. Twenty beam end connector tests were carried out to study the effect of loading and unloading on joint behaviour. In each test two methods of displacement measurements were used in obtaining the moment curvature relationship. The same joints were tested under hogging and sagging moments to obtain a fuJI understanding of the joint behaviour. Initial moment-curvature diagrams showed different stiffness values between vertical and horizontal directions of measurement. The horizontal method gave higher stiffness values than the vertical method. This difference was investigated and two correction equations were derived reducing this difference to a small value. Results showed that under loading, the moment-curvature curve could be modelled by a multilinear curve. Reversing the direction of loading unloads the joint and the curve follows a different path which is linear until loading in opposite direction takes place. A theoretical procedure for the analysis of frames with semi-rigid connections was proposed. The procedure is based on the idea of linearizing the moment-curvature relationship as a tri-linear curve following the experimental curve in both loading and unloading directions. A computer program was written for the analysis of the frame structure based on a non-linear elastic analysis incorporating stability functions. Different frame examples have been used to fully test the program. The results have been successfully compared with other research and with a finite element model demonstrating the full capabilities of the computer program. The research further investigated the joint behaviour and the sway deflection behaviour of a full racking frame structure. Eight pin-based racking frame structures of one storey and one bay were tested under the effect of loading and unloading with two different percentages of lateral loading. The tested frames were laid horizontally applying a mirror image system to enable the frames to sway freely without friction and with load position maintained at the same point. A theoretical analysis was undertaken using three different stiffness models to compare the results against the tested frames. A tri-linear stiffness model and two bi-linear stiffness models were included in the study; the first model was based on the Federation Europeene de la Manutention code (FEM code) requirements while the second model was calculated using the storage equipment manufacturers association code (SEMA code). Results showed that the tri-linear stiffness model managed to predict a similar behaviour to the tested frames under the effect of loading and unloading and while the two bi-linear models failed to predict the frame unloading behaviour. For the monotonic loading case the FEM code stiffness model was capable of achieving a good correlation in analysis with the tested frames and with the tri-linear model, providing simplicity in analysis. The SEMA code model failed to predict the frame sway behaviour as it led to lower deflection values. Under the combination of side and vertical loading, the tested samples as well as the analytical solution had shown that unloading of a connection occurred. The theoretical study was extended to cover the effects of joint type on frame behaviour also to study the effects of loading and unloading on multi-bay and multi-storey frames. Results of the portal frames showed a good correlation between theoretical and experimental predictions. Suggestions and recommendations are provided to achieve safe and simple analysis models.
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