ELASTIC POST-BUCKLING BEHAVIOR OF SEMI-RIGID CONNECTIONS PLANE STEEL FRAMES USING FINITE ELEMENT METHOD

Abstract

ABSTRACT Estimating post-buckling behavior for structures have slender elements is very important task, since post-buckling state means loss the structures stability related with large displacement and that lead to destruction the structures. On the other hand, in the design and analysis of steel frame, the beamcolumns connection is assumed perfect pin or fully rigid, this assumption leads to incorrect estimation of the structural behavior. Practically, beamcolumn connection is between these two assumptions and this type of connection is called semi-rigid. This study presents a numerical analysis using finite element method by ANSYS program to study the influence of the semi-rigid connections on post-buckling behavior of plane frames subjected to static loads. The influence of the semi-rigid beam-columns connections was studied on different frame cases such as: supporting type, number of frame stories, state of bracing (braced and unbraced), presence of lateral loading, length of frame span, depth of the frame beam and modulus of elasticity of the frame elements, also a comparison was achieved between the variables of each case. The semi-rigid connections were modeled as linear elastic rotational spring, using COMBIN14 element which permit to change its rotational stiffness value. The material of frames is assumed to be in elastic stage. Arc-length technique was used to predict the non-linear behavior of large displacement arising from buckling. The numerical analysis contains two basic parts, the first part includes modeling of three frame types with semi-rigid connections as well as the rigid connection case and the numerical results of these frames were compared with that of the previous studies to check the validity and accuracy of these finite element models. The results of the numerical study showed a good agreement with that of the previous studies. In the second part, the effects of the semi-rigid beam-columns connections were studied on different frame cases. ii The numerical results showed that; the effect of changing the beam-column connections from rigid to semi rigid with rotational joint stiffness 25EI/L to 15EI/L and 10EI/L leads to decrease the initial peak load of the frames of fixed-fixed supports with percentages 3.36 %, 5.6% and 8.95% respectively as compared with that of the rigid connection frame, While, the initial peak load not affected by changing the joint stiffness for fix-pin and pin-pin support cases. Also the results of analysis showed that, the effect of changing joint stiffness on vertical displacement in three-storey frame is significant more than in two-storey. Increasing depth of beam and modulus of elasticity lead to increase the initial peak load and decrease vertical displacement and make the frame more affected by changing joint stiffness. Bracing the frame by single diagonal and presence of lateral load make the frame less affected by changing joint stiffness. Thus, the semi-rigid connections should be considered in analysis and design of the steel frame to obtain more realistic results.