Non-linear analysis of infilled frames
This thesis is concerned with the analysis of building frames acting compositely with infilling wall panels. The significance of the composite action is emphasized and previous work on infilled frames is reviewed. The existing methods of analysis are categorized and their analytical assumptions are highlighted. It is concluded that more accurate results may be obtained from the development of a non-linear finite element analysis. The finite element method is reviewed and new elements for representing beams, interfaces and loading are developed. Failure criteria for concrete under multiaxial stress and also failure criteria for masonry under uniaxial compression are developed. The non-linear elastoplastic behaviour of concrete is modelled using the concept of equivalent uniaxial strain and the model is extended for cracked materials. Elastoplastic models are also developed for ductile materials(steel) for secant and incremental changes of stresses and strains. These models and the newly developed elements are incorporated into the finite element analysis which is numerically implemented by a new computer program, NEPAL. A number of steel frames with concrete inf ills covering the practical range of beam, column and infill strengths and also wall panel aspect ratios, are analysed using this program. The finite element results are compared with the predictions of a range of existing methods of analysis and their limitations are discussed in detail. A new method of hand analysis is developed, based on a rational elastic and plastic analysis allowing for limited ductility of the infill and also limited deflection of the frame at the peak load. The new method is shown to be capable of providing the necessary information for design purposes with reasonable accuracy, taking into account the effects of strength and stiffness of the beams and columns, the aspect ratio for the infill, the semi-rigid joints and the condition of the frame-infill interfaces (co-efficient of friction and lack of fit). It is concluded that simple and economical design approaches can be established for frames with infilling walls.