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.