Title:
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The ultimate strength of uniformly loaded laterally restrained rectangular two-way concrete slabs
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The thesis describes a theoretical and experimental
investigation into the ultimate flexural strength of uniformly
loaded two-way rectangular concrete slabs (reinforced and unreinforced)
with membrane action induced by restraint against
lateral displacement at the boundaries. The experimental part
of the programme involved the making up and testing of 45
concrete slabs. In addition, the results of 22 slabs tested by
other investigators are analysed. The investigation is divided
into four main parts.
Firstly, a yield-line theory incorporating a rigid-plastic
strip approximation is developed which defines the loaddeflection
curve at and after the ultimate flexural load for
slabs with rigid boundaries during the compressive membrane
action stage. Cases of boundary restraints which produce
compressive membrane action in either one or two directions are
considered and the accuracy of approximate yield-line patterns
are investigated. On the basis of an empirical value~first for
the width of the yield bands and thence for the central
deflection at the ultimateload~expressions are obtained for the
ultimate flexural strength of slabs which compare well with
results obtained from slabs tested under short-term loading in
very stiff surrounding frames.
Secondly, the theory is generalized to take into account
the effects of axial strains in the slab and of small lateral displacements at the boundaries,in order that the etfects
of long-term loading and elastic boundary restraints may be
included •. The theory is checked against test results obtained
:f'rom slabs tested under periods of sustained loading. It is
shown that axial strains and small lateral displacements at
the boundaries can significantly reduce the compressive
membrane action, especially in the case of thin slabs.
Thirdly, the behaviour of a slab as a tensile membrane
at large deflections beyond that at the ultimate :f'lexural load
is examined. A theory is developed to de:f'ine the loaddeflection
characteristics ,assuming that the reinforcement
acts as a plastic membrane. It is shown that heavily reinforced
slabs can carry loads by tensile membrane action which exceed
the ultimate :f'lexural load.
Finally, the laterally sti:f'fness and strength required
01' surrounding beams and panels of slab and beam floors in
order to entorce membrane action in interior panels is
investigated. Results obtained from tests on the interior
panels of 9 panel floors are analysed and show good agreement
with the theory.
An appendix containing some results obtained by the
conventional Johansents yield-line theory is also included
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