In a final-year project at Cambridge, Trim (2001) tested
several cantilevers, representing a beam end framing in to a column, under
cyclic loading with a reversing plastic hinge and appreciable imposed deformation
both ways, as occurs in an earthquake. This loading was followed by a static
shear test with low moment at the hinge position. Most of his specimens had
conventional shear stirrups, and showed marked damage and appreciable loss
of shear strength. However, in one last test, with spiral binding top and
bottom in place of stirrups, confining the compression zones and holding on
the damaged concrete, much less concrete fell away during the cyclic imposed–deformation
tests, and the residual shear strength was appreciably greater.
The study presented in this thesis is a continuation and exploration of Trim’s work. Since the vast majority of structures, even in high-risk zones, will not actually be subjected to earthquake during their design lives, it is felt that spiral binding is only likely to be used in practice if it can contribute to the static shear strength of the structure, and so replace at least some of the stirrups. Binding of the compression zone, or of entire columns, is of course fairly common already in seismic design of concrete structures. Often spirals of steel, even interlocking spirals, are used; to quote from Tsitotas (1996) “ Columns of rectangular cross-section with interlocking spirals are the recent improvement in bridge building, where they are applied on piers with high earthquake requirements. This method can be applied even of soft stories of buildings, ... but mainly their behaviour due to shear must be further investigated ”. The question addressed here is whether such spirals are useful to carry shear in beams, and in particular whether they can usefully improve the residual shear strength at hinges after severe cyclic bending.
First, to investigate the effect of confinement geometry on concrete properties, cubes confined with spirals and normal links were tested and compared. As expected, the confinement effect proved to be much more marked with spirals than with normal links. It has to be stated that 90 degree, not 135 end hooks, were used and if the latter had been used normal links might have been more efficient. Then simply supported beams were tested with shear link type (spiral and normal links), configuration and layout as variables, giving insight into beam behaviour, peak load and mode of failure. The shear contribution and confinement effect of spirals were separately identified and studied under both static and cyclic loading. The average integration method used by CALTRANS and New Zealand codes to calculate the spiral shear contribution was revised; both theoretical and practical recommendations were given regarding their design factor. A simple crack sectional analysis, finite elements, and a program based on modified compression field theory, were used to analyse the experimental beams under static loading. Later, crack shape, spiral geometry, and concrete shear contribution were included in a simplified crack sectional analysis to assess the shear behaviour of beams reinforced with spirals. Agreement between experiments and this theory is reasonably good.
The project has shown that spirals can effectively replace a considerable proportion of the normal shear links in a plastic hinge zone, while maintaining the integrity of the hinge region during heavy cyclic loading, which normal links cannot do.
Keywords: Concrete, Beams, Shear, Spiral links, Confinement, Cyclic loading