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

This page is maintained by rcb@eng.cam.ac.uk (last update 4 October 2004)