PRELOADED CAST-IN-PLACE FASTENERS UNDER PULSATING TENSILE LOADING

In industrial buildings, machinery is clamped to reinforced concrete elements by means of preloaded hold-down bolts. Since they are not commercial products, fasteners with bearing heads of various shapes and sizes are installed mainly depending on the local construction practice. Sizes are linked to the magnitude of the design loads and to the more stringent safety requirements, particularly for strategic buildings. Typical geometries are larger than most of the common solutions and far beyond the range of applicability of the existing design methods. Available models, in fact, do not explicitly account for the differences in the developed bearing pressure at the fastener head. Most of the research studies, which form the theory background of concrete cone failure, barely considered fasteners differing from headed studs. In fact, larger size fasteners were only tested in a few campaigns. This paper presents an experimental campaign on large cast-in-place anchor bolts, with head size up to 270 mm side. Single fasteners were embedded in the centre of concrete elements reinforced to prevent bending failure. Two test series were carried out: (i) monotonic pull-out to investigate the influence of the fastener head size, (ii) pulsating tension, for a reduced number of configurations, to study the effects of repeated loading up to 2 million of cycles. Anchor bolts were slightly prestressed to reproduce real site conditions. For industrial machinery with rotating parts, in fact, anchor bolts are usually prestressed to reduce the effects of fatigue. From the experiments, preload relaxation and axial stiffness are proved to be related to the considered fastener head size, while the application of repeated loading seems to have a negligible influence, as long as the external load does not release the clamping force.