| Résumé | Columns are the most critical elements in structural systems. They may fail catastrophically in buildings exposed to bomb blast, which could lead to progressive collapse of the entire structure. Past global blast events have shown that preventing progressive collapse would significantly reduce the number of casualties and minimize structural damage. Hence, columns in existing critical facilities must be upgraded to resist the effects of blast loads in order to minimize the risk of fatalities, injuries, and damage costs. To predict the behavior of reinforced concrete (RC) structures under blast load, single-degree-of-freedom (SDOF) dynamic analysis can be used. However, in this analysis the force-displacement relationship (also called resistance function) of the structural member must be identified. An idealized resistance function can be established by simply specifying the moment capacity and the corresponding curvature for the section at different load levels, like yield level (My) and ultimate level (Mu ) and by knowing the equivalent plastic hinge length (Lp). This length can be predicted using one of the models available in the literature. In RC columns exposed to catastrophic events (e.g., significant earthquake, significant blast or impact) plastic hinges form at maximum moment regions where the most damage occurs. Plastic hinge length (Lp) can be defined as the virtual length over which the plastic curvature is assumed to be constant. The rotation over a cantilever RC member can be computed by integrating curvatures along the member length if Lp is known, from which the tip displacement can be computed; and vice versa. For this reason it is essential to predict the plastic hinge length with acceptable accuracy since it is a key step in correlating the section-level response to the member level response of a concrete column. Indeed, the plasticity spreads over a larger physical length, referred to as the yield length (Ly). The yield length is the distance between the critical section and the location where the tension steel reaches its yield stress. Hence, the plastic hinge length can be expressed as Lp= βLy, where β is a dimensionless reduction factor for the curvature distribution near the support and it is always smaller than 1.0. While a large number of studies were carried out to estimate the plastic hinge length formed in un-retrofitted RC beams and columns subjected to monotonic or cyclic loading, only a very limited studies were conducted to investigate the plastic hinge formation mechanism in FRP-jacketed RC columns. No experimental investigation has been conducted so far to compute the plastic hinge length of RC concrete elements exposed to blast effects despite the importance of this parameter in establishing the resistance function. This function in turn is essential in obtaining the blast response of the member using SDOF model. The objective of this paper is to present the experimental evidence of the enhancement of the structural performance of RC columns when retrofitted by CFRP laminate. The focus will be on the ductility enhancement by increasing the deformation capacities and plastic hinges length. This paper presents part of the results of a large study to develop high performance protection systems for concrete columns subjected to blast loads. The results show that columns retrofitted with CFRP laminate have higher blast resistance, significantly higher ductility in terms of longer plastic hinge compared to non-retrofitted columns. Columns retrofitted with CFRP laminate containing woven ±45˚ CFRP fabrics developed longer yield lengths than those that had unidirectional fabrics only when subjected to the same blast loads. Hence, CFRP strengthening laminate including woven ±45˚ CFRP fabric improved the ductility of the column. It can also be concluded that when the applied lateral load is less than the column lateral capacity, the residual axial load reached 95% of the initial applied axial load and the residual deformation at mid height is very small. |
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