PSI - Issue 44
A. Viskovic et al. / Procedia Structural Integrity 44 (2023) 1348–1355 A. Viskovic et al. / Structural Integrity Procedia 00 (2022) 000–000
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concerns. In this case masonry confinement is commonly resorted to. Proposals range from invasive FRP warping to hooping to confine the mortar joints. The paper deals with the presentation of a novel confinement approach capable to overcome the drawbacks of the existing techniques either in terms of mechanical behavior or technological aspects. The purpose of the work is to study the improvements of the performances of masonry columns reinforced through non-conventional hooping system using “Dyneema ribbons”. Apart strengthening, this system is endowed with ease of installation and removal, as compared with steel or FRP based systems, and is fully compliant with the requirements of fitting, deformability and reversibility. The research develops along with two main phases: numerical (completed) and experimental (in progress). The numerical models are realized using 3D solid f.e. with dimensions adjusted to fit the deformation gradients. The nonlinearity is concentrated at the mortar-brick joints. The bonding between ribbons and mortar is modelled using appropriate interface elements aimed at reproducing the stress transfer during installation and in working condition. A prototype column with square section has been considered so that exploiting the column and load symmetries it is possible to manage with unscaled samples, but with small dimensions. Incremental non linear static-equivalent analyses have been performed with the models subjected to axial compression according to a field of imposed displacements. The results of the numerical simulations have been used to optimize the design of the column samples to be tested. A device has been properly designed to install the ribbons with the correct hooping traction. Incremental non-linear static-equivalent analyses have been performed. The numerical results, compared to similar solutions but with different strength and stiffness (Nylon ribbons), allow to draw the following conclusions: (i) the Dyneema ribbon offers better mechanical performances than Nylon ribbon, (ii) the hooping provided by the ribbons noticeably increases the stiffness and the load carrying capacity of the column, (iii) the failure mode tends to change from brittle to ductile. 2. State of the art The practice of circling columns and pillars to improve their bearing capacity has been in use since ancient times. Experience and intuition have made it clear that any suitably confined material, as well as sand in a closed bag, improves its ability to withstand compressive loads. As it is known, the experimental tests confirm this fact: the cylindrical concrete specimens, if confined, improve the bearing capacity and the ductility also. In recent decades, research and professional practice have introduced the use of confinement systems using synthetic or steel nets, first with resin bonding (FRP) and later with inorganic matrices (FRCM). At the same time, research continues to explore alternative solutions, such as the use of natural fibers instead of synthetic or steel ones (for example the use of bamboo in [Hongyao] is proposed) or the use of fiber-reinforced concrete in paste and with no added nets [Mingke]. Others, considering that, in a wall structure, the mortar joints are the critical point of the "masonry" structural set, have studied the improvement produced by the partial replacement of the same joints with higher quality mortar [Chen]. At the same time, the research also aimed at trying to evaluate the effectiveness of the confinements carried out on masonry columns using FRP [Ramaglia] or FRCM, both with synthetic fibers [Yilmaz] and with steel fibers [Borri 1]. However, it has been seen that a continuous bandage not only works to provide a confinement effect, but it is also involved in absorbing the axial compression load, with the risk of instability and buckling of the bandage itself. This led to the experimentation of discontinuous bandages [Cascardi] [Aiello]. The discontinuous bandages, however, leave some horizontal mortar joints without confinement, therefore free to flatten due to the Poisson effect and cause the break, by horizontal traction, of the bricks of the layers immediately above and below to mortar joint. Furthermore, both the continuous and discontinuous bandages are not "active", in the sense that they cannot be adequately put in tension, therefore it is necessary the start of the transversal bulging of the masonry, under the action of axial compression, so that these confinements come into action. A possible solution to this problem is to concentrate the discontinuous confinement on all the horizontal mortar joints and initially the research focused on the use of appropriately post tensioned steel cables [Borri 2]. However, to be adequately flexible and turn with small radii of curvature at the corners of a column, the steel cables must have a relatively small diameter and inevitably much smaller than the usual thicknesses of the mortar joints. This fact reduces the effectiveness of confinement and led to the idea, presented in this paper, of the use of very flexible synthetic tapes (or ribbons) with a width close to the thickness of the mortar joint.
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