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A study on the strengthening of reinforced concrete structures with emphasis on the use of tension-ties (Doctoral thesis)

Λιώλιος, Άγγελος/ Liolios, Angelos

This PhD Thesis is a contribution to the contemporary study and solution of the problem of seismic reinforcement of Reinforced Concrete Structures frames (RCS) using tension ties. It is considered that after the static and seismic evaluation of the existing or new RCS, using known methods, has already established the need for reinforcement and seismic upgrading of the structure as a whole and that from the available methods of strengthening have been selected the tie strengthening method. Tie reinforcement is known to be advantageous compared to traditional reinforcement methods (e.g., mantles, walls, FRP) in terms of ease of installation, reduction of constructional nuisance, respect for the principles of sustainable construction, maintenance, etc. However, in the relevant calculations of the RCS RESPONSE with ties, a realistic analysis of the use of ties as a means of strengthening must be taken into account, taking strict account of their unilateral behavior, which is the ability to receive only tensile forces and the inability to absorb compressive forces. Initially, the basics from the theory of structures with unilateral links are explained and are made the mathematical formulation of the problem as an inequality problem of the Structural Engineering. In the Inequality Problems of the Structural Analysis, the conditions that govern the problem are equalities and inequalities. These are nonlinear problems of structures with particular difficulties, both in their mathematical formulation and in their numerical solution. Ties are structural elements acting as one-sided couplings: they can only receive axial tensile forces while they cannot receive compressive forces. Therefore, the conditions of non-negative tension on ties are inequality conditions. This non-linearity due to ties augments the known non-linearity governing the RCS problems. Generally, the use of ties imposes on the RCS a variable mode of operation, that is, its function varies according to the cause. As it is known, the same phenomenon of the variable mode of operation occurs in RCS due to cracking under a changing operating cause, e.g. seismic stress. Then, for the numerical solution of the general problem (static and dynamic), two-fold differentiation is used in the space with the Finite Element Method (FEM) and in time with direct methods of integration, and on the other hand methods of optimization for management of anisotropic conditions. Thus, in each time step, a linear complementarity problem (LCP) is solved. In addition, an alternative augmentation approach to the problem is developed, on which the appropriate implementation of the RUAUMOKO computing code is based. The credibility of this code is documented by experimental results. These experiments were carried out at the Reinforced Concrete and Antiseismic Structures (RCAS) Laboratory of the Democritus University of Thrace (DUTH) for a typical one-storey, two-column reinforced concrete frame, first bare and then reinforced by ties. This framework has bent-shear features, which appear in old RCS, studied and constructed before the application of the newest anti-seismic codes. In these experiments, it was emphasized to be studied the influence of alternating axial force on the shear strength of structural elements, thus taking into account the influence of the xiii vertical component of the earthquake on the behavior of low buildings. Experimental monitoring of structural integrity and fault diagnosis in shear-type tests is done by applying a new real-time diagnosis and fault detection system. This system is described in detail in the works (Voutekaki, 2009), (Voutetaki et al., 2018) and is based on the method of Electromechanical Impression and the use of "intelligent" piezoelectric sensors. In addition, the documentation of the code is also made with experimental results available in the international bibliography, concerning specimens-frames with bending behavior reinforced with steel ties. In the computational simulations of both the above experimental procedures a very satisfactory convergence of experimental and computational results was found. We then investigate the effect of earthquake sequences (multiple earthquakes) on the RESPONSE of RCS, whether or not reinforced by ties. For seismic sequences recorded and selected as extreme for the tested RCS, their cumulative effect on the damage for critical RESPONSE sizes is calculated. Relatively, damage indices are introduced and calculated, local based on ductility, and global for the whole body. Critical dimensions relate to intensive and distortive features of the RESPONSE, e.g. maximum horizontal upper floor movements, relative slopes between floors, plastic joints formations, etc. Thus, with the above damage indices, the potential increase of the seismic risk for RCS without reinforcement is quantitatively and reliably quantified, the need for reinforcement is assessed quantitatively, and the optimal arrangement of the ties configuration is chosen. Finally, some typical practical cases of ties-reinforced RCS are examined and their seismic RESPONSE under seismic sequences, with or without reinforcement, is compared. The comparison is based on the damage indices (local or global) for various critical RESPONSE characteristics. This enables the engineer to assess in a realistic way the need to reinforce an existing RC construction or not and, in the positive case, to choose the optimal way of reinforcing with ties. The main originality points of the dissertation are, among others, the realistic and numerical solution of the static and dynamic problem of RCS with ties and the emphasis given on seismic sequences and the realistic assessment of their cumulative effect on damage. Thus, the engineer has a reliable computational tool to assess the need for antiseismic reinforcement of the RCS and to select the optimal arrangement of ties under seismic sequences, something not covered by the current seismic codes.