| An innovative strengthening technique for RC beams strengthened with composite bonding steel plates is presented to strengthen RC structure under the conditions of heavy loads or the loads which can not be unloaded. The strengthened beams have the advantages of excellent loading behaviors, good ductility and feasible construction. The thick composite bonding steelplates are bonded at the same time on the tensile and compression surfaces of the members by using this technology, so that the prestresing force can be generated by tensioning and compressing plates, which can overcome the stress hysteresis of the added materials and improve the efficiency of such material. The horizontal U shaped closed steelplate jackets are applied along the beam length for further strengthening. The anchorage of steelplate jackets can transfer the shear force between steelplates and concrete and prevent plates from being separated to enable the thick steelplates working jointly with the concrete. Flexural and shearing strengthen is combined for the strengthened beams to improve the load capacity and the normal performance of the strengthened beams. The main research contents of this paper are as follows:(1) The basic principle of RC beams strengthened with composite bonding steelplates is analyzed, the prestressing methods and steps are indicated and the calculation formula for conquering strain hysteresis is derived. Based on the energy variation principle, taking the loading boundary as the movable boundary, the calculation formula for local buckling strength of the compression plates is derived and the relevant construction requirements are presented.(2) The large size members for the RC beams strengthened with composite bonding steel plates are tested. Based on the strengthening technology principle, four T shaped RC beams with large sizes are prepared,3 tested beams and 1 contrast beam are used for the static load tests to verify the strengthening methods and obtain the test basis. The test results show that the failure potentially occurred in the strengthened beams is in bending failure mode and flexural failure mode on the oblique section. The combined section formed by steelplate and concrete can work effectively. The strength of the applied material can be fully exerted. The strengthen mechanism indicates that the jacket formed by double steel plates can share the load of the original beam to further improve the bearing capacity, sectional stiffness and anti-cracking performance of the beam. The mechanical property of RC beam and the applied steel plates can fully function to cope with the poor ductility and limited strengthening with the steelplates on a single surface.(3) The validated finite element models of the beams strengthened with composite bonded steelplates are used; the adhesive interfaces of the beam are simulated using the unique cohesive force model of ABAQUS to analyze the load and deformation behaviors. The load transfer mechanism in the shear-span zone is analyzed based on distribution characteristics of the stress field, with the shear span ratio is greater than 2, the path of the main compressive stress transferring to the support is discontinuous, the action of strengthened part makes the stress distribution "deflecting", the straight struts centered on the steelplate jackets works more gradually, to form the indirect force transfer mechanism. Analysis on stress behaviors of adhesive layers indicates that the stress state is within its failure bearing limitation. The basic parameters influencing the shearing behaviors of the strengthened beam are obtained. The influence rank is shear-span ratio> stirrup ratio> longitudinal steelplate ratio> concrete strength. When the total jacket ratio exceeds 2.5%, the effect of the steel plate jacket on the deformation ability is unobvious.(4) Based on the flexural mechanism of the strengthened beam, the procedure of the section analysis is compiled to analyze the whole nonlinearity bending process of the nominal section of RC beam, to discuss the rule of strength and areas of the upper and lower steelplates, the strength and prestress of the concrete influencing to the bearing capacity and ductility. When the area of the upper and lower steel plate is equal, the best ductility emerges. When the prestressing range is over 1.2, the effect of the ductility increase is unobvious. It is unnecessary to exert excessive prestress. The steel plate quantity and arrangement mode for strengthening can be adjusted in terms of the composite strengthening characteristics to achieve the ductility strengthening and bearing capacity strengthening. Hereby, a simplified calculation formula of the bearing capacity for flexural strengthening is presented. The calculated results are in good agreement with the test and numerical simulated ones.(5) The shear mechanism of the strengthened beam is analyzed based on the principle of the strut and tie model, and the universal criteria taking the shortest path principle as force flow method is proposed. The precise strut and tie model in the shear-span zone is configured in terms of the force flow chart of the zone, and then translated into a simplified model. The strengthened beams are classified according to different shear span ratios and configured with different struts and ties to take the key parameters of the individual simplified model determined using the constrained optimization method, to determine the angles of diagonal struts in BD zone and the load transfer mechanism in different shear span ratios and to obtain a complete idea on the optimal configuration of the strut and tie model with the consideration of the influence of main parameters on shearing mechanism. It can be used as the theoretical basis of shearing calculation. The simplified calculation formula of the shearing strengthen bearing capacity is presented with the calculation in high accuracy. The calculation results are in good agreement with the test results and the numerically simulated results.(6) The deformation performance of the strengthened beams under normal conditions is analyzed. The proposed statically determinate truss model can distinguish the different inclination angles of the diagonal ties in B and D zones. By sectioning B zone into the basic units, the angles of the diagonal ties in B zone are derived in terms of the principle of minimum strain energy, and the angles of those ties in D zone are calculated using the topology optimization to establish the entire truss models with the consideration of coupling action of bending shear. The bending deformation can be simulated by means of tensile compression of chord members and the shearing deformation can be simulated by means of the tensile compression of the diagonal web members and the straight struts. In order to avoid the underestimate of deformation of the beams strengthened with composite bonding steel plates when the stress is high, the elastic constitutive relation of the web straight struts should be reduced. For rectangle beams under the action of symmetrical concentrated loads at two points, when the reinforcement parameter is λs≥5, the method in the current code is inapplicable, the influence of shear deformation in the shear-span zone and that of oblique cracking on the mid-span deflection should be considered.(7) On the basis of theoretical analysis and experimental research, the design method, steps, simplified formulas and construction requirements of beams strengthened with composite bonding steelplates are presented. |
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