رفتار ساختاری از غشاهای RC با داشتن تمایل میله های فولادی

In seismic design, special care is needed when the shear force governs the response of a reinforced concrete (RC) element because this element undergoes stiffness degradation, strength deterioration and reduction in the energy dissipation capacity, as the cyclic loading increases beyond the yielding level. Experimental and analytical research has shown that this undesirable response can be controlled and even eliminated in the hysteretic load–deformation curves of a shear-dominant element if the steel orientation within the element is aligned in the direction of the applied principal stresses. However, in practice, it is quite difficult to orient the steel bars parallel to the principal stress directions due to geometric and construction limitations. In this study, the effectiveness of the steel reinforcement orientation on the structural response of RC shear membrane elements was investigated by analyzing the test results of four panels previously reported in the technical literature. The test results were also analyzed by using a compatibility-aided truss model. The experimental and analytical results indicated that the ductility and energy dissipation capacity of RC panels strongly depended on the reinforcement orientation. The results also showed that the hysteretic response of an RC panel did not vary linearly with the steel grid’s orientation within the panel and there was a boundary where the deformability and energy dissipation of the RC panel increased rapidly.

Reinforced concrete (RC) structures, when located in earthquake regions, are designed to withstand moderate seismic loading within the elastic range, and to absorb the energy of severe seismic loading using the plastic range. Therefore, to protect the life of occupants and prevent the collapse of structural elements, RC structures should be designed and proportioned to maintain their initial strengths and stiffnesses as well as show high energy dissipation capacitates in the event of high seismic excitations. Consequently, it becomes necessary to evaluate the inelastic responses and energy dissipation capacities of RC structures and to determine practical methods to enhance their seismic responses under earthquake loading. Experimental and analytical studies have shown that when the shear force governs the response of an RC element, as in the case of an RC low-rise shear wall or RC beam with short-span-to-depth-ratio, the effect of shear on the element’s cyclic response is thought to be responsible for the “pinching effect” in the hysteretic loops. This in turn causes the RC element to undergo stiffness degradation, strength deterioration and reduction in the energy dissipation capacity, as the cyclic loading increases beyond the yielding level. However, experimental research has shown that this undesirable “pinching effect” can be controlled and even eliminated in the hysteretic load–deformation curves of a shear-dominant element if the steel orientation within the element is aligned in the direction of the applied principal stresses. With such an orientation, RC shear-dominant elements can be designed to possess high energy dissipation capacities, similarly to RC flexural-dominant elements. The effect of the steel bar orientation on the structural response of RC structures was first experimentally investigated by Park and Paulay [1] and Paulay et al. [2] who showed that the pinching effect in the hysteretic loops of coupling beams can be controlled by adding inclined shear reinforcement. Following this work, many experimental studies have been conducted in an attempt to assess the effects of the steel orientation on the cyclic response of RC shear-dominant elements. For example, Minami and Wakabayashi [3] and Tegos and Penelis’ [4] conducted experimental tests which showed that short columns with inclined reinforcing bars had improved structural responses when compared to specimens with conventional reinforcement. In addition, Tsonos et al. [5] tested RC beam-column joint specimens with crossed inclined reinforcing bars. Their test results showed no appreciable strength degradation, and showed “spindle-shaped” hysteresis loops with large energy dissipation capacities. Hsiao and Chiou [6] tested low-rise shear walls with various steel grid orientations. Their experimental results showed that the pinching effect was remarkably reduced by reinforcement orientation. Shaingchin et al. [7] also conducted experimental study about shear walls to figure out influence of diagonal web. From the test, the specimens with diagonal web reinforcement exhibit less pinching effect in the hysteresis curves than the conventional one. Analytical research has also been conducted to predict the effects of the steel grid orientation on the cyclic response of shear-dominant RC elements. For example, Mansour et al. [8] and Lee and Kim [9] analyzed the structural response of RC membrane elements (panels) having inclined steel bars. The analytical results indicated that the structural responses of RC panels are influenced by the orientation of the steel bars with respect to the directions of the applied principal stresses. Hindi and Hassan [10] recommended a theoretical model to predict the behavior of diagonally reinforced coupling beams based on the results of technical literatures. It is generally accepted that the inclined reinforcement improves the ductility and increases the energy dissipation capacity of an RC member. Experimental and analytical research [8] and [9] have confirmed that if the steel grid in a shear RC element is set parallel to the direction of the applied principal stresses, the cyclic response of the shear element is maximized to a limit that the shear-dominant element behaves similar to a flexural-dominant element. However, in practice, it is quite difficult to orient the steel bars parallel to the principal stress directions due to geometric and construction limitations (such as the shear span-to-depth ratio of a member). This study investigates the effectiveness of the steel reinforcement orientation on the structural response of RC shear membrane elements (or panels) by analyzing the test results of four panels previously reported in the technical literature. The test results are analyzed by using a compatibility-aided truss model. Comparison between the experimental and predicted responses of the RC panels is also carried out. The results, reported herein, indicate that the ductility and energy dissipation capacity of RC panels strongly depend on the reinforcement orientation. The results also show that the hysteretic response of an RC panel does not vary linearly with the steel grid’s orientation within the panel; in other words changing the steel grid orientation from an angle of 45° to 90° does not cause the energy dissipation or ductility of the panel to increase twofold. As such, this research also aims to find a trend, if any, between the variation in the energy dissipation capacity and ductility of RC shear-dominant elements with steel grid orientation.

In this paper, the effect of the steel bar orientation on the structural response of RC structures was investigated. Based on this study, the following conclusions and observations were made: (1) The test results of the four RC panels subjected to cyclic shear showed that orienting the steel grid in the direction of the principal applied stresses eliminates the undesirable pinching effect and increases the energy dissipation capacity and ductility of RC elements subjected to cyclic shear. However, when the inclined steel bars were arranged at a α < 68.2° to the principal direction, the inclined steel bars did not improve effectively the energy dissipation of the RC panels. (2) The analytical results of the panels indicated that the strength and the deformability of the RC panels with various orientations of reinforcing bars increased as the orientation of reinforcing bars approached the axis of the principal compressive stress (α = 90°). However, the shear strength does not increase proportionally with the increase of α . The reason for this tendency was due to the resistance contributions of concrete and steel bars. The deformability of the panel with α⩾78.9°α⩾78.9° is much greater than the panel with α = 45.0°, because the stress (fy ) is resisted by both materials (concrete (View the MathML sourcef2c) and steel (ρtft )) and the View the MathML sourcef2c arrives at the View the MathML sourcevfc′ in the large stage of deformability. However, the deformability of the panel with α < 78.9° is almost the same as that of the panel with α = 45.0°, because the stress (fy ) is resisted by one material (concrete (View the MathML sourcef2c)) and the View the MathML sourcef2c arrives at the View the MathML sourcevfc′ in the large stage of deformability. (3) The analytical results of the panels indicated that the pinching effect is influenced by the orientation of the steel grid in RC elements; this undesirable pinching mechanism can be eliminated if the steel reinforcement is placed parallel to the direction of the applied principal stresses. However, the energy dissipation of the panels did not increase proportionally with the increase of α, because the energy dissipation in the stress vs. strain curves of steel bars does not increase proportionally with the increase of the angle α. The experimental test observations and the analytical results indicated the steel bar orientation, in order to effect the structural response of RC structures, was about 80° because before α ≈ 80°, the increasing rate of the energy dissipation capacity and deformability was very low, while after α ≈ 80°, it showed a rapid increase. However, it is worth mentioning that more additional analytical and experimental works on the structural behavior of RC members with inclined steel bars, especially the RC members having different loading conditions or boundary conditions, are necessary to find a more rational evaluation method for the effects of the inclined steel bars. In addition, it is important to investigate the possibility of bucking of the inclined rebars, the further research on the buckling problem is needed.