Theoretical Analysis And Experimental Study On Coupling Beam With Small Span-to-Depth Ratio In Seismic Coupled Shear Walls Structure

It's well known that RC coupled shear walls structure has superior earthquake resistant behavior, which comes from not only its strong wall legs but also its coupling beams. Coupling beams are the link of wall legs. Under earthquake loading, the most important parts of coupled shear walls structure, load-bearing walls, will be protected if coupling beams form plastic hinge which dissipate earthquake energy. One hand, the coupling beam with small span-to-depth ratio often occurs in architectural layout. On the other hand, shear walls of frame-shear structure and core tube of core tube structure resist main horizontal loading. So the span-to-depth ratio of coupling beam in them should be smaller to increase their lateral rigidity. The coupling beam with small span-to-depth ratio (the span-to-depth ratio is not greater than 2.5) has larger nominal shear stress. It belongs to counter flexural deep beam whose both ends are restrained to the walls and whose contra-flexure point is at the mid-span. Its seismic response and deformation have much difference with common frame beam and simply supported deep beam. Therefore, the coupling beam with small span-to-depth ratio was widely researched at home and abroad. It is proved in experimental study that it will most probably fail to shear failure and can't meet the seismic performance required by the structure if it is designed according to ordinary frame beam theory. The topic is overviewed throughout home and abroad. A new reinforcement arrangement scheme hasn't appeared so far, which is convenient for construction and which has superior earthquake resistant performance. The problem on seismic design method of coupling beam with small span-to-depth ratio was put out again when the structure design codes were newly revised. Technical Specification for Concrete Structures of Tall Buildings (JGJ 3-2002) temporarily adopt the design method of decreasing the maximum shear force and increasing the amount of stirrups under the condition that experimental results are not adequate. While China Code for Design ofConcrete Structures (GB50010-2002) leave the content of earthquake resistant design method for the kind of coupling beam blank.Seeing that coupling beam with large span-to-depth ratio (the span-to-depth ratio is greater than 2.5) has been studied extensively so far at home and abroad, while the research on the coupling beam with small span-to-depth ratio is inadequate. Its shear strength formula is acquired through that of the coupling beam with large span-to-depth ratio multiplied by 0.9 reduction factor and the treatment is insufficient in theory foundation. Therefore the perfect calculating formula of its shear strength hasn't been put out till now. The softened truss model associate with stress equilibrium equations, strain compatibility and constitutive laws of materials to analysis the loading response of RC members subjected to shear and torsion. The coupling with small span-to-depth ratio is analyzed with softened truss model hasn't been discussed at home. In this dissertation, its shear strength and loading response is analyzed with softened truss model for RC membrane element. Compared with experimental results in references, it is shown that softened truss model can be applied to analyze coupling beam throughout loading history and the analytical results are in good agreement with the experimental results. Furthermore, it is shown that three different failure modes (under-reinforcement, balanced and over-reinforcement) can be clearly identified, depending on the relative contribution of stirrup to that of concrete. For each failure mode, an explicit formula is derived for prediction of shear strength. At last, the simplified and practical formula for design is presented.In recent years, the strut-and-tie model method based on the finite element analysis is proposed and applied in the local regions of reinforced concrete structure (The local region is sometimes referred as the D region. The prefix D indicates that the stresses and strains in the region are disturbed or that the region is discontinuous) in the European and American codes. The method can allow the designers and engineers to achieve an audio-visual and clear knowledge about the interior force transmission mechanism of RC local region. But the strut-and-tie model method only meet stresses equilibrium equations, which gets the upper limit value of D region. Therefore, the calculated results tend to be unsafe. The scholar Shyh-Jiann Hwang improved the strut-and-tie model method and proposed the softened strut-and-tie model method which satisfies stresses equilibrium equations, strains compatibility equations and constitutive laws of materials. Its application in beam-column joints, deep beams, corbels and squat walls shows that the softened strut-and-tie model can predict shear strength of these kinds of D region enough accuracy. The coupling beam with span-to-depth ratio belongs to D region. By analogical analysis, the author intuitivelydiscovered that the interior force transmission mechanism of the coupling beam with small span-to-depth ratio is the same as that of the proceeding kinds of D region. So the author applied the method to analyze its shear capacities and reasonable agreement was obtained. From the softened truss model to the softened strut-and-tie model, the author deepened the understanding to the shear properties of the coupling beam with small span-to-depth ratio further. On the basis of the previous analysis, the author proposed a new reinforcement arrangement scheme of RC coupling beam with small span-to-depth ratio.According to the shear transmission mechanism obtained from the softened truss model and the softened strut-and-tie model and current research results at home and abroad, a new reinforcement arrangement scheme of coupling beam with small span-to-depth ratio in which the stirrup is divided into three rings like a chain along the depth of coupling beam is proposed by the author. Tentative test for three short coupling beams specimens has been carried out. The whole experimental procedure is introduced and experimental phenomenon is described in detail in this dissertation. Some factors such as shear-compression ratio, shear- shear resistance ratio and characteristic value of stirrup have been studied, which have effect on ductility, energy dissipation and failure mode. Compared with the other short coupling beams schemes, the new reinforcement arrangement scheme is superior in seismic performance and is easy for construction, so it is desirable to study further. The new reinforcement arrangement scheme of the short coupling beam is an effective way to solve its seismic performance problem. The formulas in other literatures for flexural strength of the short coupling beams are referred. A simple formula is given, which is suitable for engineering design and whose calculated value is in good agreement with experimental value in references.To save experimental funds and to compensate for the drawback of the experimental set-up, which can't reflect the real loading state of the coupling beams in structure, the short coupling beam specimens were numerically simulated with the ANSYS program and the simulation results were compared with the experimental results.The short coupling beam with larger flexural rigidity can provide huge restraining effect on the wall legs. It is suitable to the coupled walls structure which is inferior to resist lateral loading as an integral whole. The structure is termed as the coupled walls structure with stiffening coupling beams where the stiffening coupling beams are made of the short coupling beams. It's proved in experiment in this dissertation that the short coupling beam with the new reinforcement arrangement scheme has superior deformation and energy dissipation capacity. Therefore, the coupled shear walls structure with the kind of beams will have good earthquake resistant performance. The structure has more ductile charactersin earthquake bearing behavior than the frame structure with strong columns and weak beams. In view of the available method of determining the optimum locations and number of the stiffening coupling beams is not presented currently, several hypotheses based on uniformly coupled shear walls are made in this dissertation. The restraining effect of weak coupling beams is incorporated to wall legs whose rigidity is expressed with equivalent stiffness. Thus the coupled shear walls structure is simplified to a structure which only includes the stiffening coupling beams. The restraining moment applied to walls are acquired through the deformation compatibility equations of the joints at the locations of the stiffening coupling beams. Then the top displacement of the structure is got with unit-load method. The optimum locations are obtained by maximizing the top displacement reduction value caused by the stiffening coupling beams. While the needed number of the stiffening coupled beams is worked out according to the top displacement of the structure meeting the serviceability limit state. The tables are given for the coupled shear walls structure with two stiffening coupling beams design and some factors are discussed in optimum design in this dissertation, as can be referred and applied at the stage of preliminary design.Considering that the numbers of specimens are not ample and experimental set-up has drawback, the general rules of some factors which have influence on the seismic performance of the short coupling beam cannot be easily acquired. Although non-linear FEM can take place of some experimental research in certain range to analyze the parameters, the simulation results of specimens are not perfect under cyclic loading. To fully understand the seismic performance of the short coupling beam with the new reinforcement arrangement, theory analysis and experimental study must be extensively conducted further.