Introduction: Horizontal loading experiments were performed with shear walls which show shear failure without undergoing flexural yield. Based on the experimental results, this paper discusses the effects of the wall reinforcement upon the ultimate strength. Furthermore, the conditions of shear failure of the columns are established to modify the macro model demonstrated in References 1) to 3). Using the modified macro model, the effects of the vertical and horizontal wall-reinforcement upon shear walls' ultimate strength are studied comprehensively. Experiment on the Ultimate Strength of Shear Walls: The shape of the test specimens and the arrangement of reinforcement are shown in Fig.1 and Fig.2, respectively. Ten specimens were tested in total. They are different in height-span ratio and arrangement of reinforcement as shown in Table 1. The loading and measurement schemes are shown in Figs.3 to 5, and the mechanical characteristics of concrete and reinforcement are shown in Table 2. The final crack pattern of each specimen is illustrated in Fig.6. More cracks are produced along with the increase of the wall reinforcement ratio. For every specimen, compression failure occurs, under a maximum load, along the diagonal line of a wall between compression-side foot and tension-side top, whereas the main bars of the tension-side column do not yield. The load-displacement response of each specimen is represented in Fig.7. The test results are summarized in Table 3. Comparison of the ultimate strengths is given in Fig.9. In the case of the specimens with horizontal and vertical reinforcement, higher ultimate strength is obtained by increasing the horizontal reinforcement ratio only, than by solely increasing the vertical reinforcement ratio. The larger is the height-span ratio, the more remarkable is such enhancement of ultimate strength caused by the increase in horizontal reinforcement ratio. On the other hand, in the diagonally (45°) reinforced specimens, the augmentation of ultimate strength due to the increase of wall reinforcement is more remarkable than in the specimens with horizontal and vertical reinforcements. Referring to the principal strain distributions in the wall panel under the maximum load (Fig.10), it is confirmed that the increase in the horizontal bar results in larger principal compressive strain in an area distant from the diagonal. This agrees with the fact that a larger ratio of horizontal reinforcement gives a higher ultimate strength of the shear wall. The Shear Forces Contributed by the Wall Panel and Column: Using the measured strains in the wall-panel concrete, the main bars of columns and the vertical bars of wall, the stress distributions over the X-X section in Fig.13 are calculated. Taking account of the equilibrium of moments about the point A, the efficiency factor of the wall-panel concrete is estimated. The relationship between principal compressive stress and principal compressive strain in the wall-panel concrete are assumed to be a parabola-type expressed by the formula (1). The average efficiency factor ν is 0.7, as shown in Fig.14. The compressive strength of the wall-panel concrete is lower than the uniaxial compressive strength. The shear forces contributed by the wall panel and columns are evaluated, using the efficiency factor calculated. The obtained result is given in Fig.16. As indicated in this figure, the shear force contributed by the columns do not vary largely with the change in the wall reinforcement ratio, but the increase in the horizontal reinforcement ratio causes an enlargement of shear force contributed by the wall panel. This result agrees satisfactorily with the results of the numerical analysis by FEM described in Reference 1). The Shear Failure Condition of the Columns: Based on the equilibrium of horizontal forces as shown in Fig.17, the conditions of shear failure of the columns (Eq.(2), Eq.(3)) are

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