Document Type : Review Article
Authors
1
Department of Civil Engineering, Faculty of Technology and Engineering, Razi University, Kermanshah, Iran
2
Department of Civil Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran
3
Director of Earthquake Engineering Department, Paydar Behtab Davam Consulting Firm Engineers
Abstract
The desirable mechanism in reinforced concrete frames involves the formation of plastic flexural hinges in the beams, and if the columns yield, the hinges are also of the flexural type. In other words, in reinforced concrete frames, the ideal design of the columns is based on the flexural failure mode. To achieve this goal, all effective actions on column behavior must be considered. The methods for calculating the design shear of columns in the ACI318-19 standard have significant flaws that increase the probability of collapse in reinforced concrete moment frames. The results of laboratory and numerical studies on special moment frame systems indicate that the shear design of columns in spectral dynamic analysis is significantly lower than reality. Additionally, the shear calculated from the mechanism of beams in the design of columns is completely incorrect and independent of the chosen method for distributing the flexural moments of the beams between the columns connected to the joint, and should be avoided. However, the proposed methods for the shear design of the aforementioned columns are quite similar to the process of calculating the shear design of shear walls, which includes an over Strength factor, a dynamic amplification factor, along with the effects of column positioning. In the design of columns, in addition to the mentioned coefficients, another important parameter that is not addressed in the ACI318-19 standard is the phenomenon of elongation of reinforced concrete beams. Due to the elongation of the beams, the maximum displacement occurs in the columns of the first story. This is because the bases of the first story columns are fixed to the foundation, and as the elongation of the beams, the lower supports of the first story columns cannot displace the foundation. Consequently, this causes movement in the upper part of the columns, resulting in a significant rotation in the exterior columns of the first story. The position of the columns is also very important in calculating the design shear force. Exterior columns on the first floor experience a significant shear difference compared to other columns due to the phenomenon of elongation of reinforced concrete beams, which necessitates the use of a column mechanism in their design shear calculations. The failure to apply the mentioned factors in conventional structural designs leads to a change in the failure mode of columns from flexural to flexural-shear or even shear, affecting the overall behavior of the structure and the design assumptions. According to ASCE7-22, the use of intermediate reinforced concrete moment-resisting frame is prohibited throughout Iran. The Chinese reinforced concrete building standard GB 50011-2010 limits the maximum allowable height of special reinforced concrete moment-resisting frame systems in very high intensity of seismicity to 24 meters. In this paper, using various standards and research by other researchers, computational approaches for shear design of reinforced concrete columns in moment frames are investigated, and the accuracy of each model is compared with numerical and experimental results, and necessary suggestions are provided. The high shear value in columns probably makes the use of modern materials, including high-strength reinforcement, inevitable in the design and retrofitting of reinforced concrete moment frames.
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