The influence of reinforcement on load carrying capacity and cracking of the reinforced concrete deep beam joint

Abstract

The paper presents the results of experimental research of the spatial reinforced concrete deep beam systems orthogonally reinforced and with additional inclined bars. Joint of the deep beams in this research was composed of the longitudinal deep beam with a cantilever suspended at the transversal deep beam. The cantilever deep beam was loaded throughout the depth and the transversal deep beam was loaded at the mid-span by longitudinal deep beam attached to it. Morphology of cracking and stresses in the reinforcing steel, as well as the load distribution in the cantilever deep beams using Strut-and-Tie model taking into account an effort of concrete compression strut and efficiency of softening coefficient are presented and discussed. In the paper, the effectiveness of the mixed reinforcement in both tested deep beam systems, as referred to the design recommendation proposed in the published papers is also verified. It is also demonstrated that the inclined reinforcement favorably influences the width of cracks in cantilever and transversal deep beams and ensures the increase of the load carrying capacity.

5. Experimental results

Due to the non-linear distribution of stresses in the reinforced concrete deep beams, the most appropriate for the design of such structural elements is a model Strut-and-Tie or nonlinear analysis. Strut-and-Tie model analysis of reinforced concrete elements is considered an alternative to the usual approaches of analysis and design, and is applied effectively in regions of discontinuity. In the previous literature, we find a lot of interesting scientific publi- cations relating to the analysis of the load carrying capacity of deep beams with single or twin span using the S–T model and the theory of stress fields [19–22]. The comparison S–T model with analysis of nonlinear finite element method of Vollum’s [23] for deep beams with single and twin span is definitely worth attention.

In the presented analysis, distribution of load in the deep beams DBI and DBII based on the measured strains of steel and concrete obtained from own experimental tests and analytical using Strut- and-Tie model was determined. The study analyzed the flow of forces in the cantilever and transversal deep beams in relation to the recommendations by [15,16], in which assumptions are based on the setting the force resistance limits in the reinforcement using the S–T model. Additionally, the S–T model for corbels loaded throughout the depth according to Nagrodzka-Godycka [24] was also verified.

In the diagrams (Figs. 10 and 11), the strains in the main steel reinforcement in both cantilever deep beams, together with a loca- tion of gauges arrangement, were shown. To calculate the stresses of steel used the average modulus of elasticity of steel Es = 200 GPa on the basis of the Certificate of Material Properties. Based on the extensometer measurements of strains, the essential differences in the reinforcement strains distribution between the deep beams DBI and DBII may be noticed.