Experimental investigation on the seismic performance of beam-column joints in low-strength reinforced concrete structures

A considerable portion of the building stock in our country is in earthquake-prone zones. Depending on this fact and structural deficiencies, there is a risk of significant damage or collapse on these structures when encountering possible ground motion. The beam-column joint regions are one of the most affected regions of a structure from the seismic effects. These lateral loads acting on the structures cause cyclic loadings in the beam-column joint regions which are also called "critical regions". The regulations aim at designing these regions with adequate structural safety to safely transfer the exposed loads. 135-degree hook arrangement on the beam longitudinal reinforcements in beam-column joint regions, usage of standard/high-strength concrete and continuum of column stirrups in joint regions with an adequate spacing are the most important facts that should be considered for beam-column joint regions to safely resist the cyclic loads caused by the seismic effects with sufficient strength. However, in many existing buildings located in our country, the desired confinement effect is not satisfied because of the lack of transverse reinforcement or an insufficient amount of transverse reinforcement in the beam-column joint regions. Also in many existing structures, the beam longitudinal reinforcements are not hooked in joint regions or hooked 90 degrees. Moreover, the use of plain reinforcement bars causes easily debonding of reinforcement from concrete under cyclic loads. Considering the periods in which these structures' built, the use of low-strength concrete (fc ≤ 10) in constructions were widespread. Therefore, collapse occurs without reaching the bending capacities of the reinforced concrete members connected to the joint regions as a result of the deficiencies mentioned above under these conditions. In developed countries with seismic risk, higher concrete strength and the use of ribbed bars as reinforcements are typical in the structures built in similar periods concerning our nation. Although the use of plain bars as reinforcements banned in our country, such deficiencies are still encountered in our old structures. Thus, the studies on this topic in literature including the weaknesses mentioned above is rare. It is essential to conduct new studies on this subject considering the deficiencies and weaknesses of our structures for estimating the seismic performance of existing buildings more realistically and preventing the collapse that may occur due to this. It is also necessary to conduct new studies on strengthening the weak beam-column joint regions of our existing structures with new material and methods appropriate to preclude the effects of the deficiencies mentioned above. In this Ph.D. study, the experimental investigation on the performance of beam-column joints having low-strength concrete and plain reinforcements with various configurations (hook status of beam longitudinal reinforcements, the existence of lateral reinforcements in joint regions, etc.) under cyclic loads simulating the seismic effects is aimed. It is desired to form the specimens having similarities with the existing building stock in our country. It is also wanted to improve the seismic performance of the mentioned joint regions by strengthening with FRP (fibre reinforced polymers) materials. It is intended to try more economical and easily applicable strengthening technique and to assess the effect of this technique on overall structural performance. Besides, an analytical study is carried out including the application of test results on structural frames built in the analysis software. The applied results are obtained from the actual beam-column joints tests. The reflection of mentioned deficiencies for beam-column joint regions in the numerical analyzes is investigated. Finally, the perspective of building codes and regulations on this subject is presented. The calculations are done for the specimens in this thesis based on the provisions of various regulations. In this context, preliminarily an experimental study is planned including the tests of 10 different specimens. The produced specimens are grouped into two series; In the first group, five different specimens are produced which are consist of joint regions formed with a reinforced concrete beam (150 x 200 mm cross-section) and a reinforced concrete column (150 x150 mm cross-section). One of them is produced as the reference specimen. The reinforcement configuration of these specimens is varying in the beam-column joint regions. In the second group, five more specimens are produced having the same cross-section dimensions and reinforcement configuration with the first group. In other words, the specimens in the second group are the pairs of the ones in the first group. The specimens in the second group are strengthened with fibre reinforced polymer (FRP) materials. This Ph.D. study is presented under five main chapters: 1. Chapter - Introduction: The importance of the thesis subject and the aim and scope of the thesis is presented in this chapter. The details of the specimens and which type of materials will be used in the strengthening are briefly explained. 2. Chapter – General Information: In this chapter, general information is given about the deficiencies encountered in the beam-column joint regions of the existing buildings in our country. The damages occurred from the previous earthquakes are visually presented. The studies related to beam-column joint behaviour in the literature are also summarised. A literature review of the load transfer mechanism in the joint region, experimental studies on the strengthened and unstrengthened joint specimens and some analytical methods about joint regions are given. Some recent work on the strengthened specimens with different and new materials are also presented. 3. Chapter – Materials and Methods: This chapter is divided into three sections as experimental studies, analytical studies and regulations' approaches. In experimental studies; design details of test specimens, the strengthening application, material tests, test setup, measuring systems and loading pattern are presented. In analytical studies; nonlinear static analysis model and assumptions made in the modelling phase, material models for concrete and reinforcement steel, strength evaluation for column and beam sections and joint regions are explained. In the section where regulations' approaches are summarised, the provisions about joint regions in DBYBHY (2007), Eurocode-8 (2004), ACI-318 (2011), ACI-352R-02 (2002), ACI-369R-11 (2011) and ASCE/SEI-41-13 (2013) regulations are presented. 4. Chapter – The Findings: In this chapter, the results and the findings obtained from experimental studies, analytical studies and regulations' approaches are given. Firstly, the behaviour of the specimens is presented by load-displacement-drift ratio graphs in the findings obtained from the experimental studies. Each test's damage assessment was done for different drift ratios. The findings obtained during the tests about the shear strain of the joint regions and strain of the reinforcement bars are evaluated. Finally, the total energy dissipation of the specimens is calculated, and the findings gathered about the energy dissipation and rigidity change during the tests are given. The findings obtained from the analytical studies are divided into three main sections. The results gathered from the related software about the cross-sectional strength of the beam and columns are given. The seismic performance of the frames is evaluated by comparison of the curves obtained from the nonlinear static analyzes. Some coefficients are proposed for the use of the numerical analyzes of the joint regions with this kind of deficiencies as a result of the analytical studies. Finally, the results of the strength calculations made for the specimens according to the regulations are presented. 5. Chapter – Discussion and Results: This chapter is divided into two sections as the conclusions obtained from this Ph.D. study and the proposals. In the conclusions section, the results from the experimental and analytical studies are summarised. In the proposals section, general information about the coefficients proposed as a result of the analytical studies and future work suggested for this subject is given. Also, suggestions are made about the parts considered necessary to be included in the regulations.