Browsing by Author "Sharma, Akanshu (Prof. Dr.-Ing.)"

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Open Access
Assessment and strengthening of reinforced concrete slabs against punching shear failure
(2024) Biyan, Berhanemeskel E.; Sharma, Akanshu (Prof. Dr.-Ing.)
In this thesis punching shear behavior of reinforced concrete slabs is studied in detail. The study is mainly organized in two parts. The first part mainly focuses on provision of simple, safe, economical and realistic design equation formulation for the design of slabs against punching shear. The second part proposes an innovative strengthening approach for existing RC slabs to enhance their capacity against punching shear. For this, a low-invasive, efficient and effective system of post-installed reinforcement as punching shear reinforcement is utilized. Reinforced concrete slabs resting directly on column are popular slab systems for reinforced concrete structures. Their usage is quite common in industrial structures (typically with post-tensioning) and in buildings used for parking purposes, since they allow higher head room for the same story height compared to traditional beam-slab systems. However, these slabs are susceptible against punching failure at column locations and catastrophic failures of slabs have been reported in the past due to punching. This highlights a lack of understanding and calls for a rational approach which can effectively describe and quantify the mechanism of punching shear. Almost all codes propose empirical equations based on statistical evaluation of existing experimental data to estimate the punching shear capacity of the reinforced concrete slabs. In the first part of this thesis, through an extensive numerical parametric study a new, simple, and realistic punching shear predictive equation is proposed. The numerical study is carried out using the software MASA developed at the University of Stuttgart, which is known for its capabilities to simulate the nonlinear concrete behavior quite well. The numerical modeling approach is first verified against existing experimental results and then used to carry out a detailed parametric study. The proposed equation considers the quantitative contribution of concrete grade, flexural reinforcement ratio, member depth and loading column size. The prediction capacity of the proposed equation is tested by comparing it with existing 235 test data bank. It has been shown that the proposed equation is able to predict the punching shear capacity of the RC slabs with high accuracy and significantly better than the current formulations given in different codes and standards. The second part of the thesis is invested on studying the strengthening of existing reinforced concrete slabs by introducing post installed reinforcing bars as punching shear reinforcement, through numerical and experimental study. The novelty lies in the fact that for the first time the effectiveness and efficiency of plan and vertical arrangement of post installed shear reinforcement is evaluated in this work. A well-designed experimental program that evaluates the effect of arrangement of post installed shear reinforcement is developed and executed. It is shown that the punching shear strength depends not only on the amount of introduced shear reinforcement but also on their plan and vertical arrangement. Furthermore, the effect of member depth on the effectiveness of post installed shear reinforcement is analyzed and it is shown that the percentage utilization of the reinforcement is significantly affected by the member depth. Eventually, an empirical equation that predicts the punching shear capacity of slab strengthened with post installed shear reinforcement is proposed. The degree of accuracy and the reliability of the proposed equation is verified using experimentally tested slabs in this thesis, previous experiment as well as numerically tested slabs, there is a good agreement between the calculated value and experimentally and numerically tested slabs.
Open Access
Enhanced performance of anchorages in uncracked concrete with steel fibres or post-installed reinforcement
(2024) Vita, Norbert; Sharma, Akanshu (Prof. Dr.-Ing.)
Due to the development of anchor technology over the last few decades, it is no longer the anchor but often the concrete that is the decisive factor in the design. In most cases of anchor design, concrete failure is the dominant failure mode, which limits the load-bearing capacity of the anchor. This work examines two different approaches to increase the capacity anchorages, namely by improving the material properties of the base material (concrete), and by supplementing the tensile resistance of the concrete member by providing post-installed reinforcement. The first approach can be used for the new designs, while the second approach is useful for both new and existing anchorages. Part I of the thesis focuses on the performance of bonded anchors in steel fibre reinforced concrete (SFRC). In the performed tests, the influence of the steel fibres (with hooked-end) on the load-bearing behaviour of the post-installed anchorages was investigated. Concrete parameters such as concrete strength, fibre content, as well as the installation parameters such as anchor diameter, effective depth of the anchor, edge distance, anchor spacing, and eccentricity were investigated. The tests were carried out on single anchors and anchor groups under tension and under shear loads. In addition, the hardened concrete properties (such as compressive-, splitting and flexural tensile strength and fracture energy) of the steel fibre reinforced concrete used were determined. The test results confirm the positive influence of the steel fibres (e.g. crack-bridging effect) on the load-displacement behaviour of the anchor, provided certain requirements of the size and quantity of the fibres are fulfilled. A significant load increase and a more ductile failure mode were observed. Based on the detailed evaluation of the test results, a new model is proposed on the basis of the CCD method for anchors in plain concrete, that takes into account the positive influence of the steel fibres on the concrete breakout resistance of anchors in SFRC. Part II of the thesis presents the method of adding post-installed supplementary reinforcement (PISR) to enhance the concrete cone or concrete edge resistance of the anchorages. The tests were carried out on single anchors and anchor groups formed with bonded anchors. This approach combines the benefits of post-installed reinforcement with the function of supplementary reinforcement to enhance the resistance of anchorages against concrete breakout failure modes. The experimental investigations (tensile and shear tests) examined various variables such as the amount of reinforcement, the position and number of reinforcements, the spacing of the reinforcements and the anchorages. The results showed that the addition of post-installed reinforcement can lead to a significant increase in the load carrying capacity and displacements at peak load. A new approach for calculating the failure loads of PISR-reinforced anchors is proposed that considers different failure modes such as reinforcement yielding, bond failure and a new approach to consider the possible strut failure.
Open Access
Post installed rebar end anchorages in reinforced concrete structural connections
(2022) Mahadik, Vinay; Sharma, Akanshu (Prof. Dr.-Ing.)
This thesis is an attempt to integrate the different available perspectives for rebar end anchorage design in general and post installed rebar in particular, and work towards a general framework for design of end anchorage zones in reinforced concrete (RC) connection applicable irrespective of the detailing (straight or with bends) and type (cast-in or post installed) of the connection. It builds on comprehensive discussions of the current state-of-the-art consideration of anchorages in reinforced concrete construction. The discussions are aligned in a way to highlight the diversity of the different perspectives of assessment and design of rebar end anchorages and at the same time identify the context in which the perspectives could be integrated. The mechanical state of loading and the possible load resisting mechanisms are identified as pivotal for this purpose, and hence provide the rational background platform necessary for comparison of different approaches for assessment of rebar end anchorages. The differences in the assessment approaches are better understood in the light of the possible load resistance mechanisms and the potential of the models used for their realistic assessment. The system (or product) specific definition of bond resistance in fastenings technology in contrast to the general empirical definition of bond resistance in conventional RC practice is found to be one primary aspect responsible for the evident differences in the assessment approaches. The recent developments of system specific models for bond resistance coherent with RC practice (PO+SP model framework) that resulted from research projects conducted at IWB, University of Stuttgart, Germany are discussed. These models form an important input for the general assessment framework for rebar end anchorages based on failure hierarchy of all possible load resisting mechanisms (the Failure Hierarchy (FH) approach) developed in this thesis. The different mechanics in RC connections: column-foundation joints (CFJ), wall-foundation joints (WFJ) and beam-column joints (BCJ) strongly affects the behaviour rebar end anchorage zone. Hence a possibility of connection specific consideration in assessment of the rebar end anchorage zones is identified based on the discussion of the available response database in literature for the different joint types. This thesis develops an assessment framework (Failure Hierarchy Approach) for realistic evaluation of rebar end anchorage zones giving due consideration to (i) system specific characteristics of different types of anchorages (cast in and post installed) and (ii) different possible mechanical states of the connection (CFJ, WFJ, BCJ). A need for generating benchmark test data for observing the transition of different failure modes possible in rebar end anchorage zones is highlighted in these discussions. To this end, the design of test specimens is performed for obtaining response database specific to the range of parameters over which a transition of different possible failure modes in the rebar end anchorage zones can be observed. The test program on column-foundation joints provides the necessary database for overall validation of the FH approach. A comparison of different assessment approaches in the light of test results from this thesis and from the literature show the potential of the FH approach for realistic assessment of the behaviour of rebar end anchorage zones in RC connections. For highlighting the importance of connection specific consideration, experiments on wall-foundation joints and beam-column joints are conducted. The generated database provides useful insights towards harmonization of co-existing concrete failure theories. The thesis concludes with an overview of the extent to which the objective of development of general assessment framework for rebar end anchorage design could be reached. Contribution from this thesis towards design concepts for rebar end anchorage anchorages are discussed. The thesis closes with open questions that need further investigations and recommendation for the way forward based on insights it has developed.
Open Access
Post-installed fastening solution for strengthening of RC frames with seismic bracing : a displacement based approach
(2024) Stehle, Erik Johannes; Sharma, Akanshu (Prof. Dr.-Ing.)
Steel bracing is a popular solution for strengthening of reinforced concrete (RC) frame structures against seismic hazards. The new structural brace elements can be directly connected to the existing frame structure by means of post-installed anchors. Such a connection offers a low invasive, economical, and practical solution. However, there are also certain challenges which arise when using post-installed anchors. Different types of post-installed anchors exhibit different displacement and hysteretic behavior. Since the displacement (and hysteretic) behavior of the anchorage has a marked influence on the performance of a strengthening solution, it is important to understand the actual behavior of the anchors and to take their behavior into account in the design of a strengthening solution. Therefore, it is required to assess the suitability of different types of anchors for such structural applications. This is associated with an adequate procedure for the assessment of the hysteretic and displacement behavior of the same. When post-installed anchors are used to form the connection between the steel bracing and the RC frame, they might be subjected to geometric restrictions that negatively affect the performance of the anchors. For instance, the limited dimensions of the structural members, such as beams and columns, in which the anchors are installed in. Or the arrangement of anchors in a spatial corner configuration in case the steel braces are to be fastened to the corner of a RC frame. These challenges call for a displacement based design solution for post-installed anchors, which accounts for the actual performance of the anchors and anchor connections in terms of the displacement and hysteretic behavior. The first part of this thesis investigates the hysteretic and displacement behavior of tension loaded single anchors. For this purpose, a new displacement-based testing procedure is presented. An experimental program was carried out, in which five different types of post-installed anchors were tested following the new testing procedure and the current testing approaches for qualification of anchors against seismic actions. Based on the evaluation of the single anchor behavior, a hysteretic model was developed to simulate the behavior of anchors under pulsating tension load. The model includes the unloading and reloading behavior of the anchors, strength degradation in subsequent cycles at the same displacement level, and accounts for the residual displacements of the anchors after unloading. The second part of this thesis deals with the behavior of anchor groups. Two focal points are investigated. One is the hysteretic behavior of anchors groups. For this purpose, an experimental program is carried using the displacement-controlled testing procedure previously applied to single anchors. Based on the experimental results the hysteretic model for single anchors is extended to anchor groups. Second is the behavior of tension loaded anchor groups in narrow concrete members. Thus, anchor groups in the vicinity of two parallel close edges. The results of the experimental results highlight the need for a modified analytical design approach for anchor groups in narrow members to overcome the current over-conservatism. In the third part of the thesis, the connection between steel bracing and RC frame corner is investigated. The experimental and numerical analysis of the spatial corner configuration highlights the feasibility of such a connection approach. Based on the results, an analytical and a displacement-based design solution for the connection is proposed. The displacement-based solution is based on the nonlinear spring modelling approach and allows the assessment of the complete connection.