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Dr. Ahmed Saudi Abdel-Maula El-Sayed :: Theses :

Title Evaluation of Shear Strength of Reinforced Concrete Beam-Column Joints
Type MSc
Supervisors Youssef M. H. Hammad, Fouad B. A. Beshara, and Ibrahim M. Bazan
Year 2003
Abstract Under seismic excitations, the beam-column connections are subjected to horizontal and vertical shear forces whose magnitudes are higher than those within the adjacent beams and columns. To overcome the shortcomings of the current semi-empirical design methods for the connections, the present thesis is concerned with the development of comprehensive and rational design approach for evaluating the shear strength and failure modes of the beam-column joint cores in reinforced concrete ductile frames under seismic loads. Based on the discrete modeling of shear resistance mechanisms, a nonlinear softened strut-and-tie model, and a plastic strut-and-tie model have been proposed for the exterior and interior joints. These models account for different material, geometry, and loading conditions. The proposed approaches have been successfully employed to perform several validation and parametric studies of the shear response and design of beam-column joints. The nonlinear softened strut-and-tie model was derived to satisfy the conditions of equilibrium and strain compatibility, and the constitutive laws of cracked concrete and steel. The intended approach addresses all critical shear components within the joint, and a statically indeterminate load path has been introduced before and after yielding of the reinforcement within the joint. The macro-model of the diagonal compression strut of concrete depends on the effective joint dimensions and the level and type of column load. The horizontal and vertical ties are made-up of the joint hoops, the column intermediate bars, and the inclined joint bars. Depending on the distribution pattern and bond condition, the model accounts for the unequal participation of joint reinforcement in shear resistance. The nonlinear compression law for concrete considers the effects of hoops-induced confinement and cracking-related softening. For reinforced concrete in tension, the composite law accounts for the influence of tension stiffening and yielding of steel ties. Following an incremental-iterative technique for the solution procedure of the nonlinear problem, the proposed model was incorporated into a computer code to trace the joint shear strength and different failure modes due to concrete crushing and steel yielding. The accuracy of the proposed procedure was checked by comparing the calculated shear strengths with the experimental data reported in literature, and a satisfactory correlation was found. Extensive parametric studies were performed to provide valuable insights into the behavior and design of exterior and interior joints under seismic loading. By simplifying the nonlinear model, a plastic strut-and-tie model was derived as a direct design tool for the shear strength of connections. The spatial discretization of the analogous truss is similar to that of nonlinear model. The proposed design approach is simple, however it accounts for various factors such as equilibrium conditions, geometrical configurations, effective concrete strength, compression softening, vertical and horizontal steel quantities, failure modes, and column load level and type. A good correlation was obtained between the predicted joint shear of several beam-column connections and the existing experimental results. The amount of joint hoops is determined according to the selected limit state. The joints in ductile moment-resisting frames, where there is a need for sustained strength under deformation reversals into the inelastic range, the designed for the limit state of tie yielding. By using illustrative design examples, the provisions of the joint shear strength and transverse hoop reinforcement of design codes are compared with the proposed design method. Finally, case studies were performed in order to demonstrate the variations in hoop requirements caused by the control parameters, and also to provide design guidelines for the beam-column joints under different conditions.
Keywords beam-column connections
University Benha University
Country Egypt
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Title Behavior of Epoxy-Modified Reinforced Concrete Beams
Type PhD
Supervisors Youssef M. H. Hammad, Fouad B. A. Beshara
Year 2011
Abstract Epoxy-modified concrete is made by partially replacing ordinary Portland cement with epoxy by weight. It is considered to be high durable material with relatively good mechanical properties. It has been progressively used in reinforced concrete structures under normal and severe environmental conditions. The present research is concerned with the behavior and performance of epoxy-modified reinforced concrete (EMRC) beams. To study the response characteristics and failure mechanisms in flexure and shear, comprehensive experimental programs were carried out on simply-supported EMRC beam specimens. All beams were made from high strength concrete with 70 MPa as the compressive strength. Different material and structural variables were considered in the testing programs. In addition, nonlinear finite element analysis program was modified in order to predict the load-deflection curves of tested beams. Finally, proposed design equations were formulated to predict the ultimate moment capacity and ultimate shear strength of EMRC beams. The predicted results of finite element and design studies correlated well with measured results. In the flexure experimental program, ten under-reinforced beams with shear span-to-depth ratio (a/d) as 7, were casted. Using different epoxy weight ratios as 0%, 5% and 10%, the flexural beams were mainly divided into three groups. In each group, two different values were used for the tension steel ratio (1.1% & 1.97%) and the un-bonded length (200 & 400 mm) of longitudinal tension bars. In the shear experimental program, eighteen over-reinforced beams were casted, and were equally divided into three groups with 0%, 5%, and 10% epoxy weight ratios. In each group, three different values were employed for the transverse stirrups ratio (0%, 0.24% & 0.48%) and shear span-to-depth ratio (2.5, 3.5, and 4.5). All beams were tested under monotonic static loading till failure. The results indicate that beams containing higher epoxy content tend to produce less widened, and more spreader cracks which can be attributed to the ability of polymer-modified concrete in arresting the growth and widening or of cracks. Also EMRC beams exhibit less deformation, less steel strain, less stirrups strain, and higher load-carrying capacity at different levels. The increase of epoxy weight ratio, results in increase in the cracking and ultimate moment and shear capacities. For 1.1% steel ratio, the use of epoxy weight ratio 5% and to 10% increases the cracking moment by 5% and 9.3%, and increases the ultimate moment by 4% and 7.5%. The corresponding increases for 1.97% steel ratio were 4.6% and 8.7% for cracking moment and 2.3% and 5% for ultimate moment. The moment capacities of EMRC beams with un-bonded length are higher than of beams with 0% epoxy. The use of epoxy weight ratio 5% and to 10% increases the cracking shear strength by 14% and 20% for a/d= 2.5, by 13% and 15.4% for a/d= 3.5, and by 5% and 11% for a/d= 4.5. The corresponding increases in the ultimate shear strength are 3% and 10% for a/d= 2.5, 7% and 13.6% for a/d= 3.5, and 6.7% and 11.11% for a/d= 4.5. For nonlinear finite element analysis of EMRC beams, the existing constitutive model for conventional concrete was modified to include the material properties of EMRC in compression and tension. The epoxy-modified concrete is modeled strain-induced orthotropic material with nonlinear stress-strain relations in tension and compression, and Kupfer’s biaxial strength envelope. The steel behavior is represented by the bilinear elasto-plastic model. The load-deflection curves predicted by the finite element for all beams were in a good agreement to the experimental curves. As an extension of ACI approach for high strength concrete, proposed flexural design equations were developed on the basis of strain compatibility and equilibrium conditions. For predicting the ultimate shear strength of EMRC, semi-empirical equation was derived as a function of concrete strength, steel ratio and (a/d) ratio. The predicted ultimate moments and ultimate shear strength of several reinforced concrete beams were compared with the experimental results in the present work and in other several sources of literature. Despite the difference in test specimens, the proposed approaches predict their ultimate moment and ultimate shear strength capacity reasonably well.
Keywords epoxy-modified reinforced concrete beams
University Benha University
Country Egypt
Full Paper -

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