Ananth Ramaswamy

A finite element assessment of flexural strength of prestressed concrete beams with fiber reinforcement

Abstract This paper presents an assessment of the flexural behavior of 15 fully/partially prestressed high strength concrete beams containing steel fibers investigated using three-dimensional nonlinear finite elemental analysis. The experimental results consisted of eight fully and seven partially prestressed beams, which were designed to be flexure dominant in the absence of fibers. The main parameters varied in the tests were: the levels of prestressing force (i.e, in partially prestressed beams 50% of the prestress was reduced with the introduction of two high strength deformed bars instead), fiber volume fractions (0%, 0.5%, 1.0% and 1.5%), fiber location (full depth and partial depth over full length and half the depth over the shear span only). A three-dimensional nonlinear finite element analysis was conducted using ANSYS 5.5 [Theory Reference Manual. In: Kohnke P, editor. Elements Reference Manual. 8th ed. September 1998] general purpose finite element software to study the flexural behavior of both fully and partially prestressed fiber reinforced concrete beams. Influence of fibers on the concrete failure surface and stress–strain response of high strength concrete and the nonlinear stress–strain curves of prestressing wire and deformed bar were considered in the present analysis. In the finite element model, tension stiffening and bond slip between concrete and reinforcement (fibers, prestressing wire, and conventional reinforcing steel bar) have also been considered explicitly. The fraction of the entire volume of the fiber present along the longitudinal axis of the prestressed beams alone has been modeled explicitly as it is expected that these fibers would contribute to the mobilization of forces required to sustain the applied loads across the crack interfaces through their bridging action. A comparison of results from both tests and analysis on all 15 specimens confirm that, inclusion of fibers over a partial depth in the tensile side of the prestressed flexural structural members was economical and led to considerable cost saving without sacrificing on the desired performance. However, beams having fibers over half the depth in only the shear span, did not show any increase in the ultimate load or deformational characteristics when compared to plain concrete beams.

Optimal fuzzy logic control for MDOF structural systems using evolutionary algorithms

The paper presents an optimal fuzzy logic control algorithm for vibration mitigation of buildings using magneto-rheological (MR) dampers. MR dampers are semi-active devices and are monitored using external voltage supply. The voltage monitoring of MR damper is accomplished using evolutionary fuzzy system, where the fuzzy system is optimized using evolutionary algorithms (EAs). A micro-genetic algorithm (μ-GA) and a particle swarm optimization (PSO) are used to optimize the FLC parameters. Two cases of optimal FLCs are shown. One where FLC is optimized keeping the rule base predefined and in the other case, FLC rule base is also optimized along with other FLC parameters. The FLC rule base and membership function parameters are optimized using 10 variables. Fuzzy controllers with a predefined rule base and with an optimal rule base are applied to a single degree of freedom (SDOF) and a multi-degree of freedom (MDOF) system. Finally, the study evaluates the performance of the fuzzy controller optimized off-line, on a three storey building model under seismic excitations. The main advantage of using FLC to drive the MR damper voltage is that it provides a gradual and smooth change in voltage. Consequently, the present approach provides a better vibration control for structures under earthquake excitations.

Hybrid structural control using magnetorheological dampers for base?isolated structures

In this paper two nonlinear model based control algorithms have been developed to monitor the magnetorheological (MR) damper voltage. The main advantage of the proposed algorithms is that it is possible to directly monitor the voltage required to control the structural vibration considering the effect of the supplied and commanded voltage dynamics of the damper. The efficiency of the proposed techniques has been shown and compared taking an example of a base isolated three-storey building under a set of seismic excitations. Comparison of the performances with a fuzzy based intelligent control algorithm and a widely used clipped optimal strategy has also been shown.

Flexural strength predictions of steel fiber reinforced high-strength concrete in fully/partially prestressed beam specimens

Abstract This study presents results from an experimental program for eight fully prestressed beams and seven partially prestressed beams made with high strength fiber-reinforced concrete (plain concrete strength of 65 MPa). These studies mainly attempted to determine the influence of trough-shaped steel fibers in altering the flexural strength at first crack and ultimate, the load–deflection and moment–curvature characteristics, ductility and energy absorption capacity of the beams. The magnitude of the prestress, volume fraction of the fibers ranging from 0% to 1.5% and the location of fibers were the variables in the test program. Analytical models to determine the load–deflection and moment–curvature relationships as a function of the fiber volume fraction have been formulated. Empirical relationships for the ultimate strength, first crack load level, load versus deflection and moment versus curvature as a function of fiber content have been proposed by making use of force equilibrium and compatibility considerations. A primary finding emerging from the experimental program was that the placement of fibers over a partial depth in the tensile side of the prestressed flexural structural members provided equivalent flexural capacity as in a beam having the same amount of fiber over the full cross-section. In large scale precast concrete applications it is expected that this would be economical and lead to considerable cost saving in the design without sacrificing on the desired structural performance. The analytical models proposed in this study predicts the test results closely.

Behavior of Fiber-Reinforced Prestressed and Reinforced High-Strength Concrete Beams Subjected to Shear

This paper presents experimental and analytical results of 13 fully/partially prestressed high-strength concrete (plain concrete strength of 65 MPa) beams having fibers, under four points loading. The behavior of prestressed beams, which were designed to be shear dominant in the absence of fibers, has been discussed and the influence of fiber content and fiber location on the shear behavior of the beams has been examined. The levels of prestressing force, fiber volume fraction (0, 0.5, 1.0, and 1.5%), fiber location (full length, and shear span only), the presence/absence of stirrups in the shear span, and the shear span-depth ratio were the variables considered in the experimental program. A rigorous analytical model is proposed to predict the shear strength of prestressed high-strength concrete beams containing steel fibers. Effects of fiber content, level of the prestressing force, shear span-depth ratio, aggregate size, and the compressive and tensile strength of the plain concrete are accounted for in the model. Test results of this study and those reported in the literature were used for the verification of the proposed model. For beams designed to be shear predominant, a significant conclusion emerging from this study is that the beams having fibers located only within the shear span over the full cross section had load deformational response and peak load values that were comparable to the response of beams having fibers over the entire beam. Fibers in the shear span altered the failure mode from one of brittle shear to one of ductile flexure. This study also indicates that stirrups may be replaced by an equivalent amount of fibers without compromising the overall structural performance. A nominal minimum shear reinforcement may be provided with fibers for safety.

Testing and Modeling of MR Damper and Its Application to SDOF Systems Using Integral Backstepping Technique

Magnetorheological dampers are intrinsically nonlinear devices, which make the modeling and design of a suitable control algorithm an interesting and challenging task. To evaluate the potential of magnetorheological (MR) dampers in control applications and to take full advantages of its unique features, a mathematical model to accurately reproduce its dynamic behavior has to be developed and then a proper control strategy has to be taken that is implementable and can fully utilize their capabilities as a semi-active control device. The present paper focuses on both the aspects. First, the paper reports the testing of a magnetorheological damper with an universal testing machine, for a set of frequency, amplitude, and current. A modified Bouc-Wen model considering the amplitude and input current dependence of the damper parameters has been proposed. It has been shown that the damper response can be satisfactorily predicted with this model. Second, a backstepping based nonlinear current monitoring of magnetorheological dampers for semi-active control of structures under earthquakes has been developed. It provides a stable nonlinear magnetorheological damper current monitoring directly based on system feedback such that current change in magnetorheological damper is gradual. Unlike other MR damper control techniques available in literature, the main advantage of the proposed technique lies in its current input prediction directly based on system feedback and smooth update of input current. Furthermore, while developing the proposed semi-active algorithm, the dynamics of the supplied and commanded current to the damper has been considered. The efficiency of the proposed technique has been shown taking a base isolated three story building under a set of seismic excitation. Comparison with widely used clipped-optimal strategy has also been shown.

Shear Strength of Prestressed Concrete T-Beams with Steel Fibers Over Partial/Full Depth

The test results of nine shear-critical partially prestressed concrete flanged beams with and without steel fibers are presented in this paper. T-beam specimens were cast with three grades of concrete: normal strength (35 MPa [5.07 ksi]), moderately high strength (65 MPa [9.42 ksi]), and high strength (85 MPa [12.32 ksi]). For each grade of concrete, three beams were cast: a control beam without fiber reinforcement, a beam with fiber reinforcement over the full depth of the cross section, and a beam with fiber reinforcement only in the web portion. Test results indicated that the provision of fiber reinforcement only in the web portion appreciably improved the shear-resisting capacity of the partially prestressed) beams. A model to predict the shear strength of prestressed and reinforced (nonprestressed) concrete beams has been proposed. The proposed model is expected to predict the test results of reinforced concrete beams having steel fibers over partial and full depth.

Safety Assessment of a Masonry Arch Bridge: Field Testing and Simulations

AbstractThe safety of an in-service brick arch railway bridge is assessed through field testing and finite-element analysis. Different loading test train configurations have been used in the field testing. The response of the bridge in terms of displacements, strains, and accelerations is measured under the ambient and design train traffic loading conditions. Nonlinear fracture mechanics–based finite-element analyses are performed to assess the margin of safety. A parametric study is done to study the effects of tensile strength on the progress of cracking in the arch. Furthermore, a stability analysis to assess collapse of the arch caused by lateral movement at the springing of one of the abutments that is elastically supported is carried out. The margin of safety with respect to cracking and stability failure is computed. Conclusions are drawn with some remarks on the state of the bridge within the framework of the information available and inferred information.

Simplified Mixture Design for Production of Self- Consolidating Concrete

In this work, a methodology to achieve ordinary-, medium-, and high-strength self-consolidating concrete (SCC) with and without mineral additions is proposed. The inclusion of Class F fly ash increases the density of SCC but retards the hydration rate, resulting in substantial strength gain only after 28 days. This delayed strength gain due to the use of fly ash has been considered in the mixture design model. The accuracy of the proposed mixture design model is validated with the present test data and mixture and strength data obtained from diverse sources reported in the literature.

Crack-Width Prediction for High-Strength Concrete Fully and Partially Prestressed Beam Specimens Containing Steel Fibers

This paper describes an experimental and analytical comparison of crack widths in eight fully and seven partially prestressed high-strength concrete beam specimens containing fibers(plain concrete strength 65 MPa). The variables considered in the experimental program were the magnitude of prestress, the volume fraction of fiber ranging from 0 to 1.5%, and the location of the fibers. The analytical model proposed in this study to compute the crack width includes the effect of fiber content expressed in terms of its volume fraction and aspect ratio, magnitude of the longitudinal steel strain, and the interfacial bond stress between the concrete and steel (prestressing wires, deformed bar, and fibers). It has been found that the analytical model is able to predict the crack width satisfactorily, when compared with the measured crack width, for each of the 15 beams tested in this study, from the cracking stage up to the stage just prior to failure.