Seismic Behavior of Flexural Dominated Reinforced Concrete Bridge Columns at Low Temperatures

This paper presents the results from Phase II of an experimental study on the behavior of reinforced concrete bridge columns in cold seismicly active regions. Six half-scale circular reinforced concrete columns, designed to be flexural dominated, were tested under reversed cyclic loading while subjected to temperatures ranging from ?36°C (?33°F) to 22°C (72°F). Four of the units tested were reinforced concrete filled steel tube (RCFST) columns and the other two were ordinary reinforced concrete columns. Results obtained reiterated the observations made in Phase I, which is that low temperatures cause an increase in the flexural strength and initial stiffness as well as a reduction in the spread of plasticity and displacement capacity of the column. Another important observation made was that the plastic hinge length is drastically reduced in the RCFST units compromising the displacement capacity of this type of column even at room temperature conditions. Current predictive models were revised and modified to account for the low-temperature effect.     

Seismic Behavior of Posttensioned Concrete-Filled Fiber Tubes

Precast segmental construction technique is an excellent candidate for economic rapid bridge construction in highly congested urban environments and environmentally sensitive regions. This paper presents the seismic behavior of four hybrid segmental columns consisting of precast posttensioned concrete-filled fiber tubes (PPT-CFFTs). A fifth monolithic column was also tested as a reference specimen. The columns were tested under increasing lateral loading cycles in a displacement control. The columns had circular cross section diameters of 203 mm and heights of 1,524 mm each. The parameters investigated included different construction details and energy dissipation systems. The PPT-CFFT columns developed lateral strength and deformation capacity comparable to those of the monolithic reinforced concrete column. However, the PPT-CFFT columns dissipated smaller hysteretic energy compared to that of the monolithic reinforced concrete column. Finally, a simple model was used to predict the backbone curves of segmental columns. The model was conservative and it predicted approximately 75% of the measured ultimate strength and displacement.     

Testing and Modeling of a New Moment Connection of Concrete-Filled FRP Tubes to Footings under Monotonic and Cyclic Loadings

This study explores a new moment connection of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) to concrete footing. The tube is tightly fitted and adhesively bonded to a short reinforced concrete stub protruding from the footing, which facilitates concrete filling of the tube without the need for shoring. To establish the critical stub length (Xcr), specimens with heavily steel-reinforced stubs varying in length from 0.5D to 2.0D, where D = diameter of the CFFT, were fabricated and tested in flexure by using a cantilever setup. The Xcr required to achieve flexural failure of the CFFT was 1.05D. Additional specimens with a sufficient stub length of 1.5D were then fabricated to examine the effect on strength and ductility of the steel reinforcement ratio (ρs) in the stub. The optimal ρs of the stub required for the CFFT to reach flexural failure was 3.2%. Finally, the effect of low-cycle reversed bending fatigue was studied with and without an axial compression load. Remarkable ductility associated with the formation of a plastic hinge was observed at ρs of 2%. An analytical model capable of predicting (1)?moment capacity of the connection, (2)?whether failure is governed by flexure or bond, and (3)?Xcr was developed and validated. It was then used in a parametric study to explore the effects of CFFT mechanical and geometric properties on Xcr.     

Confinement Effectiveness of FRP in Retrofitting Circular Concrete Columns under Simulated Seismic Load

The results of a research program that evaluated the confinement effectiveness of the type and the amount of fiber-reinforced polymer (FRP) used to retrofit circular concrete columns are presented. A total of 17 circular concrete columns were tested under combined lateral cyclic displacement excursions and constant axial load. It is demonstrated that a high axial load level has a detrimental effect and that a large aspect ratio has a positive effect on drift capacity. Compared with the performance of columns that are monotonically loaded until failure, three cycles of every displacement excursion significantly affect drift capacity. The energy dissipation capacity is controlled by FRP jacket confinement stiffness, especially under a high axial load level. The fracture strain of FRP material has no significant impact on the drift capacity of retrofitted circular concrete columns as long as the same confining pressure is provided, which differs from the common opinion that a larger FRP fracture strain is advantageous in seismic retrofitting. The amount of confining FRP greatly affects the length of the plastic hinge region and the drift capacity of FRP-retrofitted columns. A further increase in confinement after a critical value causes a reduction in the deformation capacity of the columns.     

Seismic Performance Assessment of Damage-Controlled FRP-Retrofitted RC Bridge Columns Using Residual Deformations

Despite the improved performance of fiber-reinforced plastic (FRP)-retrofitted bridges, residual deformations in the event of an earthquake are inevitable. Little consideration is currently given to these deformations when assessing seismic performance. Moreover, important structures are currently required not only to have high strength and high ductility but also to be usable and repairable after high intensity earthquakes. This paper presents a definition of an FRP-RC damage-controllable structure. An intensive study of 109 bridge columns, extracted from recent research literature on the inelastic performance of FRP retrofitted columns with lap-splice deficiencies, flexural deficiencies, or shear deficiencies, is used to evaluate the recoverability of such retrofitted columns. The residual deformation, as a seismic performance measure, is used to evaluate the performance of 39 FRP-retrofitted RC columns from the available database. Based on this evaluation, a requirement for the recoverable and irrecoverable states of FRP-RC bridges is specified. Finally, the Seismic Design Specifications of Highway Bridges for RC piers is adapted to predict the residual deformations of FRP-RC columns.     

Active Confinement of Reinforced Concrete Bridge Columns Using Shape Memory Alloys

This paper presents experimental and analytical work conducted to explore the feasibility of using an innovative technique for seismic retrofitting of RC bridge columns using shape memory alloys (SMAs) spirals. The high recovery stress associated with the shape recovery of SMAs is being sought in this study as an easy and reliable method to apply external active confining pressure on RC bridge columns to improve their ductility. Uniaxial compression tests of concrete cylinders confined with SMA spirals show a significant improvement in the concrete strength and ductility even under small confining pressure. The experimental results are used to calibrate the concrete constitutive model used in the analytical study. Analytical models of bridge columns retrofitted with SMA spirals and carbon fiber-reinforced polymer (CFRP) sheets are studied under displacement-controlled cyclic loading and a suite of strong earthquake records. The analytical results proves the superiority of the proposed technique using SMA spirals to CFRP sheets in terms of enhancing the strength and effective stiffness and reducing the concrete damage and residual drifts of retrofitted columns.     

Experimental Investigation on Seismic Retrofitting of Square RC Columns by Carbon FRP Sheet Confinement Combined with Transverse Short Glass FRP Bars in Bored Holes

Jacketing is less effective to large square/rectangular RC columns due to the inability of the rectangular-shaped jacket in restraining the dilation of concrete in the middle of a straight side. A new retrofit method is proposed in this work by fiber reinforcing the surface concrete in the middle of a straight side. Fiber reinforcing is achieved by inserting small fiber-reinforced polymer (FRP) bars into the concrete in the plastic hinge zone. The inserted FRP bars act as horizontal reinforcement to increase the ductility of the concrete in a similar way as that in normal fiber-reinforced concrete. When this fiber reinforcing technique is combined with the conventional jacketing, the concrete in all parts of a cross section may be effectively confined. In this work, experimental tests were undertaken to investigate the effectiveness of this new retrofit technique. Six half-scaled columns were tested and the test results demonstrated the effectiveness of the method.     

Seismic Performance of Hollow High-Strength Concrete Bridge Columns

The seismic performance of rectangular hollow bridge columns has been a significant issue with the high-speed rail project in Taiwan, because the ductility of these columns is unknown. To investigate the seismic performance of hollow high-strength concrete bridge columns, six specimens were tested under a constant axial load and a cyclically reversed horizontal load. Based on the results of these tests an analytical model was developed in order to predict the moment-curvature curve of sections and the load-displacement relationship of the bridge columns. The seismic performance of the test specimens are presented in this paper. The test results were compared to both the analytical model and the shear capacity specified in the codes. It was found that the ductility factors of the tested columns range from 3.9 to 4.7, and that the analytical model predicts the load-displacement relationship of such columns with acceptable accuracy.     

Fiber-Reinforced Plastic Jackets for Ductility Enhancement of Reinforced Concrete Bridge Columns with Poor Lap-Splice Detailing

This paper presents an inclusive testing program conducted on scaled models of reinforced concrete (RC) bridge columns with insufficient lap-splice length. Thirteen half-scale circular and square column samples were tested in flexure under lateral cyclic loading. Three columns were tested in the as-built configuration whereas ten samples were tested after being retrofitted with different composite-jacket systems. A brittle failure was observed in the as-built samples due to bond deterioration of the lap-spliced longitudinal reinforcement. The jacketed circular columns demonstrated a significant improvement in their cyclic performance. Yet, tests conducted on square jacketed columns showed a limited improvement in clamping on the lap-splice region and for enhancing the ductility of the column.     

Bond-Slip Analytical Formulation toward Optimal Embedment of Concrete-Filled Circular FRP Tubes into Concrete Footings

This paper presents a robust analytical model for a moment connection of concrete-filled fiber reinforced-polymer (FRP) tubes (CFFTs) to concrete footings. The CFFT connection is based on a simple approach of direct embedment into the footing, thereby eliminating the need for connection rebar or mechanical devices. The CFFT is externally subjected to lateral and axial loads, resembling practical applications such as piles affixed to pile caps, bridge columns, or utility poles. The model adopts the concepts of equilibrium, deformations compatibility, and nonlinear concrete stress-strain behavior. It also employs a “bond stress-slip” relation that can be obtained from simple push-through tests on some of the commercially used tubes. The model can predict the critical embedment length Xcr, which is the minimum length required to achieve material failure of the CFFT outside the footing, and bond failure inside the footing, simultaneously. If the actual embedment length is less than Xcr, bond failure occurs prematurely at a lower strength that can also be predicted by the model. The model was verified using experimental data and showed that Xcr was only 0.7 of the diameter for that case. A sensitivity parametric analysis was carried out that led to some approximations. Based on which, a simple closed-form expression was established for Xcr in the case of lateral loading only.     

Closed-Form Model and Parametric Study on Connection of Concrete-Filled FRP Tubes to Concrete Footings by Direct Embedment

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been introduced as a new system for piles, columns, and poles. A simple moment connection based on direct embedment of the CFFT into concrete footings or pile caps, without using dowel-bar reinforcement, has been proposed by the authors. Robust analytical models to predict the critical embedment length (Xcr) were also developed and experimentally validated. In this paper, a comprehensive parametric study is carried out using the models developed earlier along with a newly developed closed-form model for the general case of axial loading, bending, and shear applied to the CFFT member. The parameters studied are the diameter (D), thickness (t), length outside the footing (L), and laminate structure of the FRP tube, as well as the tube-concrete interface bond strength (τmax?), concrete compressive strength in the CFFT (fct′) and footing (fc′), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasing D, L/D, τmax?, and fc′ of the footing, or the axial compression load, reduces (X/D)cr, whereas increasing t and fct′ of the CFFT, the fraction of longitudinal fibers in the tube, or the axial tension load, increases Xcr. As the axial load eccentricity increases, Xcr reduces for tension loads and increases for compression loads until both cases converge asymptotically to the same Xcr value, essentially that of pure bending.     

Seismic Design of Concrete-Filled Circular Steel Bridge Piers

The adequacy of the existing design provisions for concrete-filled steel pipes subjected to axial forces and flexure is reviewed by comparing the strengths predicted by the CAN/CSA-S16.1-M94, AISC LRFD 1994, and the Eurocode 4 1994 codes and standards against experimental data from a number of investigators. New proposed design equations are then developed, in a format compatible with North American practice. The new equations, based on a simple plasticity model calibrated using experimental data, are shown to provide improved correlation between predicted strength and experimental data. This paper provides information and data in support of the proposed design equations, which have already been implemented in the 2001 edition of the CSA-S16-01 “limit state design of steel structures” (CSA 2001) and in the “Recommended LRFD Guidelines for the Seismic Design of Highway Bridges” (MCEER/ATC 2003).     

Fragility Increment Functions for Deteriorating Reinforced Concrete Bridge Columns

The increased deformation and shear fragilities of corroding RC bridge columns subject to seismic excitations are modeled as functions of time using fragility increment functions. These functions can be applied to various environmental and material conditions by means of controlling parameters that correspond to the specific condition. For each mode of failure, the fragility of a deteriorated column at any given time is obtained by simply multiplying the initial fragility of the pristine/nondeteriorated column by the corresponding function developed in this paper. The developed increment functions account for the effects of the time-dependent uncertainties that are present in the corrosion model as well as in the structural capacity models. The proposed formulation is a useful tool for engineering practice because the fragility of deteriorated columns is obtained without any extra reliability analysis once the fragility of the pristine column is known. The fragility increment functions are expressed as functions of time t and a given deformation or shear demand. Unknown parameters involved in the models are estimated using a Bayesian updating framework. A model selection is conducted during the assessment of the unknown parameters using the Akaike information criterion and the Bayesian information criterion. For the estimation of the parameters, a set of data are obtained by first-order reliability method analysis using existing probabilistic capacity models for corroding RC bridge columns. Example fragilities of a deteriorated bridge column typical of current California’s practice are presented to demonstrate the developed methodology. The increment functions suggested in this paper can be used to assess the time-variant fragility for application to life cycle cost analysis and risk analysis.     

Axial Load Capacity of Concrete-Filled FRP Tube Columns: Experimental versus Theoretical Predictions

This paper presents the experimental and theoretical results of small and medium-scale concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) columns. A total of 23 CFFT specimens were tested under axial compression load. Five different types of new FRP tubes were used as stay-in-place formwork for the columns. The effects of the following parameters were examined: the FRP-confinement ratio, the unconfined concrete compressive strength, the presence of longitudinal steel reinforcement, and the height-to-diameter ratio. Comparisons between the experimental test results and the theoretical prediction values by the three North American codes and design guidelines (ACI 440.2R-08, CSA-S6-06, and CSA-S806-02) are performed in terms of confined concrete strength and ultimate load carrying capacity. The results of this investigation indicate that the design equations of the ACI 440.2R-08, CAN/CSA-S6-06, and CAN/CSA-S806-02 overestimate the factored axial load capacity of the short CFFT columns as compared to the yield and crack load levels. Also, the CAN/CSA-S6-06 and CAN/CSA-S806-02 confinement models showed conservative predictions, while the ACI 440.2R-08 was slightly less conservative. A new confinement model is proposed for the confined concrete compressive strength of the CFFT cylinders. Also, the design equations are modified to accurately predict the ultimate and yield load capacities of internally reinforced and unreinforced short CFFT columns. Two new factors are introduced in the modified equations, (kcc) accounts for the in-place-strength of CFFT columns to CFFT cylinder strength, and (kcr) accounts for the initiation of the steel yielding and concrete cracking for the FRP-confined columns.     

Flexural Load Testing of Concrete-Filled FRP Tubes with Longitudinal Steel and FRP Rebar

The flexural performance of reinforced concrete-filled glass-fiber reinforced polymer (GFRP) tubes (CFFTs) has been investigated using seven specimens, 220?mm in diameter and 2.43?m long. Specimens were reinforced with either steel, GFRP, or carbon–fiber reinforced polymer (FRP) rebar of various sizes. Prefabricated GFRP tubes with most of the fibers oriented in the hoop direction were used in five specimens. One control specimen included conventional steel spirals of stiffness comparable to the GFRP tube and the other had no transverse reinforcement. Test results have shown that CFFT beams performed substantially better than beams with a steel spiral. Unlike CFFTs with FRP rebar, CFFTs with steel rebar failed in a sequential progressive manner, leading to considerable ductility. An analytical model capable of predicting the full response of reinforced CFFT beams, including the sequential progressive failure, has been developed, verified, and used in a parametric study. It is shown that laminate structure of the tube affects the behavior, only after yielding of the steel rebar. Steel reinforcement ratio significantly affects stiffness and strength, whereas concrete strength has an insignificant effect on the overall performance.     

Deflection Prediction of Steel and FRP-Reinforced Concrete-Filled FRP Tube Beams

This paper presents the results of experimental and theoretical investigations that study the flexural behavior of reinforced concrete-filled fiber-reinforced polymer (FRP) tubes (RCFFTs) beams. The experimental program consists of 10 circular beams [6 RCFFT and 4 control reinforced concrete (RC) beams] with a total length of 2,000?mm, tested under four-point bending load. The experimental results were used to review and verify the applicability of various North American code provisions and some available equations in the literature to predict deflection of RCFFT beams. The measured deflections and the experimental values of the effective moment of inertia were analyzed and compared with those predicted using available models. The results of the analysis indicated that the behavior of steel and FRP-RCFFT beams under the flexural load was significantly different than that of steel and FRP-RC members. This is attributed to the confining effect of the FRP tubes and their axial contribution. This confining behavior in turn enhanced the overall flexural behavior and improved the tension stiffening of RCFFT beams. For that, the predicted tension stiffening of steel and FRP-RCFFT beams using the conventional equations (steel or FRP-RC member) underestimates the flexural response; therefore, the predicted deflections are overestimated. Based on the analysis of the test results, the Branson’s equation for the effective moment of inertia of RC structures is modified, and new equations are developed to accurately predict the deflection of concrete-filled FRP tube (CFFT) beams reinforced with steel or FRP bars.     

Large-Bar Connection for Precast Bridge Bents in Seismic Regions

A new beam-to-column connection has been developed for assembling precast concrete bridge bents in regions of high seismicity. The connection is made with a small number of large column bars, which are grouted into large corrugated-metal ducts embedded in the cap beam. Bents built with these connections can be erected quickly and permit generous construction tolerances. To evaluate the seismic performance of the proposed connection, lateral-load tests were performed on three manifestations of the connection, as well as on a comparable cast-in-place connection. The tests demonstrated that the force-displacement response and damage progression in the precast connection are similar to those of typical cast-in-place concrete connections. Deliberate partial debonding of the longitudinal reinforcement only slightly affected the force-displacement response and observed damage.     

Textile-Reinforced Mortar versus FRP Jacketing in Seismic Retrofitting of RC Columns with Continuous or Lap-Spliced Deformed Bars

The effectiveness of a new structural material, namely, textile-reinforced mortar (TRM), was investigated experimentally in this study as a means of confining oldtype reinforced concrete (RC) columns with limited capacity due to bar buckling or due to bond failure at lap splice regions. Comparisons with equal stiffness and strength fiber-reinforced polymer (FRP) jackets allow for the evaluation of the effectiveness of TRM versus FRP. Tests were carried out on nearly full scale nonseismically detailed RC columns subjected to cyclic uniaxial flexure under constant axial load. Ten cantilevertype specimens with either continuous or lap-spliced deformed longitudinal reinforcement at the floor level were constructed and tested. Experimental results indicated that TRM jacketing is quite effective as a means of increasing the cyclic deformation capacity of oldtype RC columns with poor detailing, by delaying bar buckling and by preventing splitting bond failures in columns with lap-spliced bars. Compared with their FRP counterparts, the TRM jackets used in this study were found to be equally effective in terms of increasing both the strength and deformation capacity of the retrofitted columns. From the response of specimens tested in this study, it can be concluded that TRM jacketing is an extremely promising solution for the confinement of reinforced concrete columns, including poorly detailed ones with or without lap splices in seismic regions.     

Comprehensive Composite Inelastic Fiber Element for Cyclic Analysis of Concrete-Filled Steel Tube Columns

This paper presents the development of a comprehensive composite beam-column fiber element for large displacement nonlinear inelastic analysis of concrete-filled steel tube (CFT) columns. The bond/slip formulation represents the interaction between concrete and steel over the entire contact surface between the two materials. Thus, the modeling accounts for the two factors that cause the slippage between steel shell and concrete core. The first factor is the difference between axial elongation of the steel shell and the concrete core, and the second is the difference between curvatures in the cross section for the concrete core and the steel shell. These effects are integrated over the perimeter and are added to the virtual work expression of the basic element. Furthermore, the constitutive models employed for concrete and steel are based on the results of a recent study and include the confinement and biaxial effects. A 13 degree of freedom (DOF) element with three nodes, which has five DOF per end node and three DOF on the middle node, has been chosen. The quadratic Lagrangian shape functions for axial deformation and the quartic Hermitian shape functions for the transverse directions are used. The model is implemented to analyze several CFT columns under constant concentric axial load and cyclic lateral load. The effect of semi- and perfect bond is investigated and compared with experiments. Good correlation has been found between experimental results and theoretical analyses. The results show that the use of a studded or ribbed steel shell causes greater ultimate strength and higher dissipation of energy than the columns with nonstudded steel shells.     

Concrete-Filled Square and Rectangular FRP Tubes under Axial Compression

This paper presents results of an experimental study on the behavior of square and rectangular concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) under concentric compression. FRP tubes were designed as column confinement reinforcement and were manufactured using unidirectional carbon fiber sheets with fibers oriented in the hoop direction. The effects of the thickness and corner radius of the tube, sectional aspect ratio, and concrete strength on the axial behavior of CFFTs were investigated experimentally. Test results indicate that FRP confinement leads to substantial improvement in the ductility of both square and rectangular columns. Confinement provided by the FRP tube may also improve the axial load-carrying capacity of the square and rectangular columns if the confinement effectiveness of the FRP tube is sufficiently high. The results also indicate that the confinement effectiveness of FRP tubes is higher in square columns than in rectangular columns, and in both sections the effectiveness of confinement increases with the corner radius. Furthermore, for a given confinement level, improvement observed on the axial behavior of concrete due to confinement decreases with increasing concrete strength.