Development, Shake Table Testing, and Design of FRP Seismic Restrainers

The development and performance of fiber-reinforced polymer (FRP) fabrics as an alternative to steel for restrainers to reduce bridge relative movements at hinges during earthquakes was explored. Glass, carbon, and hybrid (glass/carbon) restrainers were developed and tested on a representative in-span hinge using a shake table at the large-scale structures laboratory at the University of Nevada, Reno. The components of the study presented in this article are: (1) the FRP restrainer development and testing; (2) comparisons among FRP, steel, and shape memory alloy restrainers; and (3) development and evaluation of a simple restrainer design method and a numerical example. Important findings of the study were that compared to steel restrainers, the FRP restrainers were effective in substantially reducing the relative hinge displacements and pounding at hinges. The method to develop the flexible portion of the FRP restrainers and the bond to superstructure was successful in accomplishing the target performance. The proposed restrainer design method provides a rational yet simple tool to design FRP or other types of restrainers.     

Seismic Testing of a Two-Span Reinforced Concrete Bridge

A quarter-scale, two-span reinforced concrete bridge was tested using the shake-table system at the University of Nevada, Reno. The shake-table tests were part of a multiuniversity, multidisciplinary project utilizing the network for earthquake engineering simulation, with the objective of investigating the effects of soil-foundation-structure interaction on bridges. This paper discusses the development and testing of the bridge model, and selected experimental results, including those that demonstrate the effects of incoherent motions and stiffness irregularities on the distribution of forces and deformations within the bridge system. Motion incoherency affected the asymmetric bridge response (planar torsion of the superstructure), but had little effect on the symmetric bridge response (center-of-mass displacement of the superstructure). These experimental findings are consistent with conclusions from numerical analyses conducted by other researchers. During a 2.0?g PGA earthquake excitation, numerous longitudinal bars buckled and fractured at a drift ratio between 5.5 and 7.9%. Despite the level of damage, detailing of the column transverse reinforcement according to NCHRP 12-49 guidelines provided sufficient column ductility to prevent collapse during a subsequent 1.4?g PGA earthquake excitation.     

Assessing the Strengthening Effect of Various Near-Surface-Mounted FRP Reinforcements on Concrete Bridge Slab Overhangs

The near-surface-mounted (NSM) method has proved to be a reliable alternative to the existing externally bonded (EB) method for the repair and strengthening of concrete structures using fiber-reinforced polymer (FRP) composites. This technique is especially advantageous in bridge deck upgrades for larger barriers and slab overhang strengthening. This paper presents the results of a comparison of the flexural behavior of bridge slab overhangs strengthened in negative bending moment regions with various types of NSM reinforcement that differ in surface condition (e.g., textured and sand coated), cross-sectional shape (e.g., round and square), material type (carbon and glass), and prestressing effect. Eleven full-scale overhang specimens (1.524 m long in overhang and 0.914 m wide) were tested under a cantilever condition. Test results showed that the FRP NSM reinforcements were effective in increasing both yield and ultimate strength of predamaged slab overhangs. All surface treatments were more beneficial than the smooth condition, and the square-shaped reinforcement displayed better performance than the round shape. The prestressing unit developed in this study is simple to apply and could be further explored for field applications.     

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.     

Identification of the Mechanical Subsystem of the NEES-UCSD Shake Table by a Least-Squares Approach

A least-squares method is used to determine the fundamental parameters of a simple mathematical model for the mechanical subsystem of the NEES-UCSD large high performance outdoor shaking table. The parameters identified include the effective horizontal mass, the effective horizontal stiffness, and the coefficient of the classical Coulomb friction and viscous damping elements representing the various dissipative forces in the system. The values obtained for these parameters are validated by comparisons with previous results based on an alternative identification method applicable only to periodic tests and by comparisons with experimental data obtained during earthquake simulation tests and harmonic steady-state tests. The proposed identification approach works well for periodic sinusoidal and triangular tests, earthquake simulation tests, and white noise tests with table root mean square above 10% of gravity.     

Evaluation of Seismic Performance of Gravity Load Designed Reinforced Concrete Frames

The seismic performance of reinforced concrete frames designed for gravity loads is evaluated experimentally using a shake table. Two 1:3 scale models of one-bay, three-storied space frames, one without infill and the other with a brick masonry infill in the first and second floors, are tested under excitation equivalent to the spectrum given in IS 1893-2002. From the measured response of the models during excitation, the shear force, interstory drift, and stiffness are evaluated. The effect of masonry infill on the seismic performance of reinforced concrete frames is also investigated. Then, the frames are tested to failure. Severe damage is observed in the columns in the ground floor. The damaged columns are strengthened by a reinforced concrete jacket. The frames are again tested under the same earthquake excitations. The test results showed that the retrofitted frames could sustain low to medium seismic forces due to a significant increase in strength and stiffness.     

Behavior and Effectiveness of FRP Wrap in the Confinement of Large Concrete Cylinders

This paper presents the results of an experimental investigation on the strength and behavior of large 250 mm diameter concentrically loaded unreinforced fiber-reinforced polymer (FRP) confined concrete cylinders. In this study, the effect of the number of layers of the FRP and different overlap locations on the effectiveness of the FRP wrap is determined. Discontinuous versus continuous wrapping configurations to confine the cylinder are also investigated. To quantify the level of strain in the wrap and to aid in developing a deeper understanding of the behavior of these larger sized test specimens, an extensive array of electrical resistance strain gauges is used in addition to electronic speckle pattern interferometry (ESPI) optical measurement at selected locations. The ESPI results prove especially powerful in confirming the existence of strain concentrations at the ends of the overlap region, which may contribute to rupture failure of the wrap. The strain gauges in turn enable the effectiveness of the FRP to be quantified in addition to the distribution of hoop strain in the overlap and nonoverlap regions. Also of interest in these tests is identification of the occurrence of interfacial failure between the FRP and concrete at the FRP rupture failure position. Finally, the test results are found to correlate reasonably well with the ACI 440.2R-08 predictions for FRP-confined concrete columns.     

Analytical Models for Concrete Confined with FRP Tubes

Concrete columns encased in fiber-reinforced polymer (FRP) tubes offer an attractive solution to enhance behavior of concrete in terms of strength as well as ductility. Analytical models for development of stress-strain curves for concrete confined with FRP are proposed in this paper. The predicted stress-strain curves for confined concrete using the proposed models are compared with those of tests for concrete specimens confined with FRP. It is demonstrated that the proposed models predict the stress-strain behavior of confined concrete very well. Based on the confidence gained in the proposed models, the effects of using different fibers, the presence of voids, and the number of layers are established.     

Seismic Behavior of Concrete Columns Reinforced by Steel-FRP Composite Bars

Steel-fiber-reinforced polymer (FRP) composite bars (SFCBs) are a novel reinforcement for concrete structures. Because of the FRP’s linear elastic characteristic and high ultimate strength, they can achieve a stable postyield stiffness even after the inner steel bar has yielded, which subsequently enables a performance-based seismic design to easily be implemented. In this study, lateral cyclic loading tests of concrete columns reinforced either by SFCBs or by ordinary steel bars were conducted with axial compression ratios of 0.12. The main variable parameters were the FRP type (basalt or carbon FRP) and the steel/FRP ratio of the SFCBs. The test results showed the following: (1)?compared with ordinary RC columns, SFCB-reinforced concrete columns had a stable postyield stiffness after the SFCB’s inner steel bar yielded; (2)?because of the postyield stiffness of the SFCB, the SFCB-reinforced concrete columns exhibited less column-base curvature demand than ordinary RC columns for a given column cap lateral deformation. Thus, reduced unloading residual deformation (i.e., higher postearthquake reparability) of SFCB columns could be achieved; (3)?the outer FRP type of SFCB had a direct influence on the performance of SFCB-reinforced concrete columns, and concrete columns reinforced with steel-basalt FRP (BFRP) composite bars exhibited better ductility (i.e., a longer effective length of postyield stiffness) and a smaller unloading residual deformation under the same unloading displacement when compared with steel-carbon FRP (CFRP) composite bar columns; (4)?the degradation of the unloading stiffness by an ordinary RC column based on the Takeda (TK) model was only suitable at a certain lateral displacement. In evaluating the reparability of important structures at the small plastic deformation stage, the TK model estimated a much smaller residual displacement, which is unsafe for important structures.     

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 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.     

Flexural Performance of Carbon Fiber-Reinforced Polymer Prestressed Concrete Side-by-Side Box Beam Bridge

The effect of varying transverse posttensioning levels and arrangements on the load response of a one-half scale 30° skewed seven box beam bridge model was investigated. The effective span of the bridge model was 9.45?m (31?ft) with a width of 3.35?m (11?ft) and depth of 355.6?mm (14?in.). The bridge model was prestressed and reinforced with carbon fiber composite cables (CFCCs). CFCCs were also used as shear reinforcement. The bridge model was provided with five transverse diaphragms equally spaced along the length of the bridge. The experimental investigation included load and strain distribution tests and a flexural ultimate load test. The load and strain distribution tests were conducted on the bridge model with and without full-depth longitudinal cracks at the shear-key locations. The investigation showed that the application of an adequate transverse posttensioning force was successful in restoring the load distribution of the bridge model with full-depth longitudinal deck cracks to that of the case without deck cracks. The ultimate load and the associated compression-controlled failure mode of the bridge model agreed well with that predicted according to ACI 440.4R-04 and numerical analysis. The behavior of the bonded pretensioned and reinforced CFCC strands was linear elastic and remained intact throughout the collapse of the bridge model. The unbonded transverse posttensioned CFCC strand also remained intact.     

Design and Construction of a Bridge in Very High Performance Fiber-Reinforced Concrete

The design, technology, and construction of a small road bridge made of very high performance fiber-reinforced concrete is described in this paper. The bridge consists of precast prestressed concrete beams with a cast-in-place ordinary concrete deck. A preliminary experimental investigation was conducted to define the mix design, to establish the properties of the material and its durability, and to study the flexural behavior of the prestressed concrete beams with and without the concrete deck. The effect of steel fibers at the structural level, where there is an influence of constitutive behavior and size effects, was analyzed by testing a prestressed beam using very high performance fiber-reinforced concrete without fibers. The establishment of the structural properties of the material then allowed the design of the final section of the bridge beams and the definition of a model to justify the design rules adopted. This project represents an attempt to demonstrate the industrial feasibility of very high performance concrete structural elements manufactured with conventional raw materials and usual production techniques and to evaluate the production technology when utilizing steel fibers.     

Seismic Behavior of Reinforced Concrete Interior Beam-Wide Column Joints Repaired Using FRP

To prevent the casualties that can result from the collapse of earthquake-damaged structures, it is important that structures be rehabilitated as soon as possible. This paper proposes a rapid rehabilitation scheme for repairing moderately damaged reinforced concrete (RC) beam-wide column joints. Four nonseismically detailed interior beam-wide column joints were used as control specimens. All four subassemblages were subjected to similar cyclic lateral displacement to provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks with epoxy and externally bonding them with carbon-fiber-reinforced polymer (CFRP) sheets and glass-fiber-reinforced polymer (GFRP) sheets. These repaired specimens were then retested and their performance compared with that of the control specimens. This paper demonstrates that the repair of damaged RC beam-wide column joints by using FRP can restore the performance of damaged RC joints with relative ease, suggesting that the repair of beam-column joints is a cost-effective alternative to complete demolition and replacement     

Experimental Investigations on the Seismic Response of a Base-Isolated Reinforced Concrete Frame Model

The existing knowledge based on the physical characteristics of rubber-based seismic isolators and the validity of assumptions involved in design can be strengthened with shake table tests and field monitoring during earthquakes. Existing structures can be retrofitted with seismic isolators and successful examples are coming up day by day. This paper presents the results of analytical and experimental investigations of a one-third scaled model of a reinforced concrete soft first-storied structure mounted on natural rubber-based isolators and subjected to uniaxial seismic motion. Translational and rocking responses of the scaled model were measured under four different synthetic time histories, covering a wide range of frequencies. Through base isolation, the time period of the horizontal mode is far removed from the peak spectrum but the rocking modes may well be entrapped in the peak region. Hence, a simplified fail-safe system for protection against overturning was also studied.     

Analytical Model for FRP Confinement of Concrete Columns with and without Internal Steel Reinforcement

The paper aims to contribute to a better understanding of the behavior of reinforced concrete columns confined with fiber-reinforced polymer (FRP) sheets. In particular, some new insights on interaction mechanisms between internal steel reinforcement and external FRP strengthening and their influence on efficiency of FRP confinement technique are given. In this context a procedure to generate the complete stress-strain response including new analytical proposals for (1) effective confinement pressure at failure; (2) peak stress; (3) ultimate stress; (4) ultimate axial strain; and (5) axial strain corresponding to peak stress for FRP confined elements with circular and rectangular cross sections, with and without internal steel reinforcement, is presented. Interaction mechanisms between internal steel reinforcement and external FRP strengthening, shown by some experimental results obtained at the University of Padova with accurate measurements, are taken into account in the analytical model. Four experimental databases regarding FRP confined concrete columns, with circular and rectangular cross section with and without steel reinforcement, are gathered for the assessment of some of the confinement models shown in literature and the new proposed model. The proposed model shows a good performance and analytical stress-strain curves approximate some available test results quite well.     

Size Effect of Concrete Short Columns Confined with Aramid FRP Jackets

Most previous studies on concrete short columns confined with fiber-reinforced polymer (FRP) composites were based on small-scale testing, and size effect of the columns still has not been studied thoroughly. In this study, 99 confined concrete short columns wrapped with aramid FRP (AFRP) jackets and 36 unconfined concrete short columns with circular and square cross sections were tested under axial compressive loading. The circular specimens were divided into six groups, and the square specimens were divided into five groups, with each group containing different levels of the AFRP’s confinement. In each group, the specimens were geometrically similar to one another and had three different scaling dimensions. Statistical analyses were used to evaluate the size and interaction effects between the specimen size and the AFRP’s confinement, and a size-dependent model for predicting the strength of the columns was developed by modifying Baz?nt’s size-effect law. The experimental results showed that the size of a specimen had a significant effect on the strength of AFRP-confined concrete short columns, lesser effect on the axial stress-strain curves, and slight effect on the failure modes. The modified Baz?nt model was in good agreement with the experimental data.     

Unified Strength Model Based on Hoek-Brown Failure Criterion for Circular and Square Concrete Columns Confined by FRP

Most existing models for evaluating the strength of fiber-reinforced polymer (FRP)-confined concrete columns are based in an early work. In this paper, a new model based on the Hoek-Brown failure criterion is proposed. The existing strength models for FRP-confined circular and square concrete columns are reviewed, evaluated, and compared with the proposed model. An updated database that includes a large number of test data are then used to evaluate the models. Comparisons between the models and the test results demonstrate the accuracy of the proposed model. Apart from this improved accuracy, the proposed model also has a unified form for both circular and square columns, and can be used to predict the strength of columns that have existing damage or cracks. Test data for FRP-repaired concrete columns are collected from the literature and used to evaluate and demonstrate the performance of the proposed model in predicting the strength of FRP-confined deficient columns.     

Nonlinear Finite Element Analysis of a FRP-Strengthened Reinforced Concrete Bridge

Three-dimensional nonlinear finite element (FE) models are developed to examine the structural behavior of the Horsetail Creek Bridge strengthened by fiber-reinforced polymers (FRPs). A sensitivity study is performed varying bridge geometry, precracking load, strength of concrete, and stiffness of the soil foundation to establish a FE model that best represents the actual bridge. Truck loadings are applied to the FE bridge model at different locations, as in an actual bridge test. Comparisons between FE model predictions and field data are made in terms of strains in the beams for various truck load locations. It is found that all the parameters examined can potentially influence the bridge response and are needed for selection of the optimal model which predicts the magnitudes and trends in the strains accurately. Then, using the optimal model, performance evaluation of the bridge based on scaled truck and mass-proportional loadings is conducted. Each loading type is gradually increased until failure occurs. Structural responses are compared for strengthened and unstrengthened bridge models to evaluate the FRP retrofit. The models predict a significant improvement in structural performance due to the FRP retrofit.     

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.