Seismic Response of FRP-Upgraded Exterior RC Beam-Column Joints

Shear failure of exterior beam-column joints is identified as the principal cause of collapse of many moment-resisting frame buildings during recent earthquakes. Effective and economical strengthening techniques to upgrade joint shear resistance and ductility in existing structures are needed. In this paper, efficiency and effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in upgrading the shear strength and ductility of seismically deficient exterior beam-column joints have been studied. Four as-built joints were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing preseismic code design construction practice of joints and encompassing most of existing beam-column connections. Out of these four as-built specimens, two specimens were used as baseline specimens (control specimens) and other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epoxy bonded to joint, beams, and part of the column regions. In the second scheme, however, sheets were epoxy bonded to joint region only but they were effectively prevented against any possible debonding through mechanical anchorages. All of these four subassemblages were subjected to cyclic lateral load histories so as to provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks through epoxy and externally bonding them with CFRP sheets under the same above two schemes. These repaired specimens were subjected to the similar cyclic lateral load history and their response histories were obtained. Response histories of control, repaired, and strengthened specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, column profiles, joint shear distortion, ductility, and stiffness degradation. The comparison shows that CFRP sheets are very effective in improving shear resistance and deformation capacity of the exterior beam-column joints and delaying their stiffness degradation.     

Experimental Studies on Seismic Behavior of Steel Pile-to-Pile-Cap Connections

To investigate the seismic behavior of bridge pile-to-pile-cap connections, five full-scale H-shaped steel pile-to-pile-cap connection subassemblies representing a portion of a typical bridge pile foundation were tested. Two of the full-scale subassembly specimens were subjected to a vertical cyclic load simulating axial forces in a pile caused by footing overturning during an earthquake attack. Two others were loaded with cyclic lateral force and constant vertical load. One specimen was tested under proportionally varied vertical and horizontal forces. It was found that, although it was designed as a pinned connection following the current design standard, the pile-to-pile-cap connection can sustain a significant amount of moment. Localized brittle failure was observed in the vicinity of the pile-to-pile-cap connection, as the results of unexpected moment resistance. Test results also showed that the anchorage details using two V-shape bars could not develop the full-design ultimate tensile capacity. Analytical methods are developed and found adequate to evaluate the strength of the pile-to-pile-cap connection.     

Seismic Behavior of As-Built, ACI-Complying, and CFRP-Repaired Exterior RC Beam-Column Joints

In this paper, the efficiency and effectiveness of carbon-fiber-reinforced polymer (CFRP) sheets for upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint were studied and compared with an American Concrete Institute (ACI)-based design joint specimen. One as-built joint specimen, representing the preseismic code design and construction practice for joints and one ACI-based design joint specimen, satisfying the seismic design requirements of the current code of practice were cast. The as-built specimen was used as baseline (control) specimen. These two specimens (i.e., the as-built control and the ACI-based specimens) were subjected to cyclic lateral load histories to induce damage equivalent to damage expected from a severe earthquake. The damaged control specimen was then repaired by filling its cracks with epoxy and externally bonding CFRP sheets to the joint, the beam, and part of the column regions. This specimen was identified as the repaired specimen. The repaired specimen was subjected to a similar cyclic lateral load history, and its response history was recorded. The response histories of the as-built control, the repaired, and the ACI-based design specimen were then compared. The test results demonstrated that externally bonded CFRP sheets can effectively improve both the shear strength and the deformation capacity of seismically deficient and damaged beam-column joints to a state comparable to the ACI-based design joint.     

Seismic Rehabilitation of Corner RC Beam-Column Joints Using CFRP Composites

In this paper, efficiency and effectiveness of carbon fiber reinforced polymers (CFRPs) in upgrading the shear strength and ductility of seismically deficient corner or knee reinforced concrete beam-column joints have been studied. For this purpose, four as-built corner/knee joints were constructed with no transverse reinforcement, representing extreme case of preseismic code design construction practice of joints and encompassing many existing beam-column corner joints. Out of these four as-built specimens, two specimens were used as baseline specimens (control specimens) and other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epoxy bonded to joint, beams, and part of the column regions. In the second scheme, however, sheets were epoxy bonded to joint region only but they were effectively prevented against any possible debonding through mechanical anchorages. All these four subassemblages were subjected to cyclic lateral load histories to simulate loading due to earthquake and provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks through epoxy and externally bonding them with CFRP sheets under the same above two schemes. These repaired specimens were subjected to the similar cyclic lateral load history and their response histories were obtained. Response histories of control, repaired, and strengthened specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, column profiles, ductility, and stiffness degradation. The comparison shows that CFRP sheets are very effective in improving shear resistance and deformation capacity of the corner beam-column joints and delaying their stiffness degradation. Shear capacities of control, repaired, and strengthened specimens were also predicted using writers’ published formulation. The predicted shear capacities were in a good agreement with the experimental values.     

Seismic Response of Interior RC Beam-Column Joints Upgraded with FRP Sheets. I: Experimental Study

In this paper, efficiency and effectiveness of carbon fiber-reinforced polymers (CFRP) in upgrading the shear strength and ductility of seismically deficient beam-column joints have been studied. For this purpose, four reinforced concrete interior beam-column sub-assemblages were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing preseismic code design construction practice of joints and encompassing the vast majority of existing beam-column connections. Out of these four, two specimens were used as baseline specimens (control specimens) and the other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epoxy bonded to the joint, beams, and part of the column regions. In the second scheme, however, sheets were epoxy bonded to the joint region only but they were effectively prevented against any possible debonding through mechanical anchorages. All four subassemblages were subjected to cyclic lateral load histories so as to provide the equivalent of severe earthquake damage. Further, the damaged control specimens were repaired after filling the cracks through epoxy and wrapping them with CFRP sheets under the same two above-mentioned schemes. These repaired specimens were subjected to the similar cyclic lateral load history and their response histories were obtained. Hence, a total of six specimens were tested: two control; two strengthened; and two repaired. Response histories of control, repaired, and strengthened specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, column profiles (maximum horizontal displacements of column along its height), joint shear distortion, ductility, and stiffness degradation. The comparison shows that CFRP sheets improve the shear resistance of the joint and increase its ductility. Results of two chosen schemes of strengthening were also compared and the importance of beam upgrading was highlighted.     

Repair of Steel Composite Beams with Carbon Fiber-Reinforced Polymer Plates

One significant cause of deterioration of steel bridge structures is the corrosion due to extensive use of deicing salts in winter weather. The investigation presented in this paper focused on the behavior of steel composite beams damaged intentionally at their tension flange to simulate corrosion and then repaired with carbon fiber-reinforced polymer (CFRP) plates attached to their tension areas side. Damage to the beams was induced by removing part of the bottom flange, which was varied between no damage and loss of 75% of the bottom flange. All beams were tested to failure to observe their behavior in the elastic, inelastic, and ultimate states. To help implement this strengthening technique, a nonlinear analytical procedure was also developed to predict the behavior of the section/member in the elastic, inelastic, and ultimate states. The test results showed a significant increase in the strength and stiffness of the repaired beams. Through the use of CFRP plates, all damaged beams were fully restored to their original (undamaged state) strength.     

Simplified Methodology for the Evaluation of the Residual Strength of Corroded Reinforced Concrete Beams

Steel corrosion in reinforced concrete (RC) structures leads to change of steel mechanical properties, longitudinal cracking in the concrete cover, and other related effects that weaken the serviceability and load capacity of the composites. It is therefore extremely important to have methods targeted to the evaluation of the structural damage induced by corrosion for estimating the residual load capacity of a structure, and then for inspection procedures and strengthening the maintenance interventions. This paper presents a simplified methodology capable of providing estimates of the residual life of corroded RC beams. The proposed method uses damaged material properties, and accounts for the length of partial corrosion and the amount of corrosion, concrete loss and change of bond strength within this specified length. A comparison of the model predictions with the experimental results published in the literature shows the validity of the model. It is also concluded that the ultimate flexural moment of corroded RC beam will not significantly influenced by the partially corroded or unbonded length and the bond characteristics over this partial length as long as the tensile steel of the beam can reach its yield strength. In addition, although complete loss of bond over the partial length is assumed to asses the residual strength of corrosion-damaged RC beam, neglecting the influence of bond strength within the corroded length may lead to underestimate the ultimate flexural capacity of the damaged beam, especially when the corrosion level of tensile steel of the RC beam is not very high.     

外加强环式H型钢梁-方钢管角柱节点抗震性能

通过对5个试件进行拟静力加载试验,研究了加载方式对角柱和边柱节点抗震性能的影响.试验通过对加载方式（单向加载、双向轴对称加载和双向中心对称加载）和钢管柱宽厚比（D/t=22和33）主要参数的变化分析,着重研究了试件的滞回性能、刚度退化和耗能性能等特性.试验结果表明：加载方式对试件刚度及承载力影响十分明显.在双向中心对称荷载作用下,试件的承载力比在单向荷载作用下试件的承载力降低约20%;而在双向轴对称荷载作用下,试件的承载力与在单向荷载作用下试件的承载力基本相同.方钢管柱宽厚比是影响试件承载力的主要因素之一,随着宽厚比的增加,试件承载力逐步减小.所有试件的滞回曲线均呈饱满的纺锤体状,等效黏滞阻尼系数在0.2左右,具有稳定的耗能能力.     

Shear Strengthening of Interior Slab–Column Connections Using Carbon Fiber-Reinforced Polymer Sheets

This paper presents the results of an experimental investigation undertaken to evaluate the punching shear capacity of interior slab–column connections, strengthened using flexible carbon fiber-reinforced polymer (CFRP) sheets. Sixteen square (670×670?mm) slab–column connections with different slab thicknesses (55 and 75 mm) and reinforcement ratios (1 and 1.5%) were tested. Twelve specimens were strengthened using CFRP sheets and the remaining four specimens were kept as controls. Without strengthening, all specimens were designed to experience punching shear failure. The CFRP sheets were bonded to the tension face of the specimens in two perpendicular directions parallel to the internal ordinary steel reinforcement. The test results clearly demonstrate that using CFRP leads to significant improvements in the flexural stiffness, flexural strength, and shear capacity of beam–column connections. Depending on the content of the ordinary reinforcement, thickness of the slab, and area of CFRP sheet, the flexural strength increased between 26 and 73% and the shear capacity increased between 17 and 45%. The measured stress in the CFRP sheets at nominal strength varied between 22 and 69% of the ultimate tensile strength of the fibers. Comparison with available prediction equations showed that the punching shear capacity can be predicted with reasonable accuracy if the contribution of CFRP reinforcement to the increase in flexural strength is accounted for. On the other hand, the code design expressions were conservative in predicting the capacity observed in the tests.     

Experimental Testing of Pile-to-Cap Connections for Embedded Pipe Piles

For bridges supported by piles, acceptable system performance under seismic loading depends on effective pile-to-cap connections. A fixed pile-to-cap connection is often desirable to help control deflections during lateral loading when soft soils are present. While reinforcement bar cages that extend from the pile into the cap are effective in providing a fixed pile-to-cap connection, it is more economical to rely on pile embedment to provide fixity and moment resistance. This study investigated embedded pile-to-cap connections for concrete-filled pipe piles. Four full-scale specimens, each consisting of a cap with two piles, were investigated in the field under cyclic loading. The specimens had minimal reinforcement and varying amounts of pile embedment. Results show that the moment resistance of pile-to-cap connections can be significantly greater than what is typically calculated based on the flexural reinforcement and embedment bearing. Excess moment capacity may be explained by friction between the pile and the cap at the connection. This friction mechanism is described and discussed in the context of experimental results from other studies.     

Residual Performance of FRP-Retrofitted RC Columns after Being Subjected to Cyclic Loading Damage

Numerous recent research findings evidenced the success of retrofitting existing RC columns using fiber-reinforced plastic (FRP) jacketing. However, little is known about the residual performance of FRP-retrofitted RC columns following limited seismic damage. In this paper, the residual performance of FRP-retrofitted columns damaged after simulated seismic loading is studied. Eight model columns with a shear aspect ratio of 5.0 were tested first under cyclic lateral force and a constant axial load equal to 20% of the column gross axial load capacity. The main parameters considered were the type of FRP jacket and peak drift ratio where the lateral loading was interrupted. Glass fiber-reinforced plastic (GFRP) and carbon fiber-reinforced plastic (CFRP) were both used for retrofitting. Five of the model columns were subjected to long-term axial loading after being subjected to limited damage by lateral cyclic loading. From the results of long-term loading test, it was found that FRP-retrofitted columns had much smaller creep deformation than the counterpart as-built model. The deformation of retrofitted columns under long-term axial loading depended on the previous damage intensity and the modulus of elasticity of FRP. The effective creep Poisson’s ratios of the retrofitted columns were much smaller than the as-built column but identical for GFRP and CFRP retrofitted columns. Under the testing conditions of this study, the long-term axial deformation of retrofitted columns tends to be sufficiently stable, despite the simulated earthquake damage.     

轻质混凝土拼装墙板填充钢框架协同抗震性能试验研究

为了研究墙板与钢框架结构之间的协同抗震性能,对采用不同墙框连接节点的轻质混凝土拼装墙板填充钢框架进行了低周往复荷载试验。通过对比试件的承载力、滞回性能、刚度、耗能以及延性性能,探讨了轻质混凝土拼装墙板及其整体性对结构抗震性能的影响。结果表明:填充墙板钢框架结构的最终破坏形态以墙板挤压开裂,框架梁柱端部翼缘屈曲为主;轻质混凝土拼装墙板与钢框架协同工作,有利于提高结构整体的承载力和变形能力,减轻钢框架在平面内的屈曲破坏;与刚性节点相比,采用柔性节点连接墙板与钢框架对结构的承载力、层间刚度和耗能能力更为有利;增强拼装墙板的整体性,有助于提高结构整体刚度、变形和耗能能力。研究结果可为轻质混凝土拼装墙板填充钢框架结构的抗震设计提供参考。      

Structural Resistance of SRC Column-Steel Beam Joint Developed for Innovative Construction of Buildings

This paper presents an experimental study investigating factors influencing the effective width of steel reinforced concrete (SRC) column-steel beam joints of building in order to calculate its resisting moment.Five 1/2 scaled interior SRC column-steel beam joint specimens were made with considering parameters such as beam flange width,beam depth and SRC column width.One directional increasing moment was applied to the joint by acting forces to each ends of beam and the structural behavior of joint was studied.And previous design method suggested by Deierlein was reviewed and a modified equation was proposed from the analysis of test result.Test result indicated that the equation to calculate the effective width in Deierlein’s design method didn’t consider effectively the influence caused by the variation of beam depth so that a modified equation was suggested and the validation of it was confirmed in this paper.     

Static Cyclic Response of Masonry Walls Retrofitted with Fiber-Reinforced Polymers

The behavior of seven one-half scale masonry specimens before and after retrofitting using fiber-reinforced polymer (FRP) is investigated. Four walls were built using one-half scale hollow clay masonry units and weak mortar to simulate walls built in central Europe in the mid-20th century. Three walls were first tested as unreinforced masonry walls; then, the seismically damaged specimens were retrofitted using FRPs. The fourth wall was directly upgraded after construction using FRP. Each specimen was retrofitted on the entire surface of a single side. All the specimens were tested under constant gravity load and incrementally increasing in-plane loading cycles. The tested specimens had two effective moment/shear ratio, namely, 0.5 and 0.7. The key parameter was the amount of FRP axial rigidity, which is defined as the amount of FRP reinforcement ratio times its E modulus. The single-side retrofitting/upgrading significantly improved the lateral strength, stiffness, and energy dissipation of the test specimens. The increase in the lateral strength was proportional to the amount of FRP axial rigidity. However, using high amount of FRP axial rigidity led to very brittle failure. Finally, simple existing analytical models estimated the ultimate lateral strengths of the test specimens reasonably well.     

Damage Analysis of Hanshin Expressway Viaducts during 1995 Kobe Earthquake. I: Residual Inclination of Reinforced Concrete Piers

The damage suffered by elevated viaducts of the Hanshin Expressway Kobe Route during the 1995 Kobe earthquake is described with emphasis on reinforced concrete (RC) piers. Although many piers were severely damaged, it is also true that the damage to many piers appeared moderate or even mild. On the other hand, a number of piers suffered from large residual inclination in spite of the apparently light damage. By considering that the large residual inclination of piers included severe earthquake-induced damage, it is pointed out that almost all the RC single piers from P35 to P350 received consistently severe damage. The cause of large residual inclination, especially in apparently nondamaged piers, is studied. A dynamic analysis of a single RC pier is conducted to study the relationship between residual inclination and residual deformation of a pier. As a result, we find that the flexural residual deformation of a pier cannot explain the observed large residual inclination, but it is suggested that the pulling out of reinforcing bar from the footing can be a primary cause of the observed large residual inclination.     

Investigation of the load sequence effects in fatigue of damaged composite laminate & characterization of fatigue damage

Carbon fiber reinforced composites are widely used today in various areas and specially in aerospace industry for structural applications. This investigation focuses on the effect of different load sequencing and impact damage on the fatigue behaviour of CFC laminates. The specimens made from plain CFC laminates and low energy impact damaged CFC laminates were subjected to a typical flight block loading sequence and the fatigue strength degradation was monitored through stiffness measurement using load displacement data obtained during block loading. Three different stress/strain levels were used in testing. All the tests were performed using a computer controlled 100 kN servo-hydraulic test machine in load mode at room temperature and in lab air atmosphere on undamaged and low energy impact damaged composite laminates. Fatigue tests were performed with a sinusoidal waveform at 3 Hz. It was observed that lower strain levels did not show any significant effect on the fatigue properties in both the type of loading i.e. low to high and in high to low block loading in case of both the undamaged and impact damaged CFC specimens. Significant.reduction in stiffness was seen at higher strain level i.e. 6500me in both the undamaged and impact damaged CFC specimens. The low energy impact damaged specimens showed early failure at higher strain levels compared to undamaged specimens. The specimens were observed to have delaminated in the high stress fatigue cycling. The observed stiffness reduction due to fatigue cycling and the presence of delamination provide a means of macroscopic identification of fatigue strength degradation in composite materials. The energy plots appear useful tool to assess the damage growth.     

Moment-Inelastic Rotation Behavior of Longitudinally Stiffened Beams

The significance for inelastic design of moment-inelastic rotation behavior with respect to interior pier sections of steel girder bridges is experimentally investigated. Under center span loading conditions, 12 welded, built-up, simply supported beams with various slenderness ratios of the flange and web plates are tested. In this test, lengths and locations for partial longitudinal stiffeners on the web plates are varied, and the results are then compared with the inelastic deformation capacity of beams without longitudinally stiffened web plates. The results are also compared with the inelastic design code in AASHTO LRFD bridge design specifications. It is concluded that (1) the ultimate strength of stiffened beams is governed by the local buckling at the compression flange of the far end from the loading point due to the presence of a partial longitudinal stiffener; and (2) the inelastic rotation capacity and ultimate strength of a beam with a stiffened web plate are remarkably improved. The optimum length and location of stiffeners on the plates are given.     

Fiber-Reinforced Polymer Retrofitting of Predamaged Substandard RC Prismatic Members

An experimental study was conducted to investigate the efficiency of FRP jackets in upgrading the seismic behavior of lightly reinforced concrete prismatic members previously damaged under a combination of axial compression and a reversed cyclic lateral displacement history simulating earthquake effects. The test program comprises 13 cantilever prismatic specimens, which, owing to substandard reinforcing details representative of older construction practices in southern Europe, were susceptible to various undesirable modes of damage such as web-shear cracking, longitudinal bar buckling, or lap-splice failure. After repair, the specimens were retested using the same load combination. The efficiency of the repair options considered in the study, which refer to alternative strengthening systems (with glass or carbon wraps), was investigated with reference to the design parameters of the intervention, the type of the applied lateral displacement history, and the mode of failure that had occurred previously in the initial phase of the tests. The results provided valuable insight regarding participation of the FRP jackets in the various mechanisms of resistance, their ability to reverse the effect of initial damage, and to impart deformation capacity to the structural member.     

Fabrication and Testing of Cuff Connections for GFRP Box Sections

A new type of connection between beams and columns has been designed and fabricated specifically for use with glass fiber reinforced plastic (GFRP) pultruded box members. The work is built on previous efforts in the area of GFRP connections, which demonstrated that innovative connections between box sections are superior to connections based on concepts from steel construction for connecting I-beams. The new connection element is designed as a monolithic connection for frame members and is fabricated using a vacuum assisted resin transfer molding process. Individual connection specimens have been fabricated and tested to verify their performance under cyclic static loads in a test frame designed to simulate conditions in a moment resisting frame. The connection configuration was found to fare better from the standpoint of both strength and stiffness in comparison with previous attempts at developing GFRP beam-to-column connections.     

Parapet Strength and Contribution to Live-Load Response for Superload Passages

The main objective of this study is to evaluate the effects of parapets on the live-load response of slab-on-girder steel bridges subjected to superload vehicles and the effects of these loads on the parapets. A superload is a special permit truck that exceeds the predefined weight limitation. The presence of parapets can result in reduced girder distribution factors (GDFs) for critical girders, and this reserve strength can be considered for passage of a superload truck. This reduction is investigated, as well as the effects of discontinuous parapets and the capacity of parapets. Two steel bridges with significantly different geometric proportions were analyzed to evaluate the sensitivity of the structure to the effects of parapets. It was found that the GDFs can be decreased by as much as 30%, depending on the stiffness of the girders and the transverse truck position if the parapets are included in the analysis. The axial forces and bending moments resisted by the parapets were compared with the capacity of the parapets. The parapets and their connection with the deck were found to have adequate strength to accommodate the demand imposed by the superload trucks included in the study. For the discontinuous parapets, the open joint was determined to be acting like a notch, which increases the bottom flange stresses in the positive moment region and the tensile deck stresses in the negative moment region.