Confinement Of Columns Research Articles

Effects of increased slenderness in short heat-damaged RC columns confined with FRP composites

Twenty-six circular RC columns were tested under axial compression to explore the confinement effects of fiber reinforced polymer (FRP) sheets on the performance of heat-damaged short columns. The effects of increased column slenderness, level of heat-induced damage and stiffness of the FRP wrapping system were examined. Columns with l/r ratios of 12.5 and 18.75 were subjected to 500 °C for 1, 2 and 3 h. Eighteen heat-damaged columns were wrapped using unidirectional carbon and glass FRP sheets over their full heights. Test results showed that FRP wrapping significantly enhanced the axial resistance, toughness, ductility and deformation capacity of the heat-damaged columns but failed to restore their axial stiffness. Increasing the stiffness of the FRP jacket resulted in further enhancements in axial resistance, stiffness and toughness of the heat-damaged columns while adversely affecting their ductility. Carbon fiber reinforced polymer jackets with a confinement modulus of 0.65 GPa managed to reinstate, and even exceed, the original strength of heat-damaged columns. The effectiveness of FRP confinement of heat-damaged columns increased significantly with increasing damage level and decreasing column slenderness. The heat- and confinement-induced increases in column slenderness had no tangible effects on buckling stability of the test columns that were originally of low slenderness.

Experimental study on modulus of elasticity of steel tube-confined concrete stub columns with active and passive confinement

This study presents the results of an experimental investigation on the initial and reloading elastic modulus of steel tube-confined concrete (STCC) stub columns with active and passive confinement. Here, a new method was used to create an active confinement in the STCC columns, in which the fresh concrete was compressed by applying an external pressure and subsequently the steel tube was pretensioned in the circumferential direction. The prestressed steel tube-confined compressed concrete (PSTC) was produced in two ways: long- and short-term pressure to reach high and low prestressing levels, respectively. A total of 135 confined concrete specimens including non-prestressed steel tube-confined concrete (NPSTC), short-term prestressed steel tube-confined compressed concrete (SPSTC), and long-term prestressed steel tube-confined compressed concrete (LPSTC) along with 45 reference concrete specimens in 15 experimental groups were tested to investigate the effect of reference concrete strength, diameter to thickness ratio of the steel tube, and prestressing level on the initial and reloading modulus of elasticity. The results showed that prestressing the STCC specimens by the present method significantly enhanced the initial modulus of elasticity between 46% and 184% for the SPSTC and between 51% and 280% for the LPSTC. Moreover, the reloading elastic modulus increased considerably compared with the first loading elastic modulus for the passive specimens while the opposite trend was observed for the active specimens. Finally, by separating the contribution of prestressing to increasing the elastic modulus of the active specimens from that of compressing the fresh concrete, a significant contribution of prestressing was indicated.

Enhancing the confinement of ultra-high-strength concrete columns using headed crossties

This study investigates the influence of headed crossties on the confinement of 200MPa ultra-high-strength concrete (UHSC) columns. A Total of six UHSC columns confined by headed crossties (deformed reinforcing bars with heads welded on both ends) and seismic crossties, detailed with 90° bend anchorages alternated end to end, were constructed and tested under pure axial load. The amount and yield strength of transverse reinforcement were varied to determine the efficiency of headed crossties on the confinement of the UHSC columns. The columns with seismic crossties exhibited some loss in confinement due to the loss of anchorage of the 90° hooks that opened up before significant yielding of the crossties could occur. The relatively small heads used are capable of developing the yield force of high-strength transverse reinforcement in the UHSC columns. The use of high-strength headed crossties resulted in improved confinement compared to columns with high-strength seismic crossties resulting in improved post-peak performance of UHSC columns. A prediction model that accounts for the additional confinement pressure due to the bearing of the heads, brittle cover spalling and the stress-strain relationship of UHSC was used to predict the axial load-strain behavior of the UHSC columns.

Evaluation of FRP Confinement Models for Substandard Rectangular RC Columns Based on Full-Scale Reversed Cyclic Lateral Loading Tests in Strong and Weak Directions.

Although many theoretical and experimental studies are available on external confinement of columns using fiber-reinforced polymer (FRP) jackets, as well as numerous models proposed for the axial stress-axial strain relation of concrete confined with FRP jackets, they have not been validated with a sufficient amount and variety of experimental data obtained through full-scale tests of reinforced concrete (RC) columns with different geometrical and mechanical characteristics. Particularly, no systematical experimental data have been presented on full-scale rectangular substandard RC columns subjected to reversed cyclic lateral loads along either their strong or weak axes. In this study, firstly, test results of five full-scale rectangular substandard RC columns with a cross-sectional aspect ratio of two (300 mm × 600 mm) are briefly summarized. The columns were tested under constant axial load and reversed cyclic lateral loads along their strong or weak axes before and after retrofitting with external FRP jackets. In the second stage, inelastic lateral force-displacement relationships of the columns are obtained analytically, making use of the plastic hinge assumption and different FRP confinement models available in the literature. Finally, the analytical findings are compared with the test results for both strong and weak directions of the columns. Comparisons showed that use of different models for the stress-strain relationship of FRP-confined concrete can yield significantly non-conservative or too conservative retrofit designs, particularly in terms of deformation capacity.

Seismic Performance of Full-Scale FRP Retrofitted Substandard RC Columns Loaded in the Weak Direction

FRP confinement of sub-standard columns with low quality concrete, light transverse reinforcement and improper reinforcement detailing is widely accepted as an efficient retrofitting strategy. This paper introduces an improved method using carbon fiber reinforced polymers (CFRP) and external steel ties for seismic retrofitting of full-scale rectangular reinforced concrete columns loaded in their weak directions. Three cantilever columns with a cross-sectional aspect ratio of two (600 mm x 300 mm) are tested under constant axial load and reversed cyclic lateral loads. The columns are representative of existing substandard members with characteristics such as low concrete quality, low transverse reinforcement ratio, plain bars and high axial load level. The test results indicate that columns retrofitted with FRP jacketing and external steel ties significantly benefit from the applied retrofit scheme particularly in terms of ductility and energy dissipation. Additionally, the experimental results are compared with the performance predictions of seismic assessment and design documents.

Strength Models of Axial Capacity of FRP-Confined Circular Concrete Columns

Several numbers of strength models of FRP-confined circular concrete columns have been published by several researchers. In this study, strength models were proposed based on experimental data. Nine existing FRP-confined strength models of column confinement were reviewed in this study and two forms of strength models of FRP-confined circular concrete columns were proposed. The models were proposed based on different number of experimental data collected from literature. Based on the number of experimental data, i.e. 64-, 106-, 156- and 192-, the strength models were developed. The performance of the existing and the proposed strength models were evaluated and compared. The evaluation results showed that the strength models proposed based on 192-data had the best performance among other models proposed with smaller number of data. It was also shown that the proposed strength models had better performance than the existing strength models.

SHORT TERM DURABILITY OF STEEL-STRAP CONFINED CONCRETE

Steel strapping tensioning technique (SSTT) confinement has been proven as a type of external confinement to retrofit and upgrade concrete structures. Their remarkable enhancement to concrete mechanical properties, practicing active lateral confinement, low cost, easy to operate, time saving and no structure interruption during application have brought this technique as of the most affordable technique for column confinement, especially for high strength concrete (HSC) which naturally behaves insufficient in ductility. Although a great number of previous studies have demonstrated the performance of SSTT confinement, there are several anxieties related to the long term behaviour of SSTT confined columns which have yet been implemented especially in real hostile condition. Environmental effects, such as ultraviolet radiation exposure and saline solution (sea water) may affect the confining material properties, and therefore decrease the performance of the confined columns. In this study a total of twenty HSC specimens with dimension of 100 mm and 200 mm in diameter and height respectively were prepared, volumetric-identically pre-tensioned with steel strap and exposed to three conditions: indoor, outdoor and saline water for three months. The exposed specimens were monotonically tested and their performance was studied through the stress-strain relationship, mode of failure and degree of corrosion. The results show that none of the forenamed environmental conditions have exaggerating influence on the performance of the SSTT confined concrete specimens even some material has been severely corroded. It is then parallel compared with existing FRP confinement studies and the result proves that SSTT confinement able to possess the highest strength performance under similar environmental exposure.

CFRP confinement of circular concrete columns affected by alkali-aggregate reaction

Alkali-aggregate reaction (AAR) is known to cause expansion and cracking in concrete, which adversely affect the mechanical properties of concrete, leading to a decrease in the service life of structures. Wrapping damaged elements with epoxy-bonded carbon fiber reinforced polymer (CFRP) is a method of rehabilitation. This paper investigates the level of expansion in concrete caused by AAR and its effects on the mechanical properties of concrete during a period of one year. The decreases in compressive strength, flexural strength, splitting tensile strength and modulus of elasticity reached 74%, 81%, 76% and 93% respectively. Twenty six reinforced and plain columns of dimensions 200mm in diameter and 500mm in height were fabricated from normal and reactive concretes. The latter contained fused silica at 7.5% of total aggregate as reactive component. The columns developed high expansion levels due to AAR in the storage period between 120 and 180days, which caused severe cracking in the affected columns. One, two and three layers of CFRP were used to confine the affected and unaffected columns. Results indicated that CFRP wrapping increased the strength and strain capacities of confined columns compared to unconfined columns. Larger number of applied CFRP layers also increased the capacity of the affected columns.

Unified model for hollow columns externally confined by FRP

Nowadays, the employment of Fibre Reinforced Polymer (FRP) composites in civil engineering field has been successfully experienced. Different potential applications of solid concrete-filled FRP tubes are exploitable in marine piles, overhead sign structures, poles and posts, bridge columns and piers, girders, large pipes and tunnels (mainly circular cross-section). This technique is also used for the confinement of masonry columns, typically encountered in monuments and historical buildings (both rectangular and circular cross-section); hence the need of providing formulas for the design of an appropriate strengthening. Several analytical models are available in the scientific literature for assessing the increase of strength and ductility of concrete or stone solid elements externally confined with FRP or for hollow columns internally steal enclosed and externally FRP-confined but, there is still a lack of research about hollow columns only externally confined. The presence of an empty core implies a different stress state in the inner cylindrical surface with respect to the outer one. Inwards deformations are more significant and this behaviour is not taken into account by available models.The present study aims to illustrate a detailed summary of the existing analytical models and to provide a unified procedure for concrete and masonry hollow columns valid for both circular and square cross-sections. An iterative method that updates the geometrical parameters according to step-by-step uniaxial compressed column is shown in order to capture the real deforming behaviour of the compressed solid. This analytical approach has two important implications: the first is the ability of calculating the stress state of the column and of the external FRP reinforcement at each step; the second is that of theorizing a procedure, which is independent on the type of material used for the column. The outputs of the proposed method are then compared with experimental results currently available in the scientific literature. A good matching is obtained between the available experimental results and the analytical predictions in term of axial stress–strain curves.

Role of concrete confinement of wide-flange structural steel shape in steel reinforced concrete columns under cyclic loading

In this study, effects of wide-flange structural steel shapes on concrete confinement of steel reinforced concrete (SRC) columns were examined by conducting structural testing on six large-scale SRC columns under cyclic loading. Specifically, effects of the major- and minor-axis bending relative to the steel shape on the ductility and confinement effectiveness of SRC columns were investigated. Experimental results showed that concrete confinement of SRC columns were highly influenced by the bending direction. Details of the underlying confining mechanisms of SRC columns were uncovered. Findings from this study indicated that the required confinement reinforcement for the SRC columns subjected to major-axis bending could be significantly reduced, compared to that of the SRC columns subjected to minor-axis bending.

Analytical Fragility Functions for Horizontally Curved Steel I-Girder Highway Bridges

Horizontally curved bridges were investigated following a statistical evaluation of typical details commonly used in the United States. Both seismically and non-seismically designed bridges are considered where the primary differences are in column confinement, type of bearings and abutment support length. Columns and bearings were found to be the most seismically vulnerable components for both categories. Central angle was identified as an important factor that increases the demand on various components, particularly columns. Furthermore, larger angles lead to increased deformations at the supports which adversely affect the seismic vulnerability. Consistent with the fragility curves that account for the central angle explicitly, a second set of system fragility curves were introduced for cases when central angle is not specified such as the case in the National Bridge Inventory. Comparison of fragility parameters to those suggested by HAZUS-MH highlighted the need for revisions to account for current design practices and central angle.

아라미드 시트와 에너지 소산 장치에 의한 기존 골조의 능력 향상

Abstract: In this paper, the strengthening method was proposed for improving the seismic performance of the vulnerable structural frames. To improve the brittle characteristics of columns, aramid fiber sheet was used for the lateral confinement of columns. And to introduce th e energy dissipation capacity, a steel damper with S-shaped struts was installed. By making the unreinforced and reinforced specimens with full size specimens were evaluated for lateral load resistance capacity. It was confirmed the strengthening effects by the evaluation of failure shape, strength, stif fness degradation, and energy dissipation capacity. Also from the FE analysis using ABAQUS, the hysteretic behavior of the specimens were predicted and evalu ated.Keywords: Aramid sheet, Fe analysis, Lateral confinement, Metallic damper, Seismic strengthening 1 정회원, 동양대학교 건축소방행정학과 교수, 교신저자*Corresponding author: hhlee@dyu.ac.kr∙본 논문에 대한 토의를 2015년 10월 1일까지 학회로 보내주시면 2015년 11월호에 토론결과를 게재하겠습니다.Copyright Ⓒ 2015 by The Korea Institute for Structural Maintenance and Inspection. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0)which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Modeling of passive and active external confinement of RC columns with elastic material

One of the most attractive applications of composite materials is their use as confining devices for concrete columns, which may result in remarkable increases of strength and ductility. The current investigation focuses on the modeling of reinforced concrete columns passively or actively confined by composites, under axial load. Furthermore, the research highlights the effectiveness and modeling of ultra high extension capacity fiber ropes implemented as external confining reinforcement so as to upgrade ductility and strength of concrete columns. It concerns columns of square cross‐section with plain or steel reinforced concrete. The novel confining technique uses composite rope made of polypropylene fibers as passive or pretensioned reinforcement while it presents linear elastic behavior up to failure and is applied by hand. It may enable confined columns to dissipate enormous amounts of earthquake induced energy through concrete deformation. The proposed constitutive model is compared against available experimental results involving circular or square columns with passive or active FRP confinement.

Construction and design defects in the residential buildings and observed earthquake damage types in Turkey

Abstract. Turkey is situated in a very active earthquake region. In the last century, several earthquakes resulted in thousands of deaths and enormous economic losses. In 1999, the Kocaeli earthquake had an approximate death toll of more than 20 000, and in 2011 the Van earthquake killed 604 people. In general, Turkish residential buildings have reinforced concrete structural systems. These reinforced concrete structures have several deficiencies, such as low concrete quality, non-seismic steel detailing and inappropriate structural systems including several architectural irregularities. In this study, the general characteristics of Turkish building stock and the deficiencies observed in structural systems are explained, and illustrative figures are given with reference to the Turkish Earthquake Code 2007. The poor concrete quality, lack of lateral or transverse reinforcement in beam–column joints and column confinement zones, high stirrup spacings, under-reinforced columns and over-reinforced beams are the primary causes of failures. Other deficiencies include weak-column–stronger-beam formations, insufficient seismic joint separations, soft-story or weak-story irregularities and short columns. Similar construction and design mistakes are also observed in other countries situated on active earthquake belts. Existing buildings still have these undesirable characteristics, and so to prepare for future earthquakes they must be rehabilitated.

Fire resistance of double-skinned composite tubular columns including concrete confinement

Double-skinned composite tubular (DSCT) columns consisting of concrete cast between outer and inner tubes have been developed to overcome certain limitations of other columns, such as the large self-weight of concrete-filled tubular columns and lack of concrete confinement of hollow concrete-filled tubular columns. The strength and ductility of the column are enhanced by the continuous confining stress provided by the inner tube. Their excellent structural performances make them particularly suitable for applications in high-rise buildings. However, if a high-rise building is damaged by fire, the economic costs associated with building repair can be high. It is very important to put the fire-damaged building back into service with the minimum post-fire repair. Thus, to predict the status of a structure under fire, its behavior should be evaluated based on the fire duration. Studies on the fire resistance of DSCT columns have been carried out for this purpose. However, they have involved the performance of the entire system without considering the effects of the DSCT column’s components on the fire resistance. In this paper, the behavior of a DSCT column is investigated under an ISO-834 standard fire using an analytical method. The evaluation method for the fire resistance of a DSCT column utilizes a thermal analysis and Eurocode. In addition, the relationship between the DSCT column’s components and the fire resistance is investigated, considering the confining effect. Moreover, the behavior of the DSCT column is evaluated through parametric studies of the hollow ratio, thickness of the outer tube, and thickness of the inner tube.

Analysis-Oriented Stress-Strain Model for Concrete under Combined FRP-Steel Confinement

AbstractExtensive research has been conducted on fiber-reinforced polymer (FRP)-confined plain and RC columns, leading to a large number of stress–strain models. Most of these models have been developed for FRP-confined plain concrete and are thus applicable only to concrete in FRP-confined RC columns with a negligible amount of transverse steel reinforcement. The few models that have been developed for concrete under the combined confinement of FRP and transverse steel reinforcement are either inaccurate or too complex for direct use in design. This paper presents an accurate design-oriented stress–strain model for concrete under combined FRP-steel confinement in FRP-confined circular RC columns. The proposed model is formulated on the basis of extensive numerical results generated using an analysis-oriented stress–strain model recently proposed by the authors and properly captures the key characteristics of FRP-steel-confined concrete as revealed by existing test results. The model strikes a good balanc...

Seismic performance of post-mainshock FRP/steel repaired RC bridge columns subjected to aftershocks

This study compares the performances of three types of repair jackets on mainshock (MS) earthquake-damaged RC bridge columns subjected to aftershock (AS) attacks. These repair jackets include fiber reinforced polymers (FRP), thick steel, and thin steel wrapped with prestressing strands. Results obtained from incremental dynamic time history analyses on refined numerical finite element bridge models were utilized to evaluate the efficacy of different repair jackets application on the post-MS collapse safety of RC bridges subjected to AS attacks of various intensities. Numerical results indicated that the three repair jackets can effectively improve the bridge collapse capacity by approximately 20% under severe MS-severe AS even though they cannot restore the initial stiffness of damaged columns. Repair jackets for the severe MS-damaged columns were ineffective under moderate AS events and thus not required. Steel repair jackets exhibited higher energy dissipation under MS–AS sequences than FRP jackets. In the case of FRP jackets, bidirectional fiber wraps are recommended for plastic hinge confinement of MS-damaged bridge columns subjected to aftershocks.

Cyclic Stress–Strain Relationships of FRP Confined Concrete Members

External confinement of columns of existing reinforced concrete buildings using fiber-reinforced polymer (FRP) composites can significantly increase the ductility and compressive strength of structural members against seismic actions. Therefore, knowledge on the behavior of FRP confined concrete/reinforced concrete members under cyclic stresses is vitally important for realistic prediction of seismic performance of retrofitted structures. In this study, available experimental data on the cyclic stress–strain behavior of FRP confined concrete/reinforced concrete members are evaluated in a comparative manner, and accuracy of several stress–strain models proposed for FRP confined concrete is discussed in terms of prediction of the skeleton curves, as well as the unloading–reloading branches of the cyclic stress–strain relationships of the tested samples. Furthermore, a new model is proposed for the prediction of unloading and reloading branches based on the cyclic tests carried out specifically for this purpose. The predictions of the proposed model and two other available models proposed for cyclic loading are compared with experimental data, and a satisfactory agreement is observed between the predictions and experimental data.

Strengthening Strategies of a Historical Masonry Construction

In the last years, great attention has been given to the preservation of cultural heritage, and different research areas investigated on restoration, conservation and strengthening of monumental ancient constructions. This has been great relevance especially in Italy, the country which holds the greatest cultural heritage in the world. In this context, the use of fiber-reinforced composites as strengthening materials of existing structures revealed to be an efficient, low cost and low impact technique, due to their advantages compared to the use of traditional materials. In the present work, the convent of San Bernardino, a historical construction which belongs to the cultural heritage of the territory of Cosenza, has been taken into consideration, and different strengthening techniques for some of its masonry arches and columns have been proposed. After a detailed cognitive analysis of the construction complex, its structural deficiencies and their reasons have been investigated. A low level of safety of the entire construction have been found, especially in columns and arches of cloister. Starting from this points, several reinforcement interventions, using glass fiber materials, have been designed, in accordance with recent Italian guidelines. In particular, the technique of continuous and discontinuous confinement of columns through the application of composite materials and the effectiveness of the insertion of fiber-reinforced bars as reinforcement of arches have been designed. The strengthening interventions proposed entailed a significant increase of masonry elements strength and improved the overall safety of the monumental complex.

Confining of square RC columns with FRP sheets using corner strip–batten technique

The application of fiber reinforced polymer (FRP) composites has been successfully promoted for external strengthening of reinforced concrete (RC) columns. Recent studies showed that when these jackets are formed in a wet lay-up process for confinement of columns, an average rupture strain of FRP in specimens substantially fall below those of flat coupon tensile tests. The problem is more important in rectilinear columns because of premature rupture of FRP at corners due to stress concentration.In this paper, a new method is introduced for confinement of square concrete columns as using FRP strips at corners and FRP battens at sides. In this method, FRP battens do not experience any curvature in confining of section and are stretched as completely flat strips similar to flat coupons. To compare the new proposed method with other FRP-confining techniques, sixteen square RC columns with 133×133mm cross section and 500mm height were experimentally tested under uniaxial compression. The test parameters included continuity or discontinuity of corner strips along height of column, the volume of fibers used, and the number of confining layers. The experimental results clearly demonstrated that confining battens in the new method of corner strip–batten are uniformly stretched under the tension stresses of confinement; therefore, more uniform distribution of confining pressure on section occurs and the stress concentration at corners is eliminated. Thus, in the proposed method, better performance of FRP in confinement was observed and the compressive behavior of the strengthened column was significantly improved compared to those confined using conventional FRP wraps.