Column Bars Research Articles

Analytical modeling of monolithic joints in concrete bridges

The behavior of bridge monolithic connections is modeled using a simplified mathematical model that accounts for stress equilibrium, compatibility of deformations, and the state of bond of longitudinal column bars anchored through the joint panel. In this regard, a stress gradient factor is introduced, to model the profile of bar stresses along the anchorage. To establish this factor, two independent mechanisms of stress transfer are considered: a bond mechanism between the anchored bars and the surrounding concrete and a friction mechanism between the anchored bars and the transverse bars that enclose and restrain the anchorages. The model is used for calculation of the shear stress-shear strain relationship of all tests found in the international literature on bridge monolithic connections that showed shear type of failure under simulated seismic loading. Joint strength values calculated with the proposed model are compared with the experimental results. Based on this comparison the proposed model is verified for use in interpretation of bridge monolithic connection behavior and design.

REINFORCED CONCRETE BEAM-COLUMN JOINT:INTERACTION OF ULTIMATE STRENGTHS AND FORCES AT MEMBER ENDS

The author proposed a new mathematical model which predicts the ultimate behavior of two-dimensional reinforced concrete beam-column joints. The model is based on a kinematic model named nine DOF model combined with non-linear biaxial constitutive relation of concrete and steel to bear simple algebraic expressions for the ultimate moment capacity and moment at balanced failure. It is applicable to symmetric interior beam-column joints subjected four symmetric moments transmitted among the four adjacent members. This paper extends the model such that it could consider the interaction of ultimate strength and axial force, shear force transmitted from adjacent members framed into the joint as well as the amount of joint shear reinforcement in a beam-column joint designed by weak beam-strong column concept for Capacity Design. It is concluded from the theory that the factors increasing the moment capacity of beam-column joints include (1) axial force, (2) margin of moment capacity of column to beam, (3) confinement by joint shear reinforcement, (4) confinement by mid-layer bars in columns, the effect of which are followed by a simple combination rule in a simple algebraic equation form. It is also shown that the theory predicts that the factorsabove have little effect on the moment at balanced failure while (5) the concrete strength and (6) the ratio of the bar distance to the depth of member have large effect.

Seismic FRP Retrofit of Bond-Critical Regions in Circular RC Columns: Validation of Proposed Design Methods

This study experimentally evaluates the effect of fiber-reinforced polymer (FRP) confinement on the seismic response of circular reinforced concrete columns with spliced reinforcement at the critical hinging regions. A general expression previously was derived and verified for designing the thickness of external FRP jackets required for bond strengthening of spliced reinforcement within the critical hinging region of rectangular column sections. The current study applies the corresponding design expression for circular column sections. Full-scale circular columns with spliced reinforcement were tested under lateral load reversals. Findings show that all of the unconfined columns suffered premature splitting bond failure leading to almost total strength and stiffness degradation. Confining the splice zone with external FRP jackets within the range of thicknesses predicted by the proposed design expression substantially improved seismic performance. The mechanism by which the FRP confinement improves the bond strength of spliced bars in circular columns is similar to that in rectangular columns. Compared with other proposed design methods, the equation under consideration predicts a realistic and conservative jacket thickness and accounts for all parameters that are known to influence the splice strength of steel bars in tension.

R-Factor Parameterized Bridge Damage Fragility Curves

Damage fragilities describe the probability that a bridge will incur certain (discrete) damage states conditioned on the intensity of the earthquake it may experience. Reinforced concrete box girder highway overpass bridges are prevalent among the total inventory of bridges in California. For this class of bridges, a method for computing damage fragilities for three damage states (concrete cover spalling, longitudinal bar buckling, and column failure) based on the bridge force reduction factors ( R -factors) is derived in this paper. Bridge damage fragilities are described by equations relating the median intensity and uncertainty of ground motion to discrete damage states of column concrete cover spalling, column bar buckling, and column failure using the bridge R -factor as the principal parameter describing the bridge structure. Such damage fragility equations are furnished for earthquake intensities measured using pseudospectral acceleration (Sa) and cumulative absolute displacement (CAD) in both the ...

Behavior of Reinforced Concrete Column–Steel Beam Roof Level T-Connections under Displacement Reversals

Results from an experimental study on the seismic response of two hybrid RC column-to-steel S beam RCS roof level T-connection subassemblies are presented. The primary variables investigated were: 1 anchorage requirements in column longitudinal bars; and 2 confinement requirements in RCS roof level T-connections. In the first subassembly the column bars were anchored by providing a development length of 12 bar diameters and an end mechanical anchor with twice the diameter of the bar, while connection confinement was provided by steel band plates wrapping around the column regions just above and below the steel beam. This specimen exhibited a stable response up to 4.0% drift, the displacement at which a column bar anchorage failure occurred, which led to excessive bar slip through the connection with the associated decay in stiffness and strength. In the second specimen the development length for the column longitudinal bars was increased to 20 bar diameters and a larger bearing area for the mechanical anchors was provided through steel channel sections running transversely above the top flange of the steel beam. The behavior of the second specimen was characterized by large column inelastic rotations, good stiffness retention and energy dissipation capacity, and limited joint damage. Thus, experimental results indicate that properly designed RCS roof level T-connections exhibit a stable behavior under large inelastic deformation reversals, such as those induced by strong earthquakes. DOI: 10.1061/ASCE0733-94452006132:71041 CE Database subject headings: Composite structures; Earthquake resistant structures; Shear resistance; Shear deformation; Bearing capacity; Bond stress.

Effects of beam bar anchorage on beam–column joint behaviour

Reversed cyclic-load tests are carried out on full-scale reinforced concrete (RC) exterior beam–column joints, which are fabricated to simulate those in as-built RC framed buildings designed to BS 8110. Emphasis of the study is placed on the effects of the types of beam bar anchorage and location of laps in column reinforcement on the seismic behaviour and shear strength of RC exterior joints subjected to simulated earthquake load. The behaviour is investigated with different types of anchorage in beam reinforcement and laps in column bars at the lower zone of the column. It has been shown from the experiment that all the specimens have low ductility capacity and poor energy dissipation ability. The test results are compared with two preseismic design codes (BS 8110 and EC 2) and two seismic design codes (ACI-318 and NZS 3101). It is shown that the types of reinforcement anchorage in beams have significant effects on the shear capacity and hysteretic behaviour of exterior beam–column joints. Present codes of practice do not accurately predict the shear strength of the non-seismically detailed joints. It is indicated that ignoring the design of beam–column joints may lead to potential damage of RC framed buildings in an unexpected low to moderate earthquake.

Tumor Suppressor in Lung Cancer (TSLC)1 Suppresses Epithelial Cell Scattering and Tubulogenesis

The tumor suppressor in lung cancer 1 (TSLC1/IGSF4) encodes an immunoglobulin-superfamily cell adhesion molecule whose cytoplasmic domain contains a protein 4.1-binding motif (protein 4.1-BM) and a PDZ-binding motif (PDZ-BM). Loss of TSLC1 expression is frequently observed in advanced cancers implying its involvement in tumor invasion and/or metastasis. Using Madin-Darby canine kidney cells expressing a full-length TSLC1 or various cytoplasmic deletion mutants of TSLC1, we examined the role of TSLC1 in epithelial mesenchymal transitions during the hepatocyte growth factor (HGF)-induced tubulogenesis and cell scattering. In a three-dimensional culture, the full-length TSLC1, which was localized to the lateral membrane of Madin-Darby canine kidney cysts, inhibited HGF-induced tubulogenesis. In contrast, the mutants lacking either the protein 4.1-BM or the PDZ-BM abolished the inhibitory effect on tubulogenesis. In addition, these mutants showed aberrant subcellular localization indicating that lateral localization is correlated with the effect of TSLC1. In a two-dimensional culture, the full-length TSLC1, but not the mutants lacking the protein 4.1-BM or the PDZ-BM, suppressed HGF-induced cell scattering. Furthermore, the cells expressing full-length TSLC1 retained E-cadherin-based cell-cell adhesion even after being treated with HGF. These cells showed prolonged activation of Rac and low activity of Rho, whereas the HGF-treated parental cells induced transient activation of Rac and sustained activation of Rho. Prolonged Rac activation caused by the expression of TSLC1 required its cytoplasmic tail. These findings, taken together, suggest that TSLC1 plays a role in suppressing induction of epithelial mesenchymal transitions by regulating the activation of small Rho GTPases.

Seismic Behavior of a Concrete/Steel Integral Bridge Pier System

This paper summarizes an experimental study on an integral bridge pier system consisting of a concrete column, steel girders, and a box-shaped steel cap beam that is suitable for applications in seismic regions. Using two 1 / 3-scale test units, the research examined suitable connections between the concrete column and the steel cap beam and between the steel cap beam and steel girders as well as the overall seismic behavior of the proposed system. The anchorage length requirement for the column longitudinal bars governed the depths of the cap beam and girders in the first unit. Employing mechanical anchorage at the ends of the column longitudinal bars, the required anchorage length for the column bars was reduced in the second unit, thereby enabling the use of shallower and lighter cap beam and girders. Both test units exhibited satisfactory seismic response up to the target displacement ductilities with the intentional formation of plastic hinges in the columns. The description of the test units and key experimental results are included in this paper.

Seismic Retrofit of Octagonal Columns with Pedestal and One-Way Hinge at Base

Recent earthquake experience has shown that the response of retrofitted bridges is generally satisfactory. However, many seismically-deficient bridges remain, notably those that are supported on piers that lack proper detailing to resist strong earthquakes. This article reports on a study of seismic retrofit of bridge columns that are irregular both in the cross section and along the height, with the latter caused by the presence of a pedestal and a one-way hinge connection between the footing and the pedestal. Three identical quarter-scale column specimens were built for shaketable testing. One was tested as-built, the others were enhanced with seismic retrofits. The columns were subjected to the 1994 Northridge Sylmar earthquake in the strong direction until failure. Results showed that lateral steel of the as-built pedestal was highly deficient, resulting in an undesired pedestal failure. The first of the retrofitted columns was tested with a pedestal extension and a glass fiber-reinforced polymer (GFRP) pedestal jacket. The second was tested with the same retrofit as the first but several of the column bars were severed at the base of the column to reduce plastic shear demand. Results showed that the pedestal retrofit was successful in strengthening the pedestal sufficiently to shift the plastic hinging into the column. The severed column bars lowered shear demand and increased the plastic deformation. The authors also used analytical models to determine the shear strength and push-over behavior. The authors suggest that bond slip, shear deformation, and strain rate effects be accounted for in any effort to obtain a reasonable estimate of the push-over response of piers.

Seismic Retrofit of State Street Bridge on Interstate 80

The State Street Bridge, in Salt Lake City, was designed and built in 1965 according to the 1961 AASHO specifications; the design did not include earthquake-induced forces or displacements since only wind loads were considered. The bridge consists of four reinforced concrete (RC) bents supporting composite welded steel girders; the bents are supported on cast-in-place concrete piles and pile caps. A vulnerability analysis of the bridge was conducted that determined deficiencies in (1) confinement of column lap splice regions, (2) anchorage of longitudinal column bars in the bent cap, (3) confinement of column plastic hinge zones, and (4) shear capacity of columns and bent cap.column joints. Seismic retrofit designs using carbon-fiber-reinforced-polymer (CFRP) composites and steel jackets were performed and compared for three design spectra, including the 10% probability of exceedance in 250 years earthquake. The CFRP composite design was selected for implementation and application of the composite was carried out in the summer of 2000 and 2001, while the bridge was in service. The paper describes the CFRP composite design, which, in addition to column jackets, implemented an ”ankle wrap” for improving joint shear strength and a ”U-strap” for improving anchorage of column bars in the bent cap; other retrofit measures were implemented, such as bumper brackets and a deck slab retrofit. A capacity versus demand evaluation of the as-built and retrofitted bents is presented.

Seismic performance of external reinforced concrete beam-column joints

Four external beam-column sub-assemblies were tested to investigate the influence of different details on the performance of the Joint zone. All the details conformed to the current New Zealand structural concrete standard (NZS3101-1995). It is shown that anchoring beam reinforcement in external beam- column joints short of the outer column bars results in a reduction in the flexural strength of the column. In tests premature yielding of the longitudinal bars on the inside of the column occurred when bars were anchored at a distance of 1⁄4 of the column depth from the outside face of the column, as permitted by the structural concrete Standard. This yielding had an adverse effect on the performance of the joint zone. A way of detailing external joint zones to compensate for this loss in strength is described. Elongation of plastic hinges in the beams of ductile frames induces unidirectional plastic hinges in the external columns at the first floor level. While this has a strong influence on the distribution of moments and shears in the columns tests indicated that it does not adversely affect the structural performance of the joint zone. The use of continuous bars bent in the form of a U provides a simple detail that worked very effectively. There was little difference in joint behaviour between beams with main bars uniformly distributed over the beam depth to conventionally reinforced beams with their main bars positioned near the top and bottom edges.

프리캐스트 보-기둥 헤드철근 연결부 반복하중 실험

초록·키워드 목차 오류제보하기 등록된 정보가 없습니다. ABSTRACT1. 서론2. 실험체 설계3. 실험방법4. 실험결과 및 분석5. 결론참고문헌요약

Experimental study on simplified steel reinforced concrete beam-column joints in construction technology

The purpose of this paper is to propose a new type of steel reinforced concrete (SRC) beam-column joints and to examine the structural performance of the proposed joints, which simplify the construction procedure of steel fabrication, welding works, concrete casting and joint strengthening. In the proposed beam-column joints, the steel element of columns forms continuously built-in crossing of H-sections (<TEX>${\Box}$</TEX>), with adjacent flanges of column being connected by horizontal stiffeners in a joint at the level of the beam flanges. In addition, simplified lateral reinforcement (<TEX>${\Box}$</TEX>) is adopted in a joint to confine the longitudinal reinforcing bars in columns. Experimental and analytical studies have been carried out to estimate the structural performance of the proposed joints. Twelve cruciform specimens and seven SRC beam-column subassemblage specimens were prepared and tested. The following can be concluded from this study: (1) SRC subassemblages with the proposed beam-column joints show adequate seismic performances which are superior to the demand of the current code; (2) The yield and ultimate strength capacities of the beam-to-column connections can be estimated by analysis based on the yield line theory; (3) The skeleton curves and the ultimate shear capacities of the beam-column joint panel are predicted with a fair degree of accuracy by considering a simple stress transfer mechanism.

Performance based modeling and evaluation of joint mechanics

The authors are of the opinion that computational models suggested for exterior beam column connections in literature are not practicable in application as most of them are too complex to be used for hand analysis. Practicable computational models on the other hand contradict with the experimental data. In this paper, a simpler strut and tie model with a new truss model will be used to predict the failure modes and loads of the tests of an experimental database consisting of monotonically loaded exterior beam column connections. The relative contributions of the truss and strut mechanisms to joint shear will be determined and some design recommendations will be made. It is apparent from the results that the simple model predicts the failure loads and modes as accurate as other complicated models. The parametric studies carried out when developing the model shows that stirrups contribute as high as 29.7% to joint shear strength if they are provided by a stirrup ratio of at least 0.0055. It should also be noted that not all the stirrups yield within this region. In joints with stirrup ratios of 0.003 to 0.0055, all the stirrups yield and the contribution to joint shear strength is 18.14%. The stirrups of ratios less than 0.003 in the joint do not contribute to the joint shear strength of joints. The new simple strut and tie model proposed predicts the failure loads and modes very accurately. The model is applied on a database of 74 tests and the average of the ratio of the predicted failure load to the actual failure load is 0.99 for all tests. In this paper, several parametric studies will be carried out on this simple model both with and without stirrups in order to figure out this simple model's reliability. The influence of parameters such as column axial load. column bar ratio, concrete cylinder strength and the joint aspect ratio on the predicted joint shear strength of the model will be compared with the results of the authors' parametric studies on experimental data.

Seismic behavior and retrofit of pre-1970's as-built exterior beam-column joints reinforced by plain round bars

Four exterior beam-column joints reinforced by plain round bars designed according to pre-1970's codes were subjected to simulated seismic loading. Each of the test units was full size in scale. The main test variables were the manner in which the longitudinal beam bars were hooked in the joint core and the level of the axial load applied to the columns. The amount of transverse reinforcement in the beams, columns and joint cores was very small, as was typical of the pre-1970's. The tests on the units demonstrated that the available stiffness and strength was low when the axial load was zero. The final failure occurred due to opening of the beam bar hooks in tension and column bar buckling, irrespective of the hook details of the beam bars. The presence of axial compressive load of 0.25fc'Ag on the columns delayed the failure initiated by the beam bar hooks, leading to much improved stiffness and strength of the units. In one unit with beam bar hooks bent away from the joint core, the column regions adjacent to the joint core were jacketed with fibre-glass and the unit when tested showed much improved stiffness and strength. When compared with the results of similar units reinforced by deformed bars, the units reinforced by plain round bars showed less joint shear distortion but more opening of beam bar hooks in tension and column bar buckling. As a result, premature concrete tension cracking failure along the outer layer of column bars adjacent to the beam bar hooks was enhanced, and the attained stiffness and the force strength were significantly lower, especially the stiffness.

Seismic Repair of Hybrid RCS Beam-Column Connections

This paper describes the repair of 2 hybrid reinforced concrete column-to-steel beam connections severely damaged during earthquake-type displacements. The repair technique consisted primarily of wrapping the column regions just above and below the steel beam with carbon fiber sheets. The repaired specimens were subjected to lateral displacements of up to 5.0% drift. The behavior of the repaired specimens is compared to that of the original specimens in terms of load-vs.-displacement response, lateral stiffness, joint shear distortion, and strains in the joint transverse reinforcement and longitudinal column bars. The carbon fiber sheets effectively confined the joint region, leading to the same or higher shear strength and stiffness in the subassemblies. They also controlled the growth of diagonal cracks and prevented spalling of concrete in the corners of the joint.

The seismic behaviour of small reinforced concrete beam-column knee joints

The majority of research into beam-column knee joints has been conducted with monotonic loading. Many of these joints failed to reach their member moment capacity, especially under opening moments, while a few cyclic knee joint tests have been completed in the United States this decade. This paper describes the cyclic testing of 8 small knee joints designed to the 1995 New Zealand Concrete Standard. In addition two joints designed and detailed to the 1965 N.Z. Concrete Code were also tested. Joints with U-bar anchorages performed better than joints with standard 90 degree hook details on beam and column bars. The current Concrete Standard (NZS3101:1995) designs usually attained their nominal moment capacity in both directions up to and including ductility 4 displacements, but subsequently strengths fell off at higher ductilities. Joints with extra diagonal bars across the inner comer were able to sustain their nominal member strengths to higher ductility levels, especially under opening moments. A maximum horizontal joint shear stress of 0.12 f’c, for knee joints, in ductile frame buildings is recommended, where this limit is 60% of the current NZS3101:1995 Standard recommendation. An approximate 25% degradation of the joint shear stress occurred as displacement ductility factors increased from 1 to 8. The 1960's designed joints behaved poorly, as expected, with joint shear and anchorage failures occurring, in both moment directions, at strength levels below the beam's nominal strength. A maximum joint shear stress of only 0.072 f’c was reached and this fell to about a third of that stress between displacement ductility factors of 1 and 4 under closing moments.

Seismic load tests on two concrete interior beam-column joints reinforced by plain round bars designed to pre-1970s codes

Two identical reinforced concrete interior beam-column joint assemblies were tested under simulated seismic loading as part of an investigation of the behaviour of existing reinforced concrete structures designed to pre-1970s codes when subjected to severe earthquake forces. The test units were full-scale in size, and were replicas of part of the moment resisting frame of an existing building in Christchurch that was constructed in the 1950s. The longitudinal and transverse reinforcement were from plain round bars, the beams and columns had low quantities of transverse reinforcement and the joints had no shear reinforcement at all, as was typical of pre-1970s construction in New Zealand. One unit was tested with zero axial column load, and the other unit with a constant axial column load of 0.12 f’c Ag, where f’c = concrete cylinder compressive strength and Ag = the gross column section area. The units displayed very low structural stiffness, flexural strength less than conventionally calculated, and significant degradation of stiffness and strength during testing. The low structural stiffness and flexural strength could be attributed to the slip of the plain round longitudinal bars through the joint. However, the utilisation of plain round bars although leading to bond slip was found to improve the joint shear strength and to result in reduced shear distortion of the joint. Buckling of longitudinal column bars was found to initiate failure, due to inadequate ties.

Seismic Strengthening of Column-Pier-Cap Connections

Many bridges constructed in the 1960s in regions of high seismic risk have column–pier-cap connections with inadequate column bar development and no shear reinforcement in the joint region. The study described in this paper focuses on highway bridges built on Interstate 80 in the Reno, Nevada, area during the 1960s. Two 0.4-scale specimens representing the essential features of the column–pier-cap connections in these bridges were constructed and tested. One test showed that the asbuilt specimen had little energy dissipation capacity and failed at less than 1 percent drift. A second specimen was used to test a potential strengthening technique. The technique included increasing pier-cap depth, adding a concrete bolster to the joint, and placing a steel jacket around the column. After strengthening, a plastic hinge formed in the column, the joint damage was minimized, and the energy dissipation capacity increased by a factor of 5.

Intrinsic Mechanisms in Reinforced Concrete Beam-Column Connection Behavior

Laboratory tests on 17 reinforced concrete external beam-column connections are described-15 subjected to monotonic loading and two to simulated seismic loading. Parameters investigated were beam depth, beam tension steel percentage, reinforcement details for beam tension steel, and column load. Detailed measurements of strain distributions along both the beam and column bars were made using internally strain-gaged reinforcement, and results are presented to indicate influences exerted by the test parameters on reinforcement strain distributions. Bond stresses along the beam tension steel were evaluated and the means by which the reinforcement details effected load transfer by bond determined. Recommendations are made concerning the effectiveness and suitability of these details in practical connection design.