Full-scale Beams Research Articles

FLEXURAL BEHAVIOR OF OVER REINFORCED HSC BEAMS CONFINED BY RECTANGULAR TIES

This paper presents an experimental and analytical investigation on the flexural behavior of over reinforced concrete beam confined by rectangular ties. The experimental investigation was conducted through testing four full-scale beams with normal strength concrete (NSC). Three out of these four beams were confined using rectangular ties located on the compression region of the beams. The variables studied in these specimens were the volumetric ratio of transverse reinforcement, and the volumetric ratio of the main rebars. A finite element model was established with both material and geometrical non-linearity by using ANSYS software package. Accuracy of the model was assessed by applying it to the three tested beams. Comparison of analytical results with the available experimental results for ultimate load values and load– deflection relationships show a good agreement between the finite element and experimental results. After validating the accuracy of the proposed model, parametric study was undertaken to gain additional insight into the overall behaviour, failure modes, and deformation capacity for the concrete beams using high class concrete strength. The parameters studied were, the concrete compressive strength, the number and diameter of longitudinal rebars, and diameter of rectangular ties. The obtained results show that, the strength and ductility of high strength concrete (HSC) beams are enhanced through the application of rectangular tie reinforcement located in the compression region of the beams. The concrete compressive strength, the longitudinal bars number and diameter, and the diameter of rectangular ties are important parameters controlling the level of strength and ductility enhancement of overreinforced HSC beams.

A new approach to testing concrete coupling beams subjected to reversed cyclic loading

This paper presents an innovative experimental rig developed for tests of full-scale concrete coupling beams of tall shear wall buildings subjected to reversed cyclic loading. The coupling beams are generally subjected to very high bending moment and shearing force owing to rotations of the beam ends, which are forced by the deflection of shear walls under the action of lateral loading. Because no direct loads are generally applied to the beams, how to create the boundary conditions where two beam ends are forced to rotate equally and displace vertically is challenging in the laboratory tests. This can further be complicated by the requirement of applying reversed cyclic loading to the connected shear walls. The developed test rig allows for proper modelling of loading and boundary conditions in both monotonic and reversed cyclic load cases. Tests of the full-size specimens with different span-to-depth ratios can be conducted without changing the beam depth, which allows for direct comparisons of specimens and experimental results. The rig is capable of performing tests for investigating strength and ductility of the specimens with large rotational and vertical displacements. Available experimental methods to test coupled shear walls are reviewed to demonstrate the simplicity, accuracy and effectiveness of the proposed test rig and the method.

Behavior of full-scale self-consolidating concrete beams in shear

An experimental investigation was conducted to study the shear strength and cracking behavior of full-scale beams made with self-consolidating concrete (SCC) as well as normal concrete (NC). A total of 20 flexurally reinforced concrete beams, with no shear reinforcement, were tested under mid-span concentrated load until shear failure occurred. The experimental test parameters included concrete type/coarse aggregate content, beam depth and the longitudinal reinforcing steel ratio. The beam depth ranged from 150 to 750 mm while the shear span-to-depth ratio (a/d) was kept constant in all beams. The two longitudinal reinforcing steel ratios used were 1% and 2%. The performance of SCC/NC beams was evaluated based on the results of crack pattern, crack widths, loads at the first flexure/diagonal cracking, ultimate shear resistance, and failure modes. The ultimate shear strength of SCC beams was found to be slightly lower than that of NC beams and the difference was more pronounced with the reduction of longitudinal steel reinforcement and with the increase of beam depth. The performance of code based equations in predicting the shear resistance of SCC/NC beams is also presented. The recommendations of this paper can be of special interest to designers considering the use of SCC in structural applications.

유효 스터럽 개념을 이용한 전단보강근의 강도 예측

This paper presents the prediction of the actual strength of shear reinforcement on the basis of the concept of effective stirrups. The prediction method incorporating the shear cracking angle was proposed with the estimation by the Modified Compression Field Theory (MCFT). To check the validity of the method, discussion of the current ACI 318-05 and comparison of 39 test results from the literature including author`s retrospective test data were made. The influencing factors of compressive concrete strength and type of shear-reinforcement were also investigated. Furthermore, two full-scale beam specimens shear-reinforced with headed bars were tested to demonstrate the applicability of the proposed method.

헤디드 바, 고장력 철근 및 CFRP 바로 전단보강된 세장 고강도콘크리트 보의 전단 거동 평가

If conventional reinforcements are used for high-strength concrete (HSC) structures, a large amount of the reinforcement must be required to compensate for the brittleness of HSC and make the best use of HSC. This raises some structural problems such as steel congestion and an increase in self-weight. Therefore, alternative reinforcing materials and methods for HSC structures are needed. In this study, four full-scale beam specimens constructed with HSC (100 MPa) were tested to investigate the effect of the different shear reinforcements on the shear behavior. These four specimens were reinforced for shear stirrups with normal and high strength steels, headed bars, and carbon fiber-reinforced polymer (CFRP) bars, respectively. In addition, steel fibers were added to the HSC in the two of the specimens to observe their beneficial effects. The use of high strength steels resulted in the improvement of the shear capacity since the shear resistance provided by the shear reinforcements and the bond strength were increased. The specimen reinforced with headed bars also showed a superior performance to the conventional steel reinforced specimen due to the considerably high anchorage strength of headed bar. CFRP bars used in this research, however, seemed to be inadequate for shear reinforcement because of the inferior bond capacity. The presence of the steel fibers in concrete led to remarkable improvement in the ductility of the specimens as well as in the overall cracks control capability.

Seismic behavior of strengthened reinforced concrete coupling beams by bolted steel plates, Part 1: Experimental study

An experimental study of five full-scale coupling beam specimens has been conducted to investigate the seismic behavior of strengthened RC coupling beams by bolted side steel plates using a reversed cyclic loading procedure. The strengthened coupling beams are fabricated with different plate thicknesses and shear connector arrangements to study their respective effects on load-carrying capacity, strength retention, stiffness degradation, deformation capacity, and energy dissipation ability. The study revealed that putting shear connectors along the span of coupling beams produces no significant improvement to the structural performance of the strengthened beams. Translational and rotational partial interactions of the shear connectors that would weaken the load-carrying capacity of the steel plates were observed and measured. The hierarchy of failure of concrete, steel plates, and shear connectors was identified. Furthermore, detailed effects of plate buckling and various arrangements of shear connectors on the post-peak behavior of the strengthened beams are discussed.

Bond strength of hot-dip galvanized hooked bars in normal strength concrete structures

In 2001, a multi-phase research program was started at the American University of Beirut (AUB) to evaluate experimentally the effect of hot dip galvanizing on bond strength of reinforcement in concrete structures. In the first two phases, 24 full-scale beam specimens were tested in positive bending to evaluate the effect of galvanizing on bond strength of tension lap splices in normal and high strength concrete specimens. The variables were bar size, concrete strength, the amount of transverse reinforcement confining the splice region, and whether the bars were galvanized or not. Test results indicated that galvanizing had a negligible effect on bond strength of reinforcement in normal strength concrete. However, galvanizing caused an average of 20% decrease in bond strength of reinforcement in high strength concrete regardless of the amount of transverse reinforcement confining the splice region.This paper reports on a third phase of the AUB program, designed to evaluate the effect of galvanizing on anchorage performance of hooked bars in normal strength concrete structures. Up to date there has been no research work done in this area. Twelve specimens were tested. The specimen simulated the rigid connection of two cantilever beams to a column. The variables were bar size, the confinement mode of the beam bars anchored in the column, and whether the bars were galvanized or not. The effect of galvanizing was evaluated by comparing the performance of galvanized bars with black bars based on the mode of failure, ultimate load, and the general load–deflection behavior. Test results indicated that effect of galvanizing on the bond strength of hooked bars in normal strength concrete was not significant.

The effect of helical pitch on the behaviour of helically confined HSC beams

The strength and ductility of HSC beams are enhanced through the application of helical reinforcement located in the compression region of the beams. The pitch of helix is an important parameter controlling the level of strength and ductility enhancement of over-reinforced high strength concrete beams. This paper presents an experimental investigation of the effect of helix pitch on the beam behaviour through testing five helically confined full-scale beams. The helix pitches were 25, 50, 75, 100 and 160 mm. Beams’ cross section was 200 × 300 mm, and with a length of 4 m and a clear span 3.6 m subjected to four point loading, with emphasis placed on the mid-span deflection. The main results indicate that the helical effectiveness is negligible when the helical pitch is 160 mm (helix diameter), the concrete cover spalling off load increases linearly as the helical pitch increased, and the ultimate load decreases as the helical pitch increases. Finally, there is a considerable release of strain energy responsible for spalling off the concrete cover.

Shear enhancement of reinforced concrete beams strengthened with FRP composite laminates

This paper presents the results of an experimental investigation on shear strength enhancement of reinforced concrete beams externally reinforced with fiber-reinforced polymer (FRP) composites. A total of nine full-scale beam specimens of three different classes, as-built ( unstrengthened), repaired and retrofitted were tested in the experimental evaluation program. Three composite systems namely carbon/epoxy wet layup, E-glass/epoxy wet layup and carbon/epoxy precured strips were used for retrofit and repair evaluation. Experimental results indicated that the composite systems provided substantial increase in ultimate strength of repaired and strengthened beams as compared to the pre-cracked and as-built beam specimens. A comparative study of the experimental results with published analytical models, including the ACI 440 model, was also conducted in order to evaluate the different analytical models and identify the influencing factors on the shear behavior of FRP strengthened reinforced concrete beams. Comparison indicated that the shear span-to-depth ratio ( a/ d) is an important factor that actively controls the shear failure mode of beam and consequently influences on the shear strength enhancement.

Flexural Behavior of Concrete Beams Strengthened with Near-Surface-Mounted CFRP Strips

Engineers have proposed relocating externally bonded strengthening fiber reinforced polymer (FRP) material from the unprotected exterior of the concrete to the protected interior. This technology is known as near-surface mounted (NSM) strengthening. In NSM reinforcement, the FRP is surrounded by concrete on three sides so the bond and damage problems associated with externally bonded FRP strengthening systems are reduced or eliminated. This paper presents experimental results from 12 full-scale concrete beams strengthened with NSM carbon FRP (CFRP) strips. Three companion unstrengthened specimens were also tested to serve as a control. Experimental variables include three different ratios of steel reinforcement and two different ratios of CFRP reinforcement. Yield and ultimate strengths, flexural failure modes, and ductility are discussed based on measured load, deflection, and strain data. Test results show measurable increases in yield and ultimate strengths in all beams strengthened with CFRP as well as predictable nominal strengths and failure modes. Force transfer between the CFRP, epoxy grout, and surrounding concrete was able to develop the full tensile strength of the CFRP strips. Energy and deflection ductilities were reduced for CFRP strengthened beams. Future research needs are addressed.

Bolted end plate connections for steel reinforced concrete composite structures

In order to improve the constructability and meanwhile ensure excellent seismic behavior, several innovative composite connection details were conceived and studied by the authors. This paper reports experimental results and observations on seismic behavior of steel beam bolted to reinforced concrete column connections (bolted RCS or BRCS). The proposed composite connection details involve post tensioning the end plates of the steel beams to the reinforced concrete or precast concrete columns using high-strength steel rods. A rational design procedure was proposed to assure a ductile behavior of the composite structure. Strut-and-tie model analysis indicates that a bolted composite connection has a favorable stress transfer mechanism. The excellent capacity and behavior were then validated through five full-scale beam to column connection model tests.

Effect of Confinement on Bond Strength of Hot-Dip Galvanized Lap Splices in High-Strength Concrete

Galvanizing reinforcing steel is one of the methods used to protect steel bars against corrosion. In 2001, a research program was started at the American University of Beirut (AUB), Beirut, Lebanon, to experimentally evaluate the effect of hot-dip galvanizing on the bond capacity of tension lap splices anchored in full-scale beam specimens designed to fail in bond splitting mode. The test results indicated that the use of galvanized bars had a negligible effect on bond strength of reinforcement in normal-strength concrete. Galvanising, however, caused an average of 20% decrease in bond strength of reinforcement in high-strength concrete. The primary objective of the research reported in this paper was to find a solution to eliminate the bond reduction of galvanized bars in high-strength concrete. It was important to evaluate the potential positive effect of the addition of transverse reinforcement in the splice region. One hypothesis to be tested was that such transverse reinforcement would ensure uniform bond stress distribution over the entire splice length, thus mobilizing all bar lugs along the splice in the stress transfer mechanism between the bar and the surrounding concrete, and hence would lead to increasing the bond strength of galvanized bars. The second hypothesis to be checked was whether the aforementioned mechanism of mobilizing all bar lugs along the splice in the stress transfer mechanism would reduce the significant decrease in bond strength of galvanized bars relative to black bars in high-strength concrete. Eighteen, full-scale beam specimens, designed to fail in bond-splitting mode, were tested in positive bending. Each beam was reinforced with bars spliced in a constant moment region at midspan. The main variables were bar size and the amount of transverse reinforcement in the splice region. The test results indicated that, whereas confinement increased the bond strength of galvanized bars and black bars, confinement did not have a significant positive effect on the bond strength of galvanized bars relative to black bars.

Experimental testing of helically confined high-strength concrete beams

The strength and ductility of high-strength concrete (HSC) beams are enhanced through the application of helical reinforcement located in the compression region of the beams. The pitch of the helix is an important parameter controlling the level of strength and ductility enhancement of over-reinforced HSC beams. This paper presents an experimental investigation of the effect of helix pitch on the beam behaviour by testing five helically confined, full-scale beams. The helix pitches were 25, 50, 75, 100 and 160 mm. The cross-section of the beams was 200 × 300 mm, and with a length of 4 m and a clear span of 3·6 m subjected to four-point loading, with emphasis placed on the midspan deflection. The main results indicate that the helix had negligible effect when the helical pitch was 160 mm (helix diameter), the concrete cover spalling-off load increased linearly as the helical pitch increased, and the ultimate load decreased as the helical pitch increased.

Experimental testing of helically confined high-strength concrete beams

The strength and ductility of high-strength concrete (HSC) beams are enhanced through the application of helical reinforcement located in the compression region of the beams. The pitch of the helix is an important parameter controlling the level of strength and ductility enhancement of over-reinforced HSC beams. This paper presents an experimental investigation of the effect of helix pitch on the beam behaviour by testing five helically confined, full-scale beams. The helix pitches were 25, 50, 75, 100 and 160 mm. The cross-section of the beams was 200 × 300 mm, and with a length of 4 m and a clear span of 3·6 m subjected to four-point loading, with emphasis placed on the midspan deflection. The main results indicate that the helix had negligible effect when the helical pitch was 160 mm (helix diameter), the concrete cover spalling-off load increased linearly as the helical pitch increased, and the ultimate load decreased as the helical pitch increased.

Effect of Bar Diameter on the Behavior of Lightly Reinforced Concrete Beams

The transition from the uncracked to the cracked phase in lightly reinforced concrete beams was experimentally investigated by subjecting five full-scale beams to three-point bending. All beams had the same dimensions and the same percentage of flexural reinforcement, but varying diameters and number of reinforcing bars. By means of transducers, strain gauges, and geometric moire, the crack profiles and the concrete strain field around the cracks were measured. In this way, it was possible to observe and to reproduce numerically the crack onset and its stable propagation up to the unstable behavior that follows the effective cracking moment. During the growth of the crack, the moment rotation diagrams, the values of tensile strains in concrete, and the shape of crack profiles demonstrate the dependence of the mechanical response on bar diameter. Consequently, to avoid brittle failure, the minimum reinforcement ratio for lightly reinforced concrete beams should likewise be a function of bar diameter.

Shear behaviour of steel fibre reinforced concrete beams

The possibility of substituting traditional transverse reinforcement (stirrups) for steel fibres in precast elements can significantly reduce production costs. In the present paper, the shear behaviour of prestressed elements has been investigated by means of experimental tests on full scale beams. Tests concern beams with conventional as well as steel fibre reinforcement. Experimental results show that the shear behaviour of fibre reinforced concrete beams without conventional reinforcement is similar to, or even better than that of beams with stirrups. When used in beams with stirrups, steel fibres significantly improve their shear strength. A discussion on the contribution of steel fibres on the shear strength is also presented, with reference to the latest RILEM provisions.

Mechanical property of beam-to-column moment connection with hysteretic dampers for column weak axis

This paper presents a novel beam-to-column moment connection suitable for the column’s weak axis. The proposed system is developed in line with an idea to mainly utilize bolts with the number of welds minimized, thereby avoiding difficulties encountered in in situ welding. In this system, a wide-flange beam is joined to a wide-flange column by bolted splices at the top flange and newly invented hysteretic dampers at the bottom flange. The mechanical behavior of the damper and the whole connection system is described herein. The seismic performance of the proposed connection was verified through cyclic tests of six full-scale beam–column subassemblies, taking into account the effects of the geometry and arrangement of the dampers. The tests revealed that the proposed system is capable of achieving stable hysteresis behavior in a large deformation range. Yielding was limited only in the dampers while the beam and column remained essentially elastic until the ultimate state of the structure.

Ductility of carbon fiber-reinforced polymer (CFRP) strengthened reinforced concrete beams: Experimental investigation

Strength of reinforced concrete beams can easily be increased by the use of externally bonded CFRP composites. However, the mode of failure of CFRP strengthened beam is usually brittle due to tension-shear failure in the concrete substrate or bond failure near the CFRP-Concrete interface. In order to improve the ductility of CFRP strengthened concrete beams, critical variables need to be investigated. This experimental and analytical research focused on a series of reinforced concrete beams strengthened with CFRP composites to enhance the flexural capacity and ductility. The main variables were the amount of CFRP composites, the amount of longitudinal and shear reinforcement, and the effect of CFRP end diagonal anchorage system. Sixteen full-scale beams were investigated. A new design guideline was proposed according to the effects of the above-mentioned variables. The experimental and analytical results were found to be in good agreement.

Experimental study on fatigue strength of in-plane welded gusset joints

This paper presents the results of three phases of fatigue tests for the fatigue strength of in-plane welded gusset joints which are most common details inevitably existed in the region of high stress range. Totally fifty seven fatigue specimens with various thickness and strengths were made and the tensile fatigue tests were performed. One full scale beam fatigue tests was also carried out. The validity of the fatigue strengths of those details in specification was proved and the effects of thickness of flanges and welded attachments, length of attachments and the strength of applied steel were examined. The characteristics of crack initiation and propagation were also observed. The test results are evaluated by comparing with other test data and the fatigue criteria of other countries. In order to find out the degree of stress concentration in the weld toe depending on geometric configuration such as the attachment length and transition radius, analytical works were also carried out. Comparing with present specification, analytical results indicate that there are need of revising and subdividing the detail categories.

스터럽간격과 인장철근비에 따른 고강도 콘크리트 보의 파괴거동

Existing tests results have shown that confining the concrete compression region with closed stirrups improves the ductility and load-carrying capacity of beams. However, only few researchers have attempted to utilize the beneficial effects of the presence of these stirrups in design. This paper presents the result of experimental studies on the load-deflection behavior and the strengthening effect of laterally confined structural high-strength concrete beam members in which confinement stirrups have been introduced into the compression regions. Fifteen tests were conducted on full-scale beam specimens having concrete compressive strength of 41 MPa and 61 MPa. Different spacing of stirrups(0.25∼1.0d) and amount of tension steel(0.55∼0.7) as major variables were investigated. And also, this study present an appropriate shear equation for decision of ultimate failure modes of high-strength concrete beams according to stirrup spacing. The equation is based on interaction between shear strength and displacement ductility. Prediction of failure mode from presented method and comparison with test results are also presenteded