Ductility Of Reinforced Concrete Columns Research Articles

Seismic strategy of non-seismically designed reinforced concrete columns retrofitted with steel rods

This paper presents experimental results of combined cyclic load testing on a reinforced concrete (RC) column that was retrofitted with newly designed steel rods. The steel rods were installed around the column longitudinally and then anchored. The proposed steel rods utilize simple components and installation to enhance both the strength and ductility of RC columns. Cyclic lateral load tests were conducted on three specimens: one unreinforced specimen as reference, one specimen with the entire length of the column retrofitted, and one specimen with only the plastic hinge region of the column retrofitted. All specimens were tested under eccentric constant axial load and incrementally increasing lateral loading cycles with eccentricity. The implementation of steel rods resulted in significant improvement in ductility and an up to 60% increase in ultimate loading capacity.

SEISMIC performance of RC square columns strengthened by bidirectional basalt-fiber-reinforced polymer (BFRP) composite laminates with fan-shaped anchors

For seismic retrofitting of existing reinforced concrete (RC) structures, multiple strengthening demands may be required simultaneously including shear, flexural, and axial strength, as well as ductility. However, satisfying such multiple demands using conventional materials and strengthening systems may be challenging. As compared to commonly used unidirectional fibers reinforced polymers (FRP) composites laminates, bidirectional FRP laminates offer an effective methodology for providing strength and ductility in different directions. In this study, seven reinforced concrete (RC) columns specimens confined by bi-directional basalt-fiber-reinforced polymer (BFRP) composite jackets at plastic hinge regions with and without fan-shaped composite anchors are evaluated. All specimens are subjected to low-frequency full-reversal cyclic lateral loadings up to failure. Several design parameters are assessed to identify its impact on the behavior of confined columns that include: (i) composite laminate types, (ii) number of plies, (iii) strengthening scheme, and (iv) presence of anchorages. Service and ultimate seismic performance of each specimen including failure modes, rupture strain, and ductility are described and investigated. Experimental results indicated that ultimate failure mode changes from a shear failure, in the case of unidirectional composite jacket, to a combined flexure/shear failure mode when bidirectional [0°/90°] composite jackets are utilized. However, results indicated that the circumferential (hoop) fibers of a bidirectional composite jacket have lower ultimate hoop strain as compared to those obtained for columns with unidirectional composite jackets. Results also showed that the use of composite anchors effectively improved working strain of longitudinal fibers [0°], especially for columns with bidirectional composite jackets. In this case, tensile strain increased by 1.5 times as compared to specimens without composite anchors. Furthermore, the presence of anchors improved flexural capacity of confined columns. In addition, an analytical model to predict flexural capacity of RC columns confined with bidirectional laminates with anchors is proposed, which was verified using experimental results. Results of this study indicated that the use of bidirectional FRP composites laminates jackets with fan-shaped anchor enhanced both strength and ductility of RC columns.

Seismic performance of lap-spliced pre-damaged and intact concrete columns strengthened or retrofitted with UHPC and NSM

Reinforced concrete (RC) columns with deficient lap splice details such as short lap splice length and widely spaced stirrups were vulnerable to earthquakes. In the present study, to evaluate the effectiveness of an innovative retrofitting technique using the combination of ultra-high performance concrete (UHPC) and near surface mounted (NSM) technique on the seismic performance of RC columns with deficient lap splices details, cyclic loading tests were performed on four large-sized RC column specimens. Two RC columns were strengthened with UHPC or the combination of UHPC and NSM, and the other two were retrofitted with the combination of UHPC and NSM after the lateral strength decreased to 85% peak strength during the pre-damage cyclic tests. The load–displacement relationship, stiffness degradation, energy dissipation, and residual displacement of RC columns are evaluated. The test results showed that the peak strength and ductility of RC columns with deficient lap splice are significantly improved after strengthening or retrofitting with the combination of UHPC and NSM. The seismic performance of severely damaged RC columns due to deficient lap splice details can still be effectively restored after retrofitting with the combination of UHPC and NSM. Considering the adverse effect of pre-damage, a design method is proposed to predict the flexural capacity of RC columns retrofitting with UHPC or the combination of UHPC and NSM. The proposed method is also applied to predict the flexural capacity of RC beams strengthened with UHPC. The proposed method predicted well the test results of this study and existing tests.

Parametric studies on the ductility of axial loaded square reinforced concrete column made of normal-strength concrete (NSC) and high-strength steel confining rebar (HSSCR) with various ties configuration

During an earthquake, Reinforced Concrete (RC) building structures should behave in a ductile manner to prevent the structures from collapse. Therefore, the column element should have sufficient ductility to sustain an axial load at the post-peak region. Ductility of the RC column can be sufficiently provided by confinement to the RC column core. Therefore, in this paper, ductility of square RC columns made of NSC and HSSCR are analyzed using three-dimensional nonlinear finite element analysis (3D-NLFEA) with various ties configurations. In total, 12 specimens for each transverse steel rebar configuration were examined. The measurement used for ductility comparisons is the I10 index (AS 3600-2018) which is compared with the concept of ductility available in the literature (for example ACI 318-14). The study found that the computed minimum transverse steel rebar diameter based on ACI 318-14 showed larger diameter than the AS 3600:2018. From the 3DNLFEA analysis found that using a confining rebar higher than 700 MPa with the same volumetric ratio shows lower ductility for the Type I RC column configuration.

Axial strengthening of RC columns by steel encasement with direct fastening connections

Reinforced concrete (RC) columns are crucial structural members that sustain vertical loads. Their strength may deteriorate due to fire, corrosion of the reinforcements or design errors. In response, a new strengthening method that uses a steel encasement to increase the axial capacity and ductility of RC columns is proposed in this paper. This method is quick and convenient because direct fastening is used to assemble the system. Four columns including one control column and three strengthened columns have been tested to examine the reliability and effectiveness of the proposed method. Although direct fastening plays a crucial role in the structural performance of the proposed method, current design guidelines have not incorporated this new connection technique. Hence, extensive connection tests have been conducted to evaluate the key factors (e.g. fastener type, fastener spacing, protuberance, etc.) that affect the strength of the connections. New design equations are subsequently proposed based on the test results for shear connections that use direct fastening.

3D Finite element modeling of circular reinforced concrete columns confined with FRP using a plasticity based formulation

Strengthening reinforced concrete (RC) columns with external confining devices such as FRP wraps or steel tube is widely used in construction. By using external confining devices, both the strength and ductility of RC columns are significantly improved. However, numerical modeling to predict the capacity of strengthened RC columns is limited and often oversimplified. One of the biggest challenges in numerical modeling is to deal with unequal dilation between the concrete inner core (enclosed by both transverse steel and FRP wraps) and the concrete outer core (between the transverse steel and FRP wraps). Inaccurate modeling on the concrete dilatant behavior can lead to incorrect strength prediction. Sophisticated constitutive models which are able to model concrete dilation and robust modeling techniques are required. In this paper, three-dimensional non-linear finite element analysis (3D-NLFEA) of circular RC columns confined with conventional steel stirrups and FRP wraps is presented. In the FEA, the initial stiffness method with Process Modification (acceleration technique) is used to solve the equilibrium forces in the global solution. The constitutive model is based on the plasticity formulation proposed by the authors, which can capture the effective lateral modulus (EL) of the confining devices. This lateral modulus is obtained by observing the principal incremental stresses and strains at each element gauss point. It was found that, the lateral modulus is greatly affected by the boundary condition, dilatant behavior of the constitutive model and the Poisson’s ratio of the external confining device. To validate the performance of the proposed model, several comparisons of the proposed model, using 3D-NLFEA, with experimental results is presented. The comparisons show that the predicted response using 3D-NLFEA and the experimental results of RC columns confined with FRP are in a good agreement.

Axial compressive behaviour of RC columns with high-strength MTS transverse reinforcement

To improve the mechanical behaviour of reinforced concrete (RC) columns with square cross-section, an additional configuration of transverse reinforcement using high-strength multiple-tied spirals is proposed to achieve better lateral confinement to the concrete and to delay or avoid the buckling of longitudinal bars. In total, 12 specimens are designed for the axial compression test, including ten RC columns with high-strength multiple-tied-spiral transverse reinforcement (MTSTR) and two RC columns with conventional stirrups to provide a reference. The variables of specimens in the experiment are the spacing of circular spirals and rectangular hoops, the confined area of circular spirals and concrete strength. The experimental results indicate that the concrete and the longitudinal bars are effectively confined by the high-strength MTSTR, leading to considerable enhancements in both the load-carrying capacity and ductility of RC columns. On the basis of strain compatibility, an analytical approach to the axial behaviour of the RC column with high-strength MTSTR is proposed, and the analytical results using the model proposed by Bing and co-workers agree well with the experimental results.

Experimental and Analytical Study on Ultimate Load Capacity of RC Columns in the Subway Stations Under High-Temperature Gas Fume Effects

In fires, reinforced concrete (RC) columns in the subway stations were seriously threatened by high-temperature gas fumes. In this paper, an axial compressive simulation test that involved eight RC columns was undertaken to investigate the effect of high-temperature gas fume on RC columns. Two of them were control specimens with different cross sections. The others were exposed to high-temperature gas fumes. The specimens were heated to 300, 400, and 500 °C. The effects of high-temperature degree on ultimate load capacity and failure mode were investigated. As the applied load was increased, the crack gradually spread to the unheated area. Meanwhile, the initial stiffness and energy dissipation capacity were also summarized. The test results show that the ultimate load capacity and initial stiffness of RC columns decreased sharply with the increase in temperature. Compared to unexposed RC columns with circle cross section, the ultimate load capacity of specimens exposed to high temperature decreased by 47.9, 56.0 and 67.9%. The initial stiffness of RC columns with circle cross section exposed to 500 °C was 13 times lower than those of unexposed RC columns. The energy ductility of RC columns increased with the flue gas temperature. Furthermore, an analytical model was established to predict the residual axial load-carrying capacity of RC columns. It was demonstrated that the analytical model is correct and the predicted results are consistent with experimental results.

New configuration of transverse reinforcement for improved seismic resistance of rectangular RC columns: Concept and axial compressive behavior

Columns are considered to be critical members of moment-resisting structural systems. In seismically active regions, it is important to improve the ductile deformation capacity and energy dissipation capacity of columns so that the entire structure can endure severe ground motion and dissipate a considerable amount of seismic energy. Against this background, this paper presents the idea of a new transverse reinforcement configuration (TRC) for improved seismic performance of rectangular (including square) reinforced concrete (RC) columns. The novel features of the new TRC compared to conventional TRCs using rectangular hoops and cross ties mainly include the use of additional small circular spirals near the column ends (i.e., potential plastic hinge regions). Such a simple change can significantly enhance the performance of RC columns without complicating their construction. In this paper, the rationale for the new TRC together with its expected advantages is first explained. A series of axial compression tests on RC columns with the new TRC are then presented to demonstrate some of the expected advantages. The test results confirmed that the concrete is very effectively confined by the new TRC, leading to significant enhancements in both the load capacity and ductility of RC columns.

Combined strain gradient and concrete strength effects on flexural strength and ductility design of RC columns

The stress-strain relationship of concrete in flexure is one of the essential parameters in assessing the flexural strength and ductility of reinforced concrete (RC) columns. An overview of previous research studies revealed that the presence of strain gradient would affect the maximum concrete stress developed in flexure. However, no quantitative model was available to evaluate the strain gradient effect on concrete under flexure. Previously, the authors have conducted experimental studies to investigate the strain gradient effect on maximum concrete stress and respective strain and developed two strain-gradient-dependent factors k3 and ko for modifying the flexural concrete stress-strain curve. As a continued study, the authors herein will extend the investigation of strain gradient effects on flexural strength and ductility of RC columns to concrete strength up to 100 MPa by employing the strain-gradient-dependent concrete stress-strain curve using nonlinear moment-curvature analysis. It was evident from the results that both the flexural strength and ductility of RC columns are improved under strain gradient effect. Lastly, for practical engineering design purpose, a new equivalent rectangular concrete stress block incorporating the combined effects of strain gradient and concrete strength was proposed and validated. Design formulas and charts have also been presented for flexural strength and ductility of RC columns.

Controlling the damage of concrete columns through compression yielding

For reinforced concrete (RC) members, the crushing of concrete governs the ultimate failure of a cross-section. As the ultimate strain of a particular grade concrete is a certain value, the curvature capacity of an RC section depends on the tensile strain of the member at concrete crushing. Similarly, the curvature ductility depends on the extent of tensile yielding of the steel reinforcement. In RC columns, the existence of the axial force reduces the strain at the tension side of the cross-section. Therefore, the deformation capacity and the ductility of an RC column is usually smaller than that of an RC beam. The ductility is critical for RC columns, particularly for seismic structures to survive a major earthquake. As a result, the development of technology that can control the damage and increase the ductility of RC columns is of major theoretical significance and practical importance for RC structures. A novel structural concept for controlling the damage and increasing the flexural ductility of RC beams through compression yielding (CY) instead of tension yielding has recently been developed by the authors. Preliminary theoretical studies and experimental investigations have demonstrated that the CY concept is highly effective in reducing the damage and increasing the ductility of RC beams. In principle, the CY concept should be equally applicable and effective for RC columns. This work aims at controlling the damage and increasing the deformability and ductility of RC columns through the application of the newly developed CY technology.

Analytical model for prediction of load capacity of RC columns confined with CFRP under uniaxial and biaxial eccentric loading

This paper presents an analytical model for prediction of the load carrying capacity of reinforced concrete (RC) columns confined with carbon fiber reinforced polymers (CFRP) under uniaxial and biaxial eccentric loading. The model is based on realistic material laws and accounts for the non-linear stress–strain behavior of both unconfined and CFRP-confined concrete. Under uniaxial eccentric loading, the column cross-section is discretized into finite layers along the section depth. For a symmetric square cross-section subjected to biaxial eccentric loading with equal eccentricity about each principal axis, the column cross-section is discretized into finite layers along the diagonal of the column cross-section. For a given strain distribution in a direction perpendicular to the neutral axis, the sectional forces are integrated numerically and the load capacity of the column is predicted using an iterative process. For rectangular and non-symmetric square cross-sections subjected to biaxial eccentric loading, the load capacities under uniaxial eccentric loading along each principal axis are first derived independently. The column load capacity under concentric axial loading is calculated. The determined distribution of the column load capacities under uniaxial eccentric loading and concentric loading are then utilized to compute the load capacity under biaxial eccentric loading using the reciprocal load equation. An experimental study was carried out to examine the effectiveness of the CFRP-confinement to improve the load capacity and ductility of RC columns under biaxial eccentric loading. The accuracy of the proposed analytical model was demonstrated by comparing the model predictions to results of the current experimental study in addition to experimental data published in the literature.

Effects of Steel Lap Splice Locations on Strength and Ductility of Reinforced Concrete Columns

Longitudinal steel lap splice is always required in reinforced concrete (RC) columns. Normally, in countries having high seismic risk, lap splices of longitudinal steel must be located around mid-height of the storey. However, in regions of low-medium seismic risk, including Hong Kong, lap splices of longitudinal steel begin right above the beam-column interface to facilitate ease of construction. Such splicing method would undesirably cause the column critical region to move away from the beam-column interface under inelastic deformation. In this paper, the effects of different lap splice locations of longitudinal steel on flexural strength and ductility of RC columns with concrete cube strength around 100 MPa are studied. Four RC column specimens, which contained no lap splice, all lap splices within and outside critical region, as well as lap splices in staggered manner, were tested under simultaneous compressive axial load and reversed cyclic inelastic displacement. It is evident from the results that the column containing lap splices within its critical region had the largest strength but the poorest ductility performance. On the contrary, the column containing lap splices outside its critical region had strength and ductility comparable to those of the column without lap splice. Based on these observations, a recommendation is proposed for positioning longitudinal steel lap splices in RC columns.