Experimental study on ductility improvement of reinforced concrete rectangular Columns retrofitted with a new fiber reinforced plastics method

Reinforced concrete （RC） columns lacking adequately detailed transverse reinforcement do not possess the necessary ductility to dissipate seismic energy during a major earthquake without severe strength degradation. In this paper, a new retrofit method, which utilized fiber-reinforced plastics （FRP） confinement mechanism and anchorage of embedded bars, was developed aiming to retrofit non-ductile large RC rectangular columns to prevent the damage of the plastic hinges. Carbon FRP （CFRP） sheets and glass FRP （GFRP） bars were used in this test, and five scaled RC columns were tested to examine the function of this new method for improving the ductility of columns. Responses of columns were examined before and after being retrofitted. Test results indicate that this new composite method can be very effective to improve the anti-seismic behavior of non-ductile RC columns compared with normal CFRP sheets retrofitted column.

Near Surface Mounted Application for the Strengthening of Rectangular Reinforced Concrete Slender Columns under Eccentric Load

The behavior of slender columns under the effect of eccentric loading has always taken the attention of researchers. When investigating the strengthening of reinforced concrete columns, mainly short and circular columns are the targeted elements. This is why the data about slender columns with rectangular sections is limited and infrequent specially when loaded eccentrically. This paper aims to increase the available experimental data in this specific area. The experimental program consisted of twenty seven specimens. The specimens were divided into three groups; one control group and two groups strengthened using two strengthening schemes. Scheme 1 implied the use of near surface mounted （NSM） longitudinal steel bars, while in scheme 2, NSM longitudinal steel bars partially wrapped with one ply of carbon fibers reinforced polymers （CFRP） sheets was used. The test specimen had an overall length of 2000 mm and a 100 x 200 mm rectangular cross section. In addition to the strengthening schemes, the test parameters included three ratios for the internal longitudinal steel bars ＂μ＂ 1%, 1.57% and 2.26%. The parameters were extended to cover three stirrups＇ volumetric ratio ＂ρv＂ 0.73%, 0.49% and 0.37%. The specimens were tested under the effect of eccentric loading with eccentricity-to-section height e/h equals 0.25. The research revealed that the strength gain in specimens strengthened with scheme 2 was higher than with scheme 1. Analytical modeling of the stress strain relation of the strengthened RC columns considering the effect of strengthening scheme, internal reinforcement ratio μ, and stirrups＇ volumetric ratio ＂ρv＂ was proposed. Verification was made using available experimental data. The proposed model showed a reasonable agreement with the experimental results.

A new model for analyzing nonlinear torsion behavior of concrete filled steel tube columns with rectangular section

An experimental study on concrete filled steel tube columns with rectangular section subjected to compressionflexure-torsion combined action has been carried out. The failure modes and load-deformation hysteretic relations were obtained. Based on the principles of classical material mechanics, the relations between the torsion curvature of the section and the shear strain of the fiber on the section were established. Then the strain distribution on the rectangular section of concrete filled steel tube columns subjected to torsion was analyzed. The three-dimensional refined finite element model was also built, in order to make the precision verification. The matrix forms of the relation between the torsion curvature of the section and the shear strain of the fiber on the section were derived, and introduced into the fiber beam model considering nonlinear torsion effect on the section. The comparison between test results and calculation results showed that the fiber beam model considering nonlinear torsion effect had high modeling efficiency and solution precision for predicting the torsion behavior of concrete filled steel tube columns with rectangular sections, and was suitable for analyzing the dynamic response of various structures subjected to the combined cyclic load caused by the earthquake load.

Ductility and Ultimate Capacity of Concrete-Filled Lattice Rectangular Steel Tube Columns

A kind of concrete-filled lattice rectangular steel tube(CFLRST)column was put forward.The numerical simulation was modeled to analyze the mechanical characteristic of CFLRST column.By comparing the load-deformation curves from the test results,the rationality and reliability of the finite element model has been confirmed,moreover,the change of the section stiffness and stress in the forcing process and the ultimate bearing capacity of the column were analyzed.Based on the model,the comparison of ultimate bearing capacity and ductility between CFLRST column and reinforced concrete(RC)column were also analyzed.The results of the finite element analysis show that the loading process of CFLRST column consists of elastic stage,yield stage and failure stage.The failure modes are mainly strength failure and failure of elastoplastic instability.CFLRST column has higher bearing capacities in comparison with reinforced concrete columns with the same steel ratio.In addition,the stiffness degeneration of CFLRST column is slower than RC column and CFLRST column has good ductility.

Bilateral shear strength of rectangular frame column based on simplified space truss-arch model

To investigate the bilateral shear strength of rectangular frame column subjected to oblique horizontal load, we presented a simplified space truss-arch model developed from unilateral truss-arch model. Main parameters in the new model were the cross-sectional area, transverse reinforcement raito, axial load, and material strength of the column. The reduction coefficient of concrete sterength owing to the severe cracking of column was also introduced in the model. Finally, 14 specimens under oblique horizontal load were tested to verified the feasibility and applicability of the space truss-arch model.

PLASTIC POST－BUCKLING OF A SIMPLY SUPPORTED COLUMN WITH A SOLID RECTANGULAR CROSS－SECTION

The plastic post-buckling of a simply supported column with a solid rectangularcross-section is analysed by a new approach. High order terms in the asymptotic post-buckling expansions are carried out. In some aspects, the method proposed in thispaper is similar io Hutchinson's. However, the computation is simple since theintroduction is avoided of stretched coordinates. The method can be used to analyseinitial post-bifurcation of plates and shells in the plastic range.

Research on axial bearing capacity of rectangular concrete-filled steel tubular columns based on artificial neural networks

Design of rectangular concrete-filled steel tubular （CFT） columns has been a big concern owing to their complex constraint mechanism. Generally, most existing methods are based on simplified mechanical model with limited experimental data, which is not reliable under many conditions, e.g., columns using high strength materials. Artificial neural network （ANN） models have shown the effectiveness to solve complex problems in many areas of civil engineering in recent years. In this paper, ANN models were employed to predict the axial bearing capacity of rectangular CFT columns based on the experimental data. 305 experimental data from articles were collected, and 275 experimental samples were chosen to train the ANN models while 30 experimental samples were used for testing. Based on the comparison among different models, artificial neural network modell （ANN1） and artificial neural network model2 （ANN2） with a 20- neuron hidden layer were chosen as the fit prediction models. ANN1 has five inputs： the length （D） and width （B） of cross section, the thickness of steel （t）, the yield strength of steel （fy）, the cylinder strength of concrete （fc＇）- ANN2 has ten inputs： D, B, t, fy, f′, the length to width ratio （D/B）, the length to thickness ratio （D/t）, the width to thickness ratio （B/t）, restraint coefficient （ξ）, the steel ratio （α）. The axial beating capacity is the output data for both models.The outputs from ANN1 and ANN2 were verified and compared with those from EC4, ACI, GJB4142 and AISC360-10. The results show that the implemented models have good prediction and generalization capacity. Parametric study was conducted using ANN1 and ANN2 which indicates that effect law of basic parameters of columns on the axial bearing capacity of rectangular CFT columns differs from design codes.The results also provide convincing design reference to rectangular CFT columns.

Performance study on T-stub connected semi-rigid joint between rectangular tubular columns and H-shaped steel beams

This paper investigates the performance of T-stub connected semi-rigid joint of rectangular tubular columns and H-shaped steel beams.The finite element analysis software ABAQUS is used to analyze the nonlinear performance of the joint under monotonic loading.Meanwhile,the dimensions of T-stub,column and beam are considered as analytic parameters to discuss the performance of the joint.The analysis shows that the thickness and the length of T-stub webs,the height of beam section,bolt diameter,shear connector and the preloaded force affect the performance of the joint largely,and the thickness of the steel tube,the thickness and length of T-stub flange,bolt spacing have relatively little influences on the performance of the joint.The research results indicate that this joint is semi-rigid joint.

Axial compression behavior of CFRP-confined rectangular concrete-filled stainless steel tube stub column

The mechanical properties of CFRP-confined rectangular concrete-filled stainless steel tube(CFSST)stub columns under axial compression were experimentally studied.A total of 28 specimens(7 groups)were fabricated for the axial compression test to study the influences of length-to-width ratio,CFRP constraint coefficient,and the thickness of stainless steel tube on the axial compression behavior.The specimen failure modes,the stress development of stainless steel tube and CFRP wrap,and the load-strain ratio curves in the loading process were obtained.Meanwhile,the relationship between axial and transverse deformations of each specimen was analyzed through the typical relative load-strain ratio curves.A bearing capacity prediction method was proposed based on the twin-shear strength theory,combining the limit equilibrium state of the CFRP-confined CFSST stub column under axial compression.The prediction method was calibrated by the test data in this study and other literature.The results show that the prediction method is of high accuracy.