Load Capacity Of Columns Research Articles

Performance of axially loaded reinforced concrete rectangular columns strengthened with GFRP strips

Abstract Sticking of glass fibre cloth around a column using epoxy resin is one of the solutions for column strengthening. This paper details a study on short reinforced concrete (RC) rectangular columns having rounded edges with axial load strengthened using bi-directional Glass Fiber Reinforced Polymer (GFRP) cloth glued with epoxy resin. The results obtained, indicate a notable increase in axial load capacity of RC rectangular column when wrapped with GFRP fabric around the column and stick with epoxy resin as glue. Wrapping completely around the column with strips is better than wrapping with single continuous fabric. Considering the width of strip, strips having more width are giving finer results than narrow width strips. When strips of two or more layers are wrapped, then staggering of strips is even better than sticking one strip merely on another. The axial load capacity is increased by the use of two layers of GFRP strips and sticking three layers of GFRP strips showed a decrease in the axial load capacity.

Eccentric compressive behavior of steel fiber-reinforced RC columns strengthened with CFRP wraps: Experimental investigation and analytical modeling

Today, one of the most commonly used retrofitting techniques for reinforced concrete (RC) columns for enhancing their structural performance is to externally confine these columns with carbon fiber-reinforced polymers (CFRPs). From another aspect, the presence of steel fibers in the concrete mixture enhances the behavior of structural members. In this research, the eccentric compressive behavior of steel fiber-reinforced concrete columns strengthened with CFRP wraps, which has remained mainly under-researched, is explored experimentally and analytically. For that end, eighteen RC columns with plain concrete and fiber-reinforced concrete, confined and unconfined with CFRP sheets, length to width ratios (L/h) of 4.2 and 8.3, and load eccentricity ratios (e/h) of 0.29, 0.46, and 0.63 were manufactured and tested under eccentric compressive loading. The findings indicated that the CFRP sheets improved the loading capacity and ductility of the RC columns, while the presence of steel fibers in the concrete affected the loading capacity of the columns negligibly but made them more ductile. Further, the eccentricity of the applied load mitigated the effect of confinement with CFRP sheets on the reinforced concrete strength. Regarding the significance of the performance of these columns, an analytical model with the purpose of predicting the behavior of fibrous RC concrete columns strengthened with CFRP sheets under an eccentric compressive load was developed. In this model, by considering the simultaneous effects of the confinement with CFRP sheets and transverse reinforcement, the moment–curvature and the load–deflection diagrams were determined using the fiber section analysis method. Assessing the analytical findings against the experimental ones of the present work and those of others indicated that the proposed model is able to properly predict the behavior of fibrous concrete columns confined with transverse reinforcement and CFRP sheets under eccentric compressive loading.

Axial compression test of basalt fiber reinforced polymer reinforced coral reef and sand aggregate concrete column

In view of the insufficiency of durability of ordinary reinforced concrete structures in severe corrosive marine environments in South China Sea Islands, the basalt fiber reinforced polymer (BFRP) bar was proposed to reinforce coral reef and sea sand concrete. Axial compression test on 15 columns was conducted. The failure mode, load-displacement relationship and load-axial strain relationship were documented during the loading process. Test results indicate that, the failure of coral reef sand concrete initiates from the low strength coral reef aggregate, and finally experiences total failure at both aggregate and interface. The development law of column failure is characterized as three stages. The load capacity of BFRP coral reef and sea sand column is equivalent to that of steel bar.

An analytical model for the loading capacity of splice-retrofitted slender timber columns

Retrofitting timber columns in traditional timber structures with a steel jacketed splice joint has advantages of aesthetic appearance and similar mechanic performance to the intact columns as compared to conventional simple splice columns. The axial compression behavior of such retrofitted splice columns has been studied experimentally in detail. However, there is still a lack of a calculation model for their axial compressive strength and general guideline for their design. The objective of this study is to establish a theoretical calculation model for this type of retrofitted splice columns. Firstly, a theoretical model for the axial compressive strength of splice columns retrofitted with a steel jacket is proposed considering the contact stresses at a splice joint and the relevant stability theory. Secondly, the buckling modes of splice columns and the actual stress distributions at the splice joints (i.e. the compressive stresses at the steel-timber and timber-timber interfaces) are thoroughly investigated. Finally, the theoretical model is validated by the experimental data and finite element analysis results with different splice parameters. Comparisons show that the theoretical calculations in terms of the bearing capacity and stability coefficient agree well with the experimental results. The proposed theoretical model is also shown to be suitable for predicting the axial compressive strength of a retrofitted splice column with the location of the splice from the column end ranging from 1/5 to 1/2 of the column length. The relative errors in the theoretical bearing capacities with respect to the finite element results are found to be less than that using the stability coefficient. From the analysis results, the length of the splice and the total length of the steel jacket are recommended to be in the range of 0.5~1.5 and 2~4.5 times of the column diameter, respectively. This proposed theoretical model can be applied in the retrofitting design of timber columns in historical timber structures, and it can also be applied in the development of new large-space timber structures where splice columns may be incorporated.

Experimental Behavior of Cracked Reinforced Concrete Columns Strengthened with Reinforced Concrete Jacketing.

Reinforced concrete (RC) columns often need to be strengthened or rehabilitated to allow them to carry the loads applied to them. In previous studies, RC columns have been strengthened by jacketing, without considering the occurrence of cracking. In this study, the behavior of RC columns strengthened externally by jacketing after cracking is analyzed. The accuracy of the existing models was verified by analyzing the performance of fifteen RC columns with different cross-sections to determine the effect of new variables, such as the column size, amount of steel reinforcement, and whether the column was cracked or not, on the effectiveness of strengthening. The analysis demonstrated that this strengthening technique could effectively improve both the ductility and strength of RC column cross-sections. The results indicate that the model suggested by the ACI-318 code can predict the ultimate load capacity of RC columns without strengthening, or strengthened by RC jacketing before or after cracking, with higher accuracy and material efficiency. The RC columns without strengthening met the safety limit of the ACI-318 model. However, for strengthened columns, a reduction coefficient must be used to enable the columns to meet the safety limit, with values of 94% and 76% for columns strengthened before and after cracking, respectively. Furthermore, strengthening after cracking affects the ultimate load capacity of the column, with 15.7%, 14.1%, and 13.5% lower loads for square, rectangular, and circular columns than those strengthened before cracking, respectively.

Numerical modeling and design sensitivity of structural and seismic behavior of UHPC bridge piers

Advanced behavior of Ultra-high performance concrete (UHPC) is attracting a growing interest among the construction industry worldwide. Currently, UHPC is commonly used in precast bridge deck joints and connections. As the UHPC market grows, the material will have a great potential to extend to larger structural applications. However, there is no guidance yet on best practices or optimum structural designs that fully utilize the UHPC superior mechanical properties for larger components. The objective of this study is to better understand the overall behavior, failure mechanism, and effect of reinforcement and design details of UHPC bridge columns using detailed finite element modeling. The pushover response of a two-column bridge pier with the typical geometry and gravity loads obtained from a representative California bridge is investigated when UHPC is used instead of conventional concrete for the columns. A detailed sensitivity/parametric analysis is conducted to assess the effect of different steel fiber ratio, longitudinal reinforcement ratio, and steel grades of reinforcement bars on the overall structural behavior of the columns. The total strain crack model, as readily implemented in DIANA FEA, is utilized with user-defined input to model the UHPC constitutive material behavior. The UHPC stress-strain relationships in tension and compression are independently defined using uniaxial curves from the literature. For comparison, the two-column bent of conventional concrete is modeled and used as the reference case to evaluate the relative increase in load capacity of the UHPC bridge columns.

Effect of MS Rings’ Confinement on Lateral Deformations of M 25 Grade Circular RCC Columns under Axial Compression Loads

Confinement has been always a key concern area for researchers in present and in past also. The metal confinement of RCC columns has been extensively used from long time like concrete-filled steel tube (CFST) columns etc. In the present work mild steel rings have been used as confining material. In this paper based on experimental work, it is aimed to improve the axial compression strength and lateral deformation characteristics of circular RCC columns confined by mild steel (MS) rings. Total 45 nos. of specimen of size 150 mm dia. and 300 mm height were prepared during the experimental work. These specimen were tested and results were analyzed.These MS rings confined circular RCC columns of M 25 grade concrete were experimentally studied for different variables like (i) % of column main vertical steel bars (ii) thickness of MS rings (iii) spacing of MS rings. It was found that the MS ring confinement effectively helped in reducing lateral deformation of circular RCC column specimen resulting in improved axial compressive load capacity of circular RCC columns also. As MS rings are made up of conventional material i.e. mild steel pipes, the technique has a vast application area. In rural part of India this technique can be conviniently used for the efficient confinement of RCC columns.

THEORETICAL INVESTIGATION OF CONCRETE FILLED STEEL HOLLOW SECTION COLUMNS

Steel concrete composite columns are widely used in high rise buildings and bridges. Composite columns provide not only great reduction for the column size and weight, but also high columns efficiency and ductility. In Concrete Filled Steel Tube (CFST) columns, the steel tube provides formwork for the concrete; the concrete prevents local buckling of the steel tube. This paper illustrates a theoretical study of the behavior of CFST columns subjected to centric load by using ANSYS (18.1). The study divided into two parts. In the first part, the verification of the program was done using an experimental work of previous research, this verification showed that the capability of ANSYS program for analysis of the CFST columns with very good accuracy. In the second part: a parametric study was carried out to study the behavior of CFST braced, constant cross section area, centric loaded columns. The main variables were the shape of cross section (three shapes: square, rectangular and circular) and slenderness ratio (λ). The analysis shows that the increase of the slenderness ratio led to decrease the ultimate load capacity, stiffness and ductility of the CFST columns. The current ECP of concrete code underestimates the values of ultimate load capacity while, ECP of steel predicts the reasonable values for an ultimate load capacity of CFST columns.

The effect of high-temperature creep on buckling behaviour of aluminium grade EN6082AW T6 columns

The paper presents an experimental study that investigates the influence of high-temperature creep on reduction of the buckling load capacity of aluminium grade EN6082AW T6 columns. The study was performed by performing constant-temperature capacity and creep tests on 17 column specimens of approximately 2.6 m length. A total of eight capacity tests and nine creep tests were carried out. Results obtained within the study have revealed a critical temperature interval of 160–260 °C within which high-temperature creep significantly influences the columns' buckling load capacity. The load level at which high-temperature creep influences the reduction of columns' buckling-load capacity, by exhibiting low short-term creep resistance, is above 90% of the column's axial load capacity. The study provides relevant thermo-mechanical criteria for the assessment of creep-induced buckling of the tested aluminium alloy, in the form of the load level intervals at prescribed temperatures, which induce column buckling within the 240 min interval that is generally considered as a relevant fire resistance period for building structures over 18 m in height.

Partially encased composite columns using fiber reinforced concrete: experimental study

This paper addresses the results of an experimental study involving 10 partially encased composite columns under concentric and eccentric compressive loads. Parameters such as slenderness ratio, ordinary reinforced concrete and fiber reinforced concrete, load eccentricity and bending axis were investigated. The specimens were tested to investigate the effects of replacing the ordinary reinforced concrete by fiber reinforced concrete on the load capacity and behavior of short and slender composite columns. Various characteristics such as load capacity, axial strains behavior, stiffness, strains on steel and concrete and failure mode are discussed. The main conclusions that may be drawn from all the test results is that the behavior and ultimate load are rather sensitive to the slenderness of the columns and to the eccentricity of loading, specially the bending axis. Experimental results also indicate that replacing the ordinary reinforced concrete by steel fiber reinforced concrete has no considerable effects on the load capacity and behavior of the short and slender columns and the proposed replacement presented very good results.

Practical Hybrid Machine Learning Approach for Estimation of Ultimate Load of Elliptical Concrete‐Filled Steel Tubular Columns under Axial Loading

In this study, a hybrid machine learning (ML) technique was proposed to predict the bearing capacity of elliptical CFST columns under axial load. The proposed model was Adaptive Neurofuzzy Inference System (ANFIS) combined with Real Coded Genetic Algorithm (RCGA), denoted as RCGA‐ANFIS. The evaluation of the model was performed using the coefficient of determination (R2) and root mean square error (RMSE). The results showed that the RCGA‐ANFIS (R2 = 0.974) was more reliable and effective than conventional gradient descent (GD) technique (R2 = 0.952). The accuracy of the present work was found superior to the results published in the literature (R2 = 0.776 or 0.768) when predicting the load capacity of elliptical CFST columns. Finally, sensitivity analysis showed that the thickness of the steel tube and the minor axis length of the elliptical cross section were the most influential parameters. For practical application, a Graphical User Interface (GUI) was developed in MATLAB for researchers and engineers and to support the teaching and interpretation of the axial behavior of CFST columns.

Investigation on UPVC confined RC columns with Recycled Aggregate Concrete using C&D waste

Recycled Aggregate Concrete (RAC) is attracting world-wide research interest. Despite its promising impact on environment and economy, use of RAC has been limited to non-structural purposes mainly due to its poor performance (such as lower strength/stiffness, poor durability, higher creep, shrinkage). A possible approach to use RAC as a structural element is confinement of RAC. This study aims at the introduction of a new sustainable composite material which is reinforced RAC filled unplasticized polyvinyl chloride (UPVC) tubular columns. In this present investigation, 81 cylindrical specimens consisting of 54 confined specimens and 27 unconfined specimens have been examined under axial compression. It is found that UPVC tubes can be used effectively for good confinement of reinforced RAC columns as it enhances load capacity and ductility of RAC columns significantly. Capacity equation from Eurocode 4, as well as various confining models available for conventional concrete confined by steel tubes, have been modified and used to calculate peak compressive load of RAC filled UPVC tubular columns in order to compare and assess its potentiality & suitability in the understanding actual behavior of such a system.

Experimental Study on Performance of Fiber Concrete-filled Tube Columns under Axial Loading

There have been great developments in the area of civil engineering in the recent few decades and among these, construction and material innovation are quite prominent. Steel-concrete composite construction has emerged as one of the fastest methods of construction. Even though considerable research efforts on conventional reinforced concrete columns have been executed prior to now, concrete filled steel tube (CFST) composite columns however have received limited attention. This work aims to study the experimental behavior of steel tubular specimens. Plastic and steel specimens are considered with circular and square sections filled with the concrete with the steel fiber and as well, plain concrete. Four parameters are considered in this study which are sectional designs (circular and square), tube thickness (2 and 5 mm), tube material (plastic and steel) and content of steel fiber (0 and 5%). Ten concrete filled steel tubular columns were cast and tested. Two circular columns were made from plastic and the other made from steel. The main purpose of this work are study the effect of steel fiber and cross section on the ultimate load capacity of columns. It was discovered the utilization of steel fiber reinforced concrete filled steel tubular columns have comparatively substantial stiffness in comparison with plain concrete filled columns.

Assessment of high-strength concrete encased steel composite columns subject to axial compression

This paper investigates the axial load capacity of concrete encased steel composite stub columns with high strength concrete and steel materials. A total of 14 column specimens with varying material strengths and different steel section shapes were tested under concentric compression load. The test results revealed that the design methods in EN 1994-1-1 and JGJ 138-2016 overestimate the axial load capacity of high strength concrete encased steel columns. Adding a small percentage of steel fiber in the high strength concrete was found to improve the compression resistance of the composite section. A new test database consisting of 51 partially encased composite sections and 82 fully encased composite sections was established, covering a wide range of section geometric and material grades. Parametric study was then carried out to assess current design methods in predicting the axial capacity of such composite columns with respect to material strengths, steel contribution ratio, reinforcement ratio, section slenderness ratio, confined concrete area ratio, and concrete confinement efficiency. The effectiveness of concrete confinement in partially encased composite column was evaluated and a simplified method was proposed to compute the enhanced concrete strength based on regression analysis. For fully concrete encased composite columns, a concrete strength reduction factor was proposed to be used with EN 1994-1-1 to predict the compression resistance. Design recommendation was made considering the early cover spalling of high strength concrete and the material compatibility between steel and concrete in composite column design.

Mechanical behaviour of seawater sea-sand recycled coarse aggregate concrete columns under axial compressive loading

The use of locally available seawater and sea-sand to produce seawater sea-sand concrete (SSC) is promising in marine and coastal projects. This paper investigates the mechanical behaviour of SSC columns that are prepared with recycled coarse aggregates (SSRAC). The results demonstrate that the seawater-sea-sand-based concrete column behaves similarly to conventional natural aggregate concrete (NAC) when the shell content in sea-sand equals 2.31%. SSRAC columns outperform RAC columns in terms of strength and deformability with a peak load capacity that is approximately 17% higher. Based on the monolithic specimens, the SSRAC composite concrete column is realized by confining the SSRAC core using steel-reinforced NAC or an engineered cementitious composite (ECC) shell. Compared with monolithic specimens, the peak load capacity of the NAC-SSRAC composite column is similar, whereas that of the ECC-SSRAC column is 15% higher. It is feasible to use the ECC shell without stirrups to confine SSRAC. Furthermore, the applicability of available stirrup-confined concrete stress-strain models on the SSRAC and its composite concrete columns is evaluated.

Ultimate Bearing Capacities of Concrete Filled Hybrid Composite Tubular Short Columns

The new reinforcement material, Hybrid Composite (HC), is new innovative material and made of carbon fiber or other fibers in the form of tubular weave or braid. HC tubes has been used in column and pier to restrict the lateral expansion of concrete to improve the compression performance, while protecting the column as well. In this paper, the mechanical behavior of HC reinforced concrete short columns under axial and eccentric compression loading is analyzed. By testing the model and analysis the result using finite element software ABAQUS, the displacement, and load capacity of all specimens are collected and analyzed in terms of the number of HC layers and eccentricity ratios. The results show that the ultimate load capacity of columns increases with the number of HC layers under axial compression, and the ductility of the reinforced columns was significantly improved by HC reinforcement, compared with plain concrete columns.

Experimental study on high strength concrete encased steel composite short columns

This paper investigates on the axial compressive behavior of high strength Concrete Encased Steel (CES) composite short columns. Six column specimens made of high strength concrete were tested under monotonic pure compression. The structural performance was investigated including failure pattern, load-carrying capacity, load-deformation response, and post peak ductility. The major parameters affecting the ultimate strength of such composite columns are studied including concrete strength, spacing of transverse reinforcement bars, and the inclusion of steel fibers in the high strength concrete. Based on the experimental work, it is found that the failure mode of CES columns with high concrete strength C120 is characterized by concrete cover spalling, which triggers a sudden drop of compression capacity once the peak resistance is reached. With the decrease of transverse reinforcement spacing, the ultimate load capacity of high strength CES columns can be enhanced due to better concrete confinement but the failure is still brittle with abrupt unloading from the peak load. The test results also indicated that the provision of 0.5% volume of steel fiber is able to prevent the concrete cover spalling, and hence the load-carrying capacity can be enhanced. Finally, the test results together with additional test data collected from the literature are used for comparison with codes’ prediction using the EN 1994-1-1 and JGJ 138-2016. The code methods give excellent prediction of CES columns with normal strength steel and concrete grade up to C90, but they over-estimate the maximum resistance of CES columns with concrete strength higher than 90 MPa. To extend the current design methods for CES members with higher strength concrete, two analytical methods were proposed which shows good correlation with test results, thus enabling the design of CES members with higher strength of concrete.

Experimental investigation on steel-tube-confined-concrete stub column with different cross-section shapes under uniaxial-compression

An experimental investigation on steel tube confined concrete (STCC) stub columns with circular, square and octagonal cross-section shapes under monotonic axial compression was conducted. Different concrete grades were examined with measured compressive cylinder strengths ranging from 38 MPa to 112 MPa. The nominal yield strength of the steel tube was 355 MPa. The load capacity, load-axial shortening curves and axial-hoop strain behaviour of the STCC stub columns are compared to that of the counterparts, concrete filled steel tubes (CFST), with different cross-section shapes based on the same material and geometric properties. Comparisons indicated that the load capacity of STCC stub columns are very close to that in the CFST with circular and octagonal cross-sections, while the performance of square STCC is not as good as the CFST counterparts due to low effectiveness of confinement. According to the experimental results of lateral behaviours of STCC, CFST and plain concrete columns, it was concluded that with very high strength concrete (cylinder strength over 100 MPa), the axial performance of STCC stub columns is better than that in CFST. A design assessment was also conducted on STCC stub columns and shows that the current design formulae for load capacity of CFST are also applicable to that of STCC stub columns.

Experimental evaluation of high-strength concrete-filled steel rectangular tube columns under axial compression

This paper presents the effect of the high-strength concrete on the behavior and strength of concrete-filled steel tube (CFST) columns. To evaluate the structural performance, load-axial shortening responses, failure modes and axial load capacity are performed for ten high-strength concrete-filled steel tube (HSCFT) columns. All of the columns are 750 mm in height, and have compressive strength of 66 MPa. Test parameters included the wall thicknesses of the rectangular tube (3.0 mm, 4.5 mm and 6.0 mm) and three different of rectangular cross-sections (150×50 mm, 150×75 mm and 150×100 mm). From the experimental results, it is indicate that the load-axial shortening response of the columns have a linear behavior up to approximately 80-90% of their axial load capacity. Then, the behavior of the HSCFT columns is nonlinear with a very large deformation before failure. It is also shown that the axial load capacity and ductility of the HSCFT columns are increased significantly compared to the RC columns (reference columns), depending mainly on the wall thickness and cross-section of the hollow steel tube. Finally, by comparing the axial load capacity of the test results with those obtained from the AISC/LRFD design equation, the comparison results show that calculation formula in AISC/LRFD specification can be applied to compute the axial load capacity of HSCFT columns under axial compression.

Analysis of fire-damaged concrete-filled steel tube columns confined with FRP sheets

This paper presents an analysis of fire-damaged circular concrete-filled steel tube (CFST) stub columns confined with fiber-reinforced polymer (FRP) sheets under compression. Three main components in the analysis include (1) 2D heat transfer analysis to predict the temperature distributions in the CFST cross section under ISO-834 standard fire, (2) Post-fire mechanical properties of steel and concrete, and (3) Analytical model that incorporates the confinement effect to predict the load capacity of FRP-confined fire-damaged CFST columns. After validated with existing experimental results, the proposed analysis is employed to investigate the effects of heating period and number of FRP layers on the effectiveness of the FRP confinement method.