Axial Load Level Research Articles

Performance of recycled aggregate concrete-filled steel tubular columns under combined compression and shear load

This paper presents the behavior of recycled aggregate concrete-filled steel tubular columns under combined compression and shear load. A total of 30 recycled aggregate concrete-filled steel tubular specimens were tested, with their failure modes and shear capacities investigated. The main parameters considered include the cross-section type, the axial load ratio, the shear span-to-depth ratio, the steel ratio and the replacement rate of recycled coarse aggregates. A finite element analysis model is established, which is verified and then used to conduct analysis on the shear performance in terms of the failure modes, the full-range load-deflection relations, the stress distributions and the load transfer mechanism. The results show that both the axial load ratio and the shear span-to-depth ratio have influence on the failure modes. A strut-tie mechanism model is proposed to reveal these influences. Four failure modes are identified, while recycled aggregate concrete-filled steel tubular columns tend to fail in a shear manner with a smaller shear span-to-depth ratio and a higher axial load level. Finally, an existing simplified model is assessed to predict the shear capacity of recycled aggregate concrete-filled steel tubular columns under combined compression and shear load.

Investigation of Deformation-Based Damage Limits of RC Columns for Different Seismic Codes

The seismic performance of reinforced-concrete columns is related to the expected damage limits under seismic loads and how this damage relates to safety of the structure. In order to assess the performance of reinforced-concrete columns under seismic loads, performance-based deformation and damage limits are proposed by the seismic codes. Adequacy of the deformation and damage limit levels given in the codes such as Seismic Evaluation and Retrofit of Existing Buildings Standard, ASCE/SEI-41 (2017) and Turkish Building Earthquake Code (2018) were evaluated by carrying out parametric studies for RC columns. Reinforced-concrete circular columns are designed in parametric studies to present the effects of various parameters such as concrete compressive strength, axial load levels and spiral reinforcement ratio on performance-based damage limits. Performance limits corresponding to each performance levels obtained by different seismic guidelines were compared. When the results obtained from the analyzes are examined, it has been observed that there are significantly different results in the cross-section damage limits values of ASCE/SEI-41 (2017) and TBEC (2018) regulation, which can change the performance level of the building. TBEC (2018) gives approximately 50% conservative limitations when they are compared with the ASCE/SEI-41 (2017) limitations. As a result, TBDY (2018) seems to offer safer and ductile solutions than ASCE ASCE/SEI-41 (2017).

Seismic performance of square concrete-filled steel tubular columns with diagonal binding ribs

The structural performance of square concrete-filled steel tubular (SCFST) columns can be improved by stiffening the tube. Stiffening delays local buckling and can improve composite action while also facilitates the use of thinner and higher-strength materials. A particularly efficient form of stiffened SCFST column using diagonal binding ribs was proposed recently. Initial studies on the axial behavior of these diagonal ribs stiffened SCFST columns have shown excellent performance. However, further investigation is necessary to better understand their seismic performance under combined loading. In this work, four diagonal ribs stiffened SCFST columns were tested to failure under combined constant axial compression and cyclic lateral load. The main parameters in this study were the presence of holes in the diagonal rib, the width-to-thickness ratio of the steel tube, and the level of axial loading. Detailed analysis demonstrated that the effect of holes was slight, while the width-to-thickness ratio and axial load ratio affected the cyclic behavior significantly. Furthermore, based on the verified ABAQUS model, the use of the plastic stress distribution method for computing the interaction strength of short diagonal ribs stiffened SCFST columns was validated among a range of parameters. These parameters are the hole diameter, axial load ratio, material strength and width-to-thickness ratio, including for columns that would be classified as susceptible to local buckling by current design standards. These results provide new knowledge on the behavior of diagonal ribs stiffened SCFST columns and will support the use of this efficient form of composite construction.

Experimental investigation into cyclic working performances of prefabricated CFST columns with improved column-footing connections

Prefabricated concrete-filled steel tube (CFST) column systems with two types of column-footing connections were proposed. Eight circular prefabricated CFST columns with a diameter of 400 mm and a reinforced concrete column were investigated by applying the horizontal reciprocating loads. The investigation compared and analyzed the experimental phenomena, bearing capacity, ultimate drift angle, hysteresis curves, envelope curves, and dissipated energy of prefabricated CFST columns with different column-footing connections (bolt connection and embedded connection), steel tube thicknesses (2 mm, 6 mm, and 10 mm), axial compressive load levels (0.1, 0.2, 0.3) and opening plate configurations. The prefabricated CFST columns with bolt and embedded connections had similar bearing capacity, but the latter had superior ductility and energy consumption performance. The difference in bearing capacity between CFST columns with bolt and embedded connections was only 3.7%, but the ultimate drift angle and dissipated energy of the latter increased by 23.5% and 166.6%, respectively. The steel tube thickness and the axial compressive load level had significant effects on the mechanical properties of the prefabricated CFST columns. When the steel tube thickness increased from 2 mm to 6 mm, the bearing capacity, ultimate drift angle, and dissipated energy of the specimens increased by 64.6%, 34.2%, and 278.2%, respectively. When the axial compression ratio increased from 0.1 to 0.3, the bearing capacity, ultimate drift angle, and dissipated energy of the specimens increased by 18.0%, 43.2%, and 224.0%, respectively. Finally, the experimental bearing capacity of the prefabricated CFST columns was compared with that calculated by the design specification, and this comparison showed that the two column-footing connections can make the prefabricated CFST columns possess excellent integral mechanical properties.

Flexural performance of lightweight aggregate concrete columns

This study examined the effect of the concrete unit weight on the seismic performance of reinforced concrete columns. Three types of columns all-lightweight aggregate concrete, sand-lightweight aggregate concrete, and normal-weight concrete (NWC) were prepared with different amounts of transverse reinforcement and magnitudes of the applied axial load level. Nine columns were tested to failure under constant axial load and cyclic lateral loads. The axial force–moment interactions of column specimens were compared with the predictions obtained by ACI 318–19 procedure using the equivalent stress block and section lamina approach considering the actual stress–strain relationships of various concrete types. The curvature ductility ratios determined from the columns were compared with the ductility-based design procedure specified in EC 8. Test results showed that the moment capacity of lightweight aggregate concrete (LWAC) columns is lower than that of NWC columns, yielding that the safety of ACI 318–19 procedure slightly decreases for columns with a lower concrete unit weight and under lower applied axial load level. The ductility of columns also decreased with a decrease in concrete unit weight, revealing lower values of displacement ductility ratio and equivalent damping coefficient for LWAC columns than NWC columns. Thus, LWAC columns require more transverse reinforcement to achieve a similar seismic ductility to NWC columns under the same design conditions.

Behavior of T-shaped CFST column and U-shaped steel-concrete composite beam joints

This paper intends to study the seismic and shear behavior of joints for T-shaped concrete-filled steel tubular (CFST) column and U-shaped steel-concrete composite beam (USCB). In the joint details, the C-shaped slots cut in the T-shaped steel tube allowed the double-C channels used to connect the thin-walled U-shaped steel (USS) passing through the panel zone directly. A total of six full-scale specimens constructed according to the concept of strong column-weak beam were tested: two specimens were tested under the monotonic loading to obtain the load-carrying capacity and the other four specimens under cyclic loading to evaluate the seismic performance . The variables included the details of USCB, the axial load level, and the beam-column linear stiffness ratio . Test results demonstrated that the proposed joint details exhibited good seismic and shear performance. Three typical failure modes, i.e. welding fracture between the double-C channels and the steel tube, local buckling of the steel tube, and fracture of the double-C channels were observed. Some indexes of seismic performances, such as load-carrying capacity, deformability , degradation of load-carrying capacity, stiffness degradation , and energy dissipation capacity were evaluated. Finally, the joint shear resistance of the specimens was discussed according to the ASCE design guidelines. • An innovative joint details for T-shaped concrete-filled steel tubular column and U-shaped steel-concrete composite beam was proposed. • Some indexes of seismic performances for the joints, such as load-carrying capacity, deformability, degradation of load-carrying capacity, stiffness degradation, and energy dissipation capacity were evaluated. • The joint shear capacity was evaluated according the methods given by the ASCE Task Committee and by the previous findings of the authors.

Assessment of Concrete Filled Steel Tubular Members: An Experimental Review

In this world with rapid development, durable and fast construction techniques lead to the invention of composite materials that are robust and advantageous over conventional materials. Recently invented Concrete-filled steel tubular members are the composite members used in civil engineering works to replace conventional steel and concrete members. This paper deals with an overview of the experimental performance of various types of composite members such as columns, short columns, stub columns, beam-columns, and slender columns deals with various loads such as axial compression, flexural load, cyclic bending, long-term sustained load, torsional load, and impact load. Effects due to the variation in the parameters like steel and concrete strength, diameter to thickness ratio, axial load level, shapes of tubes of these composite members on load-carrying capacity, flexural stiffness, ductility, torsional capacity, and cyclic performance of these members are discussed in this paper. The future scope is mentioned for study related to these composite members in the paper.

Investigation of moment-curvature and effective section stiffness of reinforced concrete columns

In determining the seismic performance of reinforced concrete (RC) structures in national and international seismic code, it is desired to use effective section stiffness of the cracked section in RC structural elements during the design phase. Although the effective stiffness of the cracked section is not constant, it depends on parameters such as the dimension of the cross-section, concrete strength and axial force acting on the section. In this study, RC column models with different axial load levels, concrete strength, longitudinal and transverse reinforcement ratios were designed to investigate effective stiffness. Analytically investigated parameters were calculated from TBEC (2018), ACI318 (2014), ASCE/SEI41 (2017), Eurocode 2 (2004) and Eurocode8 (2004, 2005) regulations and moment-curvature relationships. From the numerical analysis results, it is obtained that the axial load level, concrete strength, longitudinal and transverse reinforcement ratios have an influence on the effective stiffness factor of RC column sections. The calculated effective stiffness for RC columns increases with increasing transverse reinforcement ratio, longitudinal reinforcement ratio and concrete strength. Due to the increase of axial force, effective stiffness values of concrete have increased.

Cast-in-place and prefabricated UHPC jackets for retrofitting shear-deficient RC columns with different axial load levels

Ultra-high performance concrete (UHPC) is a new class of concrete materials that features high strength, ductility, and damage tolerance. This study experimentally evaluated three new UHPC jacketing strategies for retrofitting shear-deficient reinforced concrete (RC) columns, including cast-in-place UHPC jackets with or without steel bar mesh and prefabricated UHPC panels with steel bar mesh. The steel mesh had a high nominal yielding strength of 550 MPa. The impact of different axial load levels on the seismic performance of the retrofitted columns was also explored. Two control columns and five retrofitted columns were tested under lateral displacement reversals. Results showed that while all retrofitting methods enhanced the strength capacity of the shear-deficient column, the UHPC jacketing without steel mesh failed to improve the shear-critical behavior and drift capacity of the column. With the inclusion of high-strength steel mesh in the UHPC jacket, the control columns’ brittle shear failure was significantly improved. The proposed novel UHPC jacketing method with on-site installation of prefabricated UHPC panels showed comparable seismic retrofitting performance to the cast-in-place UHPC jacketing. In addition to the experimental study, suitable strength models, which can reasonably evaluate the strength and failure mode of the RC columns retrofitted using the proposed UHPC jackets, were suggested.

Fire resistance of restrained circular concrete-filled double-skin steel tubular columns

In the past decade, more and more attention has been paid to the fire performance of concrete-filled double-skin steel tubular (CFDST) columns. The columns in actual structures are not isolated members and are affected by the adjacent structural members. However, little research has been conducted on the behaviour of restrained CFDST columns under fire. In this paper finite element models were developed to analyse the fire performance of axially and rotationally restrained circular CFDST columns. The predicted temperature–time curves, internal axial force–time curves and fire resistance agreed well with the related test results. Based on the verified models, a parametric study was performed to investigate the effect on the internal axial force–time curves and fire resistance. The parameters included axial load level, axial restraint ratio, diameter of the outer tube, slenderness ratio, diameter-to-thickness ratios of the outer and inner steel tubes, dimension ratio of the inner tube to the outer tube, load eccentricity, yield strengths of the outer and inner tubes, compressive strength of the concrete, fire protection thickness and rotational restraint ratio. Through regression analysis, a design formula was proposed to calculate the fire resistance of restrained circular CFDST columns.

Behaviour of fully encased composite columns under cyclic loads

AbstractConcrete and steel have different advantages, concrete has high compression strength and low cost while steel has high tensile strength, rigidity and ductility. Composite members have other beneficial qualities beside all of them. Composite columns are used extensively in modern buildings essentially in industrial buildings. The main purpose of this study is an analysis and a comparison between the responses (load‐displacements) of different types of composite columns under cyclic loads. For this purpose, a three‐dimensional (3D) finite element model is developed for the columns, the finite element analysis of composite columns is carried out by using ANSYS code. The numerical model considers the elasto‐plasticity of materials and the non‐linearity of contact friction. It is interesting to observe a large ductility of these composite elements and the efficiency of some types of the composite columns than others. The aim of the study is to analyze the encased composite columns under cyclic loads focused on the different type of steel section. The parameters studied are the influence of axial load levels, skeleton curve, ductility, energy dissipation and the peak lateral load also carried out. The results indicate that the first columns of the specimens have a low specimen energy dissipation due the durability of the columns is higher than the axial load applied, and for better ductility and energy dissipation the axial load level of fully encased composite columns should be limited to approximately a value of 0.6.

Experimental Study on Water-Sand Seepage Characteristics in Fractured Rock Mass under Rheological Effect

This study investigates water-sand bursting disasters associated with fractured rock that affect safe mining in the mining areas of Western China. A broken rock water-sand seepage rheological test device was developed, and rheological tests were conducted on multiple groups of broken rock samples with single-stage axial loading and different load levels. When the rheology of each group of broken rock samples was stable, water-sand mixed fluid was injected into the samples at a certain pressure gradient to conduct water-sand seepage tests on broken rock masses. It was found that when the porosity of a fractured rock mass is within a certain range, the water-sand mixed fluid does not completely pass through the fractured rock mass and some sand particles are filtered by the fractured rock sample. There is an exponential relationship between the sand breaking ability and the sand filtration ability of fractured rock and its initial porosity, and the permeability of fractured rock decreases by a certain extent after sand filtration. However, for different load levels, when the flow through a fractured rock mass tends to be stable, the final porosity of the fractured rock mass decreases exponentially with axial compression. Based on the classical Kelvin rheological model and the basic theory of fractional calculus, a new fractional rheological model has been proposed and the rheological parameters under different load levels were fitted to the model. The new fractional rheological model is better able to describe the rheological characteristics of broken mudstone.

Cyclic behaviour of a novel steel beam-to-prefabricated CFST column connection with threaded sleeve bolts

As the conventional connection configurations of the steel beam to concrete-filled steel tubular (CFST) column are complex and unfriendly in practice, the application of the blind bolting system in steel beam-to-CFST column connection has been investigated in recent years. However, the blind bolted connections to CFST columns are generally classified as being pinned or semi-rigid and of partial strength due to the susceptibility to outward deformation of the tube wall. This paper developed a novel rigid and full-strength bolted connection of steel beam to prefabricated CFST column embedded with threaded sleeve bolts. Tests on five connection specimens are conducted to experimentally study the cyclic behaviour of the proposed sleeve bolted connections. The experimental parameters include the anchorage details of the sleeve bolt, the thickness of the end plate and level of axial load. To investigate the seismic performance of the proposed connections, the failure modes, hysteretic curves, connection rotation and ductility, stiffness and strength degradation, and energy dissipation capacity are presented and analyzed. The testing results prove that the proposed connection details have a strong influence on the cyclic behaviour and the moment-rotation behaviour of the novel connections. According to EC3, the proposed connection configurations behave as being rigid and of full-strength for non-sway frame. The component-based method is also proposed to predict the initial stiffness of the connections. The experimental and analytical results can provide a reference to promote the application of the novel sleeve bolted connections in engineering practice.

Seismic performance of stainless-steel built-up box columns subjected to constant axial loads and cyclic lateral deformations

The seismic behavior of stainless-steel built-up box columns subjected to constant axial loads and cyclic lateral deformation that are used in special moment frames (SMFs) has been investigated through finite element (FE) analysis. A total of 208 numerical models were conducted to cover a range of parameters affecting the seismic behavior of the columns. These include the web slenderness ratio, flange slenderness ratio, global slenderness ratio, axial load level, and lateral loading protocol. The performance indices of overstrength factor, axial shortening, out-of-plane deformation, plastic hinge length, and rotation capacity were calculated and compared. Local buckling of the columns also was investigated and was shown to be responsible for a sharp rise in plastic strain and premature fracture. Predictive empirical formulas are proposed for calculating the plastic hinge length and axial shortening. All previous experimental data on built-up box columns composed of different types of steel materials were collected and added to the FE analysis results to form a reliable database. It was found that ASCE 41-17 conservatively calculated the plastic rotation angles of the built-up box columns. Furthermore, it was indicated that the highly ductile limits of AISC 341-16 for stainless-steel built-up box columns are satisfactory.

EFFECT OF ASPECT RATIO ON THE BEHAVIOR OF GFRP-RC CIRCULAR COLUMNS UNDER SIMULATED SEISMIC LOADING

The mechanical and physical properties of glass fiber-reinforced polymer (GFRP) reinforcement are different from steel, which requires independent code provisions for GFRP-reinforced concrete (RC) members. The currently available code provisions for GFRP-RC members still need more research evidence to be inclusive. For example, the available provisions for confinement reinforcement of FRP-RC columns do not consider the effects of column aspect ratio, which is not yet supported by any available research data. In this study, two full-scale spirally reinforced GFRP-RC circular columns were constructed and tested under concurrent seismic and axial loads. Both specimens had an aspect ratio (shear span-to-diameter ratio) of 7.0, while other two specimens with an aspect ratio of 5.0, from a previous stage of this study, were included for comparison purposes. For each aspect ratio, each specimen was loaded under one of two levels of axial load; 20 or 30% of the axial load capacity of the column section. All test specimens had a 35 MPa concrete compressive strength, 350-mm diameter, 85-mm spiral pitch and 1.2% longitudinal reinforcement ratio. The experimental results were analyzed in terms of hysteretic response, drift capacity and inelastic deformability hinge length. Based on the experimental results, it can be concluded that the aspect ratio affects the magnitude of secondary moments and inelastic deformability hinge length. In addition, the aspect ratio may affect drift capacity of GFRP-RC columns, depending on axial load level.

SEISMIC PERFORMANCE OF GFRP-RC RECTANGULAR COLUMNS: NUMERICAL STUDY

Composite materials like glass fiber-reinforced polymer (GFRP) is becoming widely acceptable to be used as a reinforcing material due to its high ultimate tensile strength-to-weight ratio and excellent resistance to corrosion. However, the seismic behavior of GFRP-reinforced concrete columns has not been fully investigated yet. This paper presents the results of a numerical analysis of full-size GFRP-RC rectangular columns under cyclic loading. The simulated column depicts the lower part of a building column between the foundation and the point of contra-flexure at the mid-height of the column. GFRP reinforcement properties and concrete modeling based on fracture energy have been incorporated in the numerical model. Experimental validation has been used to examine the accuracy of the constructed finite element models (FEMs) using a commercially available software. The validated FEM was used to perform a parametric study, considering several concrete strength values and axial load levels, to study its influence on the performance of the GFRP-reinforced concrete columns under cyclic loading. It was concluded that the hysteretic dissipation capacity deteriorates under high axial load level due to severe softening of the concrete. The FE results showed a substantial improvement of the lateral load-carrying capacities by increasing concrete compressive strength.

Effective flexural stiffness for reinforced concrete shear walls having confined boundary elements

In the design of reinforced concrete (RC) shear walls strength, ductility and effective stiffness of the elements must be taken into account and are important parameters in terms of structural safety. Accurate estimation of the ductility and effective stiffnesses of RC members has always been an attractive subject of study as it provides a reliable estimate of the capacity of buildings under seismic loads. In this study, RC shear wall models with different concrete strength, longitudinal and transverse reinforcement ratios were designed to investigate effective section stiffness and coefficients. The effective stiffness of the cracked section in the RC shear walls designed in different parameters were analytically obtained. Analytically investigated parameters were calculated from TBEC (2018), ACI318 (2014), ASCE/SEI41 (2017) and Eurocode8 (2004, 2005) regulations and nonlinear behaviors. The results obtained according to different design parameters were compared and examined. In the relations suggested for the effective section stiffness coefficient, the confining effect is not taken into account as in the regulations. Therefore, it means neglecting the effects of parameters such as concrete strength, confining effect and axial load levels acting on the section. This situation can lead to unrealistic results in the design and evaluation of RC elements. For this reason, determining the moment-curvature relationship in the design and evaluation of RC elements and obtaining effective section stiffness values are of great importance in order to obtain more realistic results.

Behaviour of cold-formed steel lipped channel sections subject to eccentric axial compression

The use of light-gauge steel framed (LSF) wall systems in low- and mid-rise buildings has increased due to the fast and clean construction and robust structural performance. Cold-formed steel studs used as the primary load-bearing component in LSF walls are often subjected to combined axial compression and bending actions caused by loading eccentricity. Research on the effects of loading eccentricity on the compression capacity has been limited while the currently used AS/NZS 4600 and AISI-S100 DSM design guidelines significantly underestimate the load bearing capacities. In this study, 35 lipped channel columns of 0.6 to 1.5 m lengths were tested under concentric and varying levels of eccentric axial compression loading and the resulting load versus axial shortening curves and failure modes were analysed. Non-linear finite element models of tested columns were developed and validated using the test results. Comparison of the ultimate capacities obtained from the tests and finite element analyses against those predicted by the DSM design guidelines showed that the current DSM design guidelines underestimate the capacities whereas a closer agreement was obtained from the improved DSM design guidelines proposed in a recent research study. This paper recommends the improved DSM design method to enable economical and safe designs.

Assessment of potential strain injury to rectus capitis posterior minor muscles during whiplash type distortions of the cervical spine.

Whiplash type injuries resulting from a rear end motor vehicle accident (REMVA) are thought to be caused by excessive loading and displacement of structural components of the cervical spine. On impact, the seat propels the driver's torso forward relative to the head, resulting in forced flexion of the occipitoatlantal (OA) joint, accompanied by forced stretching of the rectus capitis posterior minor (RCPm) muscles. Flexion of the OA joint and stretching of the RCPm muscles continues to increase until the vehicle's headrest strikes the back of the driver's head. It is known that externally applied forces that attempt to move the OA joint beyond its anatomic barrier can result in fracture, dislocation, or soft tissue damage to its structural components. However, the magnitude of headrest backset, defined as the distance between the driver's head and the vehicle's headrest, that would result in RCPm muscles being stretched to a length that would put them at risk for a muscle strain injury is unknown. To quantify the relationships among flexion of the OA joint, RCPm muscle stretch, and backset, and to quantify the biomechanical response of RCPm muscles to increasing levels of axial load due to stretching. Unembalmed head and neck specimens from three White females aged 85, 63, and 70 years were obtained from the Anatomical Services Division at the University of Maryland. Donors had provided written consent allowing use of their body for research purposes. Using an analytic model of the OA joint, the relationships between flexion of the OA joint and RCPm muscle stretch as a function of backset were estimated. RCPm muscles were removed from the cadavers and forcibly stretched using a servomechanism controlled hydraulic testing machine to quantify the load/displacement properties. After testing, the tissues were sectioned, mounted, and stained using Masson's trichrome to selectively stain muscle fibers red and collagen blue. Forced flexion of the OA joint was seen to be directly related to the magnitude of headrest backset. For values of backset greater than 7.2cm, biomechanical testing of the RCPm muscles revealed that strain injuries ranged from the tearing of a few muscle fibers to complete rupture of the muscle and separation of the tendon at the posterior process of C1. Results showed that headrest backset at the time of vehicle impact is an important factor in estimating the risk of muscle strain injury to RCPm muscles. Muscle strain injury would be expected to impact the functional relationship between the RCPm muscles and the pain sensitive spinal dura. Physicians should be alert to the possibility that cervicogenic pain patients who have experienced whiplash associated with REMVA may show clinically relevant structural damage to the RCPm muscles on MRI.

The Proposition of an EI Equation of Square and L–Shaped Slender Reinforced Concrete Columns under Combined Loading

The stability and strength of slender Reinforced Concrete (RC) columns depend directly on the flexural stiffness EI, which is a major parameter in strain calculations including those with bending and axial load. Due to the non-linearity of the stress-strain curve of concrete, the effective bending stiffness EI always remains variable. Numerical simulations were performed for square and L-shaped reinforced concrete sections of slender columns subjected to an eccentric axial force to estimate the variation of El resulting from the actual behavior of the column, based on the moment-curvature relationship. Seventy thousand (70000) hypothetical slender columns, each with a different combination of variables, were used to investigate the main variables that affect the EI of RC slender columns. Using linear regression analysis, a new simple and linear expression of EI was developed. Slenderness, axial load level, and concrete strength have been identified as the most important factors affecting effective stiffness. Finally, the comparison between the results of the new equation and the methods proposed by ACI-318 and Euro Code-2 was carried out in connection with the experimental results of the literature. A good agreement of the results was found.