Axial Load Level Research Articles

A numerical study on the plastic rotation capacity of CFRP-confined rectangular RC columns

To perform a nonlinear analysis of building frames in their seismic performance evaluation program, appropriate force-deformation curves of the structural members in linear and nonlinear phases which represent their actual behavior are required. Although these curves have been provided for common existing RC elements prior to retrofitting in available instructions, the codes are silent about appropriate practical models for strengthened elements. In this regard, a comprehensive numerical study is conducted in the Finite Element (FE) software VecTor2 to investigate the effects of various influential parameters on the moment-rotation behavior of CFRP-confined rectangular RC columns. The investigated parameters are cross-sectional dimensions, longitudinal and transverse reinforcement ratios, the level of column axial load, concrete compressive strength, and the effective confinement provided by external CFRP wraps. Then, through the idealization of the obtained moment-rotation curves, the influence of the aforementioned parameters on the plastic rotation capacity of the columns as one of the main required parameters to define the nonlinear behavior of the retrofitted columns is investigated. Accordingly, column axial load intensity, shear force, effective confinement, and column depth are found to be the major effective parameters on the plastic rotation capacity of the columns. Finally, a practical method consistent with ASCE 41-13 is presented to estimate the plastic rotation capacity of CFRP-confined columns.

Behavior of GFRP strengthening masonry walls using glass fiber composite anchors

This paper presents experimental and theoretical investigations on the short-term behavior of stone masonry walls constructed with lime mortar. The walls were strengthened using unidirectional GFRP (Glass fiber reinforced polymers) sheets to resist out-of-plane loads. Two methods of anchorage were used to enhance the behavior of strengthened stone masonry walls by preventing premature peeling-off of the GFRP sheets caused by GFRP debonding. Nine GFRP strengthened limestone masonry wall specimens including two control specimens were tested under combined constant low axial load levels and out-of-plane bending caused by two point monotonic loading. All the specimens have height-to-thickness (h/t) ratio of 7.1. The main variables investigated were GFRP reinforcement ratio, arrangement of GFRP, axial load level, and anchorage method. The strengthened walls showed an improvement in the out-of-plane performance which was significantly enhanced by anchorage. The increase in wall ultimate load carrying capacity reached 100%.

Analytical Estimation of the Residual Drift of Reinforced Concrete Columns under the Ultimate Displacement Capacity

Reinforced concrete columns are the most important structural elements that determine the ductility of the structures. The main parameters affecting the behavior of reinforced concrete columns are axial load level, shear span, percent of longitudinal reinforcement and percent of transverse reinforcement. The aim of this study is to examine residual damage behavior of RC columns under cyclic loading similar to the earthquake loads combined depend on variable axial load level, spanning to depth ratio, longitudinal reinforcement ratio and transverse reinforcement ratio. When the results of experiments are examined, it can be seen that the residual drift ratio of reinforced concrete columns can be used to characterize the damage occurred in the structure after earthquake or loading. In addition, the performance level of the reinforced concrete columns according to the residual drift ratio is defined in FEMA356. As a result of this study, the analytical equation that calculates the residual drift ratio of the reinforced concrete columns at the ultimate displacement limit is proposed.

Experimental study of RC columns and composite columns under low-velocity impact

In this study, thirty specimens were tested using a drop hammer impact test machine, comprising six reinforced concrete columns and twenty-four specimens of the composite columns. The composite columns include steel tubular-encased reinforced concrete (R-ST) columns, steel-reinforced concrete-filled steel tubular (S-ST) columns, hollow concrete-filled double steel tubular (H-DS) columns, and solid concrete-filled double steel tubular (S-DS) columns. The influences of different axial load levels and different column sections on the responses, such as impact force, the impact duration, the deflection, and the longitudinal strain, are investigated and discussed in detail. The results indicate that compared with the brittle failure of RC columns, all R-ST columns and composite columns behaved in a ductile manner. Moreover, the steel tube can effectively enhance the impact resistance of RC columns. The variation of axial load level has an important influence on the axial load attenuation, deflection, the impact duration, and the plateau value of the impact force. Under the same test conditions, S-DS columns have the best impact resistance with the lowest mid-span displacement, followed by H-DS columns, whereas the impact resistance of S-ST columns is slightly inferior with the highest mid-span displacement among the composite columns.

Experimental Investigation of the Shear Resistance of Concrete Hinges Under a Biaxial Load

Abstract Concrete hinges can withstand extremely high loads and rotations, while requiring only minimal maintenance. Their use is widespread, mainly in bridge construction, but they also find applications in the prefabrication of tunnel segments. With the right design and implementation, they can meet the highest requirements for the durability and resistance of a structure. However, the existing models and design procedures are relatively outdated. The models are based solely on empirical assumptions, whereas the shear resistance of the joint itself plays only a marginal role. The following paper aims to compare existing design models against experimental results in order to find the most suitable design approach that reliably captures the performance of a hinge under a shear load. An experimental program was developed in which 9 samples of concrete hinges were tested for different levels of axial loads and degrees of reinforcement. The results of the experiments were then compared with the selected design models, and a numerical nonlinear analysis was conducted.

Transverse Impact on Hollow and Concrete Filled Steel Tubular Members

This paper clarifies a review of research progress in the transverse impact process on hollow and Concrete Filled Steel Tubular (CFST) members and their dynamic response under impact loading. This review emphases analytical, numerical and experimental studies recently performed by many researchers. It clarifies the influence of some fundamental parameters on the impact performance such as; axial load level, impact energy, steel ratio, constraining (confining factor), steel tube thickness, diameter-to-thickness ratio of the steel tube. In addition to exploring the effect of concrete and steel material properties and specimen boundary conditions. It also goes through the effect of using Carbon Fiber Reinforced Polymer (CFRP) jackets. The review explained with detailed discussions the stages of impact process, impact energy, dissipated energy, failure modes, life-cycle performance of Concrete Filled Steel Tubular (CFST) members subjected to impact load. The influence of strain rates on the performances of concrete and steel are discussed as well.

In-plane seismic performance of masonry wall retrofittedwith prestressed steel-bar truss

An external prestressed steel-bar truss unit was developed as a new strengthening technology to enhance the seismic performance of an in-plane masonry wall structure while taking advantage of the benefits of a prestressed system. The presented method consists of six steel bars: two prestressed vertical bars to introduce a prestressing force on the masonry wall, two diagonal bars to resist shear deformation, and two horizontal bars to maintain the configuration. To evaluate the effects of this new technique, four full-scale specimens, including a control specimen, were tested under combined loadings that included constant-gravity axial loads and cyclic lateral loads. The experimental results were analyzed in terms of the shear strength, initial stiffness, dissipated energy, and strain history. The efficiency of the external prestressed steel-bar truss unit was validated. In particular, a retrofitted specimen with an axial load level of 0.024 exhibited a more stable post behavior and higher energy dissipation than a control specimen with an observed complete sliding failure. The four vertical bars of the adjacent retrofitting units created a virtual column, and their strain values did not change until they reached the peak shear strength. The shear capacity of the masonry wall structure with external prestressed steel-bar truss units could be predicted using the model suggested by Yang et al.

Dynamics of prestressed elastic lattices: Homogenization, instabilities, and strain localization

A lattice (or ‘grillage’) of elastic Rayleigh rods (possessing a distributed mass density, together with rotational inertia) organized in a parallelepiped geometry can be axially loaded up to an arbitrary amount without distortion and then be subject to incremental time-harmonic dynamic motion. At certain threshold levels of axial load, the grillage manifests instabilities and displays non-trivial axial and flexural incremental vibrations. Including every possible structural geometry and for an arbitrary amount of axial stretching, Floquet–Bloch wave asymptotics is used to homogenize the in-plane mechanical response, so to obtain an equivalent prestressed elastic solid subject to incremental time-harmonic vibration, which includes, as a particular case, the incremental quasi-static response. The equivalent elastic solid is obtained from its acoustic tensor, directly derived from homogenization and shown to be independent of the rods’ rotational inertia. Loss of strong ellipticity in the equivalent continuum coincides with macro-bifurcation in the lattice, while micro-bifurcation remains undetected in the continuum and corresponds to a vibration of vanishing frequency of the lowest dispersion branch of the lattice, occurring at finite wavelength. Dynamic homogenization reveals the structure of the acoustic branches close to ellipticity loss and the analysis of forced vibrations (both in physical space and Fourier space) shows low-frequency wave localizations. A perturbative approach based on dynamic Green’s function is applied to both the lattice and its equivalent continuum. This shows that only macro-instability corresponds to localization of incremental strain, while micro-instabilities occur in modes which spread throughout the whole lattice with an ‘explosive’ character. In particular, extremely localized mechanical responses are found both in the lattice and in the solid, with the advantage that the former can be easily realized, for instance via 3D printing. In this way, features such as shear band inclination, or the emergence of a single shear band, or competition between micro and macro instabilities become all designable features. The comparison between the mechanics of the lattice and its equivalent solid shows that the homogenization technique allows an almost perfect representation, except when micro-bifurcation is the first manifestation of instability. Therefore, the presented results pave the way for the design of architected cellular materials to be used in applications where extreme deformations are involved.

Finite-element modelling of concrete-filled steel tube columns wrapped with CFRP

Experimental evaluation of the structural behaviour of concrete-filled tubular (CFT) circular steel columns under the combined effects of axial and cyclic lateral loads is costly and challenging. This study provides a non-linear finite-element analysis (FEA) of 21 models of CFT circular steel columns wrapped with several carbon-fibre-reinforced polymer (CFRP) composite layers at the end region, which represents the critical location in terms of lateral load capacity. The intent is to confine the column end to avoid outward local buckling of the column and thus develop high strength, larger net drift and more energy dissipation. The non-linear FEA models were properly calibrated and validated with reputable experimental results; a parametric study was then conducted to assess the influence of the number of CFRP layers and axial load level on CFT circular steel column performance. Also, it was found that the column axial load level significantly affects the CFT circular steel column behaviour under lateral loading, behaviour improved as the axial load level increased. It takes a parametric investigation, in terms of strengthening system, and the findings of this study provide a useful guideline and methodology for similar strengthening of CFT steel columns.

Experimental and numerical investigation of the creep response of precast concrete sandwich panels

A numerical model is developed in this paper for investigating the time-dependent creep and shrinkage response of precast concrete sandwich panels made with diagonal-bar shear connectors. The material model uses the fixed smeared cracking approach and the modified principle of superposition in its integral form. It is incorporated into the finite element package ABAQUS to build a structural model that considers concrete cracking, tension stiffening, aging, creep, shrinkage and geometric nonlinearity. The time-dependent behaviour is described via a time-stepping analysis. An experimental study that includes testing of four panels under sustained loading for a period of four months is reported. Two panels were subjected to 4-point bending and the other two were loaded under eccentric axial compression. The test results show that creep has led to a reduction of up to 27% in the residual capacity of the axially loaded panels. A good correlation between the numerical model and the experimental results is obtained. The numerical model is further used for conducting parametric studies. The results provide an insight into the critical parameters that govern the time-dependent behaviour of concrete sandwich panels, which include the diameter of the shear connectors, sustained load level, loading eccentricity and concrete strength. It was found that global creep buckling of the investigated panels can occur under eccentric axial load levels that are as low as 60% the load carrying capacity.

Seismic performance of angle-steel reinforced concrete columns confined with spiral reinforcement

Angle-steel reinforced concrete columns internally confined by spiral reinforcement (SCARC) were developed to improve the seismic ductility and energy dissipation capacity. In the present study, cyclic loading tests on nine column specimens were conducted to investigate the seismic performance. The failure process and hysteretic characteristics were examined. The bending moment, deformation capacity and energy dissipation capacity were evaluated through parametric analysis of axial load level, volumetric ratio of spiral reinforcement, transverse steel configuration and types of cross section. A hysteretic model of SCARC columns based on the experimental skeleton curves and hysteretic loops was proposed. Experimental results showed that SCARC columns exhibited more satisfactory seismic performance than angle-steel reinforced concrete columns. The hysteresis characteristics, peak strength, ductility and energy dissipation capacity of SCARC columns were improved due to the increased confinement from spiral reinforcement. The developed hysteretic model provided a reliable prediction of SCARC columns under cyclic loading.

Behaviour of hollow-core and steel wire mesh reinforced ultra-high performance concrete columns under lateral impact loading

Adopting UHPC in practical construction is very expensive due to the high steel fibre content (>2.5 vol%) and passive flexure reinforcement. Aiming at balancing the performance and cost, two UHPC column designs (2000 × 168 × 168 mm) are proposed in the present study. Hollow-core components and steel wire mesh reinforced components were cast with UHPC that contained 1.5 vol% steel fibre, and the impact resistance of both structural types was studied. The test specimens included two hollow-core UHPC columns with square and circular hollow shapes, and two steel wire mesh reinforced UHPC columns with 6 and 10 layers wire mesh reinforcement. The impact scenario was modelled with a 411 kg drop hammer falling freely from 1.25 m height to the mid-span of the test specimen. The results demonstrated that all UHPC specimens remained a flexural response with minimal damage. The developed numerical model captured the impact force, structural deformation and damage with reasonable accuracy. With the validated model, the energy evolution, dynamic shear and moment distribution, residual axial capacity and damage level of post-impact columns were evaluated. The effects of hollow section shape and ratio, axial load level, and longitudinal reinforcement ratio for hollow-core UHPC columns and the effects of layers of steel wire mesh for steel wire mesh reinforced UHPC columns were investigated. Compared with other hollow-core UHPC columns under the impact velocities between 4.95 m/s – 6.64 m/s, UHPC columns with a circular hollow section and 15% hollow ratio was the most effective in balancing the cost and impact resistance. For steel wire mesh reinforced UHPC columns, the column with steel wire mesh strengthening in the whole section had better impact resistance than its counterpart that only had wire mesh reinforcement in the tensile zone.

Betonarme Dairesel Kolonların Eğilme Momenti-Eğrilik ve Hasar Sınırlarının Analizi

In this study; the effect of axial load levels, longitudinal reinforcement ratio, transverse reinforcement diameter and transverse reinforcement spacing were investigated on the moment curvature relationships of reinforced concrete columns. For this purpose, circular reinforced concrete columns having different parameters were designed considering the regulations of the Turkish Building Earthquake Code (2018). The behavior of the columns were investigated from the moment-curvature relation, by considering the nonlinear behavior of the materials taken into account. The moment-curvature relationships of the reinforced concrete column cross-sections having different axial load levels have been obtained by considering Mander model, which considers the lateral, confined concrete strength. Moment-curvature relationships were obtained by SAP2000 Software, which takes the nonlinear behavior of materials into consideration. The designed reinforced concrete cross section models are considered to be composed of three components; cover concrete, confined concrete and reinforcement steel. The examined behavioral effects of the parameters were evaluated by the curvature and moment carrying capacity of the cross-sections. From the obtained moment-curvature relationship, cracking and destruction in cover and core concrete, yield and hardening conditions in reinforcement steel were calculated and the results were presented in charts and graphs. The confining effect in the core concrete is taken into account in the calculations. The behavior of the circular column sections and the types of refraction were interpreted according to the results obtained from the moment-curvature relationship of the sections. It is observed that the variation of the axial load, longitudinal reinforcement ratio, transverse reinforcement diameter and transverse reinforcement spacing have an important effect on the moment-curvature behavior of the reinforced concrete columns. The load bearing capacity of reinforced concrete column sections ends by destruction of the core concrete. Reinforced concrete column sections damaged by reinforcement yield before crushing of cover concrete exhibit more ductile behavior.

Extended P-I diagram method

The pressure-impulse (P-I) diagram method is used in practice (for civilian and military applications) for predicting the level of damage sustained by structures when subjected to blast loads and for assessing the imposed loading regime. Each P-I curve is associated with a certain structural configuration as well as a specific form of blast load and level of damage sustained. When assessing the effect of different parameters (associated with the form of the imposed load and the design of the structure considered) on structural performance, a series of new P-I curves need to be derived. This paper presents an extended P-I diagram method, which is based on derivation of complementary diagrams that can define the effect of two parameters (e.g., the level of axial loading imposed onto a column and the level of damage sustained) on the quasi-static and impulsive asymptotes, thus governing the positions of P-I curves in the diagram plane. The extended P-I diagram method is presented in dimensional and normalised forms. The dimensional form simplifies the derivation of new P-I curves, while the normalised form simplifies the procedure adopted for assessing the behaviour of a certain structure when subjected to a new set of loads. The application of the proposed method is demonstrated in both forms using a typical reinforced concrete (RC) column subjected to a blast load. The column is modelled using finite element analysis capable of accounting for the nonlinear behaviour of concrete and steel. A novel method is proposed for material modelling of concrete. The new material model is validated at both material and structural levels against relevant experimental data. P-I diagrams are initially derived for the axially unloaded column, while complementary diagrams are derived for the column loaded by different axial forces. The framework of the extended P-I diagram method employed for the derivation of new P-I curves and the assessment of the level of damage sustained by the column when subjected to different loading conditions is provided herein.

Investigation of nonlinear behavior of high ductility reinforced concrete shear walls

In this study, the nonlinear behavior of ductile reinforced concrete (RC) shear walls having different parameters was analytically investigated. The purpose of this study is to determine the effect of axial load, longitudinal reinforcement ratio and transverse reinforcement ratios on the moment-curvature and lateral force-lateral peak displacement relationships of RC shear walls. RC shear walls that have various parameters were designed by taking into account the regulation of the Turkish Building Earthquake Code (TBEC, 2018). By considering the nonlinear behavior of the materials, behaviors of the RC shear walls were examined within the framework of the moment-curvature relation. The moment-curvature relations of RC shear walls with different parameters were obtained with the Mander model which takes into consideration the lateral confined concrete strength for different parameters. The effects of the analyzed parameters on the nonlinear behavior of the RC shear walls were evaluated in terms of curvature ductility, moment capacity, peak displacement, the angular displacement and displacement ductility values. It was seen that changes in the transverse reinforcement, longitudinal reinforcement, and axial load levels had important influence on the moment-curvature and lateral force-lateral peak displacement behavior of the RC shear walls.

Testing of URM wall-to-diaphragm through-bolt plate anchor connections

The presence of effective wall-to-diaphragm connections has been shown to significantly improve the global seismic behavior of unreinforced masonry (URM) buildings. However, despite the importance of such connections, there remains a paucity of experimental research to provide physical validation of current recommendations in design standards and guidelines. The experimental study reported herein included a total of 18 tests which were undertaken in two phases, with Phase 1 testing being undertaken on existing vintage plate anchor connections in an existing URM building and Phase 2 testing involving newly installed plate anchor connections in two additional existing URM buildings. The tested buildings offered variation in material properties, levels of axial load, and wall thickness as test parameters. Attained failure modes and corresponding force-displacement curves are presented herein, as well as comparisons regarding the influence of varying test parameters on the ultimate pull-out capacity. Prediction of plate anchor capacity was undertaken using a basic mechanics approach, and comparisons to current strength recommendations in standards and guidelines are provided.

Seismic assessment of non-engineered reinforced concrete columns in low to moderate seismic regions

In under-developed rural areas of Asia, a larger portion of the housing facilities is constructed without any intervention of engineer(s). Such non-engineered reinforced concrete (NRC) structures constitute a major portion of the housing in rural areas. The lateral load resisting columns, in these structures, are constructed by locals using poor quality concrete having compressive strength as low as of 5 MPa and the amount of reinforcement may be lower than 1%. In the case of any earthquake activity, the higher population density and vulnerability of NRC structures would result in the loss of precious human life and economic turmoil. This paper investigates the reversed cyclic behavior of such an NRC column in low to moderate seismic regions of Thailand. Four columns, representative of existing NRC structures, were tested under reversed cyclic loading. Two axial load levels were used to study the effect of axial load on the ultimate lateral load and displacement capacity of the NRC column specimens. To investigate the influence of bar type, 9 mm round rebars and 12 mm deformed bars were used to fabricate the specimens. The maximum lateral drift was found to be extremely low (i.e.,1.75%) under a higher axial load level. The type of reinforcement found to influence the extent of damage in these columns. Finally, a newly developed numerical model; to incorporate the axial-flexure-shear phenomenon, is used to simulate the reversed cyclic behavior of the tested specimen and found to be successful in replicating the observed reversed cyclic behavior.

Parameters affecting the behaviour of steel pipes under impact loading

Subsea steel pipes are often used to form extensive networks for transporting oil and gas over large distances. Such pipes can be subjected to actions characterised by high loading rates and intensities stemming from high-mass low-velocity collisions. To ensure that such networks continue to operate, even after being subjected to impact loads, it is essential that the behaviour of the pipes is characterised by a certain level of resilience. Considering that most of the available data obtained from drop-weight tests on steel pipe specimens does not provide a detailed description of the behaviour exhibited throughout the loading process, the work herein aims at investigating numerically, via dynamic nonlinear finite element analysis, the problem at hand. After validating the predictions obtained against the available test data, the numerical investigation focuses on studying the effect of a range of parameters on the exhibited behaviour. The latter parameters are associated with the shape and mass of the impacting object, the geometry and the support conditions of the pipe, the level of axial loading (applied along the axis of the pipe) as well as the level of internal and/or external pressures imposed onto the pipe walls. The numerical predictions reveal that the above parameters, most of which are associated with the in-situ conditions imposed onto subsea pipes throughout their operational life, can influence, potentially detrimentally, the exhibited behaviour. The numerical predictions obtained reveal that this effect is not accurately captured by the existing assessment methods employed in practice for predicting the level of damage suffered by pipes during impact.

Bidirectional Response of Weak-Axis End-plate Moment Connections: Numerical Approach

Brittle failure mechanisms can affect the seismic performance of structures composed of intersecting moment resisting frames, if the biaxial effects are not considered. In this research, the bidirectional cyclic response of H-columns with weak-axis moment connections was studied using numerical models. Several configurations of joints with bidirectional effects and variable axial loads were studied using the finite element method (FEM) in ANSYS v17.2 software. The results obtained showed a ductile behavior when cyclic loads are applied. No evidence of brittle failure mechanisms in the studied joint configurations was observed, in line with the design philosophy established in current seismic provisions. However, beams connected to the column minor axis reached a partially restrained behavior. Joints with four beams connected to the column exhibited a partially restrained behavior for all axial load levels. An equivalent force displacement method was used to compare the hysteretic response of 2D and 3D joints, obtaining higher deformations in 3D joints with respect to 2D joints with a similar number of connected beams. Consequently, design procedures are not capable of capturing the 3D deformation phenomenon.

Monotonic and cyclic response of hybrid fibre reinforced polymer reinforcing system for reinforced concrete columns under eccentric loading

Near-surface mounted reinforcement system using fibre reinforced polymer bars has been widely considered as an accepted system for strengthening of reinforced concrete columns, particularly with respect to increasing the flexural resistance. It involves cutting grooves into the concrete cover and bonding laminates inside the grooves with fillers (either epoxy resin or cement mortar) ensuring proper bond between fibre reinforced polymer laminate and concrete to prevent premature failure (debonding of laminate). Near-surface mounting does not require extensive surface preparation and takes minimum installation time than externally bonded fibre reinforced polymer. Unlike conventional fibre reinforced polymer jacketing technology, the efficiency of near-surface mounted bars does not depend on the geometry of the column cross-section as well. Previous experimental studies indicate that strengthening using near-surface mounting increases the lateral strength capacity and energy dissipation capacity of reinforced concrete columns. However, the scope of employing a strengthening system for structural retrofits is constrained by the limitations of the material used for strengthening. The lack of adequate confinement results in reduced ductility and energy dissipation capacity for columns strengthened using near-surface mounted technique, particularly under increased loading eccentricities. Jacketing of columns using fibre reinforced polymer increases confinement; however, the efficiency was observed to be reduced at increased loading eccentricities. Similarly, the flexural capacity and drift capacity under low levels of axial load were not observed to be significantly enhanced by the use of fibre reinforced polymer jacketing. Previous studies have indicated that a combination of these two systems could provide effective behaviour for reinforced concrete columns under eccentric loading. Therefore, this research focuses on utilizing a combination of these two methods in the form of a hybrid fibre reinforced polymer reinforcing system consisting of near-surface mounted bars and fibre reinforced polymer confinement to study the structural response of strengthened reinforced concrete columns under eccentric axial compression.