Variable Axial Load Research Articles

Hysteresis Model of RC Column Considering Cumulative Damage Effect under Variable Axial Load

Hysteresis Model of RC Column Considering Cumulative Damage Effect under Variable Axial Load

Experimental study on seismic behavior of reinforced concrete exterior beam-column joints under varying axial load

The majority of low-cycle loading tests on reinforced concrete (RC) beam-column joints were currently conducted under constant axial loads. During earthquake shaking, the axial load on the joints fluctuates due to the overturning moment and vertical ground vibration. Particularly in exterior beam-column joints, where beam shear occurs on only one side, the range of axial load variation is significantly larger. To elucidate the impact of varying axial loads on the seismic performance of exterior beam-column joints, low-cycle loading tests were conducted on four groups of full-size beam-column subassemblage specimens. Two identical specimens from each group underwent low-cycle loading tests with constant and varying axial loads, respectively. Based on the experimental findings, this paper examines the differences in cyclic behaviors among comparison specimens, focusing on damage characteristics, joint shear strength, stiffness degradation, displacement ductility, and longitudinal bar slip. The study suggests that compared to specimens subjected to constant axial loads, the comparison specimens exhibit variations in load-carrying capacity corresponding to the fluctuations in axial load. Furthermore, varying axial loads accelerate post-peak stiffness and strength degradation, leading to reduced ductility. Additionally, specimens in one of the comparison groups exhibited different failure models when subjected to varying axial loads and constant axial loads, specifically beam failure and joint failure, respectively. Numerical simulations of the corresponding beam-column subassemblages were conducted on the Opensees platform to elucidate how the two axial loading methods affect the transformation of failure modes. A subsequent parametric study focusing on the axial compression ratio was performed to further explore the impact of varying axial loads on the seismic behavior of beam-column joints.

Fire performance of aged and corroded reinforced concrete columns

The existing guidelines for determining the fire ratings of reinforced concrete (RC) columns only take into account undamaged components, without considering any deterioration caused by age or corrosion. An experimental study was conducted to examine the effect of corrosion on the fire performance of RC columns. In this study, six full-scale RC column specimens were cast, corroded, and tested in fire conditions, consisting of two made of normal strength concrete (NSC) and four made of high-strength concrete (HSC). Out of these six, three RC columns (1-NSC and 2-HSC) were corroded using a specifically designed accelerated corrosion setup for a target mass loss of 30 %. After the completion of the accelerated corrosion exposure time, these three corroded column specimens, along with three other non-corroded control column specimens, were tested in a fire furnace, simulating the ISO-834 fire curve. It was found that due to corrosion exposure, the steel reinforcement underwent significant mass loss, and severe corrosion cracks were observed. These cracks were discovered to serve as convenient channels for heat transfer. This facilitated the earlier and more severe spalling of the concrete cover, ultimately leading to premature deterioration in the strength of the corroded RC columns during the fire. Consequently, this contributed to the catastrophic failure of the corroded RC columns, with a significant reduction in fire ratings- 44 % for NSC columns and 40.49 % for HSC columns. This paper also addresses the spalling characteristics of the corroded columns and their structural responses, including axial deformations, lateral deformations, and load variations.

Seismic response assessment of ductile reinforced concrete columns affected by corrosion and axial load variations

The present study investigates the effect of reinforcement corrosion and axial compression ratio (ACR) on the seismic response of large-scale ductile reinforced concrete (RC) columns. 3D numerical models of sound and corroded RC columns were developed and validated with detailed experimentation. Various parameters, such as hysteresis and backbone curves, stiffness degradation, ductility, and energy dissipation, were evaluated for all the specimens. The numerical findings were then compared with the experimental findings. Subsequently, a thorough parametric study was executed to analyse the structural behaviour under varying corrosion degrees and ACR level combinations. It was observed that both sound and corroded specimens experienced significant degradation in terms of their strength, deformability, stiffness, and ductility due to elevated ACR levels. Moreover, increased ACR levels in both specimen types led to a decline in pre-peak and post-peak response trajectories, resulting in an early achievement of peak response at lower drift levels. The study’s outcome advocates the significance of incorporating ACR levels into the design philosophy of ductile reinforcements for RC columns.

Axial transfer characteristics of a drillstring in conjunction with bit penetration behavior in horizontal wells

Nonlinear friction and interaction between bit and rock often result in a lower weight on bit (WOB) than the intended design value. The analysis of the dynamic transmission mechanism of a drillstring in complex horizontal wells is a crucial engineering problem that aims to enhance drilling efficiency. In this paper, a mechanical model was developed, integrating nonlinear bit penetration behavior and variable axial loads, aiming to unveil the nonlinear axial vibration characteristics and elucidate the transmission mechanisms within the drillstring. Notably, the mean value of axial force distributed along the drillstring is dictated by wellhead pulling force, while the amplitude is shaped by the interaction between bit and rock, and rotary speed. In calculated examples for horizontal wells, the analysis reveals that significant losses in power axial transmission, resulting in WOB is only 38% of the intended value. And reducing the wellhead pulling force by 30 kN leads to a maximum increase in WOB only of 9.51 kN. Extended region in drillstring with negative axial forces exhibit maximum values 5–6 times higher than the WOB. Specific zones, such as 20 m–40 m and 0–20 m from the bit, experience axial force fluctuation amplitudes of 40.5% and 138.8% of the mean value respectively, with the latter identified as an intense vibration zone, while the acceleration values 0–10 m from the bit signal a high-risk zone. In prolonged horizontal section, stability issues manifest, with spiral buckling observed in the holding section and sinusoidal buckling in the horizontal section. Mitigating the synchronous vibrations between the bit and the surrounding drillstring can effectively reduce the intensity of axial vibration. To ensure successful drilling to target formation, adjusting the wellhead pulling force to keep the WOB in the range of 60–90 kN and limiting the rotary speed to 90 r/min are recommended practices. This comprehensive analysis offers valuable insights for optimizing drilling parameters, pinpointing high-vibration zones, and mitigating risks associated with drilling, providing a holistic approach to enhancing drilling efficiency.

Effect of variable axial load with different amplitude, frequency and phase on seismic behavior of RC columns: Experimental study and numerical simulation

The aim of this study is to investigate the effects of variable axial loads caused by horizontal and vertical ground motions on the seismic performance of reinforced concrete columns. First, three groups of cyclic loading tests were conducted. Further, the effects of constant axial load, axial load amplitude variation and frequency variation on the seismic performance indicators of the specimens, such as the failure mode, hysteresis curve, skeleton curve, stiffness degradation, ductility and energy dissipation, were studied. Subsequently, numerical analysis models were established using fiber finite elements, and verified their accuracies using experimental data. Finally, parametric analyses were conducted based on the verified numerical models to examine the effects of variable axial loads with different amplitudes, frequencies and phases on the columns. The results indicate that the axial load ratio of the columns changes constantly under the variable axial load, which results in readily apparent asymmetry in the hysteresis curve, ductility, and ultimate load. The hysteresis behavior of columns under variable axial load mainly depends on the axial load ratio corresponding to the positive and negative peak lateral drifts of each cycle. An equivalent axial load ratio method is proposed to determine the hysteresis model of columns under variable axial loads accurately.

Static analysis of the hysteretic performance of the central columns in subway station enhanced by engineered cementitious composite jacket under high variable axial force

Engineered cementitious composite (ECC), a cement-based composite material known for its high tensile strain and ductility, was utilized to enhance the seismic performance and operational safety of the central columns in vulnerable areas of subway stations. By establishing refined solid finite element models of ECC composite columns and ordinary concrete columns, and conducting dynamic time history analysis on the vertical axial force at the top of the column, a comparative analysis was performed using a static model to determine the difference in hysteretic performance between ECC composite columns and ordinary concrete columns. Furthermore, sensitivity analysis is conducted based on different structural parameters. The results demonstrate that ECC composite columns exhibit significant improvements in bearing capacity, ductility, and energy dissipation, while also reducing stiffness degradation compared to ordinary concrete columns under various vertical variable axial loading conditions. Increasing the diameter of longitudinal reinforcement and reducing the stirrup spacing can effectively enhance the bearing capacity and ductility of columns. Additionally, employing an ECC jacket can effectively improve the shear capacity of the columns. Furthermore, modifying the frequency, amplitude, and phase of the vertical axial force will significantly alter the stress distribution and damage failure of the columns.

Chaotic dynamics of fractional viscoelastic PET membranes subjected to combined harmonic and variable axial loads

Nonlinear chaotic vibrations of fractional viscoelastic PET (polyethylene terephthalate) membranes subjected to combined harmonic and variable axial loads is investigated in this paper. Axial tension variations arise from the machine disturbances of the processing line of roll-to-roll manufacturing. The viscoelasticity of PET membrane is characterized by the fractional Kelvin-Voigt model. Based on the Hamilton principle, the equation of motion of the membrane is established with the consideration of geometric nonlinearity, and the Galerkin procedure is employed to discretize the resulting governing equation. For the solution, the finite difference method is utilized in conjunction with the Caputo-type fractional derivative to reliably estimate the nonlinear response of fractional viscoelastic PET membrane. The reliability of this numerical strategy is proved by the available results of the fractional system and comparison examples. The influence of system parameters on chaotic behaviors is described by the bifurcation diagram and the detailed responses at the set bifurcation parameters. The fractional model together with the analysis provides a fundamental framework for the control of viscoelastic substrates in flexible manufacturing.

Seismic behavior of reinforced concrete slender walls subjected to variable axial loads

Reinforced concrete (RC) shear walls are the major lateral load-carrying structural components in high-rise buildings. Under strong earthquake motions, the axial loads of RC walls are not constant axial compression, but may be in tension, resulting in the axial load of RC walls varying from compression to tension. In this study, three RC slender wall specimens (shear-to-span ratio λ = 2.0) were tested to study the effect of the fluctuating range and loading patterns of variable axial forces on their cyclic behavior, including failure modes, hysteretic response, strength and deformation capacity. Test results indicated that the final failure of RC slender walls subjected to variable axial forces was controlled by flexural failure in the compressive-flexural direction. The hysteretic curves of RC slender walls were asymmetric and substantively different from the hysteretic curves of RC slender walls with constant axial loads. The fluctuating range of axial forces had a limited influence on the shape of hysteretic curves, while the loading patterns significantly changed the shape of these hysteretic curves. Under the variable axial forces, the lateral strength and deformation capacity of RC slender walls depended on the fluctuating range of axial forces, while loading patterns had limited influence when the fluctuating range of axial forces kept constant. The loading patterns had a limited influence on the lateral stiffness of RC slender walls, while the increase of fluctuating range of axial forces increased the difference between compressive-flexural and tensile-flexural lateral stiffness. No matter in tensile-flexural or compressive-flexural directions, the assumption that the wall section remains plane after deformation is suitable for RC slender walls under the variable axial forces, and enables reasonable estimations of yielding and peak strength. Finally, a finite element model was developed for predicting the cyclic behavior of RC walls under the variable axial loads.

Effects of pile configuration on the group behavior of semi-floating energy piles

To investigate the effects of pile configuration on the group effects of semi-floating energy piles, a series of laboratory tests were performed. The tests include three distinct pile configurations, namely, a single energy pile, a 2 × 2 energy pile group, and a 3 × 3 energy pile group. Different operation modes of energy pile groups, including all energy piles being cooled and partial energy piles being cooled, were considered. The monitoring and analysis of temperature changes in energy piles and surrounding soil, axial load variation, and displacement of energy piles and non-thermally activated piles were performed. The interactions among energy piles, non-thermally activated piles, raft, and surrounding soil within semi-floating pile groups with varying numbers of energy piles and operation modes were discussed. The findings reveal that the group effects of energy piles can be attributed to two aspects, namely pile-soil interactions and pile-raft-pile interactions. Both of these interactions contribute to reduced thermally induced axial loads and increased thermally induced displacements of energy pile groups. Under similar thermal loading, the 3 × 3 energy pile group exhibits a 19% increase in displacement and a 64% decrease in maximum thermally induced axial load compared to a single energy pile. Furthermore, the group effects of an energy pile group were affected by the pile configuration and are more pronounced with an increase in the number of energy piles within a pile group. The findings presented in this paper contribute to a more precise assessment of the thermo-mechanical response exhibited by such pile foundations.

Control of a mechanism for the application of variable axial loads in a multiaxial fatigue testing machine

Control of a mechanism for the application of variable axial loads in a multiaxial fatigue testing machine

Investigation on static stability of bidirectional FG porous beams exposed to variable axial load

Investigation on static stability of bidirectional FG porous beams exposed to variable axial load

Overturning effects on seismic performance of the hybrid friction-based seismic isolation system

The safety and reliability of the double-deck variable-depth composite truss bridge (DDVD-CTB) during earthquakes can be ensured by a hybrid friction-based seismic isolation system (HFBSIS). However, the overturning effects on seismic performance of HFBSIS is a significant concern, as it can induce vertical actions of isolation bearings. In this study, a single-degree-of-freedom (SDOF) system was first employed to describe the isolation system with the consideration of overturning effects. Following the nondimensionalization of motion equation, it indicated that the ratio of total axial load variations of spherical bearings to deadweight of the girder (γ) was a good indicator for the overturning effects on seismic performance. Then, the finite element (FE) model of SDOF system was developed and verified by the quasi-static cyclic test results. Finally, parametric sensitivity analyses were carried out to evaluate the degree of overturning effects on the seismic performance of the isolation system under different design parameters. Therefore, the maximum value γ of was less than 5 × 10−7 degree in the engineering practice. The nondimensionalization of motion equation considering overturning effects was approximately the same as that neglecting overturning effects, suggesting that overturning effects could be neglected when designing the HFBSIS in DDVD-CTB.

Numerical study on effect of constant and variable axial loads on RC column subjected to blast load

Explosion is an instant release of potential energy, heat, temperature and sound producing a pressure wave that travels away from the source radially on which the resulting force the blast wave is a blast load. Besides this, column especially external column as a main load bearing compressive structural component in buildings is the most critical structural element vulnerable to explosion and require an in-depth investigation on its performance against blast loads. Despite this, currently there is perceived gap and meagre of researches regarding numerical investigation on effect of presence of constant and variable axial loads on reinforced concrete column when subjected to blast load. In this study, the responses of reinforced concrete column under different scaled distance blast load scenarios accompanied by constant and axial loads were investigated. The finite element method of structural analysis was employed accompanied by nonlinear explicit time integration software LS-DYNA. The numerical analysis result revealed that RC columns subjected to large axial loads accompanied by small scaled distance blast scenario made the reinforced concrete column to suffer severe damages including crushing of concrete especially direct shear failures and breaking of reinforcing steel bars. In addition to this, comparing the nature and behaviour of variable axial loads with constant axial loads, the former loading case revealed larger nodal displacement values along height of column, and a higher displacement-time history curve was traced. On the other hand, the damage propagation nature of RC columns loaded with variable axial load was slow and progressive and different with RC columns with constant axial loads accompanied by small scaled blast scenario which was rendered to have a severely crushed concrete element without bending actions leaving the entire column inadequate for service.

Fatigue damage development of grouted connection under varying cyclic loading

• Axial fatigue test of grouted connection under variable load range and sequencing. • Concrete damage plasticity-based model parameters used for numerical simulation. • Damage accumulation rate increased after sudden high load. • Load sequencing affects damage accumulation rate. • The damage accumulation of the real sized model was similar to the experimental model. The jacket structures of offshore wind turbines connect the foundation piles through a grout annulus. The grouted connections of the jacket structures are predominantly under the action of low-range fluctuating axial fatigue loads. However, they are sporadically subject to sudden large loads. The process of recurrent loading of the connection can lead to lose of capacity due to the gradual grout damage. Therefore, it is essential to study the damage accumulation process under variable fatigue loading. The cyclic compression tests of grouted connection scale model were performed and the influence of load-range, loading-sequence, and sudden large load on the fatigue damage accumulation was studied. The finite element models of experimental specimens and actual sized model were numerically simulated. An initial constant low load range yielded constant value of damage index and a sudden large load applied caused damage acceleration of the grout, which enhanced the development of damage under the subsequent low-range cyclic load. The larger load range prior to smaller load range created greater damage accumulation and reduced the residual vertical bearing capacity of the connection. The work offers a numerical basis for damage assessment of grouted connections of offshore wind turbines under variable axial fatigue load using the scalar damage degradation parameter in Abaqus finite element software.

Stability and dynamic behaviour of bi-directional functionally graded beam subjected to variable axial load

The current study emphasizes static stability and dynamic characteristics of bi-directional functionally graded beams subjected to variable axial loads using the Ritz method and Reddy’s beam theory. The material property is varied as a function of the gradation pattern along with the length and thickness directions. The solutions procedures are tested against the results in the literature to show the accuracy of the present method. The influence of uniform, linear, and parabolic axial loads along the length of the beam on buckling and vibration responses are investigated. There is a remarkable variation observed in both the responses, by changing the material properties from isotropic to bi-direction functionally graded. Furthermore, the study reveals that higher stiffness is achieved by the material gradation index increment along the thickness direction compared to the lengthwise gradation index increment. Even the variations in the aspect ratios and end conditions are depicting significant variations in the buckling and vibration responses. Buckling and free vibration modes are also highly sensitive to the nature of variable axial loads and gradation index.

Bathymetry Based Modeling of Subaxial Magma Flows Under the Mid-Atlantic Ridge, 0 to 30° N

Fracture patterns of the Mid-Atlantic Ridge (MAR) provide evidence of tectonic forces related to divergence and magma upwelling at the ridge axis. In this study, we focus on the MAR from 0 to 30° N, where the N-S ridge exhibits slow spreading rates (2-4 cm/yr) and pronounced axial topography. Ridge segments and transform faults identified in bathymetry data were analyzed for strike orientation and axial depth profiles. Azimuths of transform faults and ridge segments exhibit increasing clockwise rotation with latitude, and all have left lateral displacement. Bathymetric sampling along ridge segments occurred at 9 km intervals with 20 km sampling radii, producing axial lithostatic pressure gradients. One-dimensional magma flows parallel to the ridge axis at 10 and 50 km depths were modeled using Darcy’s law based on published parameters and calculated gradients. Subaxial magma velocities of up to 4 cm/yr were predicted for horizontal flow at depth and are comparable in magnitude to upwelling rates in published literature. Average flow magnitudes (n = 422) within the melt generation region are predicted at 0.8 and 0.2 cm/yr for 10 and 50 km depths respectively. Flow velocities up to five times higher are expected with this model in the high-porosity boundary layer below the solidus. The Coriolis parameter would affect the movement of the flows predicted by our model and may be linked to rotational patterns observed at the MAR. Future research of magma migration below divergent margins would benefit from incorporating axial lithostatic load variations as a driver of flow.

Seismic behavior of slender prestressed reinforced concrete short-leg walls

To reduce the possible adverse effects of axial tensile force on reinforced concrete (RC) walls, bonded prestressed concrete (PC) walls are recommended for use in high-rise buildings. These could offer an initial axial compressive load to balance the possible axial tensile force of a RC wall induced by strong ground motions. In this study, three PC short-leg walls with a high-aspect-ratio of 2.0 were tested for various loading patterns, including constant axial forces and variable axial forces, combined with cyclic shear loading. Test results indicated that failure modes varied with loading patterns, including flexure-shear failure (coupled constant axial tension and cyclic shear loading), shear compression failure (coupled constant axial compression and cyclic shear loading), and flexure failure (coupled variable axial forces and cyclic shear loading). Variable axial forces led to the normalized tension-shear strength and compressive-shear strength of PC short-leg walls decreasing by 8.5% and 9.1%, respectively, and the tension-shear ultimate ratio decreasing by 35%. The ultimate drift ratio of PC short-leg walls ranged from 1.8% to 3.7%. Variable axial loads decreased the pre-yield secant stiffness in tension-shear and compression-shear loading, while the influence on post-yield secant stiffness was less pronounced. Variable axial forces did not increase the maximum crack width of PC short-leg walls, but clearly decreased the accumulated energy of PC short-leg walls as they changed the shape of hysteretic curves. Finally, a finite element model of PC short-leg walls was developed and the accuracy of the model was evaluated using experiment results, including its hysteretic characteristics and lateral displacement profiles.

Development of dual-parameter loading protocols for buckling-restrained braced RC frames considering variable axial loads

In the buckling-restrained braced reinforced concrete frames (BRB-RCFs), the RC columns in braced bays will undergo variable axial loads induced by the BRB axial forces, during strong seismic excitations. However, the actual axial force demands of columns have not been considered comprehensively in the existing loading protocols for seismic assessment of BRB-RCF subassemblage tests. This study aims to develop a new loading protocol considering variable axial loads to improve the accuracy of experimental investigations on the seismic performance of BRB-RCFs. To achieve these objectives, a total of 45 BRB-RCFs varying in the number of stories, story shears resisted by BRBs, and BRB configurations were first designed. Then, the matching relationship between the inter-story drift ratios (IDRs) and column axial forces was quantified. Further, the seismic responses of BRB-RCFs were systematically investigated, and the control parameters of loading histories were determined. Finally, the dual-parameter (IDRs and axial forces) loading protocols were proposed and validated by employing the cumulative distribution function of IDRs. Analytical results indicated that the proposed IDR loading protocol successfully reappeared and captured the seismic demands of the BRB-RCF subjected to the maximum considered earthquake level hazard.

Effects of near‐fault acceleration and non‐acceleration pulses on pounding between in‐plan irregular fixed‐base and base‐isolated buildings

Concave surface sliders (CSSs) are considered as an effective solution for retrofitting adjacent buildings irregular in plan that could undergo significant seismic pounding due to torsional displacements. This is because during the sliding phase there is coincidence between the projection of the gravity mass centre of the superstructure and the stiffness centre of the CSSs. However, unexpected torsional pounding, with amplification in a near-fault site, may be induced by variability of friction force and lateral stiffness of the CSSs, depending on the axial load and friction coefficient changes during an earthquake. In this work, structural pounding between fixed-base and base-isolated reinforced concrete (RC) L-shaped buildings, placed adjacent to form T- and C-shaped plans, is investigated. A simulated design of the original fixed-base framed structures is preliminarily carried out in accordance to a former Italian code, for a medium-risk seismic zone and a typical subsoil class. Then, seismic retrofitting with CSSs is carried out, to attain performance levels imposed by the current Italian code in a high-risk seismic zone and for moderately-soft subsoil. A computer code for the pounding analysis between fixed-base and base-isolated test structures is developed, in order to compare effects of nonlinear models of the CSSs that consider constant and variable axial load combined with friction coefficient at breakaway and stick–slip and as function of the sliding velocity, axial pressure and rising temperature at the sliding interface. Attention is focused on the pulse-type nature of near-fault earthquakes generally observed in the velocity time histories but largely overlooked in the acceleration ones. To this end, automated classification algorithms using wavelet analysis are adopted to compile three datasets of seismic input distinguishing between no-pulse and velocity-pulse, the latter further categorised into non-acceleration and acceleration pulses and rotated along the direction most likely to contain strong pulses.