Fiber-based Finite Element Model Research Articles

Seismic retrofitting of RC columns using stainless steel grid-reinforced UHPC jackets in plastic hinge zone

This study aims to propose a new seismic retrofitting method for reinforced concrete (RC) columns in buildings with insufficient or flawed design in early standards by locally using stainless steel grid-reinforced ultra-high performance concrete (UHPC) jackets in the plastic hinge zone. For this purpose, pseudo-static loading tests were conducted on four rectangular columns with economical reinforcement ratios, including three retrofitted columns and one traditional RC column, and the effects of the number of stainless steel grid layers and the height of UHPC jackets were investigated. Additionally, a fiber-based finite element model was developed for the retrofitted columns and its accuracy was verified. The influence of other parameters not considered in the experiment, such as the axial compression ratio, UHPC jacket thickness and concrete compressive strength on the seismic performance of the retrofitted columns were also analyzed. The results demonstrated that the proposed retrofitting method effectively improves the seismic behavior of RC columns in a quick and cost-effective manner. The combination of stainless steel grids and UHPC jackets provided stable confinement for the internal RC columns, significantly reducing the damage in the plastic hinge zone and the residual displacements. Compared with the RC column, the ductility, bearing capacity and energy dissipation of the retrofitted columns were enhanced. Especially, the ductility coefficient of the specimens using a two-layer stainless steel grid increased by 27.19 %–36.56 %.

Quasi-static tests and seismic fragility analysis of RC bridge piers with high-strength steel bars and high-strength/ultra-high performance concrete

The use of high-strength steel bars (HSSB), high-strength concrete (HSC) and ultra-high performance concrete (UHPC) in bridge piers can obviously improve bearing capacity and save material consumption. However, the use of high strength materials may detrimentally affect the seismic performance of reinforced concrete (RC) piers. This paper tries to study the seismic performance of RC piers with HSSB and HSC/UHPC by quasi-static tests and seismic fragility analysis. Three RC piers, solid pier with normal strength steel bars (NSB) and normal strength concrete (NSC), solid pier with HSSB and HSC, and hollow pier with HSSB and UHPC, were designed and tested under quasi-static loading. The failure mode, hysteretic performance, stiffness degradation and residual displacement of three specimens were compared and analyzed. A fiber-based beam-column finite element model (FEM) was developed to simulate the nonlinear response of RC piers with HSSB and HSC/UHPC. On this basis, the seismic fragility analysis was performed to study the effects of high-strength materials on the seismic performance of RC piers. Research results show that, compared with NSC piers, HSC piers reinforced with a small amount of HSSB still exhibited slightly larger lateral bearing capacity but lower deformability due to the higher brittleness of HSC. The UHPC hollow pier presented obviously larger deformation capacity, and more slow decline speed of lateral stiffness than NSC and HSC solid piers. Meanwhile, the matching use of UHPC and HSSB in hollow pier can improve the elastic deformation, and then reduce residual displacement of RC piers. The proposed fiber-based finite element model (FEM) can simulate the nonlinear response of RC piers with different high-strength materials with reasonable accuracy. Fragility results demonstrate that, HSC pier presents lower seismic fragility than NSC pier at different damage states except for collapse due to lower ductility of HSC pier. The UHPC hollow pier with HSSB has lower seismic fragility than NSC and HSC solid piers at the same seismic intensity. Overall, the hollow pier with HSSB and UHPC can maintain good seismic performance while greatly saving material consumption.

Experimental and Numerical Study on the Seismic Performances of Reinforcement-Embedded RC Column-to-Precast Cap Beams with Socket Connections

Accelerated bridge construction (ABC) has attracted much attention in China as a new and efficient construction method. However, the seismic performance of the connections between precast piers and other structures limits the application of ABC in medium and high seismic zones. In this paper, a quasi-static test was conducted to investigate the seismic performance differences between a cap–column socket connection (PSC) specimen, which reinforced an embedded RC column-to-precast cap beam with a socket connection, and a cast-in-place (CIP) cap–column specimen. A fiber-based finite element model that considers bond slippage between the connection reinforcement and wet joint concrete is proposed. The numerical simulation results compared with the experimental results show an error of about 12% in peak bearing capacity and about 2% in initial stiffness. The experimental and numerical results show that the PSC specimen demonstrates comparable seismic performance to the CIP specimen. Experimental results verified that the finite element model in this paper is adequate to predict the seismic responses of a precast column with a reinforcement-embedded socket connection. A reinforcement-embedded RC column-to-precast cap beam with socket connection can be an effective solution for construction in medium and high seismic areas.

Behavior of eccentrically-loaded tubed reinforced concrete short columns with high-strength concrete and reinforcement

Compared with traditional reinforced concrete (RC) columns, the concrete ductility in tubed reinforced concrete (TRC) columns is significantly improved and higher ultimate compressive strain matching with the high yield strain of high-strength steel rebars (HSSRs) can be attained. Therefore, high-strength concrete (HSC) and HSSR are suitable and recommended for use in TRC columns. In this study, a total of twelve circular TRC column (CTRC) specimens with the concrete compressive strength of 81 MPa and rebar yield strength above 527 MPa were tested under eccentric loading. The investigated parameters include the diameter-to-thickness ratio of steel tube and load eccentricity, whose effects on the failure mode, load capacity, and ductility are described and discussed. A fiber-based finite-element model was established and verified for conducting parametric studies and investigating the stress development of the HSSRs in CTRC columns. The study results show that all test specimens fail in a ductile manner due to the tube confinement, and the load eccentricity is a primary factor affecting the failure location, load capacity, and ductility. With the parameters considered, the longitudinal rebars in the compression region yield before the column reaches its load capacity, indicating that the strength of HSSRs can be fully utilized. Based on the study results, calculation methods are suggested to determine the strength of eccentrically-loaded CTRC columns with HSC and HSSR.

Experimental and parametric investigation on elastoplastic seismic response of CFST battened built-up columns

Previous studies have proven that CFST (concrete-filled steel tube) battened built-up columns (CFST-BBCs) exhibit good hysteresis energy dissipation capacity and deformation capacity under lateral cyclic repeated loading. To further understand the elastoplastic seismic response characteristics of such composite structures under strong earthquakes, two pseudo-dynamic tests of 1/8-scaled CFST-BBC pier specimens were first performed. The acceleration records obtained in the 1995 Kobe Earthquake and the 2008 Wenchuan Earthquake were used to investigate how ground motion intensity, ground motion characteristics, and aftershocks affect the seismic response of the CFST-BBC pier. Meanwhile, an extensive time-history analysis of 17 fiber-based finite element models (FFEMs) under 20 different ground motions was performed to reveal the influence of the ground motion intensity, the same-intensity aftershock, and the main structural parameters on the elastoplastic seismic response of the CFST-BBC pier. Finally, a two-stage checking method suitable for the seismic performance of the CFST-BBC pier under the E1-level frequent earthquake and the E2-level rare earthquake was proposed. Verified by experimental and FFEM analysis results, the proposed checking method can accurately and effectively evaluate the strength and deformation capacity of the CFST-BBC piers under strong earthquakes.

Comparative study on seismic vulnerability of RC bridge piers reinforced with normal and high-strength steel bars

Use of high-strength steel bars (HSSB) in RC bridge piers reduces the amount of reinforcement or enhances bearing capacity of RC piers. However, the use of HSSB could come with reduction in the energy dissipation of RC member, which used to be considered detrimental to the seismic performance of RC bridge piers. This paper revaluates the effects of HSSB on seismic performance of RC piers by comparing that with normal steel bars (NSB) through fragility analysis. Maximum drift ratio (MDR) and maximum section curvature (φm) are considered as engineering demand parameters (EDPs) to represent the deformation demands and section damage of RC piers, respectively. A fiber-based finite element model (FEM), calibrated by a set of shaking table tests of RC portal frames reinforced with NSB and HSSB, is adopted to simulate the nonlinear dynamic response of RC piers. Fragility curves using probabilistic seismic demand model (PSDM) are developed to compare the relative vulnerability of RC bridge piers reinforced with steel of different yield strengths. The analysis results indicate that, when using HSSB to replace NSB by equal volume replacement, the RC piers have similar deformation demands (MDR) but significantly less section damage. On the other hand, when using HSSB to replace NSB by equal strength replacement, the deformation demands (MDR) of RC piers slightly increase (about 5%) due to the decrease of post-cracking stiffness under small reinforcement ratio. In spite of slight increase of deformation demands, the use of HSSB by equal strength replacement still reduces RC piers’ section damage since the use of HSSB enhances the elastic deformation and then reduces the plastic deformation of RC piers. Moreover, as elastic deformation accounts for a higher proportion of total deformation for taller piers, the reduction amplitudes of section damage are more evident when using HSSB in taller piers. On the whole, the application of HSSB to replace NSB is more likely to be beneficial to the seismic performance of RC piers.

Experimental investigation of seismic behavior of UHPC-filled socket precast bridge column-foundation connection with shear keys

Accelerated bridge construction (ABC) has been paid great attention in China for the significant advantages compared with cast-in-place (CIP), such as higher construction speed, less environmental impact and structural members of higher quality. However, the reliability, durability and seismic behavior of the connection between prefabricated bridge column and the rest of the structure are primary factors that limit the application of the ABC in the moderate and high seismic zones. In this study, quasi-static experimental investigation of seismic performance of three 1:3-scale ultra-high performance concrete (UHPC)-filled socket reinforced concrete (RC) column-foundation connection specimens were conducted. A fiber-based finite element model is developed considering the rotation effect of the column embedded in the footing. The seismic behavior of the precast column specimens with UHPC-filled socket connection type was compared with that of CIP specimen. The experimental and numerical results show that the damage evolution, failure modes and hysteretic responses of the precast bridge column are equivalent with those of the CIP bridge column. Numerical analysis also validated that the finite element model(FEM) in this paper is adequate to predict seismic response of the precast column with UHPC-filled socket connection. The UHPC-filled socket connection is a reliable connection type in prefabricated column-foundation connection joint which is suitable for moderate and high seismic regions.

Experimental and numerical investigations on seismic performance of RC bridge piers considering buckling and low-cycle fatigue of high-strength steel bars

In order to promote the application of high-strength steel bars (HSSB) in reinforced concrete (RC) bridge structures, the mechanical properties of HSSB as well as seismic performance of high-strength RC piers were investigated by experimental study and numerical analysis. First, monotonic and low cycle fatigue tests of HRB400E and HTRB600 steel with different slenderness ratios (L/D) were conducted to comparatively study their mechanical properties. Then, quasi-static cyclic tests of seven rectangular RC piers were conducted to study the effects of concrete cover, longitudinal reinforcement yield strength, concrete strength and axial load ratio on seismic behavior of RC piers. Finally, a fiber-based beam-column element was used to simulate the mechanical behavior of steel material and nonlinear response of RC piers. Material tests indicate that HTRB600 steel has lower uniform and fracture elongations, as well as inferior low-cycle fatigue performance than HRB400E steel. Increase of the slenderness ratio (L/D) severely weakens low-cycle fatigue life of steel bars, and the low-cycle fatigue parameters of steel can be fitted for each slenderness ratio, respectively. Even though HTRB600 steel has relatively lower ductility and inferior low-cycle fatigue performance, the RC piers with HTRB600 steel still have higher or at least comparable deformation capacity than that with HRB400E steel. This results from the reduction of strain demands of longitudinal bars by the utilization of HSSB in RC piers. Removing the concrete cover neighboring the longitudinal bars in the plastic hinge region has trivial impact on bar buckling and ultimate displacement of RC piers, which confirms that the concrete cover has already spalled off prior to the initiation of longitudinal reinforcement buckling. With identical steel strength and tie configuration, the ultimate displacement of RC pier with C80 concrete is significantly lower than that with C40 and C60 concrete, while the latter two have similar deformation capacity. Increase of axial load ratio (from 0.06 to 0.18) obviously reduces ultimate displacement of RC piers, since the concrete in the compression zone endures greater compressive stress under higher axial load and thus compressive failure occurs earlier. The ReinforcingSteel material model in OpenSees is calibrated by experimental data to accurately simulate the buckling and low-cycle fatigue properties of HRB400E and HTRB600 steel. The fiber-based finite element model (FEM) can predict the strain development of longitudinal bars of RC piers under cyclic loading, and the resulting low-cycle fatigue damage of steel material as well as strength degradation of RC piers. Combined with experimental study and FEM, this paper reveals the influence mechanisms of different factors on the seismic performance of RC piers in detail.

Experimental and theoretical studies on the seismic performance of CFST battened built-up column piers

The insufficiency of seismic performance data and the lack of seismic design provisions have affected the application and promotion of the CFST battened built-up column (CFST-BBC) pier in high-intensity areas. To understand the failure mechanism, the energy dissipation capacity, and the hysteretic behavior of the CFST-BBC pier, a total of seven test specimens subjected to lateral cyclic repeated loading were first tested. Meanwhile, a fiber-based finite element model (FFEM) was established by using OpenSees to simulate the seismic behavior of the CFST-BBC pier, and an extended parametric analysis was performed to clarify the impact of key parameters such as material strength, axial compression ratio and slenderness ratio on the seismic performance of the CFST-BBC pier. Finally, theoretical calculation methods for the lateral stiffness, lateral resisting strength and allowable deformation of the CFST-BBC pier were proposed and verified by comparing with experimental and analytical results. It is proved that the calculation results obtained by the proposed methods can effectively reflect the intrinsic characteristics of the seismic performance of the CFST-BBC pier and meet the accuracy requirements of practical engineering applications.

Research on seismic resilience of prestressed precast segmental bridge piers reinforced with high-strength bars through experimental testing and numerical modelling

Precast segmental bridge piers are vigorously pursued, especially in low- and medium- seismicity regions, for their merits in accelerating construction, reducing environmental/traffic disturbances, enhancing quality control, etc. Recently, unbonded tendons are utilized to reduce the residual displacement and to facilitate fast rehabilitation of key bridges in the urban traffic system. To further enhance the resilience, prestressed precast segmentally-erected (PPC) piers were fabricated and tested, reinforced with a hybrid of unbonded tendons and high-strength (H) energy dissipation (ED) bars. In this study: (1) cyclic loading tests for the PPC piers with “H” ED bars were reported, and the seismic resilience was quantitatively evaluated and compared with the PPC piers with normal-strength (L) ED bars; (2) the computationally efficient fiber-based finite element (FE) model is developed for the tested PPC piers. Experimental results demonstrated that the “H” ED bars brought out apparent amelioration, in terms of load-carrying, ductility and self-centering capacities. In consideration of rising seismic resilience, “H” ED bars are recommended to replace currently large-scale adopted “L” ED bars in the PPC pier of vital bridges. Through validation with experimental results, the proposed fiber-based FE model is able to provide satisfactory predictions for seismic responses of the tested PPC piers.

Nonlinear Fiber Modeling of Steel-Concrete Partially Composite Beams with Channel Connectors

A simplified nonlinear fiber-based finite element model of steel-concrete partially composite beams utilizing channel type mechanical shear connectors is developed in OpenSees framework. The interaction between steel beam and concrete slab is accounted for by introducing nonlinear zero length elements and rigid links. The channel shear connector response used in numerical models is based on the previously obtained experimental response from pushout tests. Accuracy of the numerical models in predicting the response of beams is verified with the results of the previously conducted composite beam tests. The numerically predicted response agrees well with the experimental results over the entire range of load-deflection curves for both the fully composite and partially composite beams. The numerical models are also able to accurately predict the interface slip between steel beam and concrete slab when compared to the experimentally determined slip values, as well as the closed form slip predictions. Analysis results allowed the investigation of how the influence of steel and concrete damage is reflected on the overall loaddeflection response of composite beams.

Progressive collapse fragility models of European reinforced concrete framed buildings based on pushdown analysis

Structural safety for extreme loads that may cause local damage to single primary components or even the progressive collapse of the structure has been probabilistically assessed in a few studies, hence neglecting uncertainties in loads and system capacity. As such, this paper moves from a deterministic to a probabilistic framework, proposing new progressive collapse fragility models based on pushdown analysis of low-rise, reinforced concrete framed bare structures. Two building classes representative of structures designed for either gravity loads or earthquake resistance in accordance with current European codes were investigated. Monte Carlo simulation was used to generate random realizations of 2D and 3D structural models. Fiber-based finite element models were developed within an open source platform. The primary output consisted of fragility functions for each damage state of interest, given the loss of corner column at the ground floor. The fragility models were compared to those derived through incremental dynamic analysis (IDA) to assess the inaccuracy of progressive collapse fragility functions derived through pushdown analysis. Load capacity predictions provided by those analysis methods were used to develop regression models for a quick estimation of dynamic amplification factor at a given displacement/drift level. The analysis results show a significant influence of both seismic design and secondary beams on robustness of the case-study building classes.

Fragility Curves for RC Frames Subjected to Tohoku Mainshock-Aftershocks Sequences

ABSTRACTThe damaging effects of aftershocks are overlooked by current building codes and not properly accounted for in commercial seismic loss assessment software. In this paper, an evaluation of the seismic fragility relationships for reinforced concrete (RC) frame systems prone to mainshock-aftershocks sequences is conducted. Fiber-based finite element models for different types of RC frames are established and subjected to a suite of ground motions obtained from the Tohoku sequence. Fragility relationships are derived with and without consideration to multiple earthquake effects. The results from this study confirm that multiple earthquakes have significant effects on the vulnerability relationships of RC frames.

Advancements in fragility analysis using numerical calibration methods for a horizontally curved RC bridge

Horizontally curved reinforced concrete (RC) bridge piers are subjected to combined actions of axial, shear, bending and torsion. The interaction between these combined forces in the highly inelastic range significantly influences the response of the bridge piers. In this paper, a three-dimensional fiber-based finite element model of a curved bridge is developed. The model is then calibrated based on experimental data. The calibration process is conducted with the objective of developing a computationally inexpensive model that can accurately mimic the response of bridge piers subjected to complex multi-directional earthquake loading conditions. An evaluation of the seismic fragility relationships of a curved bridge is undertaken using the initial (un-calibrated) and calibrated numerical models. The results from both models indicate that combined forces interaction significantly affect fragility relationships.