Amount Of Transverse Reinforcement Research Articles

Code-based brittle capacity models for seismic assessment of pre-code RC buildings: comparison and consequences on retrofit

The existing Reinforced Concrete (RC) buildings stock is often characterized by a significant seismic vulnerability, due to the absence of capacity design principles, even in regions with high seismic hazard, such as Italy. Approximately 67% of existing RC buildings in Italy have been designed without considering seismic actions (GLD), resulting in very low transverse reinforcement amount in beams and, particularly, in columns. Additionally, beam-column joints typically totally lack stirrups. Consequently, shear failures under seismic actions are very likely for this pre-code building typology, often limiting their seismic capacity. However, the assessment of shear failures in beams/columns or joints varies significantly from code to code worldwide. The main goal of this work is to quantify the impact of different code-based brittle capacity models on the seismic capacity assessment and retrofit, focusing on GLD Italian pre-1970 RC buildings. This comparative analysis is carried out by first considering three current codes, emphasizing their, even significant, differences: European (EN 1998-3-1. 2005), Italian (D.M. 2018), and American (ASCE SEI/41 2017) standards. Then, shear capacity models prescribed by the current drafts of the next generation of Eurocodes are implemented and compared to the current models. The assessment includes: (i) a parametric comparison among models; (ii) the evaluation of case-study buildings capacity in their as-built condition and after shear strengthening interventions. The latter is performed on 3D “bare” models, due to the lack of practical guidance in most codes on modelling masonry infills.

Inelastic behavior of diagonally reinforced coupling beams confined with steel fibers

Experimental studies were conducted to analyze the plastic rotation capacity and the effect of replacing transverse reinforcements with steel fibers in diagonal reinforced concrete coupling beams. Six specimens were planned, and the steel fiber reinforcing index and total area of transverse reinforcements were the main variables. Reverse cyclic loading tests were conducted to confirm the seismic performance of the coupling beam. The results showed that the plastic hinge rotation capacity increased and the buckling point of the diagonal reinforcing bars was delayed as the steel fiber reinforcing index increased. An analysis of 78 specimens and the results of previous studies confirmed that ductility improved when the confinement effect of the steel fiber was considered. Application of an equation for ductility requirements reflecting the confining stress of steel fiber reinforced concrete confirmed that the amount of transverse reinforcement could be reduced.

Effects of enhanced confinement on cyclic behavior of concrete bridge columns with partially unbonded seven-wire steel strands

The effects of enhanced confinement on the proposed bridge columns under lateral cyclic loading were investigated in this research by testing large-scale column specimens. The proposed column used nonprestressed partially unbonded seven-wire strands as elastic elements to increase the post-yield stiffness of the column to reduce the residual displacement under seismic loads. The enhanced confinement included rectilinear transverse reinforcement with an increased amount (specimen CSCR) and reduced spacing or innovative five-spiral reinforcement (specimen CSCS). Test results showed that all the specimens exhibited a positive post-yield stiffness ratio of 3.5–6.4 %. However, the enhanced confinement reduced the post-yield stiffness by 42–45 % but increased the lateral strength of the column, which is beneficial for controlling the residual displacement. The enhanced confinement did not affect the drift ratio when the positive post-yield stiffness ended (6 %) but significantly increased the deformation capacity by 15–31 % and the energy dissipation capacity by 37–63 % of the column. Even with a smaller amount of transverse reinforcement, the specimen with the innovative five-spiral reinforcement showed deformation and energy dissipation capacities better than the other specimens with conventional rectilinear transverse reinforcement. A pushover analysis model was developed and modified to better account for the effect of the enhanced confinement and the strand slip, and to consider the end point of positive post-yield stiffness. The comparison with the test result showed that the proposed pushover model well captured the envelope responses of the specimens. Finally, a simplified method was proposed for estimating the moment strength of the proposed column.

SECTIONAL CURVATURE DUCTILTY OF REINFORECD CONCRETE COLUMNS UNDER LARGE INELASTIC DEFORMATION

SECTIONAL CURVATURE DUCTILTY OF REINFORECD CONCRETE COLUMNS UNDER LARGE INELASTIC DEFORMATION

Cyclic behavior of UHPC corner beam-column joints under bi-directional bending

Design codes contain special provisions for anchorage lengths of flexural reinforcement and amount of transverse reinforcement in beam-column joints, to safeguard against joint failure and possible structural collapse. However, those stipulations could lead to congested joints with construction problems such as segregation. This study leverages experimental testing of eight corner beam-column joints under constant axial and cyclic bi-directional bending loads. In addition to being one of few on corner joints under a complex and practical loading, the study’s main aim is to evaluate the effectiveness of ultra-high-performance concrete (UHPC) as a joint fill in lieu of normal strength concrete (NSC). Brittle failure, either by pure shear or shear combined with yielding of beam reinforcement occurred in all NSC joints and varied in shape and intensity depending on the column depth-to-beam longitudinal bar diameter (hc ⁄db) ratio and spacing of transverse beam/column reinforcement. Ductile failure characterized by the yielding of beam reinforcement and plastic hinge at the beam-joint interface occurred in all UHPC joints and was independent of the two aforementioned variables. The application of UHPC in the joint region resulted in increasing the joint ultimate strength by 27.6–54.8%, energy dissipation by 24–163%, and stiffness by 20 to 50%, compared to NSC joints. Such results were obtained when no transverse reinforcement is used in the joint, and only half transverse reinforcement is used in the beam/column members, and with hc ⁄db relaxed to 38% of the code recommended ratio, highlighting the advantages of UHPC joints. Predictions of ACI-ASCE 352R code were found to be overly unconservative and in need of future improvement.

How much code-based shear models affect seismic assessment of pre-code RC buildings?

Existing Reinforced Concrete (RC) building stock have exhibited a significant vulnerability under seismic shaking after the most recent earthquakes worldwide. They were often designed without capacity design principles, even where seismic hazard is very high, such as in Italy. Two-third of Italian existing RC buildings have been designed to sustain gravity loads only (GLD). Almost all of them lack stirrups in beam-column joints and have a minimum amount of transverse reinforcement in columns. These features make such buildings extremely prone to shear failures under seismic actions. Additionally, the same physical phenomenon, i.e., the shear failure of beams/columns or joints, is assessed differently worldwide. As a result, the same building has (even very) different seismic capacity depending on the code adopted for its assessment. The main aim of this work is quantifying such a difference with reference to RC GLD Italian case-study buildings with different number of stories located in sites with different seismic hazard, based on nonlinear static analyses. Code-based safety checks according to European (Eurocode 8-part 3, 2005; Italian DM 2018) and American (ASCE-SEI 41, 2017) approaches are carried out. The results show that capacity at Severe Damage (SD) Limit State (LS) is generally limited by joints failures. Relevant design of seismic strengthening at SD LS, thus, strongly depends on the adopted code approach, leading to generally higher strengthening costs when joints failures are (very conservatively) detected by means of the current European approach.

Assessment of plastic hinge length in reinforced concrete columns

Plastic hinge properties are crucial parameters in predicting the nonlinear response of structural elements. Because of the intricate material nonlinearity, precise determination of the plastic hinge length (PHL) has encountered several obstacles. Over the last few decades, there have been various definitions and models put forth to forecast this length, nevertheless, the outcomes displayed significant disparities. Therefore, the paper introduces a comprehensive method for determining PHL using certain criteria, including rebar strain profiles, concrete cover and core peak strains, curvature profiles, and damage observations. Furthermore, a set of four full-scale reinforced concrete (RC) columns measuring 400 mm × 400mm × 3000mm and featuring varying transverse reinforcement configurations were constructed and subjected to testing under a high axial load ratio (ALR). The tested results implied that it is necessary to separate the PHLs based on different criteria. The high axial compression load led to enhancing the PHLs, which were based on rebar compressive yield strains, curvature profiles, concrete cover and core peak strains. In contrast, it has a minor effect on PHL based on tensile yield strains. In addition, the amount of transverse reinforcement had an insignificant effect on all PHLs for tested columns. Hence, a revised equation was proposed to estimate the equivalent PHLs of rectangular RC columns based on tested results and a 114-column database. The proposed equation had better accuracy compared with some other model results in the literature.

Seismic Performance of Ordinary and Damage-tolerant Concrete Beam-column Joints under Reversed Cyclic Loading

The adoption of the capacity design principle in modern codes of practice allows for multi-storey frame structures to exhibit ductile behaviour in a major earthquake without collapsing. Whilst the risk of shear failure in regions of large inelastic deformation can be largely reduced, the detailing requirements in capacity design often leads to reinforcement congestion that adds difficulty during the construction process, due to the high amount of transverse reinforcement required to provide shear resistance and confinement. The work presented in this paper forms a part of a wider research programme that aims to simplify the detailing requirements of transverse reinforcement in the joint and plastic hinge regions of moment-resisting frame structures. To this end, damage-tolerant concrete known as engineered cementitious composite (ECC) was used to substitute concrete in these critical regions. In this article, the results of reversed cyclic tests on two half-scale exterior beam-column joints, constructed with ordinary concrete and ECC, are presented. Key features of the response such as the reversed cyclic hysteresis loops, stiffness degradation and cumulative energy dissipation are discussed, along with crack development under reversed cyclic loading in the form of digital strain maps.

Axial behavior of square steel-tubed ultra-high-strength reinforced concrete columns

Steel-tubed reinforced concrete (STRC) columns with ultra-high-strength concrete (UHSC; compressive strength greater than 120 MPa) are currently used in some super high-rise buildings in Japan and other Asian countries. To properly evaluate the seismic performance of steel-tubed reinforced ultra-high-strength concrete (STR-UHSC) columns, it is necessary to understand their behavior under axial compression. However, there are few studies on the axial behavior of square STRC columns, particularly with UHSC. Therefore, axial compression tests were conducted on four square STR-UHSC columns with the test variables being the amount of transverse reinforcement and the presence of the steel tube. The concrete compressive strength used was over 170 MPa. The test result showed that the presence of the steel tube and the amount of the transverse reinforcement had no significant effect on the load-bearing capacity but greatly improved the post-peak behavior. To discuss the influence of the size effect and confining effect on the concrete compressive strength, a database of 56 plain and reinforced concrete columns with UHSC, including four specimens of this study was constructed. A new formula was proposed to evaluate the concrete compressive strength under the size effect and confining effect based on the regression analysis of the database. The proposed formula well simulated the measured peak load with a proper amount of contributions from concrete and longitudinal reinforcement. Finally, axial load-axial strain relationships were simulated using the proposed concrete stress–strain relationship. The axial behavior of each specimen was well simulated by considering the size effect and confining effect of the steel tube as well as the transverse reinforcement on the ultimate strain.

Experimental and Analytical Study on the Axial Behavior of Circular High-Strength Concrete Columns with Hybrid Carbon Fibers and Steel Confinement System

This paper describes a work that examines a new solution to the problem that arises from the relatively high amount of transverse reinforcement required in HSC columns. It presents an alternative to common transverse steel reinforcement, a dual system comprising steel ties and a carbon-fiber mesh (CFM) applied internally together with steel ties. The behavior of the proposed system was examined in this work in a series of twelve laboratory tests of circular stub column specimens. The experiments performed in this work focused on the columns’ load and displacement capacities. The tests were planned with the aid of an analytical model that was originally developed for a hybrid system of external fiber-reinforced polymer (FRP) sheets and internal steel, and was adapted for the current system. An analysis of the results shows that for a given amount of conventional transverse steel, the application of the carbon-fiber meshes adds efficiency to the rebar confinement system, in terms of both the load bearing capacity and the ductility, and for specimens with the hybrid confinement system, the higher the carbon fiber amount the larger the ductility improvement. Furthermore, fair to good agreement was observed between the model and the measured stress–strain curves, especially those of the peak stresses. Based on the above findings and the added benefit of fire resistance, the hybrid method appears to be promising for confining HSC columns.

Lateral load resistance of reinforced concrete columns with brittle details

AbstractAnalytical models for shear strength and deformation capacity estimation of reinforced concrete members are essential for the practical implementation of performance‐based seismic evaluation of existing structures and form a key part of current seismic assessment codes (e.g., EN 1998‐3 (2005), (2022), ASCE/SEI 41‐17). Although these models have been derived from regression or calibration of large datasets collected from experimental literature, discrepancy persists between experimental and analytical estimates, especially in cases of members with old‐type detailing. The evident uncertainty about parametric dependencies of the underlying mechanistic problem motivated the present work, where a set of benchmark columns that represent older structural detailing are studied parametrically through advanced finite element simulation. Parameters considered were, the axial load ratio, transverse reinforcement amount and spacing, longitudinal reinforcement ratio, and anchorage/lap splice detailing; results were gauged in terms of the resulting drift ratio (deformation capacity) at the performance limit states and were compared with the Code estimates to evaluate their limits and conservatism. The simulation results were used to vet the assumptions that underlie the simple mechanistic models used in deriving seismic design expressions for yield and ultimate deformation capacities, shear strength, and the rate of its degradation with increasing ductility.

Failure mechanism of one-way slabs under concentrated loads after local reinforcement yielding

One-way slabs under concentrated loads may fail by one-way shear such as wide beams, punching shear, flexure, or by combining two or more of these mechanisms. Nevertheless, most publications have only addressed shear and punching failures without flexural reinforcement yielding. This study investigates the ultimate shear capacity of one-way slabs under concentrated loads with local reinforcement yielding. In total, 12 tests were conducted on six reinforced concrete slabs. Simply supported slabs of 1.60 m × 3.40 m × 0.15 m were tested. Three parameters were varied: the load position, the span length and the reinforcement ratio. All slabs failed initially by punching with limited or extensive reinforcement yielding. Due to the relatively large amount of transverse reinforcement (0.44 %), most slabs underwent a large shear redistribution around the load, resulting in a wide beam shear failure after the local punching failure. The test results were compared to the theoretical predictions of shear, punching and flexure capacity using code expressions and the Extended Strip Model (ESM). The ESM resulted in the closest predictions of the experiments. These experiments confirm that a brittle shear and punching failure mechanism can occur even after extensive reinforcement yielding. Moreover, the results indicate that the ESM can be used to assess one-way slabs under concentrated loads with local reinforcement yielding.

The Numerical Study of The Need for Transverse Reinforcement of a Spun Pile

A numerical study based on FEA was conducted to seek the optimal need for transverse reinforcement of a spun pile with 400mm-diameter and 75mm of wall thickness. Two equations from ACI 318-19 and ASCE 7-16 were compared. The parametric study was limited to a spun pile with 450mm diameter and 75mm of wall thickness. The amount of transverse reinforcement was presented as the ratio to the ACI requirement. The ratio was varied and its effect on strength and ductility was observed. Three different axial load ratios, 0.1, 0.2 and 0.3 Fc’Ag, that are usually applied on the foundation were considered. The needs for spun pile with and without concrete infill are different since it is a function of the ratio of gross area (Ag) to the confined area (Ach) of concrete. An empty spun pile require a higher amount than the filled pile. The axial load ratio is another factors that affect the need for that reinforcement. However, ACI does not recognize it. The study found that the amount of transverse reinforcement required by ACI and ASCE can be reduced to get an economical amount without significantly reducing the strength and the ductility of the spun pile.

The calibration of NLFE model for the assessment of the maximum punching capacity of flat slabs

The requirements on structural engineers to design more subtle flat slabs are still increasing and therefore they must often design a large amount of reinforcement against punching. The maximum amount of transverse reinforcement is limited by the maximum punching capacity of slab, so the precise determination of the limit is essential. In the second generation of Eurocode 2, the model for the assessment of the maximum possible increase of the punching shear resistance of slab is less empirical and it considers the performance of the punching shear reinforcement. The main objective of the paper is to develop nonlinear finite element model, which would determine as closely as possible the maximum punching capacity of flat slab, strongly reinforced with punching shear reinforcement. So that, an effect of some transverse reinforcement’s parameters on the maximum punching shear strength could be investigated quite easily. For this purpose, the software GID was used. The calibrated NLFE model showed quite good agreement with experiment. The difference is less than 8 %, even for slabs reinforced with double-headed studs it did not exceed 1%.

경량골재 콘크리트 기둥의 압축거동에 대한 횡보강근 양 영향 평가

이 연구의 목적은 경량골재 콘크리트(lightweight aggregate concrete, LWAC) 기둥의 압축연성에 대한 ACI 318-19에서 제시하는 최소 횡보강근 양(ASUBsh(ACI/SUB)의 안전성을 평가하는데에 있다. 주요변수는 콘크리트 종류와 횡보강근 양(ASUBsh/SUB))이다. 중심축하중 실험결과 LWAC 기둥은 유사한 Ash로 배근된 NWC(normal-weight concrete, NWC) 기둥에 비해 더 취성적인 파괴모드를 보였다. 결과적으로 1.0ASUBsh(ACI)/SUB로 배근된 LWAC 기둥의 압축연성비(μ) 및 강도 증가계수(KSUBs/SUB)는 NWC 기둥보다 최소 27.9 % 및 43.3 % 낮았다. 결과적으로 NWC 기둥과 동등 수준에서 LWAC 기둥의 압축거동을 고려하면 ACI 318-19에서 요구하는 최소 횡보강근 양은 LWAC 기둥에서 약 1.2~1.3배 증가시킬 필요가 있다.

Effect of connection transverse reinforcement on the behavior of GFRP-RC T-connections: Numerical investigation

This study numerically investigates the cyclic behavior of glass fiber-reinforced polymer (GFRP) reinforced concrete exterior beam-column (T-BC) connections (GFRP-RC T-BC connections) with varying amounts of transverse reinforcement (TR) in the connection panel. The amount of TR required to provide adequate confinement and shear strength to the GFRP-RC T-BC connections has not yet been fully understood. The primary objective of this study was to evaluate the effects of TR configuration and amount of TR on the load-drift ratio response, strain distributions and shear strength of the GFRP-RC T-BC connections. The numerical investigation was conducted using the numerical models developed in ATENA software and validated against the test results of four T-BC connections (one steel-RC and three GFRP-RC). The TR ratio and TR configurations (2 leg, 3 leg, and 4 leg horizontal stirrups) in the connection panel were the main parameters of the numerical study. Results of the numerical study indicated that increasing the TR ratio in the connection panel to 0.72% significantly enhanced the peak load, stiffness and drift ratio capacity of the GFRP-RC T-BC connection. Four leg TR configuration showed significantly better performance of the GFRP-RC T-BC connections due to the higher confinement factor than the other TR configurations. A shear strength equation was proposed for the GFRP-RC T-BC connections, which represents a linear relationship between shear strength and the TR ratio up to 0.60%, followed by stable shear strength of 0.85fc′ for a TR ratio up to 1.30%.

Experimental Study on Seismic Performance of Precast Pretensioned Prestressed Concrete Beam-Column Interior Joints Using UHPC for Connection.

The traditional connections and reinforcement details of precast RC frames are complex and cause difficulty in construction. Ultra-high-performance concrete (UHPC) exhibits outstanding compressive strength and bond strength with rebars and strands; thus, the usage of UHPC in the joint core area will reduce the amount of transverse reinforcement and shorten the anchoring length of beam rebars as well as strands significantly. Moreover, the lap splice connections of precast columns can be placed in the UHPC joint zone and the construction process will be simplified. This paper presented a novel joint consisting of a precast pretensioned prestressed concrete beam, an ordinary precast reinforced concrete (RC) column, and a UHPC joint zone. To study the seismic performance of the proposed joints, six novel interior joints and one monolithic RC joint were tested under low-cyclic loads. Variables such as the axial force, the compressive strength of UHPC, the stirrup ratio were considered in the tests. The test results indicate that the proposed joints exhibit comparable seismic performance of the monolithic RC joint. An anchorage length of 40 times the strands-diameter and a lap splice length of 16 times the rebar-diameter are adequate for prestressed strands and precast column rebars, respectively. A minimum column depth is suggested as 13 times the diameter of the beam-top continuous rebars passing through the joint. In addition, a nine-time rebar diameter is sufficient for the anchorage of beam bottom rebars. The shear strength of UHPC in the joint core area is suggested as 0.8 times the square root of the UHPC compressive strength.

Optimum percentage of steel fibres of HPFRCC for reduction of transverse reinforcement in beam–column joints

Reinforcement congestion causes problems during construction, including improper concrete pouring and concrete vibration. In this study, the replacement of normal concrete by high-performance fibre-reinforced cementitious composites (HPFRCC) has been investigated in order to reduce the amount of transverse reinforcement and prevent reinforcement congestion in beam–column joints. Uniaxial compression and tension tests were used to determine the mechanical properties of HPFRCC materials. In order to investigate the effects of HPFRCC on improving and upgrading the joint strength, five half-scale exterior beam–column joint connections were tested under incremental cyclic lateral loading. The tested specimens consisted of two normal concrete control specimens with and without specific seismic details and three specimens of HPFRCC with different percentages of steel fibres. The cracking patterns, hysteresis behavior, bearing capacity, energy absorption and damping percentage of all test specimens were analysed and compared with the response of normal concrete specimens. Results show that the use of HPFRCC materials in the joint area improves seismic behavior and significantly increases load-bearing capacity, energy absorption and ductility. Moreover, it prevents the shear failure of the joint and the formation of the flexural plastic hinge in the beam.

Drift limit state predictions of rectangular reinforced concrete columns with superelastic shape memory alloy rebars

This study derives empirical drift limit state expressions for rectangular concrete columns reinforced with nickel–titanium shape memory alloy (SMA) bars using numerical simulation results obtained from well-calibrated nonlinear finite element models. A parametric study is conducted to examine the effects of various design parameters, such as the axial load ratio, section information, and material properties of concrete and SMA on the drift limit states of the columns. For each limit state (LS), a multivariate stepwise regression analysis was performed. An expression was derived to estimate the associated drift capacity of the SMA-reinforced concrete columns. In addition, the precondition for the SMA-reinforced concrete columns to ensure ductile behavior was investigated. It turned out that the axial load and the amount of transverse reinforcement are the most influential parameters on the ductility of the columns. Finally, nonlinear static and dynamic analyses were conducted for an SMA-reinforced concrete building frame to demonstrate the application of the proposed drift LSs. Both analysis results revealed that the building model tends to experience core crushing or loss of lateral load capacity prior to SMA failure.

Numerical study of low confinement spun pile to pile cap connection

The numerical study has been conducted to investigate the behaviour of spun pile – pile cap connection based on the common practice in Indonesia. The piles manufactured in Indonesia have low confinement where the amount of transverse reinforcement is less than 20% than the minimum requirement set by ACI 318-19. The research objectives are to study the parameters that affect the behaviour of that connection. Three parameters investigated are: the connection details, the effect of concrete infill and the amount of transverse reinforcement. The typical connection of spun pile and pile cap in Indonesia is considered adequate and can perform as fixed connection since slip at connection region was not detected in the FE analysis. The embedment length is adequate to develop yield strength of the prestressed wire. By filling the hollow with the concrete it increases the connection strength by 15% whereas the strength improves more than 50% by adding 6D19 reinforced bar. Adequate confinement improves the performance of the connection at peak post-stage that affects ductility.