Slab Reinforcement Research Articles

Refined numerical modelling for the structural assessment of steel-concrete composite beam-to-column joints under seismic loads

This paper proposes a refined Finite-Element (FE) numerical approach to predict both global and local behaviour of steel-concrete composite welded joints subjected to seismic loads. The reference FE model is implemented in ABAQUS and first extensively validated to the full-scale experimental results of a welded steel-concrete composite specimen tested in a past research project, where the beam-to-column sub-assemblages were designed according to the prescriptions of Eurocode 4 and Eurocode 8. As shown, due to the FE modelling assumptions, a rather close agreement was generally found between the FE predictions and the corresponding test measurements, both in terms of global and local phenomena. Therefore, it is first expected that such numerical approach could be implemented as an alternative to costly and time consuming full-scale experimental tests, allowing an extensive parametric investigation of composite joints and possible design optimizations. An implicit advantage of the implemented FE model, in fact, is that according to a refined analysis of the experimental and numerical results for the welded joint object of investigation, the efficiency of the Eurocode 8 design prescriptions for steel-concrete composite joint details can be critically discussed and reviewed. In the specific case, a possible improvement of the design recommendations for the slab reinforcement around the column is proposed.

Skew decks in reinforced concrete bridges

Abstract This research investigates reinforced concrete plates and shells with skew reinforcement whose directions are not aligned with the principal internal forces. Two normal forces, one tangential force, two bending moments, and one twisting moment are defined in the plane of the element. The analysis includes two shear forces in the transverse direction. The membrane and flexural forces are distributed between two panels at the upper and lower faces of the element. The smeared cracking model, equilibrium considerations, and plasticity approach yield the design equations of the skew reinforcement. The slab reinforcement of flat bridges, with and without lateral beams and girder bridges are compared considering different skew angles. The minimum reinforcement criteria of skew meshes are discussed. The results show that skew reinforcement yields higher steel and concrete stresses.

Bond behaviour of straight, hooked, U‐shaped and headed bars in cracked concrete

AbstractMost classic investigations on bonding properties in reinforced concrete have been performed on the basis of pull‐out tests, where a reinforcement bar is pulled out from an uncracked concrete cylinder, prism or cube. In these tests, the bond is governed by the concrete strength and bar surface properties of the reinforcement (bond index, rib geometry) or by the splitting strength of the concrete (concrete cover). In the latter case, bond failure occurs due to uncontrolled cracking of the concrete specimen. In contrast to these fundamental tests, bond in many structural members is activated within already cracked concrete. This is particularly relevant for the reinforcement in beams and slabs (both for flexural and transverse reinforcement), as the reinforcing bars might be located at planes where flexural cracks develop. The opening of these cracks along the reinforcement is nevertheless not uncontrolled (as opposed to splitting failures), but it is governed by the bending deformations. The bond properties and strength of the reinforcement in actual members are therefore influenced by the opening of these cracks and are potentially different from those observed in classic pull‐out tests.The present paper aims to address this topic by presenting the results of an experimental investigation with 89 monotonic pull‐out tests performed on cracked ties. The opening of the cracks was controlled while transverse bars – located in the plane of these cracks – were pulled out from the specimens. The tests were performed for crack openings ranging from 0.2 mm to 2.0 mm in order to cover conditions both at the serviceability and ultimate limit states. The results show a very significant influence of in‐plane cracking on both strength and bond‐slip stiffness, with decreasing mechanical performance for increasing crack openings. The performance of different actual anchorage types (straight, hooked, U‐shaped and headed bars) – generally characterized through force‐slip relationships – is finally analytically investigated and compared to the test results.

Fire resilience of composite beams with simple connections: Parametric studies and design

Simple (shear) connections are designed to resist vertical shear forces resulting from gravity loading. During a fire event, floor systems expand and rotate, imposing axial forces (compressive and tensile) on simple connections. This paper presents the results of analytical parametric studies conducted to expand the experimental database and evaluate the structural response of composite beams with simple connections subjected to combined gravity loads and design fire scenarios. The parametric studies were conducted using 3D finite element models of partial frame assemblies consisting of composite beams with simple connections. These models were developed and benchmarked previously by the authors [1] using experimental data [2, 3]. These models explicitly accounted for the effects of geometric and material nonlinearity including concrete cracking and crushing, steel yielding and fracture, and the interaction between various structural components. The parameters included in the analytical studies were the connection type (shear-tab, single-angle, double-angle), deck type (flat slab, perpendicular metal deck), composite slab continuity across multiple bays, slab reinforcement type (wire mesh, mild steel), fire resistance rating of the design, and the design fire scenario (1hour fire with cooling, 2hour fire with cooling, E119). The results from the parametric studies indicated that for typical composite beams and simple connections designed with certain (n-hour) fire resistance rating, and subjected to design fire scenario with the same (n-hour) heating followed by cooling, there was no premature fracture failure of the connection leading to collapse of the composite beams. The continuity of composite slab across bays and the steel reinforcement has a significant influence on the performance of the simple connections, and the resilience of the composite beam system.

Experimental investigation of RC beam-slab substructures against progressive collapse subject to an edge-column-removal scenario

When a reinforced concrete (RC) frame subjected to an edge-column-removal scenario, its floor system exhibits a complicated mechanism against progressive collapse due to the interaction between beams and slabs and the two-way load transfer characteristics. In this study, laboratory tests of five 1/3-scaled RC frame substructure specimens, including four beam-slab specimens and one beam specimen without a slab, are reported. The effect of critical structural parameters (i.e., the beam height, slab thickness and seismic reinforcement) on the collapse resistance was investigated by analyzing the applied loads, structural deformations and material strains. The RC slabs contribute to an increased collapse resistance of the control beam-slab specimen by 146% under small deformations (i.e., the beam mechanism) and 98% under large deformations (i.e., the catenary mechanism) compared to the beam specimen. The resistances of the beam-slab specimens were mainly provided by the slabs and the beams in the direction along the free edge, more significantly by the portion close to this edge where the largest deformation was developed. Increasing the seismic reinforcement in the beams resulted in a much larger collapse resistance under both beam and catenary mechanisms. In contrast, increasing the slab reinforcement by expanding the slab thickness marginally improved the collapse resistance under the catenary mechanism but contributed little to that under the beam mechanism. Increasing the beam height also largely improved the collapse resistance under the beam mechanism, but had limited impact to that under the catenary mechanism.

Design recommendations for achieving “strong column-weak beam” in RC frames

This paper deals with the realization of ductile failure of RC frames with cast in-situ slabs. The cyclic loading experiments of two spatial reinforced concrete (RC) frames, a control specimen and a RC frame with cast in-situ slabs, are carried out. The failure pattern and the role of slabs are experimentally studied. Tests results indicate that slabs can change the failure pattern of RC frames from a typical “strong-column-weak-beam” failure to the “strong-beam-weak-column” failure. The slab reinforcement enhances the resistance moment of the beams, which is the main reason to form “strong-beam-weak-column”. The parameters such as axial compression ratio, thickness of slab, concrete strength, reinforcement ratio of slab and stiffness of transverse beam are investigated by Finite Element Analysis (FEA). Strut and tie model of slab is proposed to calculate the effective slab width. Simple equations are derived by a 95% guarantee rate based on 200 models. The required ratio of column-to-beam strength is studied based on the tests and FEA data. A reasonable required ratio is proposed to avoid the brittle failure of the RC frames for seismic design.

Design of Concrete Slabs Reinforcement – Methods of FEM Results Interpretation Incorporating Twisting Moments

There are many analytical methods to solve internal forces of slabs. But structural engineers work with structural analysis software, based mainly of finite element method (FEM). Interpretation of that analysis results may lead to errors during concrete slab reinforcement design. There are some methods to solve the problem. Most familiar are Wood-Armer equations, introduced in 1968. But other methods exists furthermore. It is common, that design engineer doesn’t know, which method his program uses. This article deals with common – not only Wood-Armer - methods of concrete slab reinforcement design incorporating twisting moments, design of skew angle reinforcement etc. Description of selected software approach.

Efficient progressive collapse analysis for robustness evaluation of buildings experiencing column removal

Progressive collapse analysis is used to evaluate robustness of buildings against unexpected extreme events and is a highly nonlinear dynamic problem. Motivated by the need for realistic and yet reasonably fast analysis, a “middle-way” approach for efficient progressive collapse analysis (ePCA) is explained in this paper, accounting for column buckling, semi-rigid connection and membrane action of slab. The accuracy of ePCA is validated by comparing with experimental results available in the literature. Application of ePCA on realistic building systems shows that robustness can be improved cost-effectively by introducing minor changes to the steel connection and slab reinforcement. The computational efficiency of this method enables incorporation of robustness design in building system in the early stage of design process. It can also be useful for researchers to more efficiently evaluate the behavior of different structural systems.

Stud Rail Systems as Shear Reinforcement in Beams and One-Way Slabs

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Design of Approach Slab of Road Bridges

This paper deals with the design and assessment of approach slabs of the transition zones of road bridges. The main goal of this article is to evaluate design criteria of this reinforced concrete member with attention to the requirements according to European standards. Up to these days, used procedure for the design of the approach slabs of the bridge construction in Slovakia is based on previous Czechoslovak CSN 73 6244 standard introduced 1981. The reason for using this standard nowadays is mainly because of its simple form as an engineering tool for the design of the geometrical dimension and also the reinforcement of the approach slab. Design procedures used for the analysis are taking into account the simplest and also the most conservative structural schemes, it is the slab with a hinge support in the place of the abutment and the second simple support on the other side substituting the gravel subsoil. This scheme should represent possible consolidation of the embankment under the slab. For more precise description of the behaviour, models with elastic support are used. Using these models, the approach slabs will be analysed for the ultimate limit state. The analysis of the results for various geometrical dimensions could serve as a template for similar design tool, to which bridge designers are used to.

Experimental investigation of progressive collapse resistance of one-way reinforced concrete beam–slab substructures under a middle-column-removal scenario

Floor systems composed of beams and slabs are critical structural elements of frame structures to resist progressive collapse. Previous experimental studies have focused mainly on beam–column or continuous-beam substructures and have ignored the influence of the slab. To study the progressive collapse-resisting mechanisms of reinforced concrete (RC) floor systems, seven 1/3-scaled one-way substructure specimens, including five beam–slab specimens and two continuous-beam specimens without slabs, were tested under a middle-column-removal scenario. The effects of various structural parameters, including sectional dimensions (beam height, slab width, and slab thickness) and seismic reinforcement, on the progressive collapse resistance were studied by analyzing material strains and load–displacement curves. Under small deformations, the progressive collapse resistance was largely affected by the beam height, slab width and seismic reinforcement in the beams. However, the effect of the slab width, upon exceeding the effective flange width, became insignificant. Note that increasing the slab thickness simultaneously increased the amount of slab reinforcement according to the minimum requirement of reinforcement ratio for slabs, such an increase will in turn enhanced the progressive collapse resistance. In addition, the existence of the slab led to an over-reinforced damage in the compressive zones of the beam ends, which accelerated the bending failure and the presence of the catenary action of the specimens. Under large deformations, the progressive collapse resistance was mainly influenced by the reinforcement area of the entire beam–slab section. The total reinforcement area of a beam–slab substructure designed to meet a higher seismic requirement was not significantly increased, and consequently, the progressive collapse resistance of the substructure under the catenary mechanism was not notably improved. This finding stands in stark contrast to those of previous tests of beam–column specimens without slabs.

Experimental Studies on Cohesion of Carbon Fibre Reinforced Polymer for Reinforcement of Bridge Deck Slabs

Bridge deck slabs reinforced by carbon fibre reinforced polymer (CFRP) are subjected to a number of problems related to cohesion, such as slippage, disengaging, and debonding. In order to address such problems, we conducted indoor shear, stretching, and debonding tests, analysed the change rule of cohesion in these three different environments, and determined the optimum construction technique for improving the cohesion of CFRP. We found that to obtain acceptable shear strength, manufactured sand should be spread on the surface of the impregnating resin adhesive. Soaking, freeze thawing, and wheel grinding processes affected the tensile strength of the interface, which was related linearly to the shear strength of the interface. Following wheel grinding on an asphalt surface at high temperatures, the test value for the bearing-debonding capacity on a test slab was unchanged, and the effectiveness of the CFRP was still apparent. The influence of high-temperature wheel grinding can be ignored. This study can serve as a reference for the design of reinforced bridge deck slabs.

Punching shear capacity of reinforced concrete slabs with headed shear studs

Punching shear in slabs is analogous to shear in beams. Despite this similarity, current design codes provide distinctly different methods for the design of shear reinforcement in the two situations. For example, the Eurocode method for beam shear design is founded on the theory of rigid plasticity. To design shear reinforcement in slabs, on the other hand, the engineer must settle for an empirical equation. The aim of the study reported is to demonstrate that it is possible in a simple manner to design shear reinforcement in slabs based on the same rigid-plasticity foundation as for beam shear design. For this purpose, an extension of the upper-bound crack sliding model is proposed. This involves analysis of sliding mechanisms in yield lines developed both within and outside the zone with shear reinforcement. Various types of headed shear studs were considered. The results obtained using the model were compared with a large number of published test results, and satisfactory agreements were found.

Optimized Reinforcement Detailing Design for Sustainable Construction: Slab Case Study

Reinforced steel rebar is commonly supplied in one-dimensional stocks and typically designed for and installed in various structural components in civil and industrial construction. Surplus reinforcement constitutes a major fraction of construction generated waste. Cutting one-dimensional stocks to suit construction project requirements results in cutting losses. Therefore, reducing steel waste (or minimizing cutting losses) has long been the focus of academic research in one-dimensional stock design and cutting problems. Previous research developed mathematical models in an attempt to analytically minimize cutting losses based on preliminary engineering designs, but little insight has been provided on how to integrate minimization of cutting losses and engineering design into an integrated optimization problem, let alone considering minimizing total steel rebar installation cost as a parallel objective. The sustainability issue in regard to balancing reinforcement waste and crew installation costs on the basis of optimized engineering design has yet to be addressed. This study introduces a Mixed Integer Programming (MIP) approach to generate optimal cutting patterns, minimum cutting losses and associated total installation cost. A reinforced concrete slab case is adopted as a test to show that the proposed methodology is capable of producing optimal tradeoff solutions in slab reinforcement detailing design, resulting in less wastage and lower crew installation cost.

도로교 RC 상판 보강을 위한 탄소섬유 기초 carbon fiber sheet와 carbon fiber strand sheet의 역학특성

Recently, a lot of interest has been shown in structural maintenance managements of civil infrastructures. Many researchers have been conducted on various maintenance techniques and repair materials. Among other fiber materials the carbon fiber materials are especially focused on maintenance management of Highway Bridges. Extensive work has been done on Carbon Fiber Sheet (CFS). Nevertheless, Carbon Fiber Strand Sheet (CFSS) is a newly developed material, on which limited work has been done until now. Therefore, in this study bonding the CFSS to RC slab specimen and fatigue resistance evaluation has been conducted. The results demonstrated an increase of 25.3 times more reinforcement of RC slab compared to non-reinforced RC slab. Moreover, compared to CFS-bonded RC slab, The CFSS-bonded RC slab showed 1.2 times greater reinforcement.

Experimental study and theoretical analysis of partially encased continuous composite beams

The mechanical behavior and the law of moment redistribution of the partially encased continuous composite beam (PECCB) are investigated in this paper. One common continuous composite beam and three PECCBs were tested. The main variable parameter of the three PECCB specimens is the amount of longitudinal reinforcement in the concrete slab of hogging moment region. Owing to the contribution of the web encasement, PECCBs have much slower crack propagation speed in the hogging moment region and higher ultimate capacity. The web encasement also contributes to postponing the yielding of the bottom flange, and therefore prevents local buckling in the steel girder. As the amount of reinforcement in the concrete slab of hogging moment region increases, the degree of moment redistribution of PECCBs decreases. On the basis of analyzing the moment redistribution requirement for plastic design and the ultimate rotation capacity of hogging moment section, the required and available moment redistribution factors of PECCBs are obtained. A parameter Rp is introduced to evaluate the relative relationship between the longitudinal reinforcement in the concrete slab and the rest part of the partially encased section, and the value of Rp should be restricted within 0.35 to enable full moment redistribution for plastic design of PECCBs.

Assessment of cover to reinforcement in slabs using different spacer and tying distances

ABSTRACTGiven that the durability of structures is directly related to the thickness of the cover to reinforcement, it is essential to ensure minimum cover is achieved when building a structure. However, studies show that this is not always the case. One of the reasons for the this problem is the lack of indication, in structural projects, of the positioning of spacers. This situation is compounded by the lack of standards and regulations on the use of spacers (i.e. minimum quantities and required layout). This study assesses three different spacer distances and two different tying distances in order to examine the influence of these factors in the cover to reinforcement. To achieve so, three slabs were prepared in situ using different arrangements. After demoulding, cover thickness was determined using a digital covermeter. It was observed that the factors investigated have a significant influence in the final cover.

Steel Fibres as Reinforcement in Post-tensioned Flat Slabs

This paper presents a numerical investigation of a post-tensioned flat slab without conventional longitudinal reinforcement; only steel fibres were employed to ensure sufficient ductility and shear capacity of the slab. Results were taken from a reported experiment conducted on a full scale steel fibre-reinforced flat slab (0.38% fibre content) which was tested until failure, undergoing a ductile bending failure. A nonlinear finite element analysis was employed to study the experiment, including the ultimate state. A parametric study was performed using the numerical model to investigate the influence of the tensile behaviour of steel fibre-reinforced concrete (SFRC) on the structural response. The model proved to be relatively sensitive to changes in the tensile behaviour, but the differences were not prominent until entering the nonlinear area of the load-displacement curve. A constant curve with tensile stress equal to the residual tensile strength of the SFRC provided a robust numerical model and results on the conservative side. Including a peak stress with a multilinear tensile curve provided a less stable analysis but more accurate results. However, the model behaviour was stiffer than the experiment, providing too small deformations at failure. Nevertheless, the numerical model was able to display the ductile bending failure mode and moment redistribution.

Seismic response of bent caps in as-built and retrofitted reinforced concrete box-girder bridges

Bridges are key components of infrastructure that are vulnerable to earthquakes and many are undergoing retrofit or complete replacement. Rigorous seismic design of new bridges and informed retrofit decisions are indispensable. A specific design issue that is concerned with the structural response of bent cap beams in as-built and retrofitted box-girder bridges under gravity and seismic loads is tackled in this paper. A combined experimental and computational research was undertaken in this study to investigate the bent cap capacity and effective slab width in reinforced concrete box-girder bridges for enhanced seismic capacity design approach. Two large-scale as-built and retrofitted column-bent cap-box-girder subassemblies were developed and tested using bidirectional quasi-static cyclic loading and hybrid simulation approach, respectively. In addition, detailed finite element models were calibrated and further used to complement the experimental programs. The study revisited the effective slab width code values for bent caps and concluded that the slab reinforcement within an effective width, especially in tension, should be included for accurate bent cap capacity estimation. Accordingly, recommendations are suggested for the relevant bridge seismic design codes.

Load-resisting mechanism to mitigate progressive collapse of flat slab structures

For reinforced concrete flat slab structures, column failure due to unexpected extreme loading may significantly increase the moment and shear force at adjacent slab–column connections, which can trigger punching shear failure at these connections and result in progressive collapse of flat slab structures. To determine the possible load-resisting mechanisms developed in flat slab structures in resisting progressive collapse, three multi-panel flat slab substructures are tested in this study. A flat slab substructure is tested as a control specimen. The remaining two specimens are strengthened by adding drop panels or post-installed shear bolts. Punching shear failure is observed in the control specimen and the specimen with post-installed shear bolts after reaching their yield loads. However, even the control specimen without any strengthening exhibited considerable post-failure resistance and deformation capacity mainly due to sufficient integrity reinforcement, which had been installed in the bottom of the slab. Test results indicated that drop panels not only increase the flexural resistance but also raise the punching shear resistance of the connections significantly. Thus, no obvious punching shear failure is observed in the specimen with drop panels. However, post-installed shear bolts can only increase the punching shear resistance. For post-failure behaviour, this is not very effective. The test results demonstrated that flexural failure dominates the performance of test specimens, as relatively low slab reinforcement ratio but high span/thickness ratio was designed.