Steel Ratio Research Articles

Compressive behavior of circular steel tubed high-strength concrete columns with embedded steel tubes

This paper presents an experimental study on circular steel tubed high-strength concrete columns embedded with steel tubes under axial compression. A total of 18 columns were tested in this study to investigate several parameters covering the continuity conditions for external steel tubes, diameters of internal steel tubes, thicknesses of external and internal tubes, and the number of CFRP layers. Test results show that the strength of specimens is improved to different levels due to the discontinuous external tubes and embedded internal tubes, and a significant enhancement can be achieved in terms of ductility and residual strength. Moreover, the concrete core as well as the sandwich concrete exhibits superior compressive behavior as the internal steel tube diameter increases under a constant steel ratio. Besides, when the external tubes are wrapped with CFRP jackets, more effective confinement can be provided for the inner concrete, while the effect of the position of internal steel tubes become less obvious. Finally, several existing models are selected to predict the axial stress-strain responses of the concrete in tubed columns. It is found that only two models can provide relatively reasonable predictions for the average axial stress-strain curves of concrete, but considerable overestimations of the ultimate stress are still observed for these two models, which indicates that further investigation is needed to develop a more accurate model.

Axial compressive property of UHPC plate-CFST laced composite columns

An experimental and analytical investigation of ultrahigh-performance concrete (UHPC) plate-concrete-filled steel tubular (CFST) laced composite columns are presented in this study. A total of 11 stub specimens under axial compression are conducted to quantify the column failure mechanism. Experimental results indicate that the compressive behavior in the UHPC plate dominated the failure mechanism of the UHPC composite columns, and the failure mode is dependent on the peak strain of the UHPC plate and CFST limb. The two main parameters in the tests are the thickness of the steel tube (t) and UHPC plates (tp), which significantly enhance the bearing capacity of UHPC composite columns. Herein, the finite element standard model is established to investigate the influence of various parameters on the compressive behavior of UHPC composite columns. The diameter of the steel tube D is the most sensitive parameter affecting the structural compressive performance, while the ratio of steel reinforcing bars has an ignorable influence on the compressive strength. Based on the ultimate limit states and parametric analysis, a unified formula for predicting the strength of UHPC composite columns is proposed, which is generally in good agreement with the experimental results.

Strut-and-tie model for FRP effectiveness in shear strengthening of RC deep beams

In this article, a strut-and-tie model (STM) is proposed for predicting the ultimate shear capacity of fiber reinforced polymers (FRP)-strengthened reinforced concrete deep beams. The proposed STM accounts for the effect of concrete strength, FRP ratio, ratio of main steel, horizontal and vertical stirrups ratio and shear span-to-depth ratio. The ultimate shear predictions of the proposed model are validated with 55 test results from the literature. The comparison showed that the proposed model performs well in predicting the ultimate shear capacity of FRP-strengthened RC deep beams. The overall average value of the ratio between the experimental capacity to the theoretical capacity of the proposed STM (Pu( EXP )/Pu( STM ) ) is of value 1.16 with a standard deviation of 0.18. Also, comparative studies between the proposed modified STM and the STM provided by the ACI code and other researchers in the literature are presented. Finally, FRP effectiveness studies are performed to study the effect of many structural parameters on the ratio between the ultimate strength for RC deep beams strengthened with FRP materials and the ultimate strength for ordinary RC deep beams. These parameters are FRP ratio (ƿ FRP), the fiber orientation angle to the longitudinal axis of the deep beam, material type of FRP, concrete compressive strength (fc ’) and the shear span-to-depth (a/d) ratio

Analysis on Seismic Performance of Steel-Reinforced Concrete-Filled Circular Steel Tubular (SRCFST) Members Subjected to Post-Fire.

Steel-reinforced concrete-filled steel tubular (SRCFST) columns have a great development prospect in engineering practice due to their high load-bearing capacity, good ductility, and energy consumption capacity. This paper established the post-fire seismic analysis model of SRCFST with a circular-cased H section using the sequential coupled thermal-stress method by ABAQUS. The P-Δ curve, stiffness, ductility, and energy dissipation were calculated. Then, the post-fire seismic performance of CFST members was compared while keeping the total steel ratio constant, and it revealed that the SRCFST had superior ductility to CFST. Finally, the ductility coefficient and skeleton curve were parametrically evaluated. The results of the study showed that the effects of heating time (th), axial compression ratio (n), slenderness ratio (λ), and steel tube ratio (αt) on the skeleton line of SRCFST columns are more significant; the axial compression ratio (n), slenderness ratio (λ), and steel tube ratio (αt) have a negative influence on the ductility subjected to post-fire.

Reverse design-based optimizations for reinforced concrete columns encasing H-shaped steel section using ANNs

ABSTRACT This study aims to propose artificial neural networks (ANNs) to solve structural engineering problems and to provide optimal designs for reinforced concrete columns with H-shaped steel sections. A reverse scenario based on preassigned safety factor () for a factored biaxial load, steel ratio (), and aspect ratio of columns () is presented, meeting code requirements. A back-substitution (BS) method using a chained training scheme with a revised sequence (CRS) is implemented to optimize training. Effects of rebar and steel ratios on objective functions, cost index (), emission, and column weight () are identified. Three-dimensional interaction diagrams are obtained based on optimized , emission, and based on preassigned equivalent to 1.0. Predictions from ANNs are ascertained using structural mechanics, demonstrating a significance of an accuracy of the optimized design obtained by the proposed ANNs. This study provides a useful and practical design for SRC columns by lessening engineer’s effort while increasing design accuracies.

Behavior of circular-steel-tube-confined square CFST short columns under axial compression

This paper proposes circular-steel-tube-confined square concrete-filled steel tube (CFST) to improve the mechanical property of the newly built and existing deficient square CFST. The external adding circular steel tube does not enter the beam-column connection area and mainly functions as lateral confinement to the inner square CFST. Twelve circular-steel-tube-confined square CFST short column specimens and twelve conventional square CFST short column specimens were designed and tested under axial compressive load. The main variables of the test include column type, steel ratio, outer circular steel tube size, and concrete strength. The overall failure modes, axial load-displacement curves, macroscopic deformation characteristics, and the inner and outer circular steel tube confinement were analyzed and compared systematically. The study revealed that the outer circular steel tube yielded horizontally in tensile strength and provided effective confinement to the inner square CFST. Therefore, the bearing capacity of the square stub column was improved significantly, and the brittle failure in the confined area was prevented. Due to the effective confinement by the outer circular steel tube, the stress state in the square steel tube was much more uniform. The column configuration led to an effective but uneven confinement in the outer circular steel tube, concentrating near the square steel tube corner. Based on the superposition method, a suitable design method is proposed to predict the compressive strength of circular-steel-tube-confined square CFST, and the theoretical predictions are in good agreement with the test results of this paper.

Effect of Cu and solid solution temperature on microstructure and mechanical properties of Fe-Mn-Al-C low-density steels

Relatively low yield strength (YS) level restricts austenitic low-density steels to some applications requiring small or no deformation than impact energy absorption. Therefore, to enhance the YS of this type of steel, this research investigates variations of microstructure and mechanical properties of two austenitic low-density steels (Fe-25Mn-10Al-1.1C-(0,0.25) Cu) with solid solution temperatures (TS, 850 °C–1050 °C). The results show that TS and Cu have significant impacts on the microstructure characteristics and mechanical properties of the two tested steels. At low TS of 850 °C, an increase of 170 MPa in YS compared with that of Cu-free steel was obtained in Cu-containing steel, which is mainly attributed to much finer grain size, more fine twins, and high dislocation density of the latter. An increase in TS caused the κ-carbides in the two steels to dissolve completely, but the austenite content in Cu-containing steel remained largely stable for the addition of Cu. As a result, the strength of the two tested steels decreased while the ductility increased, and both the strength and yield-to-tensile strength ratio of Cu-containing steel are remarkably higher than those of Cu-free steel. It is concluded that the Cu-containing steel exhibits a superior combination of YS and ductility together with a ∼17% reduction in density compared to pure iron. This study provides a promising way to further enhance the YS of austenitic low-density steels through the addition of a small amount of Cu and tailoring TS.

Experimental Study on Axial Compression Performance of Thin-Walled Steel Tube Lightweight Concrete Columns With Composite Section

To provide references for the use of fabricated thin-walled cold-formed steel tubular lightweight concrete columns, the axial compression tests were conducted on single-limb and double-limb specimens. The results were compared with hollow specimens to analyze the effects of the cross-sectional size of the members, wall thickness, and steel ratio of the sections on the axial load-bearing capacity of the specimens. The test results show the following: 1) under the axial load, the typical damage of the specimens is local buckling of the steel tube and local crushing of the concrete. 2) The ultimate bearing capacity of the double-limb specimen is the highest. For the specimen with the sectional height-width ratio of 1.43, the ultimate bearing capacity of the double-limb specimen is 275.71% higher than that of the hollow steel tubular column and 241.49% that of single-limb specimens. 3) Based on the material superposition strength theory, the calculating method of the axial load-bearing capacity of thin-walled cold-formed steel tubular lightweight concrete columns was proposed and verified by comparing with the test results, which provides the design guidance for such members.

Experimental behavior of concrete-filled thin-walled corrugated steel tubes with large helical angles under monotonic and cyclic axial compression

Concrete-filled thin-walled galvanized corrugated steel tube (CFCST) is an attractive composite member consisting of an external corrugated steel tube (CST) multi-functionally used as a confining tube, an anti-corrosive shell, and a permanent template. In practice, the CSTs with large helical angles may be employed in some projects; their working mechanism may be different from that of CSTs with small helical angles. Therefore, 26 short columns, including 17 CFCSTs with large helical angles, are tested under monotonic and cyclic axial compression. The main experimental variables are the diameter-to-thickness ratios of external CST, loading patterns, and volumetric steel ratios of transverse stirrups. The failure modes, axial load–displacement responses, plastic strains, and stress deteriorations have been discussed carefully. The test results showed that the slippage of lock-seam weakens the combined effect between the CST and core concrete, leading to a relatively lower strength and ductility. The strain and stress properties of CST having a large helical angle are analyzed, and the work mechanism of CFCST has also been examined. Based on the test results and the comparison with existing models, design suggestions are proposed eventually.

Experimental study on bearing capacity of steel and steel fiber reinforced concrete composite beams without rebar cages

In consideration of the interference between H-shaped steel and rebars and the difficulty of concrete pouring in SRC structure, steel fibers rather than the rebar cage in SRC beam are used to form the steel and steel fiber reinforced concrete (SSFRC) beams. Based on the two-point symmetrical loading experiments of 18 SSFRC specimens without rebar cages, the failure mode and load–deflection curve of each specimen were obtained. Thus, the effects of shear span ratio, H-shaped steel ratio and steel fiber volume ratio on the mechanical properties of beams were investigated. With the increase of the shear span ratio, the bearing capacity of the specimens presented a decreasing trend. Benefitting from the “bridging” effect of steel fibers, the failure mode of the specimen was less affected by the shear span ratio. When H-shaped steel ratio was greater, the bending and shear resistance of H-shaped steel could be fully utilized. Additionally, the composite action between materials was weakened when the thickness of concrete cover decreased. Increases in the steel fiber volume ratio could delay the elongation of cracks and reduce the sudden decrease of bearing capacity caused by the damage of the bonding interface between H-shaped steel and SFRC, therefore improving the bearing capacity. According to the superposition principle, the shearing capacity formula of SSFRC beam was proposed. By introducing the bond coefficient and considering the coupling effect of different parameters on the collaborative work of H-shaped steel and SFRC, the calculation formula of bending capacity with high accuracy was obtained. The coefficient could be used to objectively and quantitatively identify or predict the failure type of specimens, providing reference for engineering design.

Industrial-environmental management in China's iron and steel industry under multiple objectives and uncertainties

Industrial-environmental management is a multi-objective optimization problem plagued with multiple uncertainties. Most studies only optimize few objectives and often neglect these uncertainties. This study builds a 6-objective optimization problem to quantify energy conservation and emission reduction (ECER) potentials in China's iron and steel industry. First, uncertainties are simulated through 100,000-time random sampling, NSGA-II and the mean-effective objective mechanism are applied to calculate optimal solutions. Finally, a global sensitivity analysis is performed to classify uncertainty parameters based on their impacts on objectives' performance. Results show: (1) There exist significant discrepancies between objectives' performance under certainty and uncertainty. For example, the deterministic CO2 intensity is 1148 kg/t, which is 11.93% lower than its value under uncertainty. Therefore, neglecting uncertainty increases the risk of noncompliance with policy targets as they might be too strict; (2) Two critical uncertainty parameters (steel ratios and technology penetration rates) have the most severe impacts on objectives' performance, hence, reducing their fluctuation can minimize uncertainties when estimating ECER potentials; (3) By-product recycling and energy efficiency measures have good performance in all objectives, thus, should be prioritized. Moreover, from 77-strategies assessed, 11 are identified as key-strategies due to their large ECER effects, hence, should be strongly promoted.

Axial-load response of CFST stub columns with external stainless steel and recycled aggregate concrete: Testing, mechanism analysis and design

Recycled aggregate concrete filled stainless steel tube (RAC-FSST) is a new type of composite member combining the advantage of stainless steel and RAC. In this paper, a total of twenty-four RAC-FSST stub columns were tested under axial load, considering the influences of coarse recycled aggregate (CRA) content, steel ratios and compressive strengths of RAC. The obtained results, including the failure patterns, responses of axial load vs. deformation, stress states of external stainless steel tube and inner RAC and confinement effects, were systematically analyzed. Results indicated that all specimens presented good ductility and high residual strengths after reaching the maximum axial load. The elastic stiffness of RAC-FSSTs obviously declined with the increasing CRA content, while the strain at the ultimate load was larger. The inclusion of CRA could advance the occurrence of the confinement and lead to lower confining stress. Based on the experimental results, an analytical model with consideration of confinement action was developed to predict the axial response of RAC-FSST stub columns. Besides, the current design provisions for the normal CFST and RAC-FST members were employed to evaluate their applicability to RAC-FSSTs. In general, the design rules EN 1994-1-1:2004, GB 50936-2014 and T/CECS 625-2019 gave a conservative and relatively accurate prediction on ultimate strength of RAC-FSST stub columns.

Flexural performance and ductility of RC beams made using natural LWA

This paper evaluates experimentally the feasibility of utilizing local natural lightweight aggregates (LWA) for designing reinforced concrete (RC) beams. The aggregates investigated were of scoria origin, with an absorption capacity of 10%, and dry densities ranging between 860 and 1120 kg/m3. A series of full-scale 16 simply supported RC beams of rectangular cross section were designed using two concrete strengths, normal and high, and two tension steel ratios of the balanced reinforcement ratio, following ACI code requirements. For the lightweight concrete (LWC) mixes designed, the replacement level of normal weight aggregates (NWA) with coarse LWA was 100%, whereas the replacement level for fine LWA was 62.5%. Data presented include load-deflection, and moment-curvature graphs, cracking behavior, mode of failure and ductility indexes. Test results indicated that the flexural response of LWC beams was similar to that of normal weight concrete (NWC) beams. All the LWC beams tested failed in ductile flexural mode, exhibited relatively larger mid span deflections, higher curvatures, and wider cracks, compared to NWC beams of equal concrete strength and steel ratio. The most influential parameter on the ductility indexes of LWC beams was the steel ratio. Moreover, the scoria LWA investigated can be used successfully in structural design.

Behavior of encased steel-high strength concrete columns against axial and cyclic loading

There is limited information on the performance of encased steel-concrete (ESC) columns constructed of high-strength concrete. For instance, the analytical approach of Eurocode 4 applies only to columns made of normal weight concrete in the strength classes C20/25 to C50/60. As such, this paper presents an experimental investigation of the mechanical behavior of ESC columns made from normal strength (NS) concrete class C24/30 and high strength (HS) concrete class C56/70. The experiments comprise twelve ESC columns: four of which are axially loaded, while the other eight are subjected to axial load combined with lateral cyclic loads. The main design parameters investigated in the conducted experiments are concrete compressive strength, encased steel ratio, and axial (compression) load ratio. Relative effects of these parameters on strength enhancement, stiffness degradation, displacement ductility, and mode of failure are examined.Measured experimental results showed that ESC columns built of HS concrete failed abruptly with cracks passing through aggregates, whereas ESC columns made of NS concrete failed more gradually. For columns with encased steel ratios of 4% and 6%, the ESC columns made of HS concrete had ductility indices that are less than those made of NS concrete by up to 25% and 22%, respectively. Nevertheless, ESC columns built of HS concrete were able to withstand up to 24% more lateral loads than corresponding ESC columns made of NS concrete. Furthermore, increasing the axial load ratio resulted in an increase in the columns lateral load capacity accompanied by a loss in ductility and stiffness degeneration beyond the peak point.

Experimental and numerical study of fire performance of L-shaped concrete-filled steel tubular columns under eccentric compression

Six special-shaped columns composed of concrete-filled steel tubes (CFST) were designed and tested based on the ISO-834 temperature curve to access their fire performance under eccentric loads. Key test variables include the types of stiffener, load eccentricity, as well as load angle, and the failure modes, temperature distribution, and deformation of specimens were analyzed. The test outcomes indicate that the L-shaped CFST columns’ fire resistance declines as the load eccentricity rises, but its decrease amplitude narrows under a relatively high eccentricity. Besides, the impact exerted by the load angle on the columns’ fire resistance is also significant. A Finite Element Analysis (FEA) method was carried out afterward and compared with the test outcomes. Moreover, an extensive parametric analysis containing 2008 FEA models was developed to study the impact of major parameters on the L-shaped CFST columns’ fire resistance, namely load ratio (n), slenderness ratio (λ), column limb width (B), column limb depth-to-width ratio (D/B), load eccentricity ratio (e/r0), steel ratio (αs), steel strength (fy), concrete strength (fck), and load angle (θ). The analysis results imply that the parameter of load ratio (n) remarkably affects the columns’ fire resistance, while the other parameters exert a certain impact on the fire resistance when n < 0.5. Given the test and analysis, a simplified calculation approach for the fire resistance of L-shaped CFST columns is designed.

Energy dissipation in reinforced concrete members before or after yielding

AbstractModels for the pre‐ and post‐yield energy dissipation in reinforced concrete (RC) members are developed using a database of about 1200 tests with over 18,000 load cycles. Before yielding, the equivalent viscous damping ratio exceeds 5%, especially if the member is initially uncracked. It can be considered by using either a dedicated pre‐yield hysteretic model with parameters established in the paper or global viscous damping and special provisions against double counting of energy dissipation. The post‐yield model can be combined with various post‐yield hysteretic laws. It accounts for the member's type, shear‐span‐to‐depth ratio and likely failure mode, the continuity, ratio, and surface characteristics of longitudinal bars, and for FRP wrapping, if any. Alternatively, it recognizes only four member types but accounts also for the transverse steel ratio and the normal stress due to the axial load.

Seismic behavior of prefabricated steel reinforced concrete shear walls with new type connection mode

This paper proposed a new connection mode of prefabricated steel reinforced concrete (SRC) shear wall. The vertical rebars inside the edge concealed columns of the prefabricated SRC shear wall were welded by lap welding, the section steels inside the edge concealed columns of the prefabricated SRC shear wall were welded by ending plates, and the connection of the wall panel in the middle part of the shear wall was realized through the bolt connection between the wall footing unit and the connecting steel plate. Then the concrete was finally poured in the connecting area of the edge concealed column of the shear wall. In order to study the seismic behavior of this new type prefabricated SRC shear walls, the pseudo-static test and nonlinear finite element analysis were carried out. At the same time, parameterized analysis was carried out on the key factors affecting the new type prefabricated SRC shear wall, including axial compression ratio, concrete strength grade, steel ratio of edge concealed columns, reinforcement ratio of vertical rebar section and thickness of connecting steel plate. The research results show that the failure mode of the new type prefabricated SRC shear wall is the same as that of the integrally cast SRC shear wall, and the seismic behavior and bearing capacity are slightly lower, but the difference is very small and in an acceptable range. The increase of axial compression ratio, concrete strength grade and steel ratio of edge concealed column will increase the bearing capacity of the new type prefabricated SRC shear wall, but at the same time, it will reduce the ductility of the new type prefabricated SRC shear wall. The reinforcement ratio of vertical rebar section and the thickness of the connecting steel plate have little effect on the mechanical performance of the new type prefabricated SRC shear wall. Finally, the reasonable value range of the key parameters affecting the mechanical performance of the new type prefabricated SRC shear wall and the calculation method of the shear capacity of the horizontal connection was proposed, which provided a basis for the subsequent design.

Experimental strength evaluation of steel-polypropylene hybrid fibre reinforced concrete

Reinforced concrete structures collapse by developing tensile and flexural crack patterns. The properties of the concrete must be researched to prevent these forms of failure and enhance the overall strength and durability. The inclusion of fibres in the concrete increasing the strength of the concrete and prevents micro-cracks that lead to a major failure. In this study, two different types of fibres are considered for inclusion in the concrete as Hybrid fibre in which one has higher moduli (Steel fibre) and one has lesser moduli (Polypropylene fibre). This hybrid material formulation can reduce the probability of brittleness and micro-cracks forming in concrete. The mix design used for this investigation is M40 grade of concrete designed as per IS10262:2019. The hybrid combination of Steel and Polypropylene fibre ratio is 50:50 with different hybridization percentages as 1%, 1.5% and 2%. The compression, splitting tensile and flexural tests were carried out to study the hybrid concrete behavior for the three different hybridization dosage percentages and the same were carried out for the controlled concrete. The experimental study revealed that the inclusion of the hybrid fibre increases the strength of the concrete compared to the conventional concrete. The hybridization of 1.5 % gives the higher strength results and it is considered as an optimum percentage.

Experimental investigation on behavior of splicing glass fiber–reinforced polymer-concrete–steel double-skin tubular columns under axial compression

Splicing glass fiber–reinforced polymer (GFRP)-concrete–steel double-skin tubular column (DSTC) is to set connection component at the joint of two or more separated GFRP tubes, and then pour concrete in the double-tube interlayer to form a continuous composite member. In this paper, the splicing DSTC composite members based on steel bar connection were designed and tested under axial compression to determine its mechanical performance. The main parameters include the connection steel ratio, the hollow ratio, and the thickness of GFRP tube. The results show that the GFRP tube presents apparent constraint effect on the concrete at about 60% of the ultimate load. The failure of splicing specimen occurred in the non-splicing section at a certain distance from the splice joint, and the stirrups at the splice joint provide effective constraint effect on the internal concrete. The proposed DSTC splicing method based on steel cage connection can satisfy the strength requirements of splice joint. Nevertheless, the increase of axial steel bar ratio cannot improve the bearing capacity of the splicing column, and the steel ratio of 2.44% is suggested for the splice joint of DSTCs under axial compression. The axial bearing capacity of splicing DSTCs significantly increases with the increase of GFRP tube thickness, but the amount of stirrups should be increased properly when a larger tube thickness is used. Two models were selected to calculate the bearing capacity of splicing members and it is found that Yu’s model is more accurate in predicting splicing DSTCs.

Shear performance of reinforced self-compacting concrete beams incorporating steel and polypropylene fibers

The impact of steel and polypropylene fibers on the performance of lightweight self-compacting concrete (LWSCC) beams was investigated in this study. Seven beams with various parameters were cast and tested. Partial (50%) and full (100%) replacement of coarse aggregate with lightweight aggregate expanded clay (LECA) were considered. In addition, a 1% volumetric ratio of steel or hybrid (steel and polypropylene) fiber was added to LWSCC beams to study their effect on the shear performance. The LWSCC beams had a decrease in ultimate load and stiffness of 23 and 30% for partial and full replacement, respectively when compared to normal weight beam. The addition of steel fiber improved the efficiency of LWSCC beams in terms of crack formation, failure mode, crack width, and ultimate load, as well as changed the failure mode from shear to flexure. The ultimate load for hybrid LWSCC was increased by around 6% for a partial replacement and 13% for full replacement as compared to beams without fibers. However, hybrid beams had a larger bearing capacity, little more cracks with smaller size, and ductile failure.