Octagonal Steel Research Articles

Confinement-based direct design of axially-compressed octagonal concrete-filled double skin steel tubes

Prior research has highlighted distinctions in the confinement mechanism for concrete-filled double-skin steel tube (CFDSST) columns with octagonal sections compared to circular and square sections. Current design codes primarily address the models for the design of square and circular sections in columns made of concrete-filled steel tube (CFST). This study addresses this gap by presenting a novel confinement-based direct design model specifically developed for octagonal CFDSST columns, encompassing both unstiffened and stiffened outer steel tubes. Initial data collection involved experimental and numerical results of axially-compressed octagonal CFST and CFDSST columns sourced from literature. Subsequently, ultimate compressive resistance, as determined by international design codes (EC4, ACI-318, AISC-360, GB-50936, T/CCES-7, AS/NZS-2327, CSA-S16, AIJ) and existing analytical models, underwent comparison with test results. The results reveal that existing codes and models generally offered accurate predictions, but their consistency across the entire side width-to-thickness (w/to) ratio range varied. The study proposes a new confinement design model for predicting the compressive resistance of octagonal CFDSST columns. Although the new model does not outperform existing ones, it demonstrates satisfactory prediction scatter throughout the w/to ratio and compressive strength (fc′) range, particularly excelling in high-strength concrete (HSC) and ultra-high-strength concrete (UHSC) scenarios. Notably, the proposed formula incorporates the influence of stiffening ribs on the confinement of the octagonal steel tube on concrete, making it applicable for practical design considerations for both unstiffened and stiffened octagonal CFDSST columns.

Inelastic analysis of octagonal concrete‐filled steel tubular short columns under eccentric loading

AbstractOctagonal concrete‐filled steel tubular (OCFST) columns combine the benefits of circular and square concrete‐filled steel tubular (CFST) columns so that they not only possess higher strength and ductility but also provide the ease of connection to composite beams. However, research studies have been very limited on the performance analysis of OCFST short beam‐columns subjected to eccentric loading. In this study, a fiber‐based numerical model is developed for the performance simulation of high‐strength OCFST short beam‐columns under eccentric loading. The simulation model takes into account material nonlinearities and concrete confinement induced by the octagonal steel tube. Computational methods are given that predict the axial load–moment interaction curves and moment–curvature responses of OCFST beam‐columns. The developed fiber model is verified against available test data with good accuracy. The influences of important parameters on the responses of high‐strength OCFST short beam‐columns are studied by means of utilizing the computational model. It is found that the behavior of OCFST beam‐columns is significantly influenced by the diameter‐to‐thickness ratio of the cross‐section, concrete strength, steel yield stress, and axial load ratio. Interaction equations are proposed for expressing the axial load–moment strength envelopes of the cross‐sections of OCFST beam‐columns and validated against numerical results.

Numerical investigation and design recommendation for thin-walled octagonal concrete filled steel tubes

With the growing applications of high strength steel in modern constructions, thin-walled steel tubes attract many attentions. Local buckling failure is the most typical failure mode in a thin-walled steel tube, and concrete filled steel tubes (CFSTs) are excellent composite members in which the occurrence of local buckling is mitigated by the infilled concrete. This paper aims to investigate the compressive strength of concrete filled thin-walled octagonal steel tubes, which have the advantages of confinement efficiency over rectangular CFSTs, and constructability over circular CFSTs. Finite element models are established and validated by the experimental results from existing literatures. A parametric study is subsequently conducted and the result shows that the buckling strength of octagonal tube is significantly improved by the infilled concrete and the enhancement percentage increases with the increase of section slenderness coefficient, and the local buckling behaviour has a minor influence to the effect of confinement. In addition, a database consisting of 37 experimental specimens and 37 FE models are developed and used to assess the design methods in current codes of practice. It is found that the design methods from ANSI/AISC 360-16 and GB 50936-2014 underestimate the load capacity of octagonal CFSTs while the design formula from EN 1994-1-1 which considers the effect of confinement overestimates the capacity. Finally, a design model considering the buckling strength of octagonal steel tube, the influence of infilled concrete core and the effect of confinement is proposed and this model can well capture the compressive strength of octagonal CFSTs.

Numerical analysis of seismic behavior of octagonal multi cavity steel composite columns

Steel structures have obvious advantages and are gradually being widely used in construction. In this paper, a new type of octagonal multi cavity composite steel column is proposed for performance analysis under seismic loading. ABAQUS software is used for model building, followed by a large number of simulations under the new structural form, and finally the variables representative of the new structure are selected for in-depth analysis. Through numerical simulations, we obtained that: the main load-bearing part of the octagonal combined steel column strength is the outer octagon, with the inner combined steel controlled to work together; honeycomb holes are set in the outer channel steel, and the round honeycomb holes have the best performance. The results of the experimental study in the references are used as a reference to make the numerical simulation results more credible. This paper proposes a structural scheme and performance study of octagonal multi cavity steel combined steel column, which provides a certain structural form reference for the future development of octagonal steel column.

Experimental Study of Octagonal Steel Columns Filled with Plain and Fiber Concrete under the Influence of Compressive Axial Load with Eccentricity

In recent years, Concrete Filled Tube (CFT) columns have been very much taken into consideration due to the many advantages of the instrument. In experimental studies, the focus has been on compressive loading. Although in many cases the eccentric loading (the presence of a bending moment) has also been investigated, further research is needed in this regard. Therefore, in this study, the steel columns filled with concrete with a regular octagonal cross section were studied under the influence of pressure axial load with eccentricity. The parameters studied in this study include bearing capacity, coefficient of ductility and energy absorption. To test and compare the stated parameters, specimens of 150 cm height, which were filled with plain concrete and fiber reinforced concrete were tested. The compressive axial load has been applied to the specimens by the eccentricity of 50, 100 and 150 mm. The results show that at pure compressive loading, the increase in concrete core capacity increases the load bearing capacity of the specimens so that by increasing the concrete compressive strength by 50%, the bearing capacity of the cross section increases by about 15%. Also, based on the results, the average ductility coefficient for specimens was 7.4, and it seems that this value is independent of the type of loading. The use of concrete with intermediate grade resistance can increase the energy absorbed. However, according to the results, it seems that by increasing the bending moment the positive effects of the concrete core are greatly reduced.

Design of fixed-ended octagonal shaped steel hollow sections in compression

In recent years, octagonal steel hollow sections (OctHSs) have attracted significant interests from researchers and engineers, and OctHSs have been employed in civil engineering applications. However, designs of OctHSs are not included in structural steel design codes. This paper therefore presents comprehensive numerical analyses on the stub column behaviour of OctHSs. A database of 46 stub column tests reported in 6 sources were collected. Different fabrication routes, steel grades and cross-sectional slenderness were covered. Finite element (FE) modelling was carried out to replicate the test observations. The validated FE models were subsequently adopted to conduct an extensive parametric study to supplement the test database. Existing design methods in North American specifications, ASCE standard, Eurocode, Australian code and recently published literature were thoroughly discussed and evaluated. It was found that existing design methods are not applicable to the design of OctHSs. New cross-sectional slenderness limits and modifications to the existing design methods were then proposed. The cross-sectional resistance obtained from different design methods were compared with test and FE results. The applicability of the modified design methods was confirmed by statistical analyses. The reliability of the modified design methods was finally verified through reliability analyses. Results indicate that the modified design methods can be used to accurately predict the resistance of OctHSs with accepted safety levels.

Behavior of octagonal concrete-filled double-skin steel tube stub columns under axial compression

This paper presents an experimental investigation on the behavior of stiffened octagonal concrete-filled double-skin tube (CFDST) stub columns subjected to axial compression. A total of eight specimens were prepared and tested under axial compression aiming to study the effects of longitudinal stiffeners and hollow ratio on the elastic-plastic local buckling and ultimate load-carrying capacity of the test columns. The specimens consist of two control specimens without stiffeners (unstiffened columns) and two specimens stiffened with outer-inner tubes welded stiffeners (OI stiffeners) in four sides of the octagonal steel section. In addition, four specimens were stiffened with two types of stiffeners: OI stiffeners and outer tube welded stiffeners (O stiffeners) in eight sides of the outer steel tube section. Furthermore, two hollow ratios were employed. Based on the test results, the ultimate load-carrying capacity, load-displacement curves, load-strain response, ductility, and failure modes were discussed and clarified. The test results indicated that the strength and ductility of the specimens were improved greatly by the presence of the stiffeners. The stiffeners significantly enhanced the confinement and prevented the occurrence of elastic-plastic local buckling of the outer tube. Besides, current design equations were used to predict the ultimate load-carrying capacity of octagonal CFDST stub columns. The test results were then compared with the predicted ultimate capacities of existing design equations. These predictions showed good agreement with the test results.

Experimental behaviour of hollow reinforced concrete members with inner octagonal steel tube under lateral impact

This work investigated the impact performance of hollow reinforced concrete members with inner octagonal steel tube. Experiments on 13 specimens subjected to low-velocity drop weight impact are presented in this article, covering key parameters such as the impact height, boundary condition, axial load ratio and thickness of the inner tube. The dynamic processes, failure patterns, impact force and mid-span deflection histories, and residual mid-span deflections were obtained from the experiments. Flexure-shear was observed as the main failure pattern for all the specimens under impact. It was found that all the key parameters considered had influences on the impact performance of hollow reinforced concrete specimens with inner octagonal steel tube. Effects of these parameters on the impact performance of hollow reinforced concrete members were discussed.

Behaviour of octagonal steel-reinforced concrete box columns under compressive load

A series of tests on octagonal steel-reinforced concrete box stub columns and columns under compressive load was carried out. Material properties of the concrete, inner octagonal steel tube and reinforcement bars used in the test specimens were measured. The effects of wall thickness of inner octagonal steel tubes and eccentric distance of compressive load on the performance of octagonal steel-reinforced concrete box columns were discussed. Based on test results, a finite-element model was developed using Abaqus to simulate the experimental studies. The full stages of the behaviour of the octagonal steel-reinforced concrete box stub columns and columns were investigated. Finally, design equations for the octagonal steel tube-reinforced concrete box stub columns and columns under compressive load were proposed. The measured strengths of the tested specimens were compared with the design strengths calculated using the proposed method given in this paper.

Cyclic Loading Tests of Composite Moment Frames Using Octagonal CFTs

This research investigates the seismic performance of two interior beam-to-column connections with floor slabs. Concrete-filled octagonal steel tube column, H-section steel beam, and U-section steel beam were used for composite moment frames, respectively. To release the stress concentration at the column face, two design methods were proposed. First, H-section steel beam passed through the column, directly. Stopper plates were also used at the column faces to decrease the slip deformation. Second, top and bottom flanges of U-section beam were not welded to the column face. Instead, to transfer the tensile force of the beam flange, reinforcement bars passing through the column were welded to the beam flanges. The test results showed that the ductility of moment connections was sufficient in the case of through details; However, the load-carrying capacity was significantly affected by the details of beam-to-column connections.

Experimental and numerical studies of short concrete-filled double skin composite tube columns under axially compressive loads

This paper presents a study on the performance of concrete-filled double skin composite tube columns (CFDSCT) under axially compressive loads using both experimental and numerical methods. The CFDSCT column investigated consists of an octagonal steel tube as its outer skin layer, a circle PVC-U pipe as its inner skin layer, and high strength concrete filled in between the two layers. Influences of concrete strength, radius-to-thickness ratio, hollow section ratio, and slenderness ratio on the ultimate axial compressive capacity of the CFDSCT column are examined. It is found that the ultimate axial compressive capacity of the CFDSCT column increases with the strength of concrete, but decreases with either increased radius-to-thickness ratio or increased hollow section ratio. Based on the findings from the present experimental and numerical studies, a design formula for predicting the ultimate load of the CFDSCT column is proposed.

Axial Performance of Various Strengthening Methods Applied on Full-Scale Rectangular RC Columns

The performance of RC columns subjected to axial force is relative to the confinement. CFRP wrapping, a generally adopted retrofit method, was proved not to effectively provide confining force due to the bulging effect on the column face. Therefore, this paper is focused on the performance of the retrofitted full-scale rectangular RC columns using different retrofit schemes including the proposed CFRP wrapping conjugated with CFRP anchors method. A total of eleven rectangular RC columns with low transverse reinforcement ratio were constructed. Among them, one was tested as benchmark; one was purposely constructed with larger transverse reinforcement ratio; five were retrofitted by using CFRP wrapping and CFRP anchors; and the other four were retrofitted by using different shapes of steel jacketing alone or with adhesive anchors. All the specimens were subjected to monotonic incremental axial force until failure occurred. Experimental results demonstrated that the ductility of the specimens retrofitted by using CFRP wrapping with CFRP anchors was significantly improved compared with those retrofitted by using only CFRP wrapping. On the other hand, the specimen with octagonal steel jacketing performed better than all other specimens not only on ductility but also on strength. Finally, a novel numerical model considering the contribution of the retrofit material will be proposed and validated in the future.

Seismic steel jacketing of rectangular RC bridge columns for the mitigation of lap‐splice failures

AbstractThis paper investigated the use of external steel jacketing for seismic retrofit of non‐ductile reinforced concrete (RC) bridge columns to prevent lap‐splice failure. Three 1/2.5‐scale specimens were tested under cyclic loads. The effectiveness of two types of steel jackets for improving the ductility and strength of specimens using inadequate transverse reinforcing and lap‐splice details were examined. An octagonal steel jacketing scheme for the seismic retrofitting of rectangular RC bridge columns was proposed. In addition, the methods for seismic retrofitting rectangular columns using elliptical steel jacketing were also critically tested. The test results indicated that the octagonal steel jackets can effectively provide confinement thereby mitigating failures as a result of inadequate transverse reinforcing and inadequate lap‐splices. Tests also confirmed that the ductility performance and the energy dissipation capacity of the specimens can be significantly improved by the octagonal steel jacket. Copyright © 2010 John Wiley & Sons, Ltd.

Finite-Element Analysis of Tapered Steel and Fiber-Reinforced Plastic Bridge Camera Poles

This paper presents experimental and finite element investigations of the load-deformation behavior of tapered steel and fiber-reinforced plastic (FRP) bridge camera poles subjected to cantilever bending type loading. Three full-scale experimental tests are conducted on one tapered octagonal steel cross section and two FRP circular cross-section poles to identify their load deformation characteristics. Three-dimensional isoparametric finite-element models of the poles are developed by considering the nonlinear coupling behavior between material, contact, and geometric effects. The elastoplastic solid elements with eight nodes are employed for the effective three-dimensional finite-element modeling and analysis. A surface-to-surface contact algorithm is used to simulate the interaction between contact surfaces. An energy-based convergence criterion is adopted to obtain the converged coupled nonlinear solutions. The obtained load-deformation results from finite-element analyses are compared with those of the experiments. The behavior of the finite-element models is examined by observing the effects of individual geometric variables on the load-deformation characteristics of the poles.

Seismic jacketing of RC columns for enhanced axial load carrying performance

Axial compression test results for square RC columns incorporating Taiwanese construction practice in the placement of stirrups and various kinds of jacketing schemes are presented. The jacketing schemes include circular, octagonal and square shapes. The jacketing materials vary from steel plate to carbon fiber reinforced polymer (CFRP) composites. It is found from the monotonic axial load test results that the failure mode of the benchmark non‐retrofitted specimen is identical to that observed in real damage cases subsequent to the 1999 Chi‐Chi Taiwan earthquake. The benchmark specimen developed its design strength but a non‐ductile failure mode occurred soon after the peak load was reached. Among the retrofitted specimens, the steel jacketed specimens exhibit not only greatly enhanced load carry capacity but also excellent ductility performance. Test results show that CFRP sheets are effective in increasing the column axial strength, but the sheets could fracture suddenly in high strain conditions due to their brittle material characteristics. Test results indicate that CFRP sheet wrapping in general is not as effective as steel jacketing in improving the axial ductility capacity of RC columns. However, the proposed octagon‐shaped CFRP wrapping scheme exhibits an improved performance compared to rectangular‐wrapped columns using the same layers of CFRP sheets. Tests confirm that all octagonal steel or CFRP jacketed specimens have axial loading capacities more than 2 times the nominal capacity.

Energy dissipation and associated failure modes when axially loading polygonal thin-walled cylinders

The crumpling mechanisms of thin-walled octagonal steel tubes subjected to axial static compression and their efficiency as energy absorbing components under impact conditions are investigated and compared with the equivalent circular and square tubes of same material. Both inextensional and extensible collapse mechanisms are theoretically analysed. Theoretically predicted values based on the deformation modes encountered, as well as experimentally obtained ones, are compared and found to be in good agreement.

EXPERIMENTAL STUDIES ON ULTIMATE STRENGTH AND DUCTILITY OF REINFORCED SQUARE COLUMNS WITH CONFINING REINFORCEMENT

An experimental study on the ultimate strength and ductility of reinforced square columns with confining reinforcement is described. Sixty specimens, each 200mm square by 800mm high, containing either 8 or 12 longitudinal steel bars and different arrangement of square or octagonal steel hoops, were subjected to concentric or eccentric loads to failure under uniaxial or biaxial bending. Other variables were the spacing of hoops and the eccentricities of loads. Based on the test results and analyses, new formulas are proporsed for the ultimate strength and ductility of the columns with confinement subjected to uniaxial or biaxial bending. A new parameter which was defined, in this study, as the confining ratio of confining reinforcement ratio to the longitudinal reinforcement ratio was used in the formulas. As it is difficult to predict analytically ductilities of columns under biaxial bending, a simple design method is proposed.