Failure Modes Of Columns Research Articles

07.18: Cold‐formed C‐sections encased in ultra‐lightweight concrete: Development of a Eurocode‐based design method

ABSTRACTA new direction of development in the field of light‐gauge building systems is when the load bearing elements made of cold‐formed steel (CFS) sections are encased in an ultra‐lightweight material. This material, apart from heat insulation and fire protection effects, can provide bracing effect to the CFS elements, too, therefore can increase the resistance against stability failures. Such lightweight material is the polystyrene aggregate concrete (PAC). Based on previous experimental results this paper develops design method for calculating local, distortional and global resistance of PAC‐encased CFS C‐sections. The proposed design method is based on the specifications of Eurocode, therefore applies the effective width method for calculating local buckling. The proposed procedure considers the effect of continuous bracing of PAC by replacing it with an elastic half‐space, similar to sandwich beam theory. Close form equations are derived to be able to calculate the effective width of plate elements by hand calculation. Distortional buckling is treated as buckling of an equivalent beam resting on elastic foundation, considering the additional stiffness provided by the infill material. The global flexural buckling failure modes of columns are dealt with separately using different mechanical models. Each calculation procedure was compared to existing experimental results in order to evaluate them.

Seismic Failure Modes and Deformation Capacity of Reinforced Concrete Columns under Cyclic Loads

This paper investigates the seismic failure modes and horizontal deformation capacity of reinforced concrete square columns based on the pseudo-static test. The controlled variables include shear aspect ratio, axial load ratio and stirrup spacing. The seismic failure modes, the inelastic deformation capacity after yielding and the deformation components due to flexure, shear and anchorage slip of the RC columns were analyzed, especially flexural-shear failure. The results show that decreasing shear aspect ratio, or increasing axial load or stirrup spacing can result in the change of column failure mode from flexural failure to flexural- shear failure or shear failure, the pinching of hysteresis loops, the reductions of hysteresis loop area and deformation capacity. With the increase of total displacement, all three displacement components increased; the contribution of flexure displacement in total displacement reduced, the contribution of shear displacement increased, the contribution of anchorage slip displacement changed in the range of 30%- 40%.

Analytical study on reinforced concrete frames subject to compressive arch action

Reinforced concrete beams can resist applied vertical loads through the development of compressive arch action (CAA) when adequate horizontal restraints are provided at the ends. However, at the frame level, CAA tends to push adjoining columns outwards and may induce premature flexural or shear failure to the columns. Therefore, under CAA, flexural and shear resistances of the connecting columns need to be examined. This paper describes an analytical study on the behaviour of reinforced concrete frames subject to CAA. In the study, lateral and rotational stiffness of reinforced concrete columns is determined based on a rigid-plastic assumption. CAA of reinforced concrete frames is investigated through an analytical model, in which deformations of columns are taken into consideration. Besides, shear force and bending moment in the column are calculated from equilibrium to shed light on the possible failure mode of columns under CAA. Parametric studies are also conducted to investigate the dominant parameters on the behaviour of columns. Finally, recommendations are provided for the design of reinforced concrete columns against CAA in the connecting beam.

Optimization of the Sandwich Column with the Truss Core Which is Subjected to the Compressive Loading

Abstract The sandwich structures have multifold advantages with respect to other types of structures. Besides the architectural possibilities due to their appearance, those structures can carry the same or even higher loads than some other similar structures. Optimization of the sandwich columns with the truss core, subjected to the compressive axial load, is presented in this paper. The two types of optimization were performed: the three-parameter and the four-parameter optimization - the so called full optimization. The optimization of the column geometry (face thickness, core member height and core member diameter and core height) was performed, from the aspect of the minimal weight of the structure in terms of the load index. It was performed for four types of restrictions imposed by the corresponding column failure modes: column buckling, truss macro-buckling, local buckling of the face and face wrinkling. The tree-parameter optimization resulted in somewhat larger weight of the column than the full, four-parameter optimization.

Experimental study and analysis on the collapse behavior of an interior column in a steel structure under local fire

This article describes the experimental and analytical results of tests on two medium-scale steel frames to investigate the collapse behavior of a heated interior column in steel-framed structures and the fire-induced collapse mechanism of the structures. Because the heated column in the structure was under realistic inelastic end constraints (inelastic effects from the surrounding cool structures) as opposed to an idealized column in a furnace, the tests were designed to investigate the effect of inelastic end constraints and different load ratios. The interior column subjected to fire in the first story was connected with a force-measuring bearing, which was developed to measure the axial force in the heated column. This article presents the observations and analysis results of structural fire behavior, including the temperature distribution of the column, the development of deflection and axial forces, and the column failure modes. The results show that the effect of realistic inelastic end constraint is not significant for the column behavior during the pre-buckling stage, compared with the elastic constraint, but it is significant during the post-buckling stage. An analytical model was adopted to analyze the behavior of the column with inelastic end constraint under fire. Comparison of the theoretical and experimental results shows that the simplified analytical model can be used to predict the collapse behavior of the interior column in steel-framed structures under fire.

EXPERIMENTAL INVESTIGATION OF CIRCULAR ALUMINUM COLUMNS ENHANCED

In structural engineering, introducing an efficient structural elements with high ratio of strength or ductility to weight is a challenge.This paper is presenting and developing aluminum columns having circular hollow section strengthened with CFRP possess the highest specific (divided -by weight) mechanical properties to be advantageous in lightweight and space limited structures .The structural performance of these columns was investigated experimentally by using different strengthening orient styles and CFRP piling layers. The column specimens were subjected to uniform axial compression. The strength, ductility, axial load-shortening displacement relationships, lateral strains, and failure modes of columns were presented. Designing guideline empirical equations were derived from experimental results, the predicted unfactored strengths are found to be in a good agreement with the experimental values.

Influence of fire scenarios on progressive collapse mechanisms of steel framed structures

AbstractOpenSees is an open‐source object‐oriented software framework developed at UC Berkeley. The OpenSees framework has been recently extended to deal with structural behaviour under fire conditions. This paper presents the results of a numerical study, using OpenSees, of the progressive collapse of steel frames exposed to fire. After validating the capability of OpenSees against available analytical and experimental results of fire tests on steel members, a parametric study is carried out to find the progressive collapse mechanism of steel frames exposed to fire and corresponding influencing factors. The factors include load levels, lateral restraint, beam strength and fire scenarios. The catenary action of beams is considered by using a temperature‐dependent corotational beam/column element. The results show that different progressive collapse mechanisms can happen due to the sequence of the buckling of columns. High load levels lead to the downward collapse of the whole structure compared with the lateral collapse for low load levels. The existing of lateral restraint causes the premature buckling of bottom columns which triggers the whole frame collapse. As the beam section increases, the collapse mechanism of steel frames changes from beam failure mode to column failure mode. The fire scenarios have significant effect on the collapse mode of steel frames. The work presented in this paper is a preliminary study of the progressive collapse of steel frames. Further work is underway to combine the influence of the effect of concrete floors. A simple design method is then expected to be proposed to investigate the robustness of steel structures against progressive collapse due to fire.

Analytical behaviour of eccentrically loaded concrete-encased CFST box columns

This paper presents a theoretical study on the performance of concrete-encased concrete-filled steel tube (CFST) box columns subjected to eccentric axial loading. Finite-element analysis (FEA) was used to analyse the composite columns and experimental data reported recently by the authors was used to verify the FEA modelling. Analysis of the behaviour of the composite, including the full-range load versus lateral mid-height deflection, stress distributions, column failure modes and the interactions between steel and concrete, was carried out. The behaviour of the concrete-encased CFST box columns was compared with that of corresponding reinforced concrete columns and CFST built-up columns. Finally, a parametric study was carried out and the moment magnification approach adopted to propose design formulae for eccentrically loaded concrete-encased CFST box columns.

Experimental study on local buckling of axially compressed steel stub columns at elevated temperatures

There are few design provisions in codes and standards on local buckling of steel columns under fire conditions. To examine the local stability of steel stub columns at elevated temperatures, 12 stub columns were tested under simultaneous application of load and fire conditions. The test variables included Grade (type) of steel, buckling resistance, temperature and load levels. During fire tests, cross sectional temperatures, axial displacement, buckling deflection, and local buckling failure modes of flange and web of stub columns were recorded at various temperatures. Data from the tests is utilized to evaluate buckling resistance of flange and web both at room and elevated temperatures by applying the ultimate strain method and curve inflexion point method. Results from fire tests are utilized to validate a finite element model developed for tracing local buckling of steel columns at elevated temperatures. Results from fire tests and finite element analysis show that failure mode of columns at room and elevated temperatures follow similar pattern but the load bearing capacity of Q460 steel columns degrade much more rapidly under fire conditions than that of Q235 steel columns. Further, Eurocode 3 provisions for local buckling produce non-conservative results in certain situations.

Failure mode classification of reinforced concrete column using Fisher method

In order to apply the performance-based seismic design, an engineer must first find out whether the column is expected to fail in shear before or after flexural yielding. According to column failure characteristics and failure mode of reinforced concrete column, the UW-PEER structure performance database was discussed and analyzed. In order to investigate the relevance of failure mode and factors such as longitudinal reinforcement ratio, transverse reinforcement ratio, hoop spacing to depth ratio, aspect ratio, shearing resistance demand to shear capacity ratio and axial load ratio, Fisher’s discriminant analysis (FDA) of the above factors was carried out. A discriminant function was developed to identify column failure mode. Results show that three factors, i.e., Vp/Vn, hoop spacing to depth ratio and aspect ratio have important influence on the failure mode. The failure mode has less to do with longitudinal reinforcement ratio, transverse reinforcement ratio and axial load ratio. Through using these three factors and the model proposed, over 85.6% of the original grouped cases were correctly classified. The value of coefficient of Vp/Vn is the largest, which means that discriminant equation is most sensitive to the shearing resistance demand to shear capacity ratio.

Analysis on Bearing Capacity of Double Back-to-Back Cold-Formed Thin-Walled C Steel Composite Column with Gusset Plate

According to the composite column of double back to back cold-formed thin-walled C steel connected with gusset plate, the experiment and finite element analysis were carried out to study the stability bearing capacity and failure modes of columns under eccentric loading. The factors such as slenderness ratio, eccentricity, gusset plate thickness and spacing were discussed, which produced the impacts to bearing capacity, and the relationship between stiffness and axial load. The results revealed that the failure modes belonged to the in-plane-flexural buckling and local-buckling of the pressurized flanges and webs of the columns. Slenderness ratio, eccentricity and gusset plate spacing are the major factors that affected the mechanical behavior, while the gusset plate thickness has no obvious impact. In the end, compared with the calculation of the bearing capacity of the cold-formed thin-walled column and GB50018-2002, some suggestions based on this composite column calculation were put forward, and the correction coefficient of bearing capacity of the composite columns were given.

Evaluating the ductility and shear behaviour of carbon fibre reinforced polymer and glass fibre reinforced polymer reinforced concrete columns

AbstractBrittle shear failure in short reinforced concrete (RC) columns has been identified as one of the most dangerous failure modes that may cause collapse of bridges, based on research findings and damage reconnaissance of past earthquakes. Using ABAQUS software, the effect of carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) on retrofitting of existing circular RC columns, and also enhancing of shear strength have been studied. Five types of concrete columns were studied in the ‘as built’ condition to investigate the shear failure mode of columns. These columns were assessed again after being retrofitted by using CFRP and GFRP. Based on findings of this research, the effect of CFRP jackets on improving the shear behaviour of column is more than that of GFRP jackets, while GFRP jackets are more effective than CFRP jackets, regarding their contribution in increasing the column ductility. Therefore, these FRP composites can be utilized in different conditions, according to column demand for increasing the ductility or shear strength. Copyright © 2010 John Wiley & Sons, Ltd.

Applicability investigation of code-defined procedures on seismic performance assessment of typical school buildings in Taiwan

Seismic resistance of elementary and high school buildings is of great concern due to high earthquake risk in Taiwan. Existing school buildings are often built of similar configurations, and designed based on previous building codes, which are not in compliance with current codes in seismic performance objective. The use of current code procedures on seismic performance assessment of existing typical school buildings may not be applicable in predicting their structural behavior during earthquakes. This paper describes a full-size two-story, three-bay frame structure test of typical school building design in Taiwan, and examines the performance and failure mechanism of the frame structure with current building codes. Results from loading test show good agreement with the code-defined procedures in identifying soft story, weak story, and failure modes of columns, but show different failure mechanism in beam–column joint region, i.e. strong-column–weak-beam is expected by building code procedures, whereas the test specimen exhibits weak-column–strong-beam behavior. Although the joint shear check following current code procedures gives satisfactory results, i.e. is consistent with the experiment, the check on its own is not a reasonable one. Revisions of current checking procedures for failure mechanism in beam column joint and for joint shear are also proposed in this paper for existing non-ductile reinforced concrete school buildings. The revised procedures incorporate the effects of both weak story and weak-column–strong-beam behavior. After adopting the proposed procedures, the predicted structural behavior and failure modes are consistent with those observed from the full-size frame test.

Failure Mode of Columns of Existing R/C Building Damaged During the 2007 Niigata Chuetsu-Oki Earthquake

The school building investigated was a 3-story reinforced concrete (R/C) building built in 1963. The building suffered from a great deal of damage during both the 2004 Chuetsu Earthquake and the 2007 Chuetsu-Oki Earthquake. The damage of the first and second floors during Chuetsu Earthquake was light and that by Chuetsu-Oki Earthquake was moderate. The previous study revealed that the anticipated design failure modes of most of the columns of the building were flexure although most of them actually failed in shear during the 2007 earthquake. This is an important problem to be studied. In order to study the reason why the columns failed in shear rather than in flexural, a parametric study was conducted, paying attention to parameters including the strength of concrete, hoop spacing and subjected axial force. But those studies could not explain the real phenomenon clearly. After that the effects of cutoff location of longitudinal reinforcement bars were examined and it was concluded that the diagonal crack generated from cutoff point caused shear failure in these columns.

Concrete-filled aluminum circular hollow section column tests

An experimental investigation of concrete-filled aluminum circular hollow section (CHS) stub columns is presented in this paper. A series of tests was conducted to investigate the effects of the geometric dimension of the aluminum CHS and concrete strength on the behavior and strength of concrete-filled aluminum CHS stub columns. The structural performance of the concrete-filled aluminum CHS stub columns was investigated using different concrete cylinder strengths of 40, 70 and 100 MPa. The CHS tubes were fabricated by extrusion using 6061-T6 heat-treated aluminum alloy having nominal 0.2% proof stress of 240 MPa. The diameter-to-thickness ratio of the CHS tubes ranged from 9.7 to 59.7. The column lengths were chosen so that the length-to-diameter ratio generally remained at a constant value of 3 to prevent overall column buckling. The concrete-filled aluminum CHS specimens were subjected to uniform axial compression. The column strengths, load-axial shortening relationship, load-axial strain relationship and failure modes of columns were presented. The test strengths were compared with the design strengths calculated using the American specifications and Australian/New Zealand standards for aluminum and concrete structures. It is shown that the design strengths are generally conservative for concrete-filled aluminum CHS stub columns.

Prefabricated Composite Jacketing of RC Columns for Enhanced Shear Strength

The effectiveness of prefabricated composite jacketing to retrofit existing circular reinforced concrete (RC) bridge columns for enhanced seismic shear strength is described in this paper. The jackets consist of multilayer prefabricated glass fiber reinforced composite shells adhered together. Three half-scale circular columns were tested under cyclic shear in double curvature and constant axial load. One column was tested in the condition to investigate the shear failure mode of columns designed based on pre-1971 codes. Two other columns were tested after being retrofitted using prefabricated composite jackets. The as-built column suffered shear failure at low displacement ductility. The retrofitted columns developed significantly improved hysteretic responses with stable hysteresis loops even at displacement ductility levels as high as 10.0 corresponding to drift ratios exceeding 5.2%. Method for retrofit design using prefabricated composite jacketing is also provided and validated by the test results.

Seismic Retrofit of RC Columns with Continuous Carbon Fiber Jackets

The development, the validation, and the implementation of a new seismic retrofit system for reinforced concrete columns are described. The column jacketing system consists of continuous carbon fiber prepreg tows wound in an automated fashion onto existing circular or rectangular concrete columns, with variable jacket thickness along the column height based on experimentally validated design models. Jacket design criteria for various seismic column failure modes are described and detailed examples show their application to retrofits of columns with circular and rectangular column geometry, different reinforcement ratios, and detailing. The carbon jacket designs are validated through large-scale bridge column model tests and are found to be just as effective as steel shell jacketing in providing desired inelastic design deformation capacity levels. Furthermore, it is shown that the retrofit criteria and guidelines are also applicable to other advanced composite jacketing systems with appropriate considerat...