Torsional-flexural buckling of unevenly battened columns under eccentrical compressive loading

In this paper, an analytical model is developed to determine the torsional-flexural buckling load of a channel column braced by unevenly distributed batten plates. Solutions of the critical-buckling loads were derived for three boundary cases using the energy method in which the rotating angle between the adjacent battens was presented in the form of a piecewise cubic Hermite interpolation (PCHI) for unequally spaced battens. The validity of the PCHI method was numerically verified by the classic analytical approach for evenly battened columns and a finite-element analysis for unevenly battened ones, respectively. Parameter studies were then performed to examine the effects of loading eccentricities on the torsional-flexural buckling capacity of both evenly and unevenly battened columns. Design parameters taken into account were the ratios of pure torsional buckling load to pure flexural–buckling load, the number and position of battens, and the ratio of the relative extent of the eccentricity. Numerical results were summarized into a series of relative curves indicating the combination of the buckling load and corresponding moments for various buckling ratios.National Natural Science Foundation of China (NSFC) under grant number (No.) 51175442 and Sichuan International Cooperation Research Project under grant No. 2014HH002

Post-fire Behaviour of Innovative Shear Connection for Steel-Concrete Composite Structures

YesSteel-concrete composite structures are commonly used in buildings and bridges because it takes advantage of tensile strength of steel and compressive strength of concrete. The two components are often secured by shear connectors such as headed studs to prevent slippage and to maintain composite action. In spite of its popularity, very little research was conducted on steel-concrete composites particularly on headed stud shear connectors in regards to its post-fire behaviour. This research investigates the post-fire behaviour of innovative shear connectors for composite steel and concrete. Three type of connectors were investigated. They are headed stud shear connectors, Blind Bolt 1 and Blind Bolt 2 blind bolts. Push-out test experimental studies were conducted to look at the behaviour and failure modes for each connector. Eighteen push tests were conducted according to Eurocode 4. The push test specimens were tested under ambient temperatures and post fire condition of 200˚C, 400˚C and 600˚C. The results in ambient temperature are used to derive the residual strength of shear connectors after exposing to fire. Findings from this research will provide fundamental background in designing steel-concrete composites where there is danger of fire exposure

Seismic analysis of a tall metal wind turbine support tower with realistic geometric imperfections

The global growth in wind energy suggests that wind farms will increasingly be deployed in seismically active regions, with large arrays of similarly designed structures potentially at risk of simultaneous failure under a major earthquake. Wind turbine support towers are often constructed as thin-walled metal shell structures, well known for their imperfection sensitivity, and are susceptible to sudden buckling failure under compressive axial loading. This study presents a comprehensive analysis of the seismic response of a 1.5-MW wind turbine steel support tower modelled as a near-cylindrical shell structure with realistic axisymmetric weld depression imperfections. A selection of 20 representative earthquake ground motion records, 10 ‘near-fault’ and 10 ‘far-field’, was applied and the aggregate seismic response explored using lateral drifts and total plastic energy dissipation during the earthquake as structural demand parameters. The tower was found to exhibit high stiffness, although global collapse may occur soon after the elastic limit is exceeded through the development of a highly unstable plastic hinge under seismic excitations. Realistic imperfections were found to have a significant effect on the intensities of ground accelerations at which damage initiates and on the failure location, but only a small effect on the vibration properties and the response prior to damage. Including vertical accelerations similarly had a limited effect on the elastic response, but potentially shifts the location of the plastic hinge to a more slender and, therefore, weaker part of the tower. The aggregate response was found to be significantly more damaging under near-fault earthquakes with pulse-like effects and large vertical accelerations than far-field earthquakes without these aspects

Multilevel seismic demand prediction for acceleration-sensitive non-structural components

Existing methods to predict the seismic demand of non-structural components in current seismic design guidelines do not generally consider the intensity of the design earthquake and the expected performance level of the lateral load bearing system. This limitation is especially important in performance-based design of buildings and industrial facilities in seismic regions. In this study, a novel multilevel approach is proposed to predict the seismic demand of acceleration-sensitive non-structural components using two new parameters obtained based on site seismicity and seismic capacity of the lateral load carrying system. The main advantage of the new method is to take into account the seismic hazard level and the expected performance level of structure in the calculation of the seismic demand of non-structural components. Based on the results of a comprehensive reliability study on 5 and 10-storey steel frame structures, the efficiency of the proposed approach is demonstrated compared to the conventional seismic design methods. The results, in general, indicate that the current standards may provide inaccurate predictions and lead to unsafe design solutions for acceleration-sensitive non-structural components, especially in the case of higher seismic intensity or medium performance levels. It is shown that the estimated accelerations by NIST and ASCE suggested equations are up to 50% and 80% lower than the minimum demand accelerations calculated for the studied structures, respectively, under the selected design conditions. Based on the results of this study, a simple but efficient design equation is proposed to estimate the maximum acceleration applied to non-structural components for different earthquake intensity levels and performance targets

Seismic performance evaluation of deficient steel moment-resisting frames retrofitted by vertical link elements

In many earthquake prone regions in developing countries, substandard steel moment resisting frame (SMRF) systems pose a profound danger to people and economy in the case of a strong seismic event. Eccentric bracing systems with replaceable vertical links can be utilized as an efficient and practical seismic retrofitting technique to reduce future earthquake damages to such structures. This paper aims, for the first time, to demonstrate the efficiency of eccentric bracing systems with vertical links as a seismic retrofitting technique for the SMRF structures with WCSB and to develop fragility curves for such structures. To achieve this aim, first, the effect of the vertical links on the behaviour of 3, 5 and 7-storey frames are studied through conducting the Nonlinear Static Analyses (NSA) as well as Nonlinear Time History Analyses (NTHA) using the artificial accelerograms compatible with the target design spectrum. The analysis results indicate that, as aimed in the design stage, the seismic damage is only concentrated at the replaceable vertical links and remaining structural members work mainly in the elastic range. In addition, the proposed retrofitting technique considerably improves the performance of the deficient SMRF systems by effectively restricting the displacement response and damage distribution in such structures. Following the NTHA, Incremental Dynamic Analyses (IDA) are performed to develop the seismic fragility curves for the retrofitted SMRF systems. The results indicate that the proposed retrofitting technique significantly reduces the fragility of such systems, and therefore, can provide a simple and efficient method to improve the seismic performance of deficient steel moment resisting frames in seismic regions

Development of optimum cold-formed steel sections for maximum energy dissipation in uniaxial bending

Cold-formed steel (CFS) elements are increasingly used as load-bearing members in construction, including in seismic regions. More conventional hot-rolled steel and concrete building structures are typically allowed by the design standards to exceed their elastic limits in severe earthquakes, rendering parameters indicating ductility and energy dissipation of primordial importance. However, insufficient research has yet been conducted on the energy dissipation of CFS structures. In the majority of previous optimization research on CFS sections the ultimate capacity, as typically controlled by local, distortional and/or global buckling modes, is considered to be the sole optimization criterion. This paper aims to improve the seismic performance of CFS elements by optimising their geometric and material highly non-linear post-buckling behaviour to achieve maximum energy dissipation. A novel shape optimisation framework is presented using the Particle Swarm Optimisation (PSO) algorithm, linked to GMNIA ABAQUS finite element analyses. The relative dimensions of the cross-section, the location and number of intermediate stiffeners and the inclination of the lip stiffeners are considered to be the main design variables. All plate slenderness limit values and limits on the relative dimensions of the cross-sectional components as defined by Eurocode 3, as well as a number of practical manufacturing and construction limitations, are taken into account as constraints in the optimisation problem. It is demonstrated that a substantial improvement in energy dissipation capacity and ductility can be achieved through the proposed optimization framework. Optimized cross-sectional shapes are presented which dissipate up to 60% more energy through plastic deformations than a comparable commercially available lipped channel

Flexural behaviour of hot-finished high strength steel square and rectangular hollow sections

High strength steels, considered in the context of the structural Eurocodes, as steels with a yield strength over 460 MPa, are gaining increasing attention from structural engineers and researchers owing to their potential to enable lighter and more economic structures. This paper focuses on the bending strength of hot-finished high strength steel (HSS) square and rectangular hollow sections; the results of detailed experimental and numerical studies are presented and structural design rules for HSS cross-sections are proposed. A total of 22 in-plane bending tests, in three-point bending and four-point bending configurations, on HSS sections in grades S460 and S690 were conducted. The experimental results were replicated by means of non-linear finite element modelling. Upon validation of the finite element models, parametric studies were performed to assess the structural response of HSS sections over a wider range of cross-section slenderness, cross-section aspect ratio and moment gradient. The experimental results combined with the obtained numerical results were used to assess the suitability of the current European (EN 1993-1-1 and EN 1993-1-12) cross-section classification limits for HSS structural components. The reliability of the proposed cross-section classification limits was verified by means of the EN 1990 - Annex D method.The Research Fund for Coal and Steel (RFCS) under grant agreement No. RFSR CT 2012-00028. V&M DEUTSCHLAND GMBH, Mr. Gordon Herbert, Mr. Fillip Kirazov and Mr. Isaak Vryzidi

Elastic and inelastic buckling of steel cellular beams under strong-axis bending

This paper presents an extensive parametric study of elastic and inelastic buckling of cellular beams subjected to strong axis bending in order to investigate the effect of a variety of geometric parameters, and further generate mass data to validate and train a neural network-based formula. Python was employed to automate mass finite element (FE) analyses and reliably examine the influence of the parameters. Overall, 102,060 FE analyses were performed. The effects of the initial geometric imperfection, material nonlinearity, manufacture-introduced residual stresses, web opening diameter, web-post width, web height, flange width, web and flange thickness, end web-post width, and span of the beams and their combinations were thoroughly examined. The results are also compared with the current state-of-the-art design guidelines used in the UK. It was concluded that the critical elastic buckling load of perforated beams corresponds to the lateral movement of the compression flange while the most critical parameters are the web thickness and the geometry of the flange. However, from the inelastic analysis, the geometry and position of the web opening influence the collapse load capacity in a similar fashion to the geometry of the flange and thickness of the web. It was also concluded that the effect of the initial conditions was insignificant