Block shear failure planes of bolted connections — Direct experimental verifications

This paper presents direct experimental verifications of the active shear planes in bolted connections, previously identified by the first author for determining the block shear capacity. The laboratory test results were obtained by independent researchers for specimens where the applied loads were resisted by the block in shear only. The first set consists of five bolted connection specimens in the webs of wide flange sections where the tensile resistance planes had been sawn off. The second set consists of ten bolted connection specimens each in one leg of an angle section that had fractured completely along the net tensile plane through a block shear failure. Comparisons among the gross, net, and active shear planes against the independent laboratory test results showed that the critical shear planes of bolted connections were best represented by the active shear planes rather than either the gross or the net shear planes. It is also pointed out that full or almost full shear strain hardening was generally achieved at the ultimate limit state of block shear failure of bolted connections in hot-rolled steel plates or sections, irrespective of the connection length. Verification against independent laboratory test results of tee sections bolted at the web reinforces this point

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

Cross-section slenderness limits for columns with plastic rotations

This paper reports on a study of local inelastic buckling in square hollow section columns with large plastic rotations. The study was conducted as part of the validation of a proposed design method for discontinuous columns in braced frames in which plastic rotations in the columns are used to limit the moments in the columns. The study included both testing of full-scale columns and a parametric study by finite element analysis. The results demonstrate that current codes permit cross section slenderness in plastic sections which are likely to lead to premature buckling in structures using plastic (inelastic) design if the rotations are large. Design limits are proposed for square hollow sections relating cross-section slenderness to column end rotations

Optimum drilled flange moment resisting connections for seismic regions

Extensive damage in welded unreinforced flange (WUF) connections in previous earthquakes has led to the idea of using reduced beam section (RBS) connections to prevent brittle failure modes in welded joints. Using a similar concept, drilled flange (DF) moment resisting connections are established by a series of holes drilling on the top and the bottom flanges of the beam to create an intentional weak area to shift nonlinear deformations. DF connections are very easy-to-construct and they can also prevent the premature local buckling modes in the reduced section of RBS connections. This study aims to improve the performance of DF connections to make them viable alternatives to RBS connections for ductile steel frames in seismic regions. A wide range of experimentally validated non-linear FE models are used to investigate the effects of different design parameters such as drilled flange hole locations, hole configurations, panel zone shear strength ratio and doubler plate thickness. The results indicate that there is an optimum location and configuration for the drilled flange holes, which can reduce by up to 40% the maximum Equivalent Plastic Strain and Rupture Index of DF connections. It is shown that using strong panel zones can also improve the seismic performance of DF connections by reducing stress concentrations at the CJP groove weld lines. The results of this study are used to develop optimum design solutions for DF connections, which should prove useful in practical applications

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

Reliability analysis of structural stainless steel design provisions

Since the establishment of the Eurocode design provisions for structural stainless steel, a considerable amount of both statistical material data and experimental results on structural elements has been generated. In light of this, the current partial resistance factors recommended in EN 1993-1-4 for the design of stainless steel elements are re-evaluated. First, following an analysis of material data from key stainless steel producers, representative values of the over-strength and the coefficient of variation (COV) of the material yield strength and ultimate tensile strength were established. For yield strength, over-strength values and COVs of 1.3 and 0.060 for austenitic, 1.1 and 0.030 for duplex and 1.2 and 0.045 for ferritic stainless steels were determined. For the ultimate tensile strength, an over-strength value of 1.1 was found to be suitable for all stainless steel grades, and COV values of 0.035 for the austenitic and duplex grades and 0.05 for the ferritic grade were proposed. For the variability of the geometric properties, a COV value of 0.05 was recommended. Analysis of available experimental results based on the First Order Reliability Method (FORM), set out in EN 1990 Annex D, and utilising the derived statistical material parameters, revealed that the current recommended partial resistance factors in EN 1993-1-4 (γM0 = γM1 = 1.1 and γM2 = 1.25) cannot generally be reduced, and in some cases, modified design resistance equations are required, if the current safety factors are to be maintained