Darko Beg

Slenderness limit of Class 3 I cross-sections made of high strength steel

Abstract In this paper an attempt is made to establish a more accurate slenderness limit for Class 3 cross-section welded I beams made of high strength steel. For this purpose experimental and nonlinear numerical analysis of the local stability was performed. Ten beams with different flange slenderness were tested up to failure load. The influence of other parameters, particularly web slenderness, was analysed using finite element nonlinear numerical analysis. According to the obtained results and method proposed by B. Kato (J. Construct. Steel Res. 17 (1990) 33–94) the analytical expression for demarcation between slender and semicompact I cross-sections taking into account the flange web interaction was derived. Further research work is needed to consider also the influence of the flange to web thickness ratio.

On the rotation capacity of moment connections

Abstract The paper presents an analytical method for the determination of the rotation capacity of moment connections, based on the component method from prEN 1993-1-8. From test results and numerical simulations, simple analytical expressions for the deformation capacity of the components are derived. These values are subsequently used for the determination of the rotation capacity of the complete joint. Comparisons with tests on whole joints indicate a good agreement between analytical and experimental results. The method is fully consistent with the present rules of prEN 1993-1-8 and extends them to the numerical estimation of the rotation capacity.

Finite element simulation of slender thin-walled box columns by implementing real initial conditions

This paper deals with the numerical simulation of tests on slender thin-walled box columns, susceptible to combined instability: to global Euler buckling and to local buckling of steel plates. Eight full-scale tests were carried out with different global slenderness of welded and cold-formed columns subjected to centric and eccentric compression. Initial column geometry and residual stresses were measured and carefully implemented in numerical models. The results of FEA simulations show good agreement with the test results. On verified numerical model the influence of different initial imperfections was studied and various equivalent geometric imperfections were tested for the considered column configurations.

Assessment of the Seismic Response of Concentrically-Braced Steel Frames

The BRACED project investigated the ultimate behaviour of concentrically braced frames (CBFs). The research programme was designed to validate empirical models for the ductility capacity of hollow section bracing members and recent proposals for the improved detailing of gusset plate connections, to identify active yield mechanisms and failure modes in different brace member/connection configurations, and to provide essential data on the earthquake response of European CBFs. The central element of the integrated experimental and numerical research programme is a series of shake table experiments on full-scale model single-storey CBFs designed to Eurocode 8 (CEN EN 1998-1:2004, Eurocode 8: Design of structures for earthquake resistance—Part 1: General rules, seismic actions and rules for buildings. European committee for standardization, 2004). Twelve separate experiments were performed on the Azalee seismic testing facility at CEA Saclay. The properties of the brace members and gusset plate connections were varied between experiments to examine a range of feasible properties and to investigate the influence of conventional and improved design details on frame response. Each experiment examined the response of the test frame and brace-gusset plate specimens to table excitations scaled to produce elastic response, brace buckling/yielding and brace fracture. These experiments were supported by complementary quasi-static cyclic tests and correlative numerical simulations using pushover and time-history analysis using the OpenSees seismic analysis software. The outputs of the research programme represent a unique set of data on the ultimate earthquake response of CBFs with realistic brace members and connections. The principal experimental outcomes include measurements of elastic frame stiffness and its evolution with brace damage, measurements of the displacement ductility capacity of the brace specimens; an evaluation of the influence of brace connection configuration and gusset plate detailing on frame stiffness, damping and ductility; and observations on the contributions of brace and connection yielding to overall inelastic deformation of CBFs.

Ioannis Vayas reaches 60

Steel Construction 6 (2013), No. 4 coming from research work. His research activities span from plate buckling problems, plastic resistance of cross-sections including torsion actions, low-cycle fatigue resistance of steel frames subjected to seismic actions, rotation capacity of beam-column joints, innovative dissipative devices in seismic resistant steel frames, numerical modelling of steel concrete composite bridges, etc. As design engineer he has been involved in the design of residential, office and industrial buildings, composite bridges, towers, masts and other structures. He has employed all his experience from the research and engineering work in a number of books published in Greek, German and English. In Greek he published 8 books on steel and steel-concrete composite structures, including the application of Eurocode 3 and Eurocode 4. Three of these books were translated to German and published by Ernst & Sohn. They were well received, because they were equally appropriate for students and practising engineers mainly due to a large number of well selected numerical examples. His last book (in co-authorship with A. Iliopoulos) on Design of Steel-Concrete Composite Bridges to Eurocodes that was published this year by Taylor & Francis, represents a culmination of publishing activities of Prof. Vayas. The book brings very complete information on the design issues of steel-concrete composite bridges with a lot of detailed information on all relevant topics have so far not been available in one publication. In 1997 the Technical University of Cluj-Napoca, Romania, awarded him an honorary doctorate for establishing a model cooperation between the two universities. Those who have had the privilege to meet Prof. Vayas and work with him in European research projects, in CEN/TC 250/SC3 (Eurocode 3 Committee) or at the conferences know that he has a strong personality but at the same time a kind character. He has made a lot of friends all around Europe and when he was involved in an unfortunate dispute about the authorship of one of the text books (published in Greek by one of his academic colleagues), very prominent European academics without any hesitation took his side and helped to solve the case. This is the opportunity that on behalf of all his friends and all those who appreciate Prof. Ioannis Vayas we wish him good health, and although the academic life in Greece has seen much better times, we wish him new excitements and challenges in his academic and professional life.

Stiffness and Strength of Single Bearing Bolt Connections

This paper focuses on to single bolt connections with plate in bolt bearing that present a basic component of a multi bolt connection. The bearing strengths of mild as well as high strength steel connections are given. An extensive numeric parametric study was executed to obtain additional data. A modified bolt bearing resistance formula is presented. The stiffness of the connections according to Eurocode is also presented and compared to the experimental one.

Longitudinally Stiffened Girders Subjected to Bending Moment

The paper deals with the resistance of plated girders subjected to normal stresses due to bending moment. The elastic critical plate-like buckling stress calculated according to simplified analytical method from EN 1993-1-5 was compared against elastic critical stress calculated with EBPlate software which was further on used to determine bending resistance. This was determined by four proposed methods those were compared against numerical simulations. Finally, the results of numerical analysis were used to statistically evaluate all resistance models and to determine partial safety factors.

BENDING-SHEAR INTERACTION OF LONGITUDINALLY STIFFENED GIRDERS

To understand behaviour of longitudinally stiffened plated girders subjected to high bending moments and shear forces, four tests on large scale test specimens were performed. The results of these tests were used to verify the numerical model, which was employed for further parametric studies. With a verified simplified numerical model a parametric nonlinear analysis was systematically carried out to determine the resistance of longitudinally stiffened plated girders. Based on 630 numerical simulations a new equation for interaction at high bending moments and shear forces is proposed, as is the section, where the check should be performed. An extensive reliability analysis of five different design models was made, i.e., the EN 1993-1-5 interaction model, the proposed new model, the gross cross-section bending resistance model and two models, which are a combination of the first three.

Reliability analysis of net cross-section resistance with accidental eccentricity of holes

The aim of the research was to determine reliability function of net cross-section resistance in relation to accidental eccentricity of nominally centric holes. The response surface method was used by taking into account the relevant Eurocode design provisions and reliability requirements. Within the response surface method, the central composite design method and the least square method were used with the employment of Monte Carlo simulations. The probable distributed variables such as strength fy, breadth b, thickness t, diameter d0 and eccentricity e were determined by the central composite design method. 280 different numerical simulations were set up with varying variables. A log-normal distribution for strength (fy) and a normal distribution for geometrical variables (b, t, d0,e) were employed by taking into account the coefficients of variations: Vfy=0.07, Vb,=0.005, Vl=0.05 and Vd0=0.005. In order to determine the influence of eccentricity on partial safety factor, several normal distributions with different variation factors were applied in the analysis. The influence of the edge distance of the hole e2 over d0 ratio on the partial safety factor was determined by varying mean values of variable b. For comparison, two types of steel were used: structural steel S235 and high strength steel S690. Numerical simulations of the net cross-section resistance Fu were performed with ABAQUS 6.7. The response surface for the net cross-section resistance was determined by introducing a quadratic approximation function and by applying the least square method. The partial safely factor was then (statistically) obtained by means of robust Monte Carlo simulations on the calculated response surface.