Influence of the imperfection direction on the ultimate response of steel frames in advanced analysis

Initial geometric imperfections are unavoidable in steel members and frames due to erection and manufacturing tolerances. These include frame out-of-plumbness, member out-of-straightness and cross-sectional imperfections, and can have a significant influence on the response and resistance of steel structures. Thus, they need to be accounted for in the analysis and design of steel structures, especially when advanced design procedures are adopted. One of the easiest approaches to introduce geometric imperfections in structural finite element models is through the linear superposition of scaled eigenmodes, which are obtained from a priori elastic buckling analysis. Although the shape and magnitude of frame and member imperfections are specified in international standards, the rules for the combination of different types and directions of imperfections are unclear or impractical, and often require designers to consider many possible combinations to find the critical, or “worst case”, shape of the imperfection including the direction of each eigenmode. This paper investigates the influence of the direction of modes contributing to the imperfection on the ultimate load (i.e., resistance) of steel frames when using advanced analysis. Ultimate loads are estimated from advanced finite element simulations for 20 regular and irregular unbraced frames featuring steel and austenitic stainless steel compact sections, in which initial imperfections are modelled as linear superpositions of six scaled buckling modes considering all possible combinations of direction. The results show that the influence of the imperfection direction on the ultimate frame load is small, and that assuming a combination of all buckling modes with positive amplitudes provides a simple and accurate estimation of the critical imperfection combination.The project leading to this research has received funding from the European Union's Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 842395.Peer ReviewedPostprint (published version

Reliability of stainless steel frames designed using the Direct Design Method in serviceability limit states

Steel structures can be consistently and efficiently designed using system-based design-by-analysis approaches such as the Direct Design Method. However, since direct design approaches lead to potentially lighter structural configurations, they can also result in larger deformations under service loads. Thus, greater attention may be required to serviceability limit states in structures designed using design-by-analysis approaches than for structures designed elastically at their ultimate limit state following current two-stage approaches, especially for materials showing highly nonlinear stress vs strain responses such as stainless steel alloys. With the aim of investigating the influence of allowing larger deformations in the ultimate limit state design of stainless steel structures, this paper presents an explicit analysis framework for assessing serviceability reliability at system level. Using this framework, the paper investigates the serviceability reliability of cold-formed stainless steel portal frames designed using the Direct Design Method for different load cases, including the gravity load and the combined gravity plus wind load combinations. The study considers six baseline frames covering the most common stainless steel families and international design frameworks (i.e., Eurocode, US and Australian frameworks), for which the reliability of vertical deflection and lateral drift serviceability limit states is investigated using advanced numerical simulations and First-Order Reliability Methods. From the comparison of the calculated average annual reliability indices and the relevant target reliabilities for the different design frameworks, it was found that the reliability of stainless steel frames appears to be adequate for the serviceability limit states investigated for the Eurocode, US and Australian frameworks.Peer ReviewedPostprint (published version

On the system-based design for steel frames using inelastic analysis

Design by inelastic analysis of overall system behaviour is permitted in several steel design specifications worldwide (e.g., the American Specification AISC360-10 and the Australian Specification AS4100-1998). Advanced inelastic analysis is better able to capture the system behavioural characteristics as they currently are understood. This paper presents a case study of the design of three planar steel structures using different design methods, including the Direct Analysis method in AISC360-10, the inelastic design method in AISC360-10, and the inelastic method (“advanced analysis”) in AS4100. The effects of structural ductility (capacity of load redistribution) and failure modes on the design results are discussed

Simplified expressions for reliability assessments in code calibration

First Order Reliability Methods (FORM) have been used by specification committees in the reliability analyses required for the calibration of resistance and safety factors for the past 40 years. However, these methods are iterative, require input information that may not be readily available, and make comparisons between different approaches or design frameworks difficult. This paper presents a set of simplified equations to estimate reliability indices , resistance factors and partial safety factors based on simpler First Order Second Moment (FOSM) considerations for the US and Eurocode frameworks, which are particularized for different load cases, and on the semi-probabilistic approach prescribed in the Eurocode 0. The equations provide direct relationships between the reliability calibration results corresponding to different design frameworks, and can be used to estimate resistance factors as simple cross-checks for the US framework based on the partial safety factors derived for the Eurocode (or vice versa) from basic statistical input information and given target reliability, including when the data available in the literature is insufficient to perform FORM analyses. The accuracy of the proposed equations is assessed against reliability results derived using FORM techniques for an extensive database of steel and stainless steel frames subjected to gravity and combined gravity plus wind load cases collected from the literature, and limitations for their applicability are recommended. The results demonstrate that the set of equations proposed in this paper provides accurate estimations of the reliability index, resistance factors and partial safety factors and can assist specification committees in the process of calibrating suitable system factors.Peer ReviewedPostprint (published version

System-based reliability analysis of stainless steel frames subjected to gravity and wind loads

In the process of developing the next generation of design standards for steel structures, most relevant international structural codes including AISC 360, AISC 370, AS/NZS 4100 and Eurocode 3 already incorporate preliminary versions of system-based design-by-analysis approaches that allow a direct evaluation of the strength of steel and stainless steel structures from advanced numerical simulations. As a result, recent research works have focused on building rigorous structural reliability frameworks to investigate acceptable target reliability indices for structural systems and to develop new design methods in conjunction with adequate system safety factors and system resistance factors. Although design recommendations exist for the direct design of hot-rolled and cold-formed steel structures based on advanced finite element analysis, the extension of the method to other materials such as stainless steel is under development. This paper is part of a research effort to build a reliability framework for stainless steel structures subject to different load combinations and presents the results of system reliability calibrations carried out on six stainless steel portal frames subjected to combined gravity and wind loads. The study covers the most common stainless steel families and three international design frameworks (i.e., Eurocode, US and Australian frameworks). From the reliability calibrations derived, suitable system safety factors and system resistance factors are proposed for the direct design of stainless steel frames under combined gravity and wind loads using advanced numerical simulations.The project leading to this research has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 84239.Peer ReviewedPostprint (published version

The Shift of the Effective Centroid in Plain Channel Section Columns (No. R966)

The report derives an equation for the shift of the effective centroid that occurs in plain channel section columns as a result of local buckling. The shift causes bending when the column is compressed between pinned ends and may need to be accounted for in design. The equation is based on Stowell’s work on simply supported plate elements with a free longitudinal edge. It is shown to be in reasonable agreement with eccentricities calculated using results of geometric nonlinear shell finite element analyses of plain channel columns. It is also shown to provide more accurate values of eccentricity than the effective width method, in which the effective width of each plate element is calculated to produce an effective cross-section and an associated effective centroid

Pre-normative recommendations for the system-based direct design of stainless steel frames using advanced analysis

This document includes the pre-normative recommendations developed in the NewGeneSS research project for the design of stainless steel frames using advanced analysisThe research project NewGeneSS, the acronym of “NEW GENEration design methods for Stainless steel Structures” was financed by the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Actions (2018). The NewGeneSS project aimed at developing the basis of system-based direct design approaches for stainless steel structures in the European framework by calibrating suitable system safety factors from rigorous structural reliability considerations, and at delivering the pre-normative design recommendations included in this document.The project leading to this document has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 842395 (NewGeneSS project).Preprin

Modern temporal network theory: A colloquium

The power of any kind of network approach lies in the ability to simplify a complex system so that one can better understand its function as a whole. Sometimes it is beneficial, however, to include more information than in a simple graph of only nodes and links. Adding information about times of interactions can make predictions and mechanistic understanding more accurate. The drawback, however, is that there are not so many methods available, partly because temporal networks is a relatively young field, partly because it more difficult to develop such methods compared to for static networks. In this colloquium, we review the methods to analyze and model temporal networks and processes taking place on them, focusing mainly on the last three years. This includes the spreading of infectious disease, opinions, rumors, in social networks; information packets in computer networks; various types of signaling in biology, and more. We also discuss future directions.Comment: Final accepted versio

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR