Design and analysis of steel frames to LFRD (2010) and EC3 (2005)

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Design and analysis of steel frames to LFRD (2010) and EC3 (2005)


Author: Lin, Sang
Title: Design and analysis of steel frames to LFRD (2010) and EC3 (2005)
Degree: M.Sc.
Year: 2014
Subject: Steel, Structural.
Structural frames.
Structural analysis (Engineering)
Hong Kong Polytechnic University -- Dissertations
Department: Faculty of Construction and Environment
Pages: ix, 112, [41] p. : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: Structural nonlinearity and second-order effects can make perceptible effects on a steel frame system. Recently published steel design codes such as LFRD (2010) and EC3 (2005) have respective specifications of imperfection consideration and second-order analysis. The structural nonlinearity can be divided into geometric nonlinearity and material nonlinearity. LFRD considers L/1000 out-of-straightness imperfection of each member and 20 percent stiffness reduction of structure. Whereas EC3 assumes equivalent geometric imperfection values to cover possible effects of all types of imperfection including geometrical imperfection and residual stresses. Both LFRD (2010) and EC3 (2005) recommended the use of direct second-order analysis and design as a more accurate and reliable method in place of the first-order analysis so that the empirical design parameters such as the effective length and buckling reduction factors can be eliminated. Under a direct second-order elastic analysis, second-order effects are taken into account directly on a whole system. Meanwhile, LFRD and EC3 permit not only modeling of imperfection directly, but also a notional force method to simulate imperfection. But the notional force method may add a stress induced by fictitious load which should not exist. According to codes, both LFRD and EC3 have linear second-order design approaches by which first-order results shall be enlarged member-by-member under featured amplification factors. Those complicated and tedious design procedures consider second-order effects for members independently by indirect and empirical parameters according to design specification, which are twofold in general: 1) Flexural buckling due to P-δ effect. Determination of member effective length is indispensable for both LFRD and EC3 approached. LFRD recommends use of alignment charts for the determination, whereas EC3 have no direct guidance on effective length. 2) Sway effect due to P-Δ effect. The LFRD second-order magnification separates P-Δ effects from P-δ effects, and total performance of a structure are considered as algebraic adding of sway component and non-sway component. However, traditional Eurocode3 method is an overall amplification of structural moments under a P-Δ-only analysis. This dissertation aims to compare second-order analysis under LFRD and EC3 specifications. A practical example, a medium story steel frame model, is built by program NAF-NIDA, to find diversities of structural performance following LFRD and EC3 imperfection values. This frame is also analyzed under traditional LFRD and EC3 approaches so as to compare linear and nonlinear analysis. And two auxiliary irregular frames are launched out for comparison to find shortcomings of traditional LFRD and EC3 approaches.

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