|Title:||Theory and application of second-order direct analysis in static and dynamic design of frame structures|
|Advisors:||Chan, Siu-lai (CEE)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Structural analysis (Engineering)
|Department:||Department of Civil and Environmental Engineering|
|Pages:||13 unnumbered pages, 260 pages : color illustrations|
|Abstract:||The modern design codes such as Eurocode3 (2005), AISC360 (2016), CoPHK (2011) and GB50017 (2017) recommend the use of direct analysis method (DAM) instead of traditional effective length method for daily design. However, the research and applications of DAM mainly focus on frame structures subjected to static loads. Performance-based seismic design (PBSD) is a new trend in structural engineering and appears in most of the modern design codes or specifications such as Eurocode-8 (2005) and FEMA356 (2000). The philosophy inherent to the approach is to accurately capture the structural behavior under earthquake actions which is substantially in line with DAM, and the demanded performance according to the occupied functions is estimated. It is found that little work has been carried out on the extension of DAM to PBSD. The pushover analysis and time history analysis should be used with effective length method for PBSD in current practice as the member imperfections have not been taken into account. The DAM requires explicit consideration of member initial imperfections to suppress the use of effective length factor while PBSD needs to simulate progressive yielding along the section depth and member length. Thus, an advanced and high-performance beam-column element with member imperfection is urgently required for static and dynamic design. In the past decades, the stiffness and flexibility methods have been extensively used to derive beam-column elements. As the flexibility-based type elements can meet the compatibility and equilibrium conditions at the element level, they are more competent in direct analysis considering both geometrical and material nonlinearities. In this research project, an advanced flexibility-based beam-column element with member initial imperfections, finite joint stiffness, rigid zone and end offsets, and distributed material nonlinearity is proposed for the second-order direct analysis of frame structures under static and dynamic actions. Considerable care has been taken to verify the accuracy and efficiency of this new element. To account for material nonlinearity, the stiffness-based elements with the plastic hinge method and the flexibility-based elements with the plastic zone method are well adopted in the second-order inelastic analysis. The former emphasizes the computational efficiency with relatively less-accurate structural responses, while the latter aims to precisely simulate the structural behavior but needs typically more computer time. One proposed element is generally sufficient to model a practical member in engineering structure without the assumption of effective length. The limitations of stiffness-based elements and conventional flexibility-based elements are removed in the new element.|
To further improve the computational efficiency of the proposed element, an alternative approach following the concept of the plastic hinge method is proposed by considering the elastoplastic behavior of steel members through the integration points using the stress resultant plasticity model rather than the fiber section. This method yields high accuracy comparable to the plastic zone method but requires much less computer time and resources, without the need of a fiber mesh along the section. To consider the effect of finite joint stiffness, two zero-length springs representing moment-rotation relationships are attached at the ends of the proposed beam-column element. The contribution of axial force on the bending moments are first discussed. The proposed hybrid beam-column element can well capture the behavior of a joint under monotonic or cyclic loading. The current AISC360 (2016) specified a new effective stress-strain method for design of noncompact and slender concrete-filled steel tube (CFT) members. However, there is still lack of a practical tool for second-order direct analysis of this kind of members experiencing complicated behaviors such as local plate buckling, concrete confinement and yielding of steel tube. The stiffness change of the CFT members under combined axial force and bending moments should be considered during the incremental-iterative procedure of direct analysis. The proposed beam-column element with fiber section technique provides a new solution for the design of noncompact and slender CFT members using the effective stress-strain relationships. This method complies with the provisions in AISC360 (2016) and several recommendations are introduced for improvement of design codes. Special concentrically braced frames (SCBFs) are widely used in high seismic regions due to their structural efficiency and high ductility for energy dissipation. SCBFs are allowed for large inelastic deformation through tensile yielding, buckling and post-buckling behaviors of braces. The accurate modeling of braces with acceptable computational costs is vital to capture the real structural behavior of SCBFs subjected to earthquakes. A hybrid beam-column element with consideration of gusset plate connection is proposed for modeling of SCBFs. It shows significant improvement in the time history analysis of SCBFs. In summary, this research project proposes a comprehensive method for second-order static and dynamic analysis of frame structures with consideration of member initial imperfections, material yielding and semi-rigid connections. It will significantly improve the current design practice and help the engineers to produce a safer and more economic design.
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