Second-order design and advanced analysis of hybrid steel and concrete framed structures

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Second-order design and advanced analysis of hybrid steel and concrete framed structures


Author: Liu, Siwei
Title: Second-order design and advanced analysis of hybrid steel and concrete framed structures
Degree: Ph.D.
Year: 2014
Subject: Building, Iron and steel.
Concrete construction.
Composite construction.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xxxi, 335 leaves : ill. ; 30 cm.
InnoPac Record:
Abstract: Hybrid steel and concrete framed structures, which consist of bare steel (BS), reinforced concrete (RC) and steel-concrete composite (SCC) members, are more increasingly and extensively used in modern buildings. The structural benefit of this system is to combine the advantages of two construction materials with concrete having high compression strength, large damping ratio and good corrosion resistance and steel possessing of high tension strength, good ductility and efficiency in constructability. This structural form is superior to traditional BS and RC framed systems, and therefore, it becomes a popular selection in modern high-rise buildings. It is noted that the current design practice for this structural form is both inconvenient and inconsistent to apply. Design guidance and principles in most codes are mainly derived for first-order analysis, and the tedious and cumbersome hand calculations used in conjunction with complicated formulas are necessarily required, such as the assumptions of K-factors or the effective length for members in sway or non-sway frames. In addition, the related clauses for stability check vary in BS, RC and SCC design codes and these lead to the design procedure being inconsistent and inefficient. In this research, a unified second-order design method is proposed and it only requires section capacity check at critical locations of a member by failure surfaces without needs of using the prescriptive formulae in various codes. The philosophy of advanced analysis method is to consider the various effects inherent to real structures, namely as initial imperfections, geometric and material nonlinearities and so on. Due to the differences in material characteristics between concrete and steel, BS members are slender usually with critical stability problems, while RC members always show significant plasticity and both of these nonlinear behaviors are observed in SCC members. Moreover, frame global and member local imperfections are initially existed, which influence deflections as well as force distributions and need to be properly modeled. To these ends, a practical and efficient advanced analysis approach is proposed for designing of hybrid steel and concrete framed structure with considerations of all these vital effects. In this thesis, a beam-column finite element with an arbitrarily-located plastic hinge (ALH element) is firstly proposed for both second-order elastic and advanced plastic analysis. This element is initially curved such that member local imperfections can be directly modeled. Due to existence of the internal degree of freedoms, only one element is sufficient to simulate large deflections in members. Two plastic hinges are further incorporated into the element ends that inelastic behaviour of the element can be more accurately presented. Furthermore, the additional degrees of freedom in the proposed element are condensed and this dramatically improves numerical efficiency and brings much convenience to computer programming. Apart from the conventional formulation requiring two and more elements to model imperfections or capture the locations of plastic-hinges, one element per member is adequate in the proposed analytical model. It is believed that this numerical procedure is efficient and the saving in computer time and data manipulation efforts is considerable.
In order to evaluate a section under axial force and biaxial bending, three types of sectional yield surfaces are generated namely as initial yield, failure and concrete fracture surfaces. The initial yield and failure surface defines the elastic-limit and ultimate-limit states, respectively, while the concrete fracture surface is constructed with the use of the Branson{174}s model for simulating concrete cracking effects. In addition, a refined plastic hinge model integrated with these sectional yield surfaces for various material types of members is also proposed in this thesis. In generating of the sectional yield surfaces, an analysis technique for arbitrary sections is proposed. The quasi-Newton iterative scheme is adopted for determining the location of neutral axis of a section. Two types of stress-resultant approaches for concrete components are provided as the equivalent stress block and elaborated layer-integration methods. The former is limited to the ultimate limit states, whereas the latter can be utilized for any specified conditions. A structural steel component is automatically meshed into small fibers and each rebar is lumped into a point that occupies a certain area. The openings and voids occupied by other components are excluded by the negative area approach. The recently published codes, such as Eurocode 3 (2005) and Eurocode 4 (2004), recommend the use of second-order analysis method for different types of structural members. The corresponding second-order design perspectives in Eurocodes for RC, BS and SCC members are investigated. Furthermore, several design examples are also analysed and presented to demonstrate the feasibility of using the proposed second-order design method in modern practice. In the proposed advanced analysis approach, the material constitutive models are required and they are critical to the accuracy. Therefore, the material models from Eurocodes and available literatures are selected and discussed. Since the Eurocode 4 (2004) only permits the use of normal strength concrete in concrete-filled composite construction, an experimental investigation on the material properties of high-strength concrete (HSC) in circular and octagonal steel tubes is demonstrated. A series of benchmark examples in literatures and experiments are selected for verifying the accuracy and feasibility of the proposed analysis approach for cross sections, individual members and framed structures to elaborate the efficiency and validity of the proposed method. The distinct feature of this research includes development of an efficient curved beam-column finite element and integration with an accurate cross section analysis technique, which only requires material constitutive relations. Based on the proposed formulations, unified and practical second-order design and advanced analysis method for hybrid steel and concrete members and framed structures are developed.

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