|Advanced analysis and design of axial compression members with high-strength steel
|Chan, S. L. (CEE)
|Steel, High strength.
Hong Kong Polytechnic University -- Dissertations
|Department of Civil and Environmental Engineering
|xxviii, 298 pages : illustrations (some color)
|High-strength steel is a term usually applied to specific types of steel that have a design yield strength larger than 460 MPa, distinguishing it from the other types of steel having lower strength. Its adoption has become popular in the past decade, initiating certain benefits in fabrication, construction, cost, and so on. Not only does using high-strength steel dramatically reduce the section sizes of members associated with material consumption and member weight, it also considerably alleviates the difficulties in buildmanship and accelerates construction. This can significantly benefit construction in high density urban areas, such as Hong Kong, Shanghai, and New York, where the labor cost is usually more expensive than the material cost. Consequentially, high-strength steel can be viewed as environmental friendly because its use reduces material consumption, thereby decreasing carbon dioxide release. Although high-strength steel is mass-produced and is widely used in mechanical engineering, its utilization in building construction is still under exploration due to the lack of consummate design and analysis approaches. The current design codes for steel columns used worldwide, e.g., ANSI/AISC 360-10, Eurocode 3 Part 1-1, Hong Kong Code 2011, and China GB50017-2003, mostly provide the formulae and charts for steel with a design yield strength less than 460 MPa, resulting in obstacles in the design and application of high-strength steel members. Eurocode 3 Part 1-12 simply extends the use of high-strength steel to grade S 700. This thesis aims to propose a direct analysis and design approach for high-strength steel members, thereby eliminating the use of the tedious charts and formulae associated with linear elastic assumptions. By using high-strength steel, the sectional sizes of members can be dramatically reduced and the stability problems associated with the second-order effects would be dominated, which should be properly reflected in the design. In the present study, the pointwise equilibrium polynomial (PEP) element is employed to model the beam-column members and to simulate the P-δ effects due to initial member curvatures. Adopting the high-order shape function for the PEP element and considering the bowing effects, one element per member is sufficient and accurate for nonlinear elastic analysis. Therefore, member design can be simply executed by checking the section strength at critical locations. For a more accurate representation of the section capacity, the sectional yield surfaces are introduced, which are expressed by a series of strength points in a bi-axial loading space. Two types of surfaces are presented: initial and failure surfaces. These are generated by a cross-sectional analysis technique based on the quasi-Newton algorithm. The sections are automatically meshed into small triangular fibers. This method is valid for arbitrary shapes of the steel sections. The elastic and plastic limit states of a section can be described by the initial yield and failure surfaces, respectively. To explicitly consider the influences of residual stress, the cross-sectional analysis technique is further developed to take this effect into account. The initial stress inherent in a section is reflected by introducing the residual strains applied to each fiber; therefore, the numerical algorithm can be easily modified on the basis of the current available programs.
Consequentially, residual stress models are vital for the cross section analysis. In real-time applications, Q690 high-strength steel shows a non-negligible potential in applications; however, its residual stress distribution patterns are still being explored and they are seldom reported in the available literature. In this thesis, the magnitudes and distributions of six box and H columns, fabricated by Q690 high-strength steel, will be investigated using the sectioning method. The residual stress patterns on column sections are examined and presented. To simplify the analysis, straight-line models of Q690 high-strength steel are first proposed on the basis of the experimental investigations, which can be further applied to the analysis of the other sections with ranged width-to-thickness or height-to-thickness ratios. To fulfill the design requirements for simulating the structural behaviors under extreme scenarios, such as progressive collapse analysis, performance-based seismic design, the vital effects inherent to the structural members should be reflected. The P-Δand P-δ effects related to the frame out-of-plumpness and initial member curvatures, respectively, are directly considered by employing the curved PEP element with the high-order shape function. The influence of residual stress is reflected by explicitly calculating in the cross-section analysis. In addition, a refined plastic hinge model is introduced in order to consider the gradually yielding behaviors at the critical sections. From the present study, it can be observed that the residual stress exerts certain influences on controlling the sectional elastic limit that might cause the fiber to start yielding at a low stress state. Differing from the conventional method, where the equivalent imperfection is adopted combing initial member curvatures and residual stresses, the proposed method separately considers these two vital effects. Therefore, the design can be more accurate, safe, and economical thereby eliminating the empirical and uncertain considerations found in the conventional design method. To verify the accuracy and versatility of the proposed method, six fabricated box columns and six welded H columns with different slenderness ratios ranging from 30 to 70 are axially-loaded and studied. All of the columns were prepared by the flame-cut Q690 steel plates with a thickness equal to 16 mm. The numerical simulations of the specimens obtained by the proposed method are presented and compared with the outcomes of the experiments, showing satisfactory results in the comparisons in terms of tracing the load vs. deflection and predicting the ultimate strength. Extensive research studying the overall buckling behavior of the Q690 columns with different slenderness ratios is conducted, and 132 nos. and 192 nos. columns with box and H sections, respectively, are analyzed by the proposed method. These results are compared with the conventional buckling curves in codes, e.g., GB 50017-2003, Eurocode 3, and ANSI/AISC 360-10. The comparisons show that the axial strengths of the high-strength columns are underestimated, especially for the members with low slenderness ratios. This further proves the importance and necessity of developing an efficient and practical method for the design of high-strength members; otherwise, the material utilization efficiency could be reduced. To propose an efficient and practical method for the design of high-strength steel members, a second-order design method, based on ANSI/AISC 360-10, is proposed by revising the stiffness reduction factor for a more proper reflection of the residual stress. The optimal design formulae for high-strength steel members are proposed on the basis of an extensive study comprised of over 300 columns. In this thesis, an efficient advanced analysis and practical second-order design method for structures with high-strength steel members is proposed. A cross-section analysis technique with an explicit consideration of residual stresses is developed. Strength-line models for describing the residual stress patterns of H and box sections fabricated by Q690 steel plates are first proposed. The curved PEP element is employed to simulate the initial member curvature and capture the large deflection effect. A refined plastic hinge model using the sectional surfaces is used to model the inelastic behavior at the gradually yielding sections. Twelve columns are experimentally investigated to verify the proposed theory and extensive research on over 300 nos. columns is conducted. A refined second-order design approach, based on ANSI/AISC 360-10, for high-strength steel members is proposed.
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