Advanced Analysis with Strain Limits for the Design of Steel Structures (original) (raw)
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Steel Design by Advanced Analysis: Material Modeling and Strain Limits
Engineering
Structural analysis of steel frames is typically performed using beam elements. Since these elements are unable to explicitly capture the local buckling behavior of steel cross-sections, traditional steel design specifications use the concept of cross-section classification to determine the extent to which the strength and deformation capacity of a cross-section are affected by local buckling. The use of plastic design methods are restricted to Class 1 cross-sections, which possess sufficient rotation capacity for plastic hinges to develop and a collapse mechanism to form. Local buckling prevents the development of plastic hinges with such rotation capacity for cross-sections of higher classes and, unless computationally demanding shell elements are used, elastic analysis is required. However, this article demonstrates that local buckling can be mimicked effectively in beam elements by incorporating the continuous strength method (CSM) strain limits into the analysis. Furthermore, by performing an advanced analysis that accounts for both geometric and material nonlinearities, no additional design checks are required. The positive influence of the strain hardening observed in stocky cross-sections can also be harnessed, provided a suitably accurate stress-strain relationship is adopted; a quad-linear material model for hot-rolled steels is described for this purpose. The CSM strain limits allow cross-sections of all slenderness to be analyzed in a consistent advanced analysis framework and to benefit from the appropriate level of load redistribution. The proposed approach is applied herein to individual members, continuous beams, and frames, and is shown to bring significant benefits in terms of accuracy and consistency over current steel design specifications.
The continuous strength method for steel and composite design
Proceedings of the ICE - Structures and Buildings, 2013
Many of the principal concepts that underpin current metallic structural design codes were developed on the basis of bi-linear (elastic, perfectly-plastic) material behaviour; such material behaviour lends itself to the concept of section classification. Resistance based on the assignment of cross-sections into this discrete classification system is a useful, but artificial, simplification. The resistance of structural cross-sections is, in reality, a continuous function of the slenderness of the constituent plate elements. Although not explicitly included in the determination of resistance, strain hardening is an essential component of the section classification system, and is required, for example, to enable the attainment of the plastic moment at finite strains. The Continuous Strength Method represents an alternative treatment to cross-section classification, which is based on a continuous relationship between slenderness and (inelastic) local buckling and a rational exploitation of strain hardening. The development and application of the Continuous Strength Method to structural steel design is described herein. Materials that exhibit a high degree of non-linearity and strain hardening, such as aluminium, stainless steel and some high-strength steels, fit less appropriately into the framework of cross-section classification, and generally benefit to a greater extent from the Continuous Strength Method. The method provides better agreement with test results in comparison to existing design codes, and offers increases in member resistance and a reduction in scatter of the prediction. An additional benefit of the proposed approach is that cross-section deformation capacity is explicitly determined in the calculations, thus enabling a more sophisticated and informed assessment of ductility supply and demand. Further developments to the method are underway.
Lateral Torsional Buckling Analysis and Design of Steel Beams with Continuous Spans
2017
Design standards do not provide provisions to account for the interaction between adjacent spans of continuous beams. In the absence of such provisions, the designer may opt for calculating the lateral torsional buckling capacity for each span separately by applying the moment gradient factors provided in standards and adopting the smallest critical moment as the one governing the design. The Salvadori hypothesis of isolating a member from the rest of the structure is assessed in the present study. The elastic lateral torsional buckling resistance for continuous beams is investigated based on finite element analysis. Comparisons are made between two types of solutions: (1) those neglecting interaction effects between adjacent spans, and (2) those considering span interaction. Also examined is the effect of presence of lateral/torsional restraints at intermediate supports of continuous beams. The results illustrate the merits of adopting the FEA solution in accounting for span intera...
Laterally stable steel beams can fail only by (a) Flexure (b) Shear or (c) Bearing, assuming the local buckling of slender components does not occur. These three conditions are the criteria for limit state design of steel beams. Steel beams would also become unserviceable due to excessive deflection and this is classified as a limit state of serviceability. The factored design moment, M at any section, in a beam due to external actions shall satisfy d M M ≤ Where M d = design bending strength of the section 6.3.1 Design strength in bending (Flexure)
Deformation based design of steel and composite structural elements
Proceedings 12th international conference on Advances in Steel-Concrete Composite Structures - ASCCS 2018, 2018
Steel and composite structures are traditionally designed through strength based calculations. An alternative approach is to consider deformation capacity. Deformation based design enables a more accurate allowance to be made for the spread of plasticity and allows strain hardening to be considered in a systematic manner. Importantly, the level of deformation required by the structure at ultimate limit state to reach the required design capacity can also be assessed. In composite construction, deformation based design enables a more rigorous assessment to be made of the development of strength in the structural system taking due account of the compatibility between the constituent materials. In this paper, recent developments to the deformation based continuous strength method for steel and composite design are described. Comparisons of capacities obtained from experiments and numerical simulations with those predicted using the continuous strength method are presented and discussed. Recommendations for future work on this topic are also set out.
Development of a new design method to define the rotation capacity of steel hollow sections
2016
Curvature at the beginning of strain hardening LVDT Measured curvature from LVDTs measurements gauges Measured curvature from strain gauges recording σ Stress σ0.2 0.2% proof stress σcr Critical stress σult Ultimate stress σy Yield stress Poisson's ratio v Deflection vu Deflection at ultimate load vu,a Deflection under the loading point a of the tested specimen vu,b Deflection under the loading point b of the tested specimen Buckling reduction factor CS Cross-section reduction factor CS+MB Member reduction factor Introduction-18-1 INTRODUCTION Steel hollow sections are being more and more used in structural applications. This is due to both their aesthetic and static properties. Hence, hollow structural sections require less paint than open profile and also less maintenance cost since, for example, the water cannot accumulate on the flanges… Moreover, hollow sections possess a high torsional stiffness compared to that of wide flange beams and thus require less lateral-torsional restraints. Nowadays, in order to take advantage of the full capacity of a structure, plastic design is starting to be more and more exploited mainly in the U.K. and North America. Structures loaded in bending, and where deflections are not significant, are the structures that benefit the most from plastic design. Plastic analysis requires that a beam is able to attain its plastic moment Mpl and maintain it through a range of deformations, in order for the moment to be redistributed. This will allow a collapse mechanism to form without exhibiting local buckling in the cross sections. This thesis is related to the rotational capacity of rectangular and square hollow sections. The main aim of this research work is to investigate new ways of defining the possibility to resort to a plastic analysis in practical design, and to improve current procedures and recommendations, in order to obtain a more consistent and mechanical approach. Nowadays, major design standards allow designers to resort to a so-called "plastic analysis and design" on the sole (direct or indirect) determination of the rotation capacity of a section while disregarding the rotation demand of the structure. Furthermore, most codes suggest individual b / t ratios of the individual plates comprised within the section to give the crosssection overall response, regardless of many parameters such as moment distribution (gradient), level of shear, ultimate-to-yield stress, height-to-length ration, ductility reserves…. In addition, the section's constituent plates are being considered under ideal support conditions, i.e. webs and flanges are assumed as pinned-pinned. Current developments take place in the context of the development of the Overall Interaction Concept (O.I.C.) [1]. One of the main features of the O.I.C. is the generalised overall relative slenderness λrel (Equation 1.1), that allows to account for the behaviour of the whole crosssection, therefore taking into account its constituents' plates interaction. "class 1" in European standards Eurocode 3), compact (class 2), semi-compact (class 3) or slender (class 4). This is achieved within the O.I.C. through the generalised overall relative slenderness λrel, and through associated cross-section interaction curves that lead to a smooth and continuous definition of the cross-sectional capacity. Consequently, the classification step becomes obsolete and disappears in the O.I.C. approach, avoiding many practical difficulties, inaccuracies and inconsistencies. Therefore, the need to "re-introduce" such a criterion is clear, and is dealt with in this thesis. In current work, the generalised overall relative slenderness λrel will be referred to as the cross-section slenderness CS, since only the cross section behaviour of hollow sections is studied in simple bending. CS therefore constitutes a measure of the cross-section sensitivity to local buckling. The basic idea developed in the present thesis consists in an extended use of λCS factor to define two families of sections: sections allowing for plastic analysis and design ("class 1" sections, possessing sufficient rotational capacity for a plastic failure mechanism to develop);
Plastic-Hinge methods for advanced analysis of steel frames
Journal of Constructional Steel Research, 1993
A number of recent research efforts have focused on the development of advanced analysis techniques and their possible application in limit-states design of steel structures. The n'ew Australian Standard AS4100-1990 allows the use of this type of analysis for the design of frames in which the members are of compact section and are sufficiently restrained against lateral-torsional buckling to develop the system's in-plane capacity. The term 'advanced" is intended to indicate any method of analysis that sufficiently captures the limit states encompassed by specification equations for member proportioning such that the checking of such equations is not required. The first part of this paper presents a detailed investigation of the adequacy of two second-order plastic-hinge based approaches for use as advanced analysis techniques. This is followed by a discussion of one possible approach for consideration of geometric imperfection effects in advanced analysis/design. The paper closes with a look at some of the issues regarding consideration of out-of-plane strength and rotation capacity in frames designed based on two-dimensional advanced inelastic procedures.
Slenderness-based design criteria to allow for the plastic analysis of tubular beams
Journal of Constructional Steel Research, 2020
The present paper focuses on the rotational capacity of H.S.S. steel sections; in particular, the influence of local buckling is accounted for by means of a new generalized cross-sectional slenderness parameter, which is used to characterize the cross-sectional rotational capacity, and, by extension, the available deformation capacity. Careful shell modelling of hollow section beams in bending was used, the numerical models being previously carefully validated against more than 50 bending tests. Extensive F.E. studies were consecutively performed, including many parameters such as various material grades, load and support arrangements, length-to-height ratios, etc. Specific attention was paid to the introduction of initial geometrical (local) imperfections, as they were shown quite influential on the rotation capacity. The paper then analyses the numerical results and points out the various influences of height-to-width ratio, shear, moment gradient, yield stress and length-to-height ratio on the available rotational capacity. In a second step, the rotational capacity demand vs. stability criterion is detailed, and related to the proposed generalized cross-sectional slenderness, which is shown to be more appropriate than the b/t ratios usually proposed in design codes. Finally, code-ready recommendations for new ways of allowing for plastic analysis in practical design following the proposed approach are given.
Contribution to the Evaluation of Steel Structures Resistance to Lateral Displacement’s
International Journal of Innovative Research in Computer Science & Technology, 2017
The choice of the global analysis method of a steel structure is essentially related to its sensitivity to second order effects. This sensitivity depends on the structural strength to the lateral displacement's. The classification of structure as "flexible structure" or "rigid structure" allows choosing the required method of analysis for the latter. From a regulatory point of view, a structure can be classified as rigid, if the ratio of the value of elastic critical load for the instability into the sway mode to the value of design vertical load is greater than ten. In practice, the calculation of the elastic total vertical load is not easy. For this reason, studies have been made in this field and have accomplished the proposal of simple expressions computing as an alternative to the direct determination of the critical elastic load of the structure. The main objective of this work is to explore these alternative methods in order to extend the study in this field and to evaluate their robustness and the results of its application on different types of structures.