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|Title:||Study on structural behaviour of high strength steel s690 welded H- and I-sections||Authors:||Wang, Kai||Degree:||Ph.D.||Issue Date:||2018||Abstract:||Motivation
High strength S690 steel material possesses a yield strength of 690 N/mm² which are double or triple to those of normal strength steel materials, i.e. S235 and S355 steel materials. In recent years, massive production of these high strength steel (HSS) materials was realized in modern steel mills, and many successful applications of HSS in machinery, equipment and structures have been reported. Hence, advantages of excellent strength-to-self-weight ratios and high levels of economy of HSS have been widely recognized by researchers and engineers. It is also realized that drawbacks such as reduced ductility, stringent welding requirements as well as insufficient design guidance should be overcome. Up to the presence, wide application of HSS is hindered, particularly for building construction. Hence, it is highly desirable to experimentally understand structural behaviour of S690 welded sections. Moreover, suitable design methods should be developed to facilitate safe and efficient design for HSS structures. Objectives and scope of work In this research project, a systematic experimental and numerical investigation into structural behaviour of S690 welded H- and I-sections is conducted. This research project is comprehensive in which material properties of S690 steel material and its residual stress patterns induced by welding were accurately measured, and structural behaviour of these S690 welded H- and I-sections is examined experimentally and numerically. The scope of work covers the following tasks: - Task 1: Stocky columns of S690 welded H-sections under compression To examine section resistances of stocky columns of S690 welded H-sections through experimental and numerical studies. - Task 2: Slender columns of S690 welded H-sections To carry out a numerical investigation into overall buckling behaviour of slender columns of S690 welded H-sections. - Task 3: Restrained beams of S690 welded I-sections To examine local buckling behaviour of restrained beams of S690 welded I-sections with various section compactness through experimental and numerical studies. - Task 4: Partially restrained beams of S690 welded I-sections To examine lateral torsional buckling behaviour of partially restrained beams of S690 welded I-sections through experimental and numerical studies. The areas of interest include: - Deformation characteristics of HSS S690 welded H- and I-sections under i) compression and ii) combined compression and bending; - Comparison of welding-induced residual stresses in S690 and S355 welded H- and I-sections, and effects of residual stresses on structural behaviour of S690 welded sections; - Establishing a double Y-shaped finite element model which employs shell elements to facilitate accurate predictions on structural behaviour of welded H- and I-sections; - Applicability of current design rules in EN 1993-1-1 by comparing design resistances with measured and predicted resistances of S690 welded sections. It should be noted that four different cross-sections of welded H-sections, namely Sections C1 to C4, and six different cross-sections of welded I-sections, namely Sections B1 to B6 are fabricated with S690 steel plates of 6, 10 and 16 mm thickness. Moreover, plate thicknesses up to 40 mm are incorporated into numerical studies in order to cover a wide range of practical applications. Within this research framework, advantages of S690 steel materials and their wide application are elaborated systematically. Research methodology and key findings Based on a series of experimental and numerical investigations, all the four tasks have been successfully completed. It should be noted that: - Task 1: Stocky columns of S690 welded H-sections A total of 20 stocky columns of S690 welded H-sections were fabricated, and they were tested as follows: i) 12 stocky columns under compression, and ii) 8 stocky columns under combined compression and bending. Section resistances and deformation characteristics of the test specimens were successfully obtained. It was shown that predicted section resistances based on EN 1993-1-1 could be readily attained by all 20 test specimens. Large deformation capacity ratios as well as significant strength enhancement were obtained in S690 welded H-sections with high section compactness. Verified residual stress patterns of S690 welded H-sections are incorporated into structural finite element models of stocky columns. Generally, entire load-shortening curves of stocky columns were accurately predicted with calibrated models. Moreover, enhancement of section resistances which was highly dependent on material properties and plate local buckling was closely examined. The design rules given in EN 1993-1-1 are shown to be applicable to evaluate cross-section resistances of those stocky columns of S690 welded H-sections. In addition, it is also found that certain conservatism is embedded in interactive design curves for H-sections under combined compression and bending. - Task 2: Slender columns of S690 welded H-sections Calibrated double Y-shaped model is employed to predict overall buckling resistances of slender columns of S690 welded H-sections. Verification of the model is conducted against benchmark tests conducted by Wang et al. Through direct incorporation of residual stresses into H-sections, predicted load-shortening curves of these slender columns were found to compare very well with those of measured data. In general, current design rules in EN 1993-1-1 and -12 were found to be applicable to predict member resistances of these slender columns of S690 welded H-sections. Owing to significantly reduced compressive residual stresses in those cross-sections comparing with those in S355 steel sections, buckling curve c should be replaced by possibly curve b or even curve a based on parametric studies. Therefore, an improved design efficiency is achieved. - Task 3: Restrained beams of S690 welded I-sections In order to determine section resistances of restrained beams of S690 welded I-sections, a total of 6 beams under single-point loads were tested. Local buckling in the flange outstands was observed in all test specimens. With different cross-sectional compactness, steel sections generally attained full plastic section resistances against bending based on EN 1993-1-1. And various strength enhancement levels were obtained in steel beams, depending on section classification of these sections. Extensive studies based on verified models were conducted, and it was found that section classification rules in EN 1993-1-1 were applicable to restrained beams of S690 welded I-sections. Moreover, in order to achieve improved structural efficiency, new design criteria and methods are proposed according to numerical results of the comprehensive parametric studies.
- Task 4: Partially restrained beams of S690 welded I-sections In order to investigate lateral torsional buckling of partially restrained beams of S690 welded I-sections, a total of 12 beams under single-point loads were tested. Different failure modes, including lateral torsional buckling and plastic section failure were observed in the test specimens. It was shown that partially restrained beams are readily assessed with buckling curve b instead of curve d as suggested in EN 1993-1-1. All numerical studies were carried out successfully incorporating effects of residual stresses. It is shown that partially restrained beams of small to moderate slendernesses tend to be significantly affected by residual stresses, when compared with those of high slendernesses. Moreover, a certain conservatism embedded in the current design method in adopting curve d was highlighted according to numerical results of the comprehensive parametric studies. Key findings and their significances The major academic merits of this research project are: - A whole-process simulation on temperature distribution, residual stress distribution and structural behaviour of S690 welded sections was fully established and verified to enable effective use of S690 welded sections; - Residual stresses in S690 welded H- and I-sections were systematically investigated, and their effects on structural behaviour of columns and beams of S690 welded sections have been identified; - Current design rules in EN 1993-1-1 were justified for designing S690 welded sections according to both experimental and numerical results. Moreover, current design rules with suitably selected design parameters were proposed for improved structural design efficiency; and, - Application of HSS S690 steel materials in columns and beams were fully validated through comprehensive experimental investigation, and they are technically ready for wide application in steel structures.
|Subjects:||Hong Kong Polytechnic University -- Dissertations
Steel, High strength
|Pages:||378 pages : color illustrations|
|Appears in Collections:||Thesis|
View full-text via https://theses.lib.polyu.edu.hk/handle/200/9516
Citations as of May 15, 2022
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