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  • 1.
    Heistermann, Tim
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Lopes, Fernanda
    University of Coimbra, Department of Civil Engineering.
    Santiago, Aldina
    ISISE, Department of Civil Engineering, University of Coimbra.
    Silva, Luís Simões da
    Virginia Polytechnic Institute and State University, Universidade de Coimbra , ISISE, Department of Civil Engineering, University of Coimbra.
    Initial stiffness evaluation of reverse channel connections in tension and compression2015In: Journal of constructional steel research, ISSN 0143-974X, E-ISSN 1873-5983, Vol. 114, p. 119-128, article id 4268Article in journal (Refereed)
    Abstract [en]

    The so-called Reverse Channel connection has been conceived for the purpose of accommodating the thermal expansion of beams so that premature failure due to thermal buckling is avoided. The connection is made of a channel-shaped element, welded along the tips of its flanges onto the face of a hollow section column; an endplate welded on the beam is bolted onto the web of the channel. In a fire situation, the thermal expansion of a reverse-channel supported beam causes extensive bending deformation of the connection, therefore preventing the development of significant axial stress in the beam. Furthermore, this connection offers a high rotational capacity, if designed properly, which is beneficial in a fire situation where excessive deflections of beams can be expected. This paper aims to provide analytical stiffness assessment tools for reverse channel connections in compression and tension under uniform temperatures. The proposed analytical models are compared to results of Finite Element simulations, which in turn have been benchmarked with experiments. In addition, a comprehensive parametric study is conducted in order to identify all influencing factors on the initial stiffness response: reverse channel geometry and thickness, plate thickness, bolt position, and bolt diameter. Correction factors that account for 3D effects and bolt size are presented and discussed. The obtained expressions for the reverse channel stiffness are found to provide an accuracy that is acceptable for structural applications and can, therefore, be used as a design tool

  • 2.
    Heistermann, Tim
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Lopes, Fernanda
    University of Coimbra, Department of Civil Engineering.
    Santiago, Aldina
    ISISE, Department of Civil Engineering, University of Coimbra.
    Silva, Luís Simões da
    Universidade de Coimbra , ISISE, Department of Civil Engineering, University of Coimbra.
    Initial Stiffness of Reverse Channel Connections2014In: Eurosteel 2014: 7th European Conference on Steel and Composite Structures, September 10-12, 2014, Naples, Italy / [ed] Raffaele Landolfo; Federico M. Mazzolani, Brussels: European Convention for Constructional Steelwork, ECCS , 2014Conference paper (Refereed)
    Abstract [en]

    The robustness of a structure in a fire situation greatly depends on the rotational capacity of the connection region. High rotational capacity is required at elevated temperatures since the steel beams lose their bending stiffness and exhibit increasingly large deflections under constant load. Beam deflections result in increasing rotations at the supports and may lead to collapse due to connection failure. The reverse channel has been proposed as a practical alternative to assemble beams to tubular columns. In a simple implementation, the bending moment generated in the joint due to rotation of the beam may be neglected; however, research efforts are being attempted to quantify the level of constraint. The typical arrangement of the connection type consists of a reverse channel with its flanges welded onto the face of concrete-filled tubular columns and the web bolted to the endplate of a beam. Thicknesses and depths of the reverse channel determine the level of rotational restraint at high temperature. The reverse channel has the ability to undergo catenary deformation in the tensile zone due to the applied rotation at the support and similarly it is relatively ductile in the compression zone. Overall, the reverse channel connection response is rather ductile in terms of its ability to undergo large rotational deformation as long as bolt failure is avoided through proper design.Various tests have been performed to study the behaviour of this type of connection such as full scale buildings, sub-frames, isolated joints and individual sections. The aim of these tests was to capture the connection behaviour in relation to other structural components in fire. This paper, however, focuses on the derivation and verification of analytical models to assess the initial stiffness of reverse channel/partial-depth endplate connections. The results from finite element analyses have been utilized to validate analytical models that describe the behaviour of this type of connection at ambient and elevated temperature. Insight into the analytical models provides proper background to a structural designer to estimate the initial stiffness and understand the behaviour of the reverse channel in the connection.

  • 3.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Tubular, lattice and hybrid steel turbine towers for offshore wind energy: A numerical investigationChapter in book (Refereed)
  • 4.
    Koltsakis, Efthymios
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Noury, Pourya
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering. Delft University of Technology.
    The Contact Problem of Roller Bearings: Investigation of Observed Failures2016In: Structural Engineering International, ISSN 1016-8664, E-ISSN 1683-0350, Vol. 26, no 3, p. 207-215Article in journal (Refereed)
    Abstract [en]

    The present paper addresses how the commonly used Hertz formulas for contact stresses underestimate the actual stresses seen in practice due to temperature differentials, misalignments and other contruction-related defects. First, two failure cases of Swedish bridge roller bearings are analyzed and discussed; then, a detailed finite element (FE) model is used to investigate the accuracy of the traditional roller bearing design rules in view of issues such as abutment and girder deformability, misalignment imperfections and material nonlinearity. The bearing capacity of the studied rollers as provided by the manufacturer is used as reference. A rigorous FE model that accurately models girder, roller assembly and abutment provides the necessary information for the assessment of the related contact stresses, which were traditionally calculated by means of the Hertz analytical formulas. Numerical results first establish that roller bearings develop contact stress concentrations at the outer edges of the cylindrical drums. Second, it is established that the contact stresses are very sensitive to misalignment imperfections between the bridge girder and the abutment. Last, it is shown that the roller bearings develop inelastic deformation at relatively low loads in relation to the design load. These reasons, combined with the unlikelihood for roller bearings to shake-down, constitute the basis of the observed roller bearing failures.

  • 5.
    Manoleas, Panagiotis
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Veljkovic, Milan
    Delft University of Technology, Faculty of Civil Engineering and Geoscience.
    Multiplanar K-joints on cold-formed open sections: An experimental study with high strength steels2017In: ce/papers, E-ISSN 2509-7075, Vol. 1, no 2-3, p. 629-638Article in journal (Refereed)
    Abstract [en]

    The design, laboratory investigation and main results of an experimental programme on the joints of an innovative 3D truss made of high strength steel (HSS) is presented. An entire truss of triangular cross-section was fabricated (two compression chords and a single tension chord) to provide realistic loading conditions. Both compression and tension chords were fabricated by cold forming hot-rolled HSS plates by means of press braking. The chords were as follows:

    • an open angle-type profile with 45° contained angle as tension chord (U-chord),
    • a semi-closed octagonal profile made of 3 sectors bolted along their length every 1.2 diameters as compression chords (P-chord).

    The focus of this study is on the tension chords joints where a pair of tension and a pair of compression diagonals converge. Joints were tested to ultimate load by introducing shear to one bay of the truss at a time. Two design approaches for the joints were tested resulting in four individual tests.

    Ultimate load levels, stiffness and failure modes varied between the considered joint configurations. Stiffening increased the ultimate load by as much as 170% of that of the unstiffened version achieving higher utilisation of the tension chord, thus making more efficient use of HSS. The stiffness loss at loads close to failure varied between 60% – 80%. Two distinctive failure modes were observed, one caused by cracking of the welds connecting the diagonal's gusset plates to the tension chord while the second by cross-section tensile fracture of the circular hollow section (CHS) diagonal.

  • 6.
    Manoleas, Panagiotis
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Project: High Strength Long Span Structures2014Other (Other (popular science, discussion, etc.))
  • 7.
    Manoleas, Panagiotis
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Project: Rules on High Strength Steel2014Other (Other (popular science, discussion, etc.))
  • 8.
    Manoleas, Panagiotis
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    The Crocodile Nose Connection: An Aesthetically Appealing Joint for CHS2014In: Eurosteel 2014: 7th European Confernce on Steel and Composite Structures / [ed] Raffaele Landolfo; Frederico M. Mazzolani, Brussels, Belgium: European Convention for Constructional Steelwork, ECCS , 2014, p. 65-66Conference paper (Refereed)
    Abstract [en]

    The work carried out at LTU as a part of preparation for the experimental program of the so called Crocodile Nose (CN) connection, is presented. This work is performed in the framework of the RFCS-funded project "High Strength Steel Long Span Structures" (HILONG) that focuses on the development of applications of higher-strength steel grades for the longer spans. The advantage of this connection is in avoiding the classical abrupt right-angle termination of the CHS which, along with the protruding gusset plate, results in what is regarded by many architects as aesthetically unappealing joint. In a CN connection, the CHS member end is obliquely bevelled along two planes symmetric to its axis. On the lips of the resulting semi-elliptic cut-offs, appropriately cut cover plates are fillet-welded; past the tip of the member these are inflected and extend in parallel to the member axis forming a gap wherein a gusset plate is inserted. The inflected plates are bolted to the gusset thus securing the connection of the CHS to the other side without any eccentricity.Preliminary FE results of the crocodile nose experiments are shown

  • 9.
    Manoleas, Panagiotis
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Öhman, Kristoffer
    Luleå tekniska universitet (student).
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Reistance Results for the Crocodile Connection2015In: The 13th Nordic Steel Construction Conference: NSCC-2015 / [ed] Markku Heinisuo; Jari Mäkinen, Tampere: Tampere University of Technology, Department of Civil Engineering , 2015, p. 247-248Conference paper (Refereed)
    Abstract [en]

    A novel type of connection, referred to as the “crocodile nose” (CN) connection for circular hollow sections (CHS) is investigated in the framework High Strength Long Span Structures (HILONG) project. This connection provides an aesthetically improved alternative to the commonly used slotted-end CHS connection. The end of the CHS member is doubly bevelled and a pair of inflected plates, welded on the edges of the cuts, undertakes the load transfer. This investigation focuses on two parameters, the bevelling angle and the influence of a stiffener connecting the inflected plates. The program is completed through FE calcula-tions and laboratory tests. The design of the test specimens, preliminary FEM and test results are presented in this paper.

  • 10.
    Pavlović, Marko
    et al.
    Faculty of Civil Engineering, University of Belgrade.
    Manoleas, Panagiotis
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Veljkovic, Milan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Calibration of the Ductile Damage Material Model Parameters for a High Strength Steel2015In: The 13th Nordic Steel Construction Conference: NSCC-2015 / [ed] Markku Heinisuo; Jari Mäkinen, Tampere: Tampere University of Technology, Department of Civil Engineering , 2015, p. 231-232Conference paper (Refereed)
    Abstract [en]

    The on-going RUOSTE project aims to improve understanding of HSS by means of tests and FEA, addressing issues of ductility and stability of structures made of HSS. Various material models used in FEA are verified by tests. This paper presents calibration and verifica-tion of ductile damage material model in Abaqus FE software package referring to series of tensile test experiments on coupons and plate specimens with a single circular hole. Nominal steel grade S700MC and S960Q are used. Damage initiation criterion and evolution law are derived analysing localization of plasticity by FEA. Quasi-static analysis using explicit dy-namic solver is chosen in order to create the most realistic FEA of the specimens.

  • 11.
    Sabau, Gabriel
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Lagerqvist, Ove
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Cylindrical shell buckling under a hydraulic constraint: Numerical study2017In: ce/papers, E-ISSN 2509-7075, Vol. 1, no 2-3, p. 970-979Article in journal (Refereed)
    Abstract [en]

    Steel cylindrical shell structures are used in a large variety of civil engineering applications such as off- shore platforms, tanks, silos, wind turbine towers, etc. The local stability of such structures and their sensitivity to imperfections is a well-known problem. In current engineering practice the design method is based on the selection of an imperfection class for the shell and subsequently calculating a reduction factor,χ, to the resistance of the shell. One such methodology is supplied by the EN1993-1-6; special conditions are given to pressurized tubes subjected to meridional compression.

    Past studies have focused on the stability of cylindrical shells with internal pressure. The stability problem of a long cylinder considering the internal pressure as a simple static load was addressed. Thus, the approaches considered the fluid as compressible.

    The purpose of the present work is to investigate numerically the potential benefit of using an incompressible fluid fully enclosed in a circular cylindrical shell. The constraint imposed by the presence of the liquid in the interior of a shell will be referred to as “hydraulic constraint”. As liquids are nearly incompressible, the buckling of a liquid-filled shell has to satisfy the condition that the integral of all the displacements normal to the shell surface is equal to the volume variation of the contained liquid. The volume variation of the shell interior has to be equal to the dilation of the shell due to liquid pressure increments associated to the onset of geometrical instability. Additionally, the weight of the contained liquid causes additional circumferential tension in the cases of vertically placed cylinders.

    The methodology followed is the numerical analysis of cylindrical shells by means of the ABAQUS Finite Element code and a comparison with the methods given in the Eurocode.

  • 12.
    Sabau, Gabriel
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Lagerqvist, Ove
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
    Stability analysis of newly developed polygonal cross-sections for lattice wind towers2018In: Wind Engineering: The International Journal of Wind Power, ISSN 0309-524X, E-ISSN 2048-402X, Vol. 42, no 4, p. 353-363Article in journal (Refereed)
    Abstract [en]

    The pursuit for cheaper energy is leading the current wind tower design to increased heights. Common wind turbine tower designs would generate unjustified costs for transportation and erection leading to inefficient use of materials. In order to reduce these costs, several simplified erection methods have been proposed. One of such is the hybrid lattice-tubular steel tower. For economic feasibility, built-up cold-formed polygonal cross-sections have been proposed for the lattice part. This article presents a numerical investigation of the failure modes of closed polygonal cross-sections. The first part contains a presentation of structural systems which incorporate elements composed of plates and cold-formed members. The evaluation of the polygonal sections is done by means of finite element analysis considering local and global geometrical imperfections and residual stresses generated in the fabrication procedure. A comparative study is performed between several finite element models to propose a corresponding European buckling curve for calculating the flexural buckling resistance. The results show that the design of polygonal sections can be done according to European buckling curves methodology.

  • 13.
    Theofanous, Marios
    et al.
    Department of Civil and Environmental Engineering, Imperial College London.
    Gardner, L.
    Department of Civil and Environmental Engineering, Imperial College London.
    Koltsakis, Efthymios
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Structural response of stainless steel cross-sections under combined compression and biaxial bending2013In: Research and applications in structural engineering, mechanics and computation: proceedings of the Fifth International Conference on Structural Engineering, Mechanics and Computation, Cape Town, South Africa, 2-4 September 2013 / [ed] Alphonse Zingoni, Boca Raton: CRC Press, Taylor & Francis Group , 2013, p. 1453-1458Conference paper (Refereed)
    Abstract [en]

    The Continuous Strength Method (CSM) was developed and calibrated against experimental and numerical results as a design approach, which allows for a rational exploitation of the significant strainhardening exhibited by stocky stainless steel cross-sections, thereby leading to more economic design. In order to extend its scope of application beyond the fundamental loading cases of concentric compression and uniaxial bending, a comprehensive numerical parametric study has been conducted and the obtained numerical results have been utilized to determine suitable interaction equations for cross-sections subjected to combined loading. This paper focuses on the ultimate response of I-sections under combined loading and complements a recent relevant study on RHS under combined loading. It is concluded that the current design guidance for stainless steel cross-sections under combined compression and biaxial bending is overly conservative and significant gains can be made if the effect of strain-hardening on ultimate capacity is taken into account.

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