Change search
Link to record
Permanent link

Direct link
BETA
Koltsakis, EfthymiosORCID iD iconorcid.org/0000-0001-6150-7241
Publications (10 of 15) Show all publications
Sabau, G., Manoleas, P., Koltsakis, E. & Lagerqvist, O. (2019). Statistical evaluation of the bearingcapacity of short polygonal columns. In: PROCEEDINGS OF THE INTERNATIONAL COLLOQUIA ON STABILITY ANDDUCTILITY OF STEEL STRUCTURES: . Paper presented at PROCEEDINGS OF THE INTERNATIONAL COLLOQUIA ON STABILITY AND DUCTILITY OF STEEL STRUCTURES, PRAGUE, CZECH REPUBLIC, SEPTEMBER 11-13, 2019.
Open this publication in new window or tab >>Statistical evaluation of the bearingcapacity of short polygonal columns
2019 (English)In: PROCEEDINGS OF THE INTERNATIONAL COLLOQUIA ON STABILITY ANDDUCTILITY OF STEEL STRUCTURES, 2019Conference paper, Published paper (Refereed)
Abstract [en]

Regular convex polygon sections (RCPS) are commonly used as towers supportingtransmission lines, stadium lightning and street lamps. Their use provides advantages in the bearingcapacity and can simplify erection. Over the last 50 years experimental studies have beenconducted to check the applicability of the plate theory to stocky polygonal columns. The paperpresents the processed data gathered from compression tests found in the literature. Results from70 specimens tested under pure compression were statistically analysed. Specimens with yieldstrength varying from 235 to 700 MPa and angles varying from 144 to 175.5 (5 to 40 sides) wereinvestigated. The local non-dimensional slenderness was calculated using buckling lengths accordingto EN 1993-1-3 and EN 1993-1-5 with values ranging from 0.55 to 4.52. The objectiveof the paper was to compare the plate buckling resistance predictions to the experimental results.The paper concludes with a buckling width recommendation for evaluating the critical stress ascalculated according to EN 1993-1-3 or EN 1993-1-5.

National Category
Infrastructure Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-77723 (URN)10.1201/9780429320248 (DOI)
Conference
PROCEEDINGS OF THE INTERNATIONAL COLLOQUIA ON STABILITY AND DUCTILITY OF STEEL STRUCTURES, PRAGUE, CZECH REPUBLIC, SEPTEMBER 11-13, 2019
Available from: 2020-02-14 Created: 2020-02-14 Last updated: 2020-02-14
Stavridou, N., Koltsakis, E. & Baniotopoulos, C. (2019). Structural analysis and optimal design of steel lattice wind turbine towers. Proceedings of the Institution of Civil Engineers: Structures and buildings, 172(8), 564-579
Open this publication in new window or tab >>Structural analysis and optimal design of steel lattice wind turbine towers
2019 (English)In: Proceedings of the Institution of Civil Engineers: Structures and buildings, ISSN 0965-0911, E-ISSN 1751-7702, Vol. 172, no 8, p. 564-579Article in journal (Refereed) Published
Abstract [en]

Wind energy has shown its dominance among the means of sustainable energy production by the accelerating rise in total installed capacity and increase in size of wind energy structures. Taking into consideration the fact that the supporting structure of onshore wind power generators constitutes approximately one-third of the initial construction cost, structural optimisation of the tower is considered crucial towards the minimisation of capital expenditure during construction. Contemporary energy needs employ the construction of constantly taller and more powerful wind power converters, whose robust design in parallel with compression of initial capital expenditure cannot be neglected. The dominant structural configuration for onshore wind power generators is the tapered steel tower, but lattice towers using enhanced special cross-sections can be a rather promising solution towards economy of material use. The present paper addresses the structural performance and optimisation of tubular and lattice steel wind turbine towers, examining alternative configuration solutions for a given height and rotor characteristics. The finite-element software Abaqus has been used for the implementation of the structural models and an algorithm has been elaborated in Mathematica software in order to allow for optimisation of the use of the cross-section in the case of lattice towers.

Place, publisher, year, edition, pages
ICE Publishing, 2019
Keywords
buildings, structures & design, mathematical modelling, steel structures
National Category
Other Civil Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-75349 (URN)10.1680/jstbu.18.00074 (DOI)000476648500004 ()2-s2.0-85069482773 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-07-23 (svasva)

Available from: 2019-07-23 Created: 2019-07-23 Last updated: 2019-08-21Bibliographically approved
Koltsakis, E. (2019). Tubular, lattice and hybrid steel turbine towers for offshore wind energy: A numerical investigation. In: M.F. Randolph, Dinh Hong Doan, Anh Minh Tang, Man Bui, Van Nguyen Dinh (Ed.), Proceedings of the 1st Vietnam Symposium on Advances in Offshore Engineering: Energy and Geotechnics. Paper presented at 1st Vietnam Symposium on Advances in Offshore Engineering (VSOE2018); 1-3 November 2018; Hanoi, Vietnam (pp. 524-529). Springer Nature
Open this publication in new window or tab >>Tubular, lattice and hybrid steel turbine towers for offshore wind energy: A numerical investigation
2019 (English)In: Proceedings of the 1st Vietnam Symposium on Advances in Offshore Engineering: Energy and Geotechnics / [ed] M.F. Randolph, Dinh Hong Doan, Anh Minh Tang, Man Bui, Van Nguyen Dinh, Springer Nature , 2019, p. 524-529Conference paper, Published paper (Refereed)
Abstract [en]

The increasing world power demand combined with the need of environment protection and sustainable energy production, has led recently to the use of alternative means of energy production minimizing CO2 emissions including wind energy harvesting. The new trend to expand to offshore wind power installations in order to increase the amount of world sustainable energy production has led to the development of multiple structural solutions both for the foundations and the upper structure of wind power generators. Research on the structural optimization of wind turbine towers is of great interest and importance due to their high manufacturing and erection costs and certain transportation limitations that prevent them from reaching greater heights. The present work addresses the comparison of a classic tapered steel wind turbine tower configuration with a hybrid lattice tower of the same height and energy production potential. Aiming to contribute to better understanding of the structural behaviour of both types of wind turbine towers, the present research work focuses on the development of reliable numerical models along with the use of analytical equations in order to predict accurately and interpret the aforementioned structural response of the two tower configurations by conducting a comparative study between them.

Place, publisher, year, edition, pages
Springer Nature, 2019
Series
Lecture Notes in Civil Engineering, ISSN 2366-2557, E-ISSN 2366-2565 ; 18
Keywords
Off-shore wind turbine towers, Numerical analysis, Structural design, Steel structures
National Category
Building Technologies
Research subject
Steel Structures
Identifiers
urn:nbn:se:ltu:diva-72824 (URN)10.1007/978-981-13-2306-5_74 (DOI)2-s2.0-85060346166 (Scopus ID)978-981-13-2305-8 (ISBN)978-981-13-2306-5 (ISBN)
Conference
1st Vietnam Symposium on Advances in Offshore Engineering (VSOE2018); 1-3 November 2018; Hanoi, Vietnam
Available from: 2019-02-08 Created: 2019-02-08 Last updated: 2020-02-06Bibliographically approved
Sabau, G., Koltsakis, E. & Lagerqvist, O. (2018). Stability analysis of newly developed polygonal cross-sections for lattice wind towers. Paper presented at WINERCOST'17 – International Conference On Wind Energy Harvesting 2017, Coimbra, Portugal, 20-21 april, 2017. Wind Engineering: The International Journal of Wind Power, 42(4), 353-363
Open this publication in new window or tab >>Stability analysis of newly developed polygonal cross-sections for lattice wind towers
2018 (English)In: 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) Published
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.

Place, publisher, year, edition, pages
Sage Publications, 2018
Keywords
Cold-formed, finite element simulation, flexural buckling, lattice wind tower, polygonal sections
National Category
Building Technologies
Research subject
Steel Structures
Identifiers
urn:nbn:se:ltu:diva-70193 (URN)10.1177/0309524X18777366 (DOI)2-s2.0-85049948088 (Scopus ID)
Conference
WINERCOST'17 – International Conference On Wind Energy Harvesting 2017, Coimbra, Portugal, 20-21 april, 2017
Note

2018-08-02 (rokbeg);konferensartikel i tidskrift

Available from: 2018-08-01 Created: 2018-08-01 Last updated: 2020-02-15Bibliographically approved
Sabau, G., Koltsakis, E. & Lagerqvist, O. (2017). Cylindrical shell buckling under a hydraulic constraint: Numerical study. Paper presented at EUROSTEEL 2017, Copenhagen, Denmark, September 13–15, 2017. ce/papers, 1(2-3), 970-979
Open this publication in new window or tab >>Cylindrical shell buckling under a hydraulic constraint: Numerical study
2017 (English)In: ce/papers, E-ISSN 2509-7075, Vol. 1, no 2-3, p. 970-979Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Ernst & Sohn Verlag, 2017
National Category
Building Technologies
Research subject
Steel Structures
Identifiers
urn:nbn:se:ltu:diva-65654 (URN)10.1002/cepa.137 (DOI)
Conference
EUROSTEEL 2017, Copenhagen, Denmark, September 13–15, 2017
Note

Konferensartikel i tidskrift

Available from: 2017-09-15 Created: 2017-09-15 Last updated: 2020-02-15Bibliographically approved
Manoleas, P., Koltsakis, E. & Veljkovic, M. (2017). Multiplanar K-joints on cold-formed open sections: An experimental study with high strength steels. Paper presented at EUROSTEEL 2017, Copenhagen, Denmark, September 13–15, 2017. ce/papers, 1(2-3), 629-638
Open this publication in new window or tab >>Multiplanar K-joints on cold-formed open sections: An experimental study with high strength steels
2017 (English)In: ce/papers, E-ISSN 2509-7075, Vol. 1, no 2-3, p. 629-638Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Ernst & Sohn Verlag, 2017
National Category
Building Technologies
Research subject
Steel Structures
Identifiers
urn:nbn:se:ltu:diva-65655 (URN)10.1002/cepa.101 (DOI)
Conference
EUROSTEEL 2017, Copenhagen, Denmark, September 13–15, 2017
Note

Konferensartikel i tidskrift

Available from: 2017-09-15 Created: 2017-09-15 Last updated: 2017-11-24Bibliographically approved
Koltsakis, E., Noury, P. & Veljkovic, M. (2016). The Contact Problem of Roller Bearings: Investigation of Observed Failures. Structural Engineering International, 26(3), 207-215
Open this publication in new window or tab >>The Contact Problem of Roller Bearings: Investigation of Observed Failures
2016 (English)In: Structural Engineering International, ISSN 1016-8664, E-ISSN 1683-0350, Vol. 26, no 3, p. 207-215Article in journal (Refereed) Published
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.

National Category
Building Technologies
Research subject
Steel Structures
Identifiers
urn:nbn:se:ltu:diva-60309 (URN)10.2749/101686616X14555429843762 (DOI)000386695500003 ()2-s2.0-84997850241 (Scopus ID)
Note

Validerad; 2016; Nivå 2; 2016-11-21(andbra)

Available from: 2016-11-11 Created: 2016-11-11 Last updated: 2018-07-10Bibliographically approved
Pavlović, M., Manoleas, P., Veljkovic, M. & Koltsakis, E. (2015). Calibration of the Ductile Damage Material Model Parameters for a High Strength Steel (ed.). In: (Ed.), Markku Heinisuo; Jari Mäkinen (Ed.), The 13th Nordic Steel Construction Conference: NSCC-2015. Paper presented at Nordic Steel Construction Conference : 23/09/2015 - 25/09/2015 (pp. 231-232). Tampere: Tampere University of Technology, Department of Civil Engineering
Open this publication in new window or tab >>Calibration of the Ductile Damage Material Model Parameters for a High Strength Steel
2015 (English)In: 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, Meeting abstract (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.

Place, publisher, year, edition, pages
Tampere: Tampere University of Technology, Department of Civil Engineering, 2015
Keywords
High Strength Steel, Ductile Damage Material, Engineering mechanics - Construction engineering, Materials science - Construction materials, Teknisk mekanik - Konstruktionsteknik, Teknisk materialvetenskap - Konstruktionsmaterial
National Category
Building Technologies
Research subject
Steel Structures
Identifiers
urn:nbn:se:ltu:diva-37833 (URN)bfad9e48-b179-4c27-a574-f185cbaff20d (Local ID)978-952-15-3578-9 (ISBN)978-952-15-3579-6 (ISBN)bfad9e48-b179-4c27-a574-f185cbaff20d (Archive number)bfad9e48-b179-4c27-a574-f185cbaff20d (OAI)
Conference
Nordic Steel Construction Conference : 23/09/2015 - 25/09/2015
Note
Godkänd; 2015; 20151002 (manpan)Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2018-06-08Bibliographically approved
Heistermann, T., Koltsakis, E., Veljkovic, M., Lopes, F., Santiago, A. & Silva, L. S. (2015). Initial stiffness evaluation of reverse channel connections in tension and compression (ed.). Paper presented at . Journal of constructional steel research, 114, 119-128, Article ID 4268.
Open this publication in new window or tab >>Initial stiffness evaluation of reverse channel connections in tension and compression
Show others...
2015 (English)In: Journal of constructional steel research, ISSN 0143-974X, E-ISSN 1873-5983, Vol. 114, p. 119-128, article id 4268Article in journal (Refereed) Published
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

National Category
Building Technologies
Research subject
Steel Structures
Identifiers
urn:nbn:se:ltu:diva-10904 (URN)10.1016/j.jcsr.2015.07.006 (DOI)000364501300012 ()2-s2.0-84937848165 (Scopus ID)9c8a7835-9597-41ee-8647-866ad48ad438 (Local ID)9c8a7835-9597-41ee-8647-866ad48ad438 (Archive number)9c8a7835-9597-41ee-8647-866ad48ad438 (OAI)
Note
Validerad; 2015; Nivå 2; 20150812 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Manoleas, P., Öhman, K., Koltsakis, E. & Veljkovic, M. (2015). Reistance Results for the Crocodile Connection (ed.). In: (Ed.), Markku Heinisuo; Jari Mäkinen (Ed.), The 13th Nordic Steel Construction Conference: NSCC-2015. Paper presented at Nordic Steel Construction Conference : 23/09/2015 - 25/09/2015 (pp. 247-248). Tampere: Tampere University of Technology, Department of Civil Engineering
Open this publication in new window or tab >>Reistance Results for the Crocodile Connection
2015 (English)In: 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, Meeting abstract (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.

Place, publisher, year, edition, pages
Tampere: Tampere University of Technology, Department of Civil Engineering, 2015
Keywords
Circular hollow sections, steel connections, Engineering mechanics - Construction engineering, Teknisk mekanik - Konstruktionsteknik
National Category
Building Technologies
Research subject
Steel Structures; Attractive built environment (AERI)
Identifiers
urn:nbn:se:ltu:diva-34908 (URN)936bdf87-f69b-4adf-94fa-7bcd96134f66 (Local ID)978-952-15-3578-9 (ISBN)978-952-15-3579-6 (ISBN)936bdf87-f69b-4adf-94fa-7bcd96134f66 (Archive number)936bdf87-f69b-4adf-94fa-7bcd96134f66 (OAI)
Conference
Nordic Steel Construction Conference : 23/09/2015 - 25/09/2015
Projects
High Strength Long Span Structures
Note
Godkänd; 2015; 20151001 (manpan)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2018-06-08Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-6150-7241

Search in DiVA

Show all publications