Faglige interesser
Mekanisk oppførsel og numerisk modellering av konstruksjoner og faste stoffer generelt; elastiske og ikkelineære analyser; stabilitetsproblemer; utvikling av normrettede analyse og dimensjoneringsmetoder for søyler, rammer og plater; virkning av langtidslast og deformasjonspåførte tvangskrefter (indirekte laster) på konstruksjoner med viskoelastiske materialegenskaper (som armert betong).
Universitetsutdanning
 1970, PhD (Philosophiae Doctor), Dept. of Civil Engineering, Univ. of Waterloo, Waterloo, Ontario, Canada.
 1966, Sivilingeniør, NTH (nå NTNU (Norges teknisk naturvitenskapelige universitet)), Trondheim.
Ansettelser
Faste:
 2013.2  Professor emeritus.
 1988.1  2013.2 Professor i fastoff/konstruksjonsmekanikk, UiO.
 1984.8  1987.12 Professor II i fastoff/konstruksjonsmekanikk, UiO.
 1985.4  1987.12 Senior Principal Engineer (sjefingeniør), Veritas Offshore Technology and Services A/S (som avd.leder for Concr. Structures and Foundations) og forut for det i Industry and Offshore Division (Dept. for Structures), Det Norske Veritas (DNV), Høvik.
 1972.11985.4 Prosjektingeniør, Overingeniør, Sjefingeniør, Dr.ing. A. AasJakobsen A/S (AAJ), Oslo.
Midlertidige forskeransettelser:
 1984.68 Research Engineer, Univ. of California, Berkeley.
 1981.29 Research Associate, Univ. of Alberta, Edmonton.
 1980.71981.2 Research Engineer, Univ. of California, Berkeley.
 1966.91971.12 Research Assistant og Post Doctoral Fellow, Univ. of Waterloo.
Langvarige gjesteprofessor/forskeropphold:
 University of Texas, Austin, USA, høst 1999 (aug.jan.)
 Universitat Politecnica de Catalunya (UPC)), Barcelona, Spania, høst 2001.
Utvalgte verv o.l.
 Medlem, NTVA (Norges Tekniske Vitenskapsakademi).
 2012.11  Medlem, TG6 "Structural Analysis", av CENkomité 250/SC 2/WG 1/
 2008.11  Editorial Board member, Journal of Constructional Steel Research.
 2004  Member, Int. Struct. Steel Research Advisors (ISSRA), AISC Journ.
 1997  2001 Editorial Board member, Journal of Computer Methods in Applied Mechanics and Engineering.
 1976  1980 Norsk redaktør, Journal of the Nordic Concr. Federation.
 1976  1998 Reporter, FIP Notes, Federation Internationale de la Precontrainte.
Undervisning
Emner for lavere og høyere grad innen faststoff og konstruksjonsmekanikk.
Veiledning
 63 cand.scient./masterstudenter.
 10 doktorstipendiater (siden 1992).
Rådgivningsaktiviteter
1988  2006: Consulting and advisory services for DNV, Statoil (rehabilitation of Statpipe underwater bridge structures), the Bridge Department of the Directorate of Public Roads, Oslo (development of analysis and design specifications for bridges, and analysis and design aspects of a number of suspension bridges etc.) and Svenska vegverket (Høga Kusten bron).
1985  1987: At DNV, consultancy services to a number of petroleum companies, incl. design review and verification of offshore structures (Beryl A, Brent B, Maureen ALP, Gullfaks B, Troll GBS, Saga's deepwater tower Karin, etc.), development and updating of design spesifications, and various R&D projects.
1972  1985: At AAJ, design and consultancy services covering a great number and a wide range of structures, incl. industrial structures, various kinds of bridges and offshore structures, development of design recommendations for offshore structures (for the Norwegian Petroleum Directorate, Norsk Hydro), and a number of R&D oriented studies, etc..
Emneord:
Mekanikk,
Faststoffmekanikk
Publikasjoner

Sollund, Håvar Andreas; Vedeld, Knut; Fyrileiv, Olav & Hellesland, Jostein (2016). Improved assessments of waveinduced fatigue for free spanning pipelines. Applied Ocean Research.
ISSN 01411187.
61, s 130 147 . doi:
10.1016/j.apor.2016.10.004
Vis sammendrag
For subsea pipeline projects, the costs related to seabed correction and free span intervention are often considerable. Development of reliable methods for fatigue analyses of pipelines in free spans contributes to minimize costs without compromising pipeline integrity. Assessment of waveinduced fatigue damage on multispan pipelines is investigated, and improved analysis methods are suggested in this paper. A timedomain (TD) algorithm is developed, which accounts for nonlinear hydrodynamic loading and dynamic interaction between adjacent spans. The proposed TD approach is employed to evaluate linearized frequencydomain (FD) solutions from recognized design standards and to study the dynamic response of multispan pipelines to direct wave loading. Differences between multi and singlespan analyses are described for the first time, and the common assumption that the main fatigue damage contribution comes from the fundamental mode is demonstrated not to hold for multispans. An improved FD solution capable of predicting multimode response is derived and demonstrated to give accurate fatigue life estimates for multispan pipelines.

Vedeld, Knut; Sollund, Håvar Andreas & Hellesland, Jostein (2015). Closed Analytical Expressions for Stress Distributions in TwoLayer Cylinders and Their Application to Offshore Lined and Clad Pipes. Journal of Offshore Mechanics and Arctic Engineering.
ISSN 08927219.
137(2), s 1 9 . doi:
10.1115/1.4029357
Vis sammendrag
Closedform analytical expressions are derived for the displacement field and corresponding stress state in twolayer cylinders subjected to pressure and thermal loading. Solutions are developed both for cylinders that are fully restrained axially (plane strain) and for cylinders that are axially loaded and springmounted. In the latter case, it is assumed that the combined twolayer cross section remains plane after deformation (generalized plane strain). The analytical solutions are verified by means of detailed threedimensional finite element (FE) analyses, and they are easily implemented in, and suitable for, engineering applications. The chosen axial boundary conditions are demonstrated to be particularly relevant for pipeline and piping applications. By applying the exact solutions derived in the present study to typical offshore lined or clad pipelines, it is demonstrated that thermal expansion of the liner or clad layer may cause higher tensile hoop stresses in the pipe steel wall than accounted for in current engineering practice. It is shown that repeated cycles of startup and shutdown phases for lined or clad pipelines cause significant plastic stress cycles in liners or claddings, which may pose a risk to the integrity of such pipelines.

Brubak, Lars; Hareide, O. J. & Hellesland, Jostein (2014). Application of a SemiAnalytical Approach for Vibration Analysis of Irregularly Stiffened Plates. TritaMEK.
ISSN 0348467X.
(24), s 220 229
Vis sammendrag
Free vibration of irregularly stiffened plates is studied. The plates may be subjected to inplane loads and the plate boundaries may be simply supported or rotationally restrained. The main objective is to demonstrate and validate the applicability of an approximate, semi analytical computational model.

Sollund, Håvar Andreas; Vedeld, Knut & Hellesland, Jostein (2014). Efficient analytical solutions for heated and pressurized multilayer cylinders. Ocean Engineering.
ISSN 00298018.
92, s 285 295 . doi:
10.1016/j.oceaneng.2014.10.003
Vis sammendrag
Two independent sets of analytical solutions have been derived for calculation of displacements and stresses in elastic multilayer cylinders subjected to both pressure and thermal loading. The solutions are computationally efficient and easily implemented, and thus suitable for engineering applications. In particular, the novel solutions are relevant for offshore pipelines and risers, since the impact of internal and external coating layers on the stress state in the pipe wall often is disregarded or assessed by simplified methods in current pipeline engineering practice. Recursive solutions are obtained both for the case of uniform temperature in each cylinder layer and for the case of radially varying temperature in each cylinder layer. Both plane stress and plane strain conditions are considered. In addition, a recursive solution of the heat equation is derived for steadystate conditions. Applicability of the solutions is illustrated by using them on an offshore pipeline with a corrosionresistant liner and multilayered thermal insulation coating. It is demonstrated that coating layers, generally disregarded in conventional pipeline design, may have a nonnegligible impact on hoop stresses and the true wall axial force.

Sollund, Håvar Andreas; Vedeld, Knut; Hellesland, Jostein & Fyrileiv, Olav (2014). Dynamic response of multispan offshore pipelines. Marine Structures.
ISSN 09518339.
39, s 174 197 . doi:
10.1016/j.marstruc.2014.07.006

Vedeld, Knut; Sollund, Håvar Andreas; Hellesland, Jostein & Fyrileiv, Olav (2014). Effective axial forces in offshore lined and clad pipes. Engineering structures.
ISSN 01410296.
66, s 66 80 . doi:
10.1016/j.engstruct.2014.01.051
Vis sammendrag
To protect pipelines from corrosion attacks, some offshore pipelines have corrosion resistant liners or cladding made from stainless steel. Stainless steels have higher temperature expansion coefficients and lower Poisson’s ratios than ordinary high strength carbon manganese steels. Due to the differences in material properties, liners and cladding have a nontrivial influence on the critical design parameter called the effective axial force in offshore pipelines. Current offshore design codes do not contain guid ance on how to include the effects of liners and cladding on the effective axial force. In this paper, an exact analytical formula for the effective axial force in lined and clad pipes is deduced. A simplified approximate formula, which is more suitable for use in engineering contexts, is developed and its validity verified by comparisons to results of the complex exact one.

Brubak, Lars; Andersen, Håkon S. & Hellesland, Jostein (2013). Ultimate strength prediction by semianalytical analysis ofstiffened plates with various boundary conditions. Thinwalled structures.
ISSN 02638231.
62(January), s 28 36 . doi:
10.1016/j.tws.2012.08.005
Vis sammendrag
Semianalytical elastic methods for stiffened plate analysis are computationally very efficient. In addition to eigenvalue analysis,such methods may also offer a viable approach for the prediction of ultimate strength limits (USLs) of the plates when combined with appropriate strength criteria. In this paper,existing strength criteria are discussed,and extended criteria proposed for plates with various stiffener arrangements and boundary conditions such as full outofplane supports along all edges or plates with a free or partially stiffened edge.The extended criteria reflect in a simplified manner the effect of redistribution of stresses due to the formation of local plastic regions at stiffeners,supporting edges and in the plate interior.The equilibrium path is traced using large deflection theory and the Rayleigh–Ritz approach on an incremental form.The approach is able to account for the reserve strength of slender plates in the post buckling region. With the considered criteria included, good agreement is obtained with results from fully nonlinear finite element analyses for different support conditions and for a variety of plate and stiffener dimensions.

Brubak, Lars; Hellesland, Jostein & Hareide, Ole J. (2013). Vibration analysis of plates with arbitrary stiffener arrangements using a semianalytical approach, In
Computational Methods in Marine Engineering V.
International Center for Numerical Methods in Engineering (CIMNE).
ISBN 9788494140747.
Vitenskapeli artikkel.
s 320
 331

Challamel, Noël & Hellesland, Jostein (2013). Buckling of softening columns in a continuum damage mechanics perspective  Local versus nonlocal formulation. European journal of mechanics. A, Solids.
ISSN 09977538.
39(MayJune), s 229 242 . doi:
10.1016/j.euromechsol.2012.11.007
Vis sammendrag
This paper deals with the mathematical modelling of the buckling and postbuckling response of structural systems composed of quasibrittle materials such as concrete (as found for instance inreinforced concrete columns). More specifically, columns with softening characteristics are of concern in this study. The momentcurvature constitutive law is based on continuum damage mechanics arguments (CDM theory). Then, the instability problem can be referred to as the Elastica problem for the case with a continuum damage mechanics constitutive law, or, for short, it may be referred to as the Continuum Damage Mechanica problem. It is numerically shown for the parameters of interest, that results from geometrically exact and secondorder analyses for small rotations are almost equivalent. The instability of the imperfect softening system is associated with a limit load which decreases with the imperfection considered. Such columns are shown to be typically imperfection sensitive. Furthermore, a loadimperfection relationship is derived (similar to Koiter's power law), that can be useful in structural design contexts. An asymptotic expansion is performed to obtain a closedform analytical solution of the limit load imperfection rule. A comparison with exact numerical values for the continuous column shows good agreement with the asymptotic expression results. The need for including nonlocality formulations in a damage localization zone is finally discussed for postbuckling analysis in the presence of curvature softening. The adopted local bendingcurvature constitutive law leads to the unloading Wood's paradox. To cover the propagation phenomenon of localization, some nonlocality is included in a generalized formulation of the principle of virtual work. However, nonlocality is not necessary for limit load calculations since limit loads occur for curvatures in the local hardening regime.

Challamel, Noël; Kolvik, Gjermund & Hellesland, Jostein (2013). Plate buckling analysis using a general higher order shear deformation theory. International Journal of Structural Stability and Dynamics.
ISSN 02194554.
13(5) . doi:
10.1142/S0219455413500284
Vis sammendrag
The buckling of higherorder shear plates is studied in this paper with a unified formalism. It is shown that usual higherorder shear plate models can be classifed as gradient elasticity Mindlin plate models, by augmenting the constitutive law with the shear strain gradient. These equivalences are useful for a hierarchical classification of usual plate theories comprising Kirchhoff plate theory, Mindlin plate theory and thirdorder shear plate theories. The same conclusions were derived by Challamel(2011) for higherorder shear beam models. A consistent variational presentation is derived for all generic plate theories, leading to meaningful buckling solutions. In particular, the variationallybased boundary conditions are obtained for general loading configurations. The buckling of the isotropic or orthotropic composite plates is then investigated analytically for simply supported plates under uniaxial or hydrostatic inplane loading. An analytical buckling formula is derived that is common to all higherorder shear plate models. It is shown that cubicbased interpolation models for the displacement field are kinematically equivalent, and lead to the same buckling load results. This conclusion concerns for instance the plate models of Reddy(1984) or the one of Shi(2007), even though these models are statically distinct (leading to different stress calculations along the cross section). Finally, a numerical sensitivity study is made.

Vedeld, Knut; Sollund, Håvar Andreas & Hellesland, Jostein (2013). Free vibrations of free spanning offshore pipelines. Engineering structures.
ISSN 01410296.
56, s 68 82 . doi:
10.1016/j.engstruct.2013.04.013

Hellesland, Jostein (2012). Evaluation of effective length formulas and applications in system instability analysis. Engineering structures.
ISSN 01410296.
45(12 (Dec)), s 405 420 . doi:
10.1016/j.engstruct.2012.06.028
Vis sammendrag
The paper deals with approximate effective length formulas, and how such formulas may be applied in system instability analysis of frames. End restraint assessments in a system instability perspective are discussed, and available formulas are reviewed. The overview is limited to the important class of compression members that are either completely unbraced (freetosway, zero shear) or fully braced against lateral translation of one end relative to the other. Whereas most approximate effective length formulas have been developed for positive end restraints, buckling modes of compression members in a frame can be associated with both positive and negative end restraints. It is a main concern of the study to identify the applicability of the various approximate formulas for a reasonable wide range of positive and negative restraint combinations. Extensive comparisons with exact effective length results have been carried out for isolated members, and applications in system instability analyses are demonstrated.

Brubak, Lars & Hellesland, Jostein (2011). Semianalytical postbuckling analysis of stiﬀened imperfect plates with a free or stiﬀened edge. Computers & structures.
ISSN 00457949.
89(1718), s 1574 1585 . doi:
10.1016/j.compstruc.2011.05.007
Vis sammendrag
A large deﬂection, semianalytical method is developed for pre and postbuckling analyses of stiﬀened rectangular plates with one edge free or ﬂexibly supported, and the other three edges laterally supported. The plates can have stiﬀeners in both directions parallel and perpendicular to the free edge, and the stiﬀener spacing can be arbitrary. Both global and local bending modes are captured by using a displacement ﬁeld consisting of displacements representing a simply supported, stiﬀened plate and an unstiﬀened plate with a free edge. The outofplane and inplane displacements are represented by trigonometric functions and linearly varying functions, deﬁned over the entire plate. The formulations derived are implemented into a FORTRAN computer programme, and numerical results are compared with results by ﬁnite element analyses (FEA) for a variety of plate and stiﬀener geometries. Relatively high numerical accuracy is achieved with low computational eﬀorts.

Challamel, Noël & Hellesland, Jostein (2011). Simplified buckling analysis of imperfection sensitive reinforced concrete structures, In Jouni Freund & Reijo Kouhia (ed.),
Proceedings of the 24th Nordic Seminar on Computational Mechanics.
Alto University publication series.
ISBN 9789526043470.
Stability.
s 141
 144
Vis sammendrag
This paper is devoted to the buckling phenomenon of columns composed of a quasibrittle material such as concrete. The constitutive law at the material or the structural scale is based on continuum damage mechanics arguments (CDM theory). We investigate, using a simple onedimensional softening model based on continuum damage mechanics, how the buckling of reasonable slender reinforced concrete columns is associated with a limit load which decreases with the imperfection considered. The onedimensional continuum problem is then analytically and numerically solved for an axially loaded cantilever column with some additional geometrical imperfection. The need of including some nonlocal properties is discussed for the postbuckling analysis. Though simplified, this column model will still give some qualitatively useful results for reinforced concrete columns.

Andersen, Håkon s.; Hellesland, Jostein & Brubak, Lars (2010). Strength criterion for stiffened plates with a free or stiffened edge. TritaMEK.
ISSN 0348467X.
Vis sammendrag
Ultimate strengths of stiﬀened plates with a free or stiﬀened edge is considered. A large deﬂection, semianalytical model for pre and postbuckling calculations is used in combination with a proposed strength criterion. Strength estimates are compared with ﬁnite element analyses (FEA) results. Good agreement is found between the model and the FEA results.

Brubak, Lars & Hellesland, Jostein (2010). Response propagation analysis of imperfect stiffened plates with a free or flexible edge using a semianalytical method. TritaMEK.
ISSN 0348467X.
Vis sammendrag
A large deﬂection, semianalytical method for response propagation analysis of stiﬀened plates with a free edge is presented. The formulations derived are implemented into a FORTRAN computer program, and numerical results are compared with fully nonlinear ﬁnite element analyses. Good agreement is obtained.

Brubak, Lars; Hellesland, Jostein & Steen, Eivind (2009). Approximate postbuckling analysis of imperfect stiffened plates with a free edge, In T. Kvamsdal; B.R. Pettersen; P. Bergan; E. Onate & J. Garcia (ed.),
Computational methods in marine engineering III.
International Center for Numerical Methods in Engineering (CIMNE).
ISBN 9788496736665.
KAPITTEL.
s 336
 338

Hellesland, Jostein (2009). Extended second order approximate analysis of frames with swaybraced column interaction. Journal of constructional steel research.
ISSN 0143974X.
65(5), s 1075 1086 . doi:
10.1016/j.jcsr.2008.08.008
Vis sammendrag
Present approximate second order methods for the analysis of frames with sway are not capable of reflecting the transition from sway to partially braced, and nearly fully braced behaviour of individual columns in the frames. The main aim of the paper is to extend the approximate storey magnifier approach to account for such a transition. The key to this is in the manner local second order effects are reflected. A high order shear relationship is proposed, an general sway magnifier, critical load and effective length formulations are presented both in terms of first order lateral storey stiffness and critical, freesway column loads. Their interrelationship, and simplifications leading to existing approaches, and adaptions in present codes and standards, are discussed. Comparisons are made with exact critical loads, sway and moment magnifiers for nearly unbraced, partially braced and nearly fully braced systems. The proposed, extended approach provide predictions that generally are in very good agreement with exact results at all axial load levels. The more simplified approaches provide good agreement for low to moderate load levels for some column end restraint combinations, and up to relatively high load levels for other combinations.

Hellesland, Jostein (2009). Second order approximate analysis of unbraced multistorey frames with single curvature regions. Engineering structures.
ISSN 01410296.
31(8), s 1734 1744 . doi:
10.1016/j.engstruct.2009.02.015
Vis sammendrag
Present approximate second order methods for the analysis of unbraced multistorey frames may significantly underestimate moments in single curvature regions. To clarify reasons for this that may not be well understood, the mechanics of column interaction in single curvature regions are studied. Suitable tools for sidesway description are derived, including a nearly exact, explicit free sway effective length expression, that, when high accuracy is required, eliminates the need in many cases for cumbersome, iterative solutions of exact effective lengths from the transcendental instability equation. Two reasons for the underestimation are identified. One is related to the local second order effects, and the other to second order effects causing changes in rotational restraint stiffness at column ends due to vertical, interstorey column interaction. A modified approximate storey magnifier approach is proposed that accounts for these local second order effects through two separate ``flexibility factors''. The approach is sufficiently simple to be viable in practical analyses, and predictions are found to compare well with more accurate results.

Brubak, Lars & Hellesland, Jostein (2008). Strength criteria in semianalytical, large deflection analysis of stiffened plates in local and global bending. Thinwalled structures.
ISSN 02638231.
46(12), s 1382 1390 . doi:
10.1016/j.tws.2008.03.013
Vis sammendrag
Various strength criteria that may be used in semianalytical methods for ultimate strength prediction of arbitrarily stiffened plates are studied. The main objective is to evaluate the applicability of the criteria in ultimate strength predictions of inplane loaded plates, both in local and global bending. The equilibrium path is traced using large deflection theory and the Rayleigh–Ritz approach on an incremental form. The approach is able to account for the reserve strength of slender plates in the postbuckling region. Results are compared with fully nonlinear finite element analyses for a variety of plate dimensions and stiffeners with regular and irregular arrangements. Good agreement is obtained with a combination of a plate and a stiffener criterion. With the considered criteria included, the method is computationally very efficient and gives rather high numerical accuracy.

Hellesland, Jostein (2008). Mechanics and slenderness limits of swayrestricted reinforced concrete columns. Journal of Structural Engineering.
ISSN 07339445.
134(8), s 1300 1309 . doi:
10.1061/(ASCE)07339445(2008)134:8(1300)
Vis sammendrag
Several aspects of nonslender (short) column limits are considered. Such limits, below which it is acceptable to ignore local secondorder effects that may cause maximum moment to develop between member ends, are generally confined to columns of braced frames, but are relevant for swayrestricted columns of unbraced frames also. Local and global secondorder effects are reviewed in this context, and the effect of global secondorder effects (swaymodified moments) is discussed, and it is concluded that the end moment ratios of approximate moment magnifier expressions and slenderness limits should be expressed in terms of swaymodified end moments. The need for providing separate slenderness limits, such as in the ACI 31805 code, for columns of nonsway and sway frames, is questioned. The ACI limits are critically reviewed and several inconsistencies pointed out with respect to their differences and different basis on which they were derived. They are compared with a more comprehensive limit, that may replace both of the ACI limits, and comparisons are also made with a summary of nonlinear analysis results

Steen, Eivind; Byklum, Eirik & Hellesland, Jostein (2008). Elastic postbuckling stiffness of biaxially compressed rectangular plates. Engineering structures.
ISSN 01410296.
30(10), s 2631 2643 . doi:
10.1016/j.engstruct.2008.02.006
Vis sammendrag
The elastic buckling and postbuckling response of biaxially compressed plates is analysed with main focus on change of inplane postbuckling stiffness under prescribed plate shortenings rather than the more normal load control. An analytical closed form singledegreeoffreedom solution is derived and compared with an analytical multidegreeoffreedom model. Results for the analytical multidegreeoffreedommodel are obtained by solving numerically the equilibrium equations using the perturbation expansion technique with arc length control in an incremental scheme. Both models are applied to specific plate examples and comparisons with results obtained using a commercial nonlinear finite element program are included.

Brubak, Lars & Hellesland, Jostein (2007). Approximate buckling strength analysis of arbitrarily stiffened, stepped plates. Engineering structures.
ISSN 01410296.
29(9), s 2321 2333 . doi:
10.1016/j.engstruct.2006.12.002
Vis sammendrag
A computationally efficient method for the elastic buckling and buckling strength analysis of inplane biaxial and shear loaded stiffened plates with varying, stepwise constant thickness is presented. The main focus is on biaxial loading cases. The stiffeners may be sniped or endloaded (continuous), and their orientations may be arbitrary. Both global and local plate buckling modes are captured. The method is semianalytical and makes use of simplified displacement computations that involve the elastic buckling load (eigenvalue), determined using a Rayleigh–Ritz approach, and finally stress computations using large deflection theory in combination with strength assessment using von Mises’ yield criterion as applied to membrane stresses. The displacements are represented by trigonometric functions, defined over the entire plate. The method is implemented into a Fortran computer program, and numerical results, obtained by the method for a variety of plate and stiffener geometries, are compared with corresponding fully nonlinear finite element analysis results. An extension to strength analysis that also includes lateral loading is discussed.

Brubak, Lars & Hellesland, Jostein (2007). Semianalytical postbuckling and strength analysis of arbitrarily stiffened plates in local and global bending. Thinwalled structures.
ISSN 02638231.
45(6 (June)), s 620 633 . doi:
10.1016/j.tws.2007.04.011
Vis sammendrag
A semianalytical method for pre and postbuckling analysis of imperfect plates with arbitrary stiffener arrangements, subjected to inplane biaxial and shear loading, is presented. By using large deflection theory in combination with the Rayleigh–Ritz approach on an incremental form, the method is able to trace both local and global equilibrium paths. Ultimate strength predictions are made using the von Mises’ yield criterion applied to the membrane stresses as collapse criterion. A Fortran computer program based on the presented theory is developed and computed results are verified by comparisons with nonlinear finite element analysis. Relatively high numerical accuracy is achieved with small computational efforts. The method is therefore suited also for design optimisation and reliability studies.

Brubak, Lars; Hellesland, Jostein & Steen, Eivind (2007). Closure to discussion by O. Bedair of 'Semianalytical buckling strength analysis of plates with arbitrary stiffener arrangements'. Journal of constructional steel research.
ISSN 0143974X.
63(12), s 1618 1620 . doi:
10.1016/j.jcsr.2007.05.012

Brubak, Lars; Hellesland, Jostein & Steen, Eivind (2007). Semianalytical buckling strength analysis of plates with arbitrary stiffener arrangements. Journal of constructional steel research.
ISSN 0143974X.
63(4), s 532 543 . doi:
10.1016/j.jcsr.2006.06.002
Vis sammendrag
Buckling of plates with arbitrarily oriented, sniped stiffeners is studied. The main objective is to present and validate an approximate, semianalytical computational model for such plates subjected to inplane loading. Estimation of the buckling strength is made using the von Mises’ yield criterion for the membrane stress as the strength limit. The formulations derived are implemented in a Fortran computer code, and numerical results are obtained for a variety of plate and stiffener geometries. The model may handle complex plate geometries, by using inclined stiffeners to enclose irregular plate shapes. The method allows for a very efficient analysis. Relatively high numerical accuracy is achieved with low computational efforts. The results are, in most cases, found to be conservative compared to fully nonlinear finite element analysis results.

Hellesland, Jostein (2007). Mechanics and Effective Lengths of Columns with Positive and Negative End Restraints. Engineering structures.
ISSN 01410296.
29(12), s 3464 3474 . doi:
10.1016/j.engstruct.2007.08.021
Vis sammendrag
In approximate buckling analysis of frame structures, it is common practise to simplify by considering compression members in isolation with end restraints that supposedly reflect the interaction with the surrounding structure. Buckling modes and effective lengths of such members have traditionally been limited to modes that can be associated with positive end restraints. A wider range of realistic buckling modes, that also imply negative end restraints, are reviewed and discussed, and approximate formulas for effective lengths and inflection point locations are developed from simple physical models for unbraced and fully braced compression members. The formulas are expressed in in terms of convenient end fixity factors that allow for simple and elegant formulations. The range of applicability of the formulae are identified by comparisons with exact results for a reasonable wide range of positive and negative restraints, and applications to special structures are demonstrated.

Brubak, Lars & Hellesland, Jostein (2007). Strength criteria in stiffened plate analysis using semianalytical, large deflection methods, In E. Onate; J. Garcia; P. Bergan & T. Kvamsdal (ed.),
Computational Methods in Marine Engineering II.
International Center for Numerical Methods in Engineering (CIMNE).
ISBN 9788496736191.
Kapittel.
s 71
 74

Hellesland, Jostein (2005). Nonslender Column Limits for Braced and Unbraced Reinforced Concrete Members. ACI Structural Journal.
ISSN 08893241.
102(1), s 12 21

Mari, Antonio R. & Hellesland, Jostein (2005). Lower slenderness limits for rectangular reinforced concrete columns. Journal of Structural Engineering.
ISSN 07339445.
131(1)

Hellesland, Jostein (2000). An improved Kfactor formula. Discussion,. Journal of Structural Engineering.
ISSN 07339445.
126(5)

Hellesland, Jostein (1998). Application of the method of means to the stability analysis of unbraced frames. Journal of constructional steel research.
ISSN 0143974X.
46(13)
Se alle arbeider i Cristin

Casandjian, Charles; Challamel, Noël; Lanos, Christophe & Hellesland, Jostein (2013). R. C. Beams, Columns and Frames  Mechanics and Design.
John Wiley & Sons.
ISBN 9781848214828.
320 s.

Hellesland, Jostein; Challamel, Noël; Casandjian, Charles & Lanos, Christophe (2013). R. C. Beams, Columns and Frames  Section and Slender Member Analysis.
John Wiley & Sons.
ISBN 9781848215696.
320 s.
Se alle arbeider i Cristin

Hellesland, Jostein (2019). 2. ordens beregninger av staver og rammer  Generelt og etter Eurokode 2 (NSEN 199211).

Hellesland, Jostein (2019). Approximate end and maximum moment formulations for slender columns in frames with sway.. Research report in mechanics. March 2019.
Vis sammendrag
Based on a study of isolated restrained columns, and twocolumn panel frames, the often complicated mechanics of moment formations in the members are studdied using elastic secondorder theory for the purpose of establishing a base on which procedures for moments due to sidesway can be evaluated. A distinction is made between moment developments in “supporting (bracing) sway columns” and “supported (braced) sway columns”. The primary objective is to derive improved slender column design expressions that are suitable in approximate design code formats. Both shear, end moments and maximum moments, applicable over the full range of axial loads, are considered. Proposals are presented that will allow more economical designs of framed columns with sway. Extended application to load combinations that include both gravity and sideways loading are also suggested.

Hellesland, Jostein (2019). Mechanics of columns in sway frames  Derivation of key characteristics.. Research report in mechanics. Februar.
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Columns in sway frames may be divided into laterally ``supporting (bracing) sway columns'', which can resist applied lateral shears, and ``supported (braced) sway columns'', which must have negative shears from the rest of the frame to support them. The supported sway column acts like a braced column with an initial displacement of one end relative to the other. The maximum moment will occur at the end of a supporting sway column, but may occur between the ends of a supported sway column. Mechanics of such columns, single or part of panels, are studied using elastic secondorder theory. The main objective is to establish simple, explicit expressions for characteristic points in the moment versus axial load space. Such results may be useful in teaching, in design practice to assist in the assessment of the often complicated column response, and also useful as a supplement to full, secondorder structural analyses.

Hellesland, Jostein (2017). 2. ordens beregninger av staver og rammer  Generelt og etter Eurokode 2 (NSEN 1992).

Hellesland, Jostein (2016). Preliminary global stiffness proposal (for second order global system analysis) (Doc. TG6/N77)..

Hellesland, Jostein (2016). Second order analysis of members and systems  Revised and updated proposal for EC25.8 (Doc.TG6/N82).

Hellesland, Jostein (2016). Second order structural analysis of members and systems  Revised proposal for EC25.8 (CEN Document TG6/N107).

Hellesland, Jostein (2015). Comments on EIvalues in the momentmagnifier procedure of EC2, and in Dutch rules (Doc.TG6/N48)..

Hellesland, Jostein (2015). Flexural stiffness for slender columns  A collection of published research (Doc. TG6/N61)..

Brubak, Lars; Hareide, O. J. & Hellesland, Jostein (2014). On Application of a SemiAnalytical Approach for Vibration Analysis of Irregularly Stiffened Plates.

Hellesland, Jostein (2014). Discussion of proposal for Second order structural analysis of members and systems (doc. TG6/14).

Hellesland, Jostein (2014). Principles and basis for nominal stiffness and curvature (doc. TG6/N37).

Hellesland, Jostein (2014). Second order analysis of members and systems  Proposal for EC25.8 (doc.CEN/TC250/SC2/WG1/TG6 N14).

Brubak, Lars; Hellesland, Jostein & Hareide, Ole J. (2013). Vibration analysis of plates with arbitrary stiffener arrangement using a semianalytical approach.

Hellesland, Jostein (2013). Second order structural analysis of members and systems  Preliminary comments towards revising EC2:2004sect.5.8.

Challamel, Noël & Hellesland, Jostein (2012). Buckling of Imperfect CDM Structural Systems.

Challamel, Noël & Hellesland, Jostein (2012). Postbuckling and postfailure of CDM structural systems.

Hareide, Ole J.; Brubak, Lars; Steen, Eivind & Hellesland, Jostein (2012). Approximate buckling model for steelelastomer sandwich plates.
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The paper describes the development of a semianalytic buckling model for steelelastomer sandwich plates using the RayleighRitz method. The model includes transverse shear deformations which are important in this type of plate. The unknowns  the deﬂections and crosssectional rotations are approximated by Fourier series. All combinations of inplane tensile, compressive and shear loads are allowed. Prestressing of the plate and elastic springs on the boundary are also included. The model has been implemented in a FORTRAN programme and the results have been compared with results from the ﬁnite element programme ABAQUS.

Andersen, Håkon S.; Hellesland, Jostein & Brubak, Lars (2010). On strength criteria for stiffened plates with a free or stiffened edge.

Brubak, Lars & Hellesland, Jostein (2010). On response propagation analysis of imperfect stiffened plates with a free or flexible edge using a semianalytical method.

Brubak, Lars & Hellesland, Jostein (2010). Semianalytical large displacement analysis of stiffened imperfect plates with a free or stiffened edge. Preprint series (Universitetet i Oslo. Matematisk institutt), Mechanic. 2. Fulltekst i vitenarkiv.

Brubak, Lars; Hellesland, Jostein & Steen, Eivind (2009). Approximate postbuckling analysis of imperfect stiffened plates with a free edge.

Hellesland, Jostein (2009). Closure to "Mechanics and Slenderness Limits of SwayRestricted Reinforced Concrete Columns" by Jostein Hellesland. Journal of Structural Engineering.
ISSN 07339445.
135(11), s 1431 1433

Hellesland, Jostein (2009). Konstruksjonsmekanikk MEK3500/4500Ekasamensoppgaver med løsningsforslag.

Hellesland, Jostein (2009). Lineære og ikkelineære beregningsmetoder  2. ordens virkninger.

Hellesland, Jostein (2009). Lineære og ikkelineære beregningsmetoder  2. ordens virkninger.

Hellesland, Jostein (2009). Lineære og ikkelineære beregningsmetoder  2. ordens virkninger.

Hellesland, Jostein (2009). Lineære og ikkelinære beregningsmetoder 2. ordens lastvirkninger.

Hellesland, Jostein (2009). Todimensjonal elastisitetsteori for tynne platerIntroduksjon.

Hellesland, Jostein (2008). Approximate second order analysis of unbraced frames reflecting interstorey interaction in single curvature regions. Research report in mechanics. 082.

Hellesland, Jostein (2008). Extended second order approximate sway and critical load analysis of frames with swaybraced column interaction. Research report in mechanics. 081.

Hellesland, Jostein (2008). NSEN 1992 Lineære og ikkelineære beregningsmetoder  2. ordens lastvirkninger.

Hellesland, Jostein (2008). NSEN 1992, Eurokode2  Lineære og ikkelineære beregningsmetoder; 2. ordens lastvirkninger.

Brubak, Lars & Hellesland, Jostein (2007). Strength criteria in local and global semianalytical, large deflection analysis of arbitrarily stiffened plates. Preprint series (Universitetet i Oslo. Matematisk institutt), Mechanic. 072.

Brubak, Lars & Hellesland, Jostein (2007). Strength criteria in stiffened plate analysis using semianalytical, large deflection methods.

Brubak, Lars & Hellesland, Jostein (2006). Computational postbuckling and strength analysis of arbitrarily stiffened plates in local and global bending. Preprint series (Universitetet i Oslo. Matematisk institutt), Mechanic. 063.

Brubak, Lars & Hellesland, Jostein (2006). Semianalytical buckling strength analysis of arbitrarily stiffened plates with varying thickness. Preprint series (Universitetet i Oslo. Matematisk institutt), Mechanic. 062.

Brubak, Lars & Hellesland, Jostein (2005). Computational buckling strength analysis of stiffened plates with varying thickness.

Brubak, Lars & Hellesland, Jostein (2005). Computational buckling strength analysis of stiffened plates with varying thickness, In P. Bergan; J. Garcia; E. Onate & T. Kvamsdal (ed.),
Computational Methods in Marine Engineering.
International Center for Numerical Methods in Engineering (CIMNE).
ISBN 8495999633.
Ultimate Strength .....
s 355
 364

Brubak, Lars; Hellesland, Jostein & Steen, Eivind (2005). Semianalytical buckling strength analysis of plates with arbitrary stiffener orientations. Preprint series (Universitetet i Oslo. Matematisk institutt), Mechanic. Nr. 05  3.
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Buckling of plates with arbitrarily oriented, sniped stiffeners are studied. The main objective is to present and validate an approximate, semianalytical computational model for such plates subjected to inplane loading. Estimation of the buckling strength is made using the von Mises' yield criterion for the membrane stress as the strength limit. The formulations derived are implemented in a Fortran computer code, and numerical results are obtained for a variety of plate and stiffener geometries. The model may handle complex plate geometries, by using inclined stiffeners to enclose irregular plate shapes. The method allows for a very efficient analysis. Relatively high numerical accuracy is achieved with low computational efforts. The results are, in most cases, found to be conservative compared to fully nonlinear finite element analysis results.

Hellesland, Jostein (2005). Analysis of second order effects with axial loads, EN 1992112004 (E), 5.8  Nasjonalt tillegg (NA).

Hellesland, Jostein (2005). Buckling of plates, slenderness limits of columns, etc.  Research overview.

Hellesland, Jostein (2005). Comments to Proposed Changes to ACI 31802 Slender Column Provisions. Research report in mechanics. No. 5.

Hellesland, Jostein & Brubak, Lars (2005). Stability of Stiffened Orthotropic Plates: Performance Based Design.

Brubak, Lars & Hellesland, Jostein (2004). Approximate buckling strength analysis of plates with arbitrarily oriented stiffeners.

Brubak, Lars & Hellesland, Jostein (2004). Knekningsanalyse av plater med vilkårlig orienterte stivere.

Osnes, Harald & Hellesland, Jostein (2004). Faststoff/konstruksjonsmekanikk ved UiO.

Mari, Antonio & Hellesland, Jostein (2003). When can second order effects in columns be neglected?.

Hellesland, Jostein (2003). Nonslender Column Limits for Braced, Endloaded Members. Preprint series (Universitetet i Oslo. Matematisk institutt), Mechanic. 2.

Hellesland, Jostein (2003). Nonslender RC Column Limits. Preprint series (Universitetet i Oslo. Matematisk institutt), Mechanic. 1.
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Structural frame systems may be analyzed and designed based on forces from conventional first order theory when second order effects of axial forces on displacements in the compression members of the system are negligible. This is often the case. Slenderness limits at which this is the case are studied in this paper. Existing limits are reviewed and major factors that affect such limits are investigated. A new slenderness measure, labeled normalized slenderness, is defined based on a reduced stiffness concept. In addition to the geometrical slenderness, the normalized slenderness is a function of axial force and reinforcement. Slenderness limit predictions are obtained from numerical nonlinear analyses for unbraced (sway) members and for transversely loaded braced members. For such members, new slenderness limits are proposed and verified against the nonlinear results. They are generally found to be more reliable than existing lower limit formulations. Braced compression members subjected to end moments only are dealt with in a separate paper.

Hellesland, Jostein (2002). Lower slenderness limits for braced, end loaded r.c. compression members. Research report in mechanics. 2.
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Hellesland, Jostein (2002). Lower slenderness limits for unbraced and transversely loaded braced r.c. compression members. Research report in mechanics. 1.

Andersen, Alfred; Osnes, Harald & Hellesland, Jostein (2000). Computational analysis of computational joints.

Hellesland, Jostein; Osnes, Harald & Skeie, Geir (2000). Proceedings of the 13th Nordic Seminar on Computational Mechanics.
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These proceedings contain the papers presented at the 13th Nordic Seminar on Computational Mechanics (2021 October, 2000). The seminars are organized annually under sponsorship of the Nordic Association for Computational Mechanics (NoACM), whose member countries are the Nordic and the Baltic countries. The papers cover a wide range of subjects within computational mechanics.

Hellesland, Jostein (1999). Bridge design and construction in Norway.

Hellesland, Jostein (1999). On column slenderness limits. Fulltekst i vitenarkiv.

Hellesland, Jostein (1999). Slanke konstruksjonsdeler.

Hellesland, Jostein (1999). Slanke konstruksjonsdeler.

Hellesland, Jostein (1999). Structural engineering for the largest petroleum exploration structure in the North Sea,.

Hellesland, Jostein (1998). Andre ordens teori for armert betongstavPrinsipper og forenklinger,.

Hellesland, Jostein (1998). Dimensjoneringsgrunnlag for vertikale avstivningssystemer,.

Hellesland, Jostein (1998). Dimensjoneringsgrunnlag for vertikale avstivningssystemer,.

Hellesland, Jostein (1998). Dimensjoneringshjelpemidler for slanke trykkstaver,.

Hellesland, Jostein (1998). Kommentarer og forslag til endringer i NS 3473, pkt.A.9.4.4 (4.utg.,1992) vedr. rotasjonsbehov og rotasjonskapasitetsberegninger,.

Hellesland, Jostein (1998). Slanke betongkonstruksjoner  Oversikt over beregningsmetoder, bestemmelser i NS 3473, og aktuelle konstruksjonsdeler,.

Hellesland, Jostein (1998). Slanke trykkstaver etter NS 3473,.
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Publisert 13. nov. 2010 14:22
 Sist endret 16. juli 2015 23:22