During the incremental launching of steel and composite bridges high support reactions have to be introduced into the slender steel webs of the bridge girder. In the past, the specific characteristics of bridge launching such as long loading lengths and longitudinal stiffeners were not covered appropriately by existing design formulae for patch loading. In the frame of the research project “Competitive Steel and Composite Bridges by Improved Steel Plated Structures (COMBRI)” a clear improvement of the patch loading resistances for I-girders and box sections could be derived leading to more economic solutions for this type of loading. The paper introduces the basic concepts of the developed models for both unstiffened girders and girders with open-section and closed-section longitudinal stiffeners as well as a software tool for the advanced determination of elastic critical buckling loads and modes. A comparison of the main area of application and the resulting economic advantages concludes the survey.

Patch loading or partial edge loading of steel girder webs is a load case where a concentrated force is introduced perpendicular to the flange of a girder. This usually induces a local failure of the girder web in the vicinity of the loaded flange. In structural applications concentrated forces are a common load case for girders introduced for example; at supports, by purlins, from crane wheels and during launching of bridges. For fixed loads, the problem of concentrated forces are usually solved by transverse stiffeners but for moving loads this is nor practically possible neither an economical solution. Further, it would be possible to use longitudinal stiffeners when the load is moving but stiffeners are expensive to fit and for girders with web depth below 3 m longitudinally stiffeners are not economically justified. Instead, the girder web itself has to resist the applied load in such cases. From the fifties and later a large amount of studies on this subject have been performed, starting with investigations on the elastic buckling of plates where only a part of the edge was loaded and followed by many test series and resistance functions. The earlier proposed resistance models were usually divided into two separate checks, one for yielding and one for instability. However, the test results do not show of any clear distinction between those two cases. This thesis deals with patch loading of plated girders without longitudinal stiffeners in both the ultimate and the serviceability limit state. A resistance model in the ultimate limit state is proposed, that have a continuous transition from yielding to buckling and hence, that is harmonized with the procedure for other buckling problems. The model contains three significant parts; the yield resistance, the elastic buckling load used to establish the slenderness and a reduction factor that relates the slenderness to the actual resistance. The advantage with the design model presented herein, which is a modification of the work presented by Lagerqvist and later introduced in EN 1993-1-5, is that the same equations are used irrespective of failure mode. The in this thesis proposed design procedure for patch loading gives a better accuracy of the predicted resistance compared to the design rule in EN 1993-1-5. Most of the experimental studies performed by others contain tests with very short loaded lengths, i.e. very concentrated loads. In order to gain more knowledge of the influence of the loaded length, three patch load tests were conducted where only the loaded length was varied. By means of the test results and a parametric study with the finite element method it was concluded that the variation in loaded length could be well described by the design procedure proposed in this thesis. For a bridge girder, the problem concerning resistance to patch loading usually occurs during launching. Bridge launching is a common method to erect steel and composite bridges and means that the bridge girders are assembled on ground behind the abutment and then pushed out over launching shoes into the final position. The launching shoe on which the girder travels will introduce a concentrated force to the girder, which can be of a magnitude that governs the web thickness and even a small increase of the web thickness can add a substantial amount of steel. Therefore, it is important to find a suitable criterion for the serviceability limit state for patch loading, i.e. for bridge launching. Compared to the ultimate resistance the amount of available research considering the serviceability limit state is very small. In fact, only one serviceability limit criterion proposed by Granath, which was developed for stationary loadings, was found. A number of FE-analyses of the launching process were carried out to investigate at what loads different girder cross sections will have a repeatable behaviour. A girder section of a bridge girder was subjected to several repeated travelling loads corresponding to a long bridge girder launched over several supports together with a co-existing bending moment. By means of these FE-analyses a serviceability criterion is established with a limit criterion not allowing any effective plastic membrane strains. The bridge designer can beneficially use the proposed serviceability criterion for bridge launching.

I flertalet praktiska fall belastas stålbalkar av koncentrerade laster. Detta lastfall kallas lokal intryckning och det ger upphov till vertikala tryckspänningar i balklivet som är störst under den belastade flänsen och avtar sedan till den motsatta flänsen. Koncentrerade laster kan t ex uppträda vid stöd, under åsar, som hjultryck på kranbanebalkar och vid lansering av brobalkar. För laster som har ett bestämt läge, som t ex vid stöd, löses problemet oftast med tväravstyvningar. När rörliga laster är aktuella är det dock lite svårare. För det första är avstyvningar generellt sett dyra att montera och för det andra är det naturligtvis inte möjligt att ha avstyvningar överallt när lasten förflyttas. Ett annat alternativ är längsgående avstyvningar som höjer bärförmågan med avseende på lokal intryckning men dessa är förstås också dyra att montera. För höga balkar, över 3 m, behövs ofta längsgående avstyvningar av andra skäl men för balkar under 3 m är det inte ekonomiskt försvarbart. Med andra ord måste rörliga laster för balkar med en livhöjd under 3 m bäras av livet självt och för det krävs tillförlitliga dimensioneringsregler.Föreliggande avhandling behandlar lokal intryckning av svetsade balkar utan längsgående avstyvningar i brott- och bruksgränstillstånd. När det gäller brottgränstillstånd finns det stora mängder publicerat material från provning och inom områdena kritisk bucklingslast samt bärförmågefunktioner. Tidigare var ofta bärförmågefunktionerna uppdelade på två funktioner, en för flytning och en för instabilitet. Det är dock svårt att separera dessa och inga försök visar på en klar skillnad mellan flytning och instabilitet. Lagerqvist (1994) tog ett stort steg i detta avseende och föreslog en bärförmågefunktion harmoniserad med de modeller som används vid andra bucklings- och stabilitetsproblem. Det innebär att modellen har en gradvis övergång mellan flytning och buckling. Beräkningsmodellen består av tre olika delar, ett uttryck för den plastiska bärförmågan, den kritiska bucklingslasten och en reduktionsfaktor som är empiriskt kalibrerad mot försöksdata. Lagerqvists modell har senare blivit implementerad i Eurokod 3 del 1.5, EN 1993-1-5 (2006). I denna avhandling föreslås en modifierad variant av bärförmågefunktionen i Eurokod 3 som ger mindre spridning jämfört med försöksdata och dessutom avlägsnar den diskontinuitet som finns i beräkningsmodellen idag.Vidare har nästan all tidigare forskning fokuserats på mycket koncentrerade laster, d v s korta lastlängder. För att öka förståelsen för lastlängdens inverkan på bärförmågan och för att fylla luckorna i försöksdatabasen har tre försök utförts i detta arbete. Försöken genomfördes på tre identiska svetsade balkar med tre olika lastlängder. Utifrån försöksresultaten och en efterföljande parameterstudie med hjälp av finita elementmetoden kunde det konstateras att den föreslagna bärförmågefunktionen också kan användas för långa lastlängder. Upp till 1,2 gånger livhöjden kontrollerades med tillfredställande resultat.När det gäller bruksgränstillstånd finns det betydligt färre publicerade artiklar. Det enda egentliga kriteriet föreslogs av Granath (2000) för balkar utsatta för en stationär koncentrerad last. En brobalk utsätts normalt endast för intryckning under själva produktionen, d v s om den lanseras. Lansering av stålbroar och samverkansbroar är vanligt, speciellt långa broar som är för tunga att lyftas på plats. Vidare är det en bra metod om bron skall sträcka sig över vatten eller vägar/järnvägar och området under bron ej kan användas. Under lanseringen sammanfogas brobalkar bakom ena landfästet och skjuts sedan ut över lanseringslager till dess slutliga position. Vid lanseringen utsätts stålbalkarna för relativt stora koncentrerade laster från stöden som balken glider på. Dessa laster är alltså inte stationära utan rörliga och kan vara så stora att de bestämmer livtjockleken. Eftersom en liten ökning i livtjocklek skulle öka mängden stål avsevärt är det därför viktigt att på ett bra sätt kunna uppskatta livets bärförmåga i ett sådant fall. Eftersom detta kan ses som ett bruksgränstillstånd och inte är det lastfall som brobalken är dimensionerad för primärt, bör kravet vara att balken ej ska få kvarstående deformationer under lanseringen. Om brobalken dessutom lanseras över många stöd och om kvarstående deformationer uppstår som växer vid varje ny passage kan det bli problem att bära de laster som balken egentligen var dimensionerad för. Vidare har det blivit vanligare att lansera brobalkar där betongfarbanan gjuts innan lanseringen. Detta ökar förstås egentyngden och därigenom också de koncentrerade lasterna från lanseringslagren. I denna avhandling har detta studerats med hjälp av finita elementanalyser av broelement. Ett antal olika tvärsnitt har utsatts för ett konstant böjande moment och en koncentrerad last som vandrar längs flänsen. Detta har upprepats tre gånger och en last har bestämts när inga effektiva plastiska membrantöjningar har utvecklats i livet vilket i sin tur innebär att de kvarstående deformationerna ej växer för varje ny lastpassage. Utifrån dessa laster har ett bruksgränskriterium etablerats för brolansering, d v s för rörliga laster.

Pressure vessels have been used for a long time in various applications in oil, chemical, nuclear, and power industries. Although high-strength steels have been available in the last three decades, there are still some provisions in design codes that preclude a full exploitation of its properties. This was recognized by the European Equipment Industry and an initiative to improve economy and safe use of high-strength steels in the pressure vessel design was expressed in the evaluation report (Szusdziara, S., and McAllista, S., EPERC Report No. (97)005, Nov. 11, 1997). Duplex stainless steel (DSS) has a mixed structure which consists of ferrite and austenite stainless steels, with austenite between 40% and 60%. The current version of the European standard for unfired pressure vessels EN 13445:2002 contains an innovative design procedure based on Finite Element Analysis (FEA), called Design by Analysis-Direct Route (DBA-DR). According to EN 13445:2002 duplex stainless steels should be designed as a ferritic stainless steels. Such statement seems to penalize the DSS grades for the use in unfired pressure vessels (Bocquet, P., and Hukelmann, F., 2001, EPERC Bulletin, No. 5). The aim of this paper is to present an investigation performed by Luleå University of Technology within the ECOPRESS project (2000-2003) (http://www.ecopress.org), indicating possibilities towards economic design of pressure vessels made of the EN 1.4462, designation according to the European standard EN 10088-1 Stainless steels. The results show that FEA with von Mises yield criterion and isotropic hardening describe the material behaviour with a good agreement compared to tests and that 5% principal strain limit is too low and 12% is more appropriate.

This paper comprises a study concerning the mechanical behavior of extra high strength steel. This is investigated by means of biaxial testing of flat cross-shaped specimens in the full σ1-σ2 plane, a concept developed earlier at Steel Structures, Luleå University of Technology. Furthermore, new specimen designs had to be developed to enable testing of a material with high yield strength and low ultimate over yield strength ratio, such as the extra high strength steel Weldox 1100. The tests are performed in two steps: one initial loading followed by unloading and a subsequent loading in a new direction. The test results, containing data from 15 biaxial tests, are characterized by a slightly anisotropic initial yield criterion where the proof stress in compression is consequently somewhat higher compared to the results in tension. In the subsequent loading the observed phenomena are a Bauschinger effect in the direction opposite the initial loading direction and that the transition from elastic to plastic state in subsequent loadings is gradual and direction-dependent.

The feasibility of manufacturing pressure vessels from duplex stainless steel (grade EN 1.4462, duplex 2205: 0.02%C, 22%Cr, 5.7%Ni, 3.1%Mo, 0.17%N) was evaluated by measurements of the mechanical properties of the parent metal and welds, and by finite element modelling. The tensile stress-strain properties of 4 mm sheet, and of the weld metal and the HAZ were measured in the transverse and rolling directions. Behaviour under biaxial stress was studied by pressurising circular sheet specimens from one side only, and measuring the strain as a function of applied pressure. The specimens included parent metal, and sheet with welds and a nozzle attached by welding. These specimens, and a cylindrical pressure vessel with hemispherical ends and a central nozzle, were modelled by finite element analysis. The results are discussed in relation to the current requirement for duplex stainless steel to be classified as a ferritic steel for design purposes.

This thesis deals with the plastic behaviour of steel. It comprises an investigation with focus on biaxial tests and constitutive modelling. Tests were performed on one stainless steel grade in two different strength classes, C700 and C850, as well as one extra high strength structural steel grade. An earlier developed concept for biaxial testing of cross-shaped specimens was utilised. However, there was a demand for new specimen designs to enable testing of the extra high strength steel. A comprehensive test programme including an initial and one subsequent loading was carried out. This provides data for stress-strain curves in two steps as well as stress points describing initial and subsequent yield criteria. The Bauschinger effect was evident for all grades. Furthermore, the behaviour in subsequent loadings was found to be direction dependent and the transition from elastic to plastic state was observed as gradual. A constitutive model with the possibility of depicting the phenomenological observations from the tests was earlier developed at Steel Structures, LTU, and proposed for annealed stainless steels. Further, it is a two surface model utilising the concept of distortional hardening and it was applied to the steel grades tested in this work. The model was found to be applicable to the steels tested in this study as well and compared to simpler models the applied model clearly improves the agreement with experimental tests.

This paper presents a constitutive model proposed for stainless steel. The model is a two surface model utilizing the concept of fuzzy sets. An experimental investigation has been performed on two differentstainless steel grades as a reference to the model. The tests were performed with a procedure containing load reversal. Each specimen was initially loaded in one direction of the principal stress plane followed by unloading and subsequent loading in a new direction. The model is relatively simple but still depicts the effects of observed phenomena such as the Bauschinger effect. Hence the qualitative response to subsequent loadings can be described with the model. The proposed model has been implemented into the finite element package ABAQUS. Comparisons between test results and the response predicted utilizing the model are presented in this paper.

The main objectives of the work presented in this report are to gain knowledge and collect characteristics for the application of duplex stainless steel (EN 1.4462) in economical pressure vessel design. The work covers ECOPRESS subtasks 4.3, 5.1, 5.4 and 6.2 and have included a total of 30 uniaxial tensile tests on base material, weld material and material from HAZ, redesign of an existing test set-up for membrane tests, a total of 12 membrane tests on plain and butt welded specimens and specimens with nozzles welded to them, 3 biaxial tests, design according to the Swedish Pressure vessel code of a typical pressure vessel in duplex stainless steel and a non-linear finite element analysis of the previously designed pressure vessel using material characteristics and other necessary inputs obtained from the evaluation of the uniaxial and membrane tests.