ResearchResearch focuses
Load-bearing behaviour and stability

Load-bearing behaviour and stability

© WeserWind GmbH

Steel constructions can be designed in many different ways. In addition to the different cross-section types for the rod-shaped components of a supporting structure, the connections of these elements - points at which at least two rod-shaped components are joined - can also be designed in diverse ways by using mechanical fasteners, such as screws, welded seams or high-performance grout. Due to the constant development of material and building products the need for research work arises that deals with the load-bearing behaviour of newly introduced components and fasteners. Thus, the desired reliability for the entire supporting structure remains guaranteed. For this reason, the scientific staff of the Institute for Steel Construction are engaged in experimental and numerical analysis of the load-bearing behaviour of novel components and fasteners within the framework of various research projects.

The main areas of research in the field of load-bearing behaviour and stability are the ring-flange connections and the corresponding HV-bolting assemblies, welded connections as well as the grout connection for offshore-wind turbine support structures. 

The ring flange connection is used in a variety of steel constructions. For many years, they have been an important and indispensable connecting element between tower segments as well as between foundation components and towers for wind turbine support-structures. With the trend towards larger wind turbines and the associated larger support structures, the forces acting on the individual structural components increase. Thus, the requirements on flange connection and on the installed HV-bolting assemblies are increasing too. In the past years, the Institute for Steel Construction completed successfully several research projects evaluating the load-bearing behaviour of ring flange connections with installed HV-bolting assemblies. Among others, the research focus lied on the experimental and numerical investigations of the load-bearing behaviour of flange segments as well as the tightening behaviour of the HV-bolts in the ring flanges, which has a decisive influence on the load-bearing behaviour. A further focus of research projects was the local load-bearing behaviour in the paired thread of HV-bolting assemblies for the evaluation of the fatigue strength. Current research projects deal with the influence of geometric flange imperfections on the load-bearing behaviour of the ring flange connection, as well as the influence of the yield-strength controlled tightening process on the load-bearing and fatigue behaviour of ring flange connections and the HV-bolts, respectively.

Welded joints are typically used in steel constructions to inseparably connect several components of the construction. Especially in dynamically loaded structures such as bridges, cranes or wind energy turbines, the welds represent critical points due to the geometric and material notches, for which the fatigue limit state is design driving. In order to predict the life time of the welded joints, experimental investigations on representative test specimens are necessary. For this reason, fatigue tests are carried out at the Institute for Steel Construction on various types of welded connections.

With regard to the (offshore) wind energy sector, welded joints such as butt welds are typically used to connect bent plates for the manufacturing of towers or foundation structures such as the monopile. To predict the lifetime of these structures fatigue tests are performed at the Institute for Steel Construction by using various servo-hydraulic testing devices as well as a magnetic resonance test frame. Within the research project ventus efficiens, for example, the fatigue strength of typical butt welded joints is investigated taking various welding methods into account. However, the fatigue strength of welded joints does not only depend on the welding method itself, but also on possible mechanical or thermal post weld treatment applications to improve the fatigue resistance. For this reason, the research project DeepRolledWeld deals with deep rolling as a possible post weld treatment method for butt welds on monopile foundations.  

Geometrically more complex welded joints, so called tubular joints, are realized within the manufacturing of jacket support structures. Here several obliquely positioned pipes have to be connected. Due to the complexity of the tubular connections, these constructions have been welded manually up to now. To improve the manufacturing procedure of the jacket structures, the research project FATInWeld dealt on the one hand with the automated production of these tubular joints. On the other hand, the influence of the automated welding procedure on the fatigue strength of these tubular connections was quantified.

Grout connections are used in the offshore wind energy sector as well as in the oil and gas industry to connect two steel pipes. These steel pipes are of different diameters and infiltrate each other with a defined overlap length. The arrangement of welding beads on the steel pipe surfaces facing each other enables a better interlocking and concentrated load transfer. A common field of application are truss structures, which have to be connected to the foundation piles in submerged ambient conditions. Grouted connections have a clear advantage here over alternative steel construction connections. They are insensitive to corrosion and can be installed in submerged ambient conditions. Here, truss structures are subjected to predominantly axial loading. In addition to their use in truss structures, grout connections are also used in monopile structures, which are mainly subjected to bending loading.

As a result of the dynamic environmental influences of wind, wave and operation, grouted connections are exposed to cyclic loading over a period of 25 years. These lead to corresponding fatigue damage if the grout connections are not sufficiently designed. The Institute for Steel Construction are investigating the damage mechanism of grouted connections for about 20 years. With a realistic representation of the environmental conditions in submerged ambient conditions the Institute for Steel Construction investigated in 2016 (GROWup) deviating failure mechanisms, which so far have not been the basis of current standards. As a result of water penetrating into the connection, local grout damage nearby the shear keys and elastic deformations, continuous damage is caused by washing out the crushed particles and a corresponding increase of relative displacement between foundation pile and support structure.

Currently the institute for steel construction is investigating the influence of early age movements on the stiffening behaviour of the grout material and corresponding effects on the load-bearing capacity of the grouted connection (GREAM). In addition to the experimental implementation, a major part of the research work is the numerical investigation of the following problems.