Reinforcement of old mild steel structures with welded-on fine-grained structural steels - design, dimensioning and execution

Construction in existing buildings is becoming increasingly important compared to the construction of new buildings. This applies in particular to infrastructure structures, but also to building constructions. In the existing Deutsche Bahn network, steel bridges, truss bridges and WiB bridges account for around 40% of the total of around 25 thousand bridges. These are on average 85 years old and have a high maintenance requirement. There is a comparable age structure in the field of hydraulic engineering, where 50% of locks and weirs are over 80 years old and 30% are even over 100 years old. There are also numerous halls, railroad stations and mast constructions with a high average age in structural engineering. Overall, this shows that a significant proportion of the steel construction stock in Germany was built between 1860 and 1940.

Steels from this production period usually consist of Thomas or Siemens-Martin steel, which are generally referred to as flux-cored steels. As a result of the manufacturing processes used at the time, these steels exhibit increased impurities due to the segregation of the melt. The last solidified area inside the cross-section, the so-called segregation zone, has a higher concentration of deep-melting impurities such as nitrogen, phosphorus and sulphur. The special chemical composition leads to special material properties of mild steels compared to modern steels. This results in lower strength and reduced toughness of the steel. The reduced resistance to brittle fracture is particularly relevant with regard to the fatigue strength of old steel structures with hole weakening, as was typical for riveted joints at the time. According to the current state of the art, the weldability of mild steels is only possible to a limited extent due to the high level of impurities. Nevertheless, the use of welded joints often offers considerable technical and economic advantages over bolted and riveted joints when repairing and upgrading old mild steel structures. However, there are currently no recognized technical rules of a normative nature, meaning that this process is neither technically feasible nor legally secure.

The aim of the research project is to create a basis and recommendations for the welding repair and reinforcement of river steel structures. The results should enable a sustainable and economical approach and thus make an important contribution to the preservation of many listed buildings as well as system-relevant road and railroad bridges, hydraulic steel structures and river steel structures in building construction.

The following sub-areas are planned for the experimental and numerical investigations:

• Development of repair and reinforcement concepts for typical components and joints
• Welding processing of various seam geometries and development of process instructions
• Testing the load-bearing capacity of welded joints between mild steels and fine-grained structural steels
• Load-bearing capacity of hybrid components made of mild and fine-grained structural steels, taking into account different strength and toughness values
• Fatigue strength of weld-reinforced components with riveted joints
• Development of design regulations and preparation of a DASt guideline and basis for normative regulations

The holistic development of design and implementation regulations contributes to the improvement of technical regulations in various sectors of the economy. In particular, small and medium-sized enterprises (SMEs), engineering offices, technical departments of steel construction companies, manufacturing companies as well as steel producers and manufacturers of welding consumables benefit from the results of the research project.

© Malte Homeyer