Assessment of the brittle failure risk of riveted steel structures – further development of analytical methods

Head of Research:	Prof. Dr.-Ing. Richard Stroetmann
Research Assistant:	Dipl.-Ing. Lars Sieber
Processing Period:	06/2013 - 07/2015

An ever-growing task in the construction sector is the refurbishment and strengthening of existing structures. The almost 15.000 steel bridges of the Deutsche Bahn AG have an average age of 80 years (see figure 1). Approximately 800 of these bridges were classified into a damage category 4 according to the guideline 804.8001 and thus require urgent maintenance and / or partial renewal.

However, also the approximately 6.500 stations in Germany – about 500 of them are large or medium-size passenger stations – as well as the numerous railway operating facilities represent significant fixed assets. At the current average age of the railway stations of over 100 years, considerable maintenance and reconstruction measures are necessary with a view of a partially modified use of the buildings.

An equally inventoryof old steel structures represent the countless power poles in the power grid in Germany. About 28.000 overhead line towers have been in operation since the 1920s and 1930s in the supply network of the network operator RWE. They were verifiably built with the mild steel produced by the Thomas method. A brittle fracture failure would not only cause damages to the structures themselves, but also a significant damage of the infrastructure structures, as it was evident in the failed power poles in the Münsterland.

When assessing the safety of existing steel structures of mild steel (including steels manufactured using the Bessemer, Thomas or Siemens-Martin method) and deciding on the necessary maintenance and reinforcement measures, it is essential to check the material toughness (safety against brittle failure). This is essentially true in connection with construction or manufacturing-related notch effects, as well as damages caused by fatigue cracks, which are at an early stage and have not yet been recognized (e.g. during an inspection).

The present and normative regulated methods for the assessment of the brittle failure risk were developed for welded structures on “steels of more recent times” with a high value of toughness. The material toughness is determined by notch impact tests at defined temperatures, which are representative for the operating conditions of the steels. The relation between the notch impact energy and the fracture toughness is established by a correlation (so called Sanz correlation) which was calibrated for these steels.

The method is not valid for mild steel structures with hole weakening caused by riveted and bolted connections. The reasons for this are:

• The notch effect and the internal stress state of welded and perforated structures and thus the toughness requirements differ substantially from each other.
• The correlation between the notch impact energy and fracture toughness of new steel types cannot be transmitted to mild steels unchanged.
• The toughness characteristic of old mild steels underlies significantly larger variations.

In most cases, the notch impact toughness of mild steels is in the area of the lower shelf (see fig. 3) at room temperature. That means in the lack of expertise about the context, the structures will be rejected due to material embrittlement by aging. For the proper assessment of the brittle failure risk of riveted or bolted mild steel structures, fracture mechanical tests are valid. In this case, the construction-related toughness requirements are taken into account as well as possible damages and present toughness characteristic of the steel.

Up to now, the use of fracture mechanical test methods has been restricted to selected bridge structures. Reasons for that are on one hand the complex material analysis and FEM calculations to determine the toughness requirements, on the other hand the needed expertise which limits the investigations to only a few specialists.

The aim of the research project has benn the development of a practical method for assessing the sufficient toughness of components of mild steels with hole weakening. The basis for this are fracture mechanical calculations. Providing a practical test method to assess the brittle failure risk contributes to a better maintenance and higher conversion ability of many structures.

Participating research centers

Technische Universität Dresden, Institute of Materials Science
Helmholtz-Zentrum Dresden-Rossendorf
Ingenieurbüro für Werkstofftechnik Aachen