**1. Introduction**

Bridges are engineering structures which are subjected to dynamic actions, variable in time—repeated millions of times, and inconsistently. The endurance of steel bridges in service is determined mainly by fatigue, which usually causes catastrophic failures, and corrosion, which ordinarily results in degradation failures [1–3]. Many early welded bridges in Europe were fractured over the period 1925–1936 and later [4]. Many failures and catastrophic events happened due to fatigue and fracture despite the pioneering works of August Wöhler and other scientists [5, 6]. As a result, the limit state analysis of structures with a probabilistic approach became necessary.

Fractures led to a prohibition on using welding for early high-strength steels [7, 8]. As a result, welding was allowed only on mild steels. It was not until the late 1950s that some fatigue requirements were introduced into the design of steel railway bridges. It took another decade for these to be considered for road bridges; this started the modern approach to fatigue.

Each welded structure possesses discontinuity, or cracks and other imperfections resulting from manufacture or welding. These do not show any growth or instability while the loading rate is relatively low, below threshold fatigue strength. This means that such a stress range does not cause crack propagation. The endurance of steel structures or fatigue details with imperfections may be determined by testing small specimens or numerical analysis.

There are still many welded steel bridges in service which were manufactured in periods of poor quality of both materials and welded joints as well as few requirements for fatigue. After many years in service, each steel structure also undergoes what is known as functional aging, and its structural steel is subjected to structural aging. The mechanism of the aging process may be described by the classical separation theory or the newer dislocation blocking theory.

The quantitative results of welded butt splice quality obtained over 37 years of the testing of welded railway bridges in Poland are given. The radiographic examinations were conducted by the Steel Construction Chair at the West Pomeranian University of Technology in Szczecin [3]. The radiographic tests on the welded joints were supplemented by laboratory fatigue tests and more recently by Finite Element Method (FEM) analysis.
