Diagnostic inspections of butt joints in the oldest steel welded bridges in Poland: Fid-Bau Portal

Diagnostic inspections of butt joints in the oldest steel welded bridges in Poland

Wichtowski, Bernard / Pakos, Ryszard

X-ray tests of butt welds in the first welded bridges in Poland showed their very low quality. Based on these tests, it is not possible to specify the continued safe operating period of those constructions. However, the bridges provide a unique opportunity to gain experience on the behaviour of welds containing R4 and R5 class defects in structures operating over a long period of time. The results of periodical X-ray tests conducted 'in situ' of five bridges with cracks in the butt welds, constructed between 1935 and 1938, did not show either the development of these cracks in the period between the tests or the creation of new cracks in any of them. Each time, hidden cracks were found in the weld metal, therefore, these could be considered part of the so-called hot crystallization cracks occurring during assembly of the structure. These findings are confirmed by nearly the same number of X-ray pictures with cracks in both the compressed and tensile zones. The fact that in the period between tests, the cracks in the welds did not increase from the baseline partly explains the results obtained from the fatigue tests conducted on modelled joints of analogous properties as in the actual structures. With the number of bridge load cycles presented, each time set value of unlimited joints strength (Z_rj≈ 90 MPa) is greater than the stresses caused by the operational and steady load (? = 75 MPa). In reality, according to the tensometric tests, the stresses from operational loads are about 20% smaller than those calculated theoretically. X-ray test results confirmed that weld defects and fault class classifications alone cannot be the only measure of a welded joint's quality and the basis for its disqualification. Defects in a structure's joints are acceptable if, during the structure's use, they do not grow and do not cause exhaustion of the load capacity or stiffness of the system. A comment in PN-EN 1993-1-1:2007₁ on admissibility of operational cracks in dynamically loaded structures is fully justified. The effect of defective welding, in particular cracks in the welds, must be considered individually, taking into account the fatigue strength curve for the given defect and construction solutions of the analysed joint. Application of the same provisions and standards that apply in the design of new constructions to assess a structure's usable load capacity is economically unjustified. In the standards set for the checking of existing bridges, loads should be close to the actual masses currently undertaken by the analysed bridge.