Assessing the load-bearing capacity of closures on existing hydraulic steel structures (steel structures and corrosion protection)
The existing standards DIN EN 1993 (2010-12) and DIN 19704-1 (2014-11) set out regulations for verifying the load-bearing capacity of (new) closures on hydraulic steel structures. When these regulations are used to evaluate existing hydraulic steel structure closures, the permitted scopes of application are often exceeded. For instance, the requirements in terms of materials, tolerances and structural condition are incompatible with the properties of the closures within the remit of the German Federal Waterways and Shipping Administration (WSV), some of which are over 100 years old. This can lead to the load-bearing capacity being calculated inaccurately or incorrectly. The BAW therefore employs specific methods to take account of the unique characteristics of existing structures.
Several methods are used to determine the current condition of a structure in situ. The structural check of the load-bearing structure is carried out in several investigation steps of increasing thoroughness. Thus the assessment of load-bearing capacity makes a distinction between material behaviour at low temperatures (brittle fracture), load-bearing capacity at room temperature (forms of ductile failure) and load-bearing capacity under fatigue stress (material fatigue).
If the simplified method cannot be used to prove sufficient brittle-fracture resistance for the impact energy absorbed by the material sample taken, an assessment using fracture-mechanical methods must be carried out. The use of plastic cross-section reserves represents a verification method for existing hydraulic steel structures that, although new to the field, is necessary as far as the BAW is concerned. Compliance with the requisite material properties is established in the BAW’s laboratories. The partial safety coefficients for the stress factors are selected depending on the strength distribution (production time). The complex structures are modelled and studied using 3D finite element programs in order to produce realistic calculations of stresses and deformations.
Fatigue strength is generally evaluated by applying the idea of nominal stress in order to determine the residual useful life of the load-bearing structure. If required, the impact of corrosion is taken into account by modifying the Wöhler lines based on fatigue strength tests. The use of fracture-mechanical methods is also envisaged where components are already damaged by cracks.