Assessing the condition and structural competence of ageing structures is an area in which TWI has garnered an enviable reputation since it set up shop in the forties.
In the intervening decades however many integrity-related factors and parameters have changed. Design codes altered radically with the advent of BS5400 and limit state design considerations. Much more is also known now about corrosion rates, fatigue lives, materials properties and non destructive examination. So it was with great enthusiasm that TWI accepted the mantle offered by one Industrial Member of ‘ageing bridge assessor’.
The Industrial Member’s particular area of interest was a box girder bridge built in 1954 with an 18mm thick orthotropic steel deck and 10mm thick transverse girders.
Eight millimetre fillet welds formed their connections to the deck and these were known to contain root flaws (heel cracks) which originated during fabrication back in the fifties. These cracks had been subject to periodic non-destructive examination since construction.
TWI was approached to conduct a fatigue and fracture assessment on the degree of risk that these flaws could initiate a failure in the deck plate. After reviewing existing records a programme of strain gauge measurement, finite element analysis and NDE records analysis, some fatigue and fracture mechanics calculations were devised and recommended. These had to comply with the budgetary demands of the end user so that a fatigue and fracture analysis could be completed.
The work concerned five areas:
- Six months of strain data were monitored and recorded in situ.
- An FE model of the deck joint containing a defect was completed to obtain bridge specific stress intensity factors.
- Analysis of the historic NDT surveys was made to estimate statistically (with a high confidence limit) the largest flaw size. In total nine NDT examinations were conducted over the years, each of which varied in technique, procedure and accuracy.
- Charpy impact available data, from two different sources, were considered from which lower bound values of fracture toughness were calculated.
- From standards and codes of practice in existence at the time of construction the mechanical properties of the bridge material were established.
The risk of failure by fracture was assessed using maximum tensile stresses. The degree of crack growth under cyclic strains, based on the strains measured, was determined. The fatigue and fracture assessments followed the methodology in BS 7910, which is an accepted guide to the application of fracture mechanics for the assessment of flaws in steel.
Two levels of fracture refinement were adopted, one using the standard stress intensity factor solutions from BS 7910 (conservative), the other involving TWI’s finite element analysis of the actual joint and flaw geometry in the bridge (representative). Both results indicated that the risk of failure by fatigue and/or fracture was low.
Benefits and conclusion
Following TWI’s contribution a case was provided to the end user. Based on this the end user agreed the structural safety of the bridge deck with the legal authorities, as well as the maintenance and inspection schedules. They deduced that there was no need for a major re-fit of the bridge and consequently minimal disruption to the public.
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