Each year 50,000 groove welds are made in production of steel bridges. Under the requirements of AASHTO/AWS D1.5, Bridge Welding Code (the Code), welds are tested by non-destructive examination (NDE). The NDE acceptance criteria prescribed by the Code have effectively ensure excellent performance of welds starting in the late 1950s. However, these criteria are based on workmanship and not do not explicitly consider the factors that influence whether a given flaw is or is not critical. For example, the current AWS criteria do not consider the magnitude of the stress range, mean stress, toughness of the material, height, or location of the flaw within the weld. This results in unnecessary repairs and may also permit some flaws that should be repaired to remain. Improved criteria are based on fitness for purpose and align with actual performance needs and both ensure that the condition of the weld is suitable for performance and that the repairs made to welds in production are needed. Further, these criteria can be used for existing welds when questions arise about the welds’ condition and their suitability for continued performance.
Fitness for purpose was defined by Wells as follows (as quoted by Barsom and Rolfe): “Fitness for purpose is deemed to be that which is consciously chosen to be the right level of material [that is, having the appropriate fracture toughness, e.g., KIC, CTOD, J-integral, CVN, etc.] and fabrication quality [that is, the appropriate loading or stress level for the application], having regard to the risks and consequences of failure; it can be contracted with the best quality that can be achieved within a given set of circumstances, which may be adequate for some exacting requirements, and needlessly uneconomic for other that are less demanding.” The approach of “the best quality that can be achieved” describes the approach currently utilized for the fabrication of steel bridges. The current weld acceptance criteria is generally workmanship-based, not fitness for purposed-based. This leads to increased inspection costs, unnecessary weld repairs, delivery delays and other undesirable economic consequences.
Previous attempts to establish a fitness for purpose weld acceptance criteria for bridge groove welds has resulted in acceptance criteria that are more rigorous than the current workmanship-based criteria. While the predictions have a theoretic basis, they cannot explain why the vast majority of bridges in service, fabricated to workmanship-based standards, performance acceptably. Valid fitness-for-purpose weld acceptance criteria must consider at least the following:
• the actual stresses on the welded connection
• the actual toughness of the deposited weld (with particular focus on the toughness at the stress raiser)
• the actual stress concentration factor created by the discontinuity, and
• the actual orientation of the stress concentration relative to the stress field.
The repetition of the term “actual” is deliberate; simplifying compromises can significantly impact the validity and value of any fitness for purpose model.
A fitness for purpose model for steel bridges must consider the whole range of codes and standards that are associated with modern bridge design and construction. The design of many bridges is governed by fatigue or deflection, not by strength. Accordingly, stresses in the welded connection are often significantly less than the tensile capacity of the weld or surrounding steel. The details of welded connections are routinely selected for fatigue resistance, which is directly associated with reductions in stress concentrations (which also aids in fracture resistance). Mature welding codes govern acceptable welding filler metals, base metals, WPS qualification, welder qualification, and fabrication techniques. NDE personal are similarly qualified and procedures controlled. NDE technology has advanced and is capable of detecting and defining weld imperfections in more precise ways, including the number, shape and orientation. The fitness for purpose approach as proposed must examine all of these factors that are associated with modern steel bridge welding in a holistic manner. The resulting method will be compared to a database of defects and performance
Acceptance criteria for performance in service should align with performance expectations that result from the design. In design, fatigue is addressed by aligning loads and frequency of loading to codified fatigue performance curves based upon 97.5% survival. Further, in almost all girder and other member design, short attachments corresponding to Category C or C’ control the design reducing the stress at the groove welds in the structure, and these lower stresses increase the flaw tolerance of the weld. Acceptance criteria should consider both the design reliability and the actual demand that results from design practices.
Acceptance criteria should also consider the reliability of the NDE methods used for inspection. Ideally the judgement of whether a flaw can be tolerance should be based on the flaw’s location, size, shape, and nature – i.e., whether it is crack-like. Inspection techniques vary in how accurately they can measure these characteristics but are improving over time with advance in technology and data collection.
This research will align with the following goals of the 2018 Strategic Plan of the AASHTO Committee on Bridges and Structures:
Goal 2 – Assess Bridge Condition – Improved acceptance criteria will help bridge owner effectively assess the condition of bridge welds that are in service.
Goal 4 – Maintain and Enhance the AASHTO Specifications – In alignment with item 4.3, these criteria will be performance based.
Goal 6 – Optimize Structure Systems – This project aligns with item 6.2.5 because the new criteria will facilitate use of technology and techniques that help optimize steel bridge fabrications.