For railway bridges, fracture critical members (FCM) are those members or components of members loaded in tension whose failure would be expected to result in collapse of the bridge, or would prevent the bridge from performing its design function. If the bridge cannot carry the assigned rail traffic, it is considered not performing its design function.
Tension components include all portions of tension members and the portion of flexural members subjected to tension stresses. Attachments welded to a tension component of a FCM and having a length of 4 in or more, measured in the direction of the tension stress, should be considered a part of the tension component and thus as a FCM.
Fracture Control Plan
Provisions for a formalized Fracture Control Plan should generally be included in the design specifications. The essence of the program is stated in six areas intended to cover special requirements for materials, fabrication, welding, inspection, and testing. The plan provisions given in the AREMA Manual are as follows:
1. Assign responsibility for designating which steel railway bridge members or member components, if any, fall in the category of Fracture Critical.
2. Require that fabrication of Fracture Critical Members or member components be done in plants having personnel, organization, experience, procedures, knowledge and equipment capable of producing quality workmanship.
3. Require that all welding inspectors demonstrate their competency to assure that welds in Fracture Critical Members or member components are in compliance with this plan.
4. Require that all nondestructive testing personnel demonstrate their competency to assure that, tested elements of Fracture Critical Members or member components are in compliance with this plan.
5. Specify material toughness values for Fracture Critical Members or member components.
6. Supplement recommendations for welding contained elsewhere in [AREMA Manual] Chapter 15, Steel Structures and in [American Welding Society] AWS D 1.5.
The designer should be familiar with the AREMA definition of the Engineer as being the chief engineering officer of the owning Company, or his authorized representatives, and secondly of their assignment of design and review responsibilities. These are stated by the AREMA Manual as follows:
1. Quite apart from the Fracture Control Plan, the Engineer is responsible, for the suitability of the design of the railway bridge; for the selection of the proper materials; for choosing adequate details; for designating appropriate weld requirements; and for reviewing shop drawings and erection plans to determine conformance with the contract documents.
2. As a part of the Fracture Control Plan, the Engineer is also responsible: for determining which, if any, bridge members or member components are in the FCM category; for evaluating each bridge design to determine the location of any FCMs that may exist; for the clear delineation on the contract plans of the location of all FCMs; for reviewing shop drawings to determine that they correctly show the location and extent of FCMs;
and for verifying that this Fracture Control Plan is properly implemented in compliance with contract documents at all stages of fabrication and erection.
3. Welding procedure specifications are considered an integral part of shop drawings and shall be reviewed for each contract.
The fabricator for the structural steel should be certified under the American Institute of Steel Construction Quality Certification Program, Category III, Major Steel Bridges, or another program deemed suitable by the designer and acceptable to the owner. Welding inspector Qualifications and Certification, and Non-Destructive Testing Personnel Qualification and Certification, are detailed in the AREMA Manual and are generally known to fabricators of steel railway bridges. The designer may not participate in the fabrication or erection of the bridge; however, the calculations and plan preparation should contemplate the requirements for Fracture Critical construction.
Welding requirements should be in accord with AWS D 1.5 and the special requirements of the AREMA Manual. The designer should consider the effect of variables that pertain to Fracture Critical Fabrication, such as:
1. Minimum service temperature.
2. Material designation and grade.
3. Material thickness and requirements for Charpy V-notch impact testing.
4. Welding procedures, including preheat/ interpass temperature requirements, moisture content for electrodes, hydrogen limits for wire and coating, qualification test plates, and procedure qualification for welding and repair welding.
The AREMA Manual Commentary provides both explanatory information on various articles and also supplemental recommendations, including useful charts and tables, as well as an index to help find specific provisions on welding.