For railway bridges, fracture critical members (FCM) are those members or components ofÂ members loaded in tension whose failure would be expected to result in collapse of theÂ bridge, or would prevent the bridge from performing its design function. If the bridge cannotÂ 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 flexuralÂ members subjected to tension stresses. Attachments welded to a tension component of aÂ FCM and having a length of 4 in or more, measured in the direction of the tension stress,Â 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 designÂ specifications. The essence of the program is stated in six areas intended to cover specialÂ requirements for materials, fabrication, welding, inspection, and testing. The plan provisionsÂ given in the AREMA Manual are as follows:
1. Assign responsibility for designating which steel railway bridge members or memberÂ components, if any, fall in the category of Fracture Critical.
2. Require that fabrication of Fracture Critical Members or member components be done inÂ plants having personnel, organization, experience, procedures, knowledge and equipmentÂ capable of producing quality workmanship.
3. Require that all welding inspectors demonstrate their competency to assure that welds inÂ Fracture Critical Members or member components are in compliance with this plan.
4. Require that all nondestructive testing personnel demonstrate their competency to assureÂ that, tested elements of Fracture Critical Members or member components are in complianceÂ 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]Â Chapter 15, Steel Structures and in [American Welding Society] AWS D 1.5.
The designer should be familiar with the AREMA definition of the ââEngineerââ as beingÂ the chief engineering officer of the owning Company, or his authorized representatives, andÂ secondly of their assignment of design and review responsibilities. These are stated by theÂ AREMA Manual as follows:
1. Quite apart from the Fracture Control Plan, the Engineer is responsible, for the suitabilityÂ of the design of the railway bridge; for the selection of the proper materials; for choosingÂ adequate details; for designating appropriate weld requirements; and for reviewing shopÂ 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 determiningÂ which, if any, bridge members or member components are in the FCM category; forÂ evaluating each bridge design to determine the location of any FCMâs that may exist; forÂ the clear delineation on the contract plans of the location of all FCMâs; for reviewingÂ shop drawings to determine that they correctly show the location and extent of FCMâs;
and for verifying that this Fracture Control Plan is properly implemented in complianceÂ with contract documents at all stages of fabrication and erection.
3. Welding procedure specifications are considered an integral part of shop drawings andÂ shall be reviewed for each contract.
The fabricator for the structural steel should be certified under the American Institute ofÂ Steel Construction Quality Certification Program, Category III, Major Steel Bridges, or anotherÂ program deemed suitable by the designer and acceptable to the owner. Welding inspectorÂ Qualifications and Certification, and Non-Destructive Testing Personnel QualificationÂ and Certification, are detailed in the AREMA Manual and are generally known to fabricatorsÂ of steel railway bridges. The designer may not participate in the fabrication or erection ofÂ the bridge; however, the calculations and plan preparation should contemplate the requirementsÂ for Fracture Critical construction.
Welding requirements should be in accord with AWS D 1.5 and the special requirements ofÂ the AREMA Manual. The designer should consider the effect of variables that pertain toÂ 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 contentÂ for electrodes, hydrogen limits for wire and coating, qualification test plates, andÂ procedure qualification for welding and repair welding.
The AREMA Manual Commentary provides both explanatory information on various articlesÂ and also supplemental recommendations, including useful charts and tables, as well as anÂ index to help find specific provisions on welding.