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Bridges & Structures

Worldwide Steel Fabrication Scanning Tour

Summary Report

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The steel bridge fabrication scan included the countries of Japan, Italy, Germany, and the United Kingdom. Leading fabricators in each country were visited and technical exchanges dealing with one or more experts in the design, fabrication, steel production and welding were held. Led by Krishna Verma of the office of Bridge Technology, FHWA, the U.S. delegation was divided into five groups that specialized in the topics that follow. The major findings in each category are summarized. top-priority implementation issues are identified in each group as bullet items. Implementation of these findings will require changes in U.S. practice related to contracts, accreditation, specifications, and investment in new technology. Implementation will also require presentations to regional and national technical committees, publication of technical articles in professional magazines and journals and continuing dialog between national and international experts.

CAD/CAM

All of the fabricators had computer aided drawing (CAD) and computer aided manufacturing (CAM) software. These systems also included 3 dimensional bridge modeling that was used to verify assembly/field erection location and elevation checks. The software was purchased from a variety of vendors or developed in house. None of the software was completely integrated, and the results of one package were manually input into the next package. The generation of a digital representation of the structure is crucial to a modern fabrication shop. The information is used to verify the geometry input and simultaneously produces manufacturing numerical control data. The computer numerical control (CNC) data can be sent directly to the machine using a local area network. The U.S. practice is evolving along similar lines in a few of our larger fabrication shops. Fully integrated systems are planned and cooperative development among fabricators seems desirable. Shop detail drawings are unnecessary in the modern automated shop. Shop drawings appear to be unnecessary as the dimensional checks can identify necessary deviations on the design drawing for review by the owner. The accuracy of the fabricated pieces reduces the need for shop assembly and allows virtual assembly to be done based upon measured section dimensions in the 3 dimensional modeling program. One of the highest priority goals of U.S. and foreign fabricators is the development of a computer integrated manufacturing (CIM) software package.

* Establish a task group of owners and fabricators, possibly through the AASHTO/NSBA Steel Bridge Collaboration, to develop a documentation standard.

* Promote development of a CIM software package.

* Carry out a pilot project using digital fabrication shop documents in lieu of shop drawings.

* Develop a storage protocol for archiving as built documents.

AUTOMATED RECORDING

Automated recording of fabrication was employed by some of the fabricators visited. Automated and digitally recorded ultrasonic inspection using both pulse-echo and time-of-flight diffraction was employed in shop and field ultrasonic inspection in Japan. However, this system is still in its infancy and its reliability has not been established. One fabricator employed continuous monitoring and recording of welding variables for quality control. Various geometric measurement systems were employed by the fabricators to determine the conformance of the geometry. This ranged from simple digital surveying equipment to the more sophisticated computed assembling test system (CATS) to measure the structure and compare with the geometry generated in the computer drawing and manufacturing system. The virtual assembly provided a complete geometric record of the structure. The detailed records and information generated as well as the improved accuracy reduces the need for and the cost of owner's quality control. Owners in all of the countries visited are accepting virtual shop erection to various degrees. This ranged from 10-20% in Japan and Italy to most of the fabrication in Germany and England.

* Evaluate existing measurement technologies.

* Explore feasibility of digital geometric measurements on fabricated components for virtual assembly in lieu of preassembly.

* Carry out a pilot project, working closely with owners to ensure their confidence in the virtual approach.

HIGH-PERFORMANCE STEELS AND COATINGS

The steels used for bridges around the world are governed either by individual country specifications or by the new Eurocode. The yield strength levels are very similar to those used in the U.S. and 50 ksi is the most common strength grade except in Germany where 36 ksi is used. All of the countries have a steel grade in the 65 to 70 ksi strength ranges and report growing interest in its use. Thermo-Mechanical control processing (TMCP) is used by all countries for higher strength levels and is aggressively used in Japan to improve weldability and toughness. Quench and temper (Q & T) processing is used for steels from 70 to 100 ksi, although steels greater than 70 ksi are rarely used. Weathering steel is used in all countries visited and there are initiatives to increase its use. Japan has developed several new higher alloy "seaside" weathering steels for areas close to the coast. For painted bridges, zinc rich primers are commonly used in all countries, and the U.K. is using aluminum metallizing instead of primer. Tapered plates have been used in several countries, but it is not clear if this practice is economical.

* Study applicability of "seaside" weathering steels for U.S. marine environments.

* Promote the use of a 50 ksi low carbon HPS steel for improved weldability and toughness.

CUTTING AND JOINING

Field welding in Europe and Japan is not only considered to be an acceptable alternative to bolted connections, but in some countries is the only joining method permitted. Enclosed shelters are employed that allow the use of gas shielded welding (GMAW, FCAW-G, EGW). Field welding of webs employed automated EGW, FCAW-G or GMAW-P (pulsed). Field welding of flanges used automatic SAW or GMAW. The prevalent shop and field welding with gas shielded processes use solid, metal-cored and flux-cored wire electrodes. These processes were employed in semi-automatic, automatic and robotic welding by most Japanese and European fabricators. The benefits resulting from such usage included high production, lower rejection rates and low hydrogen weld deposits. Welding through a special wash primer with flux cored welding was done in Japan. Preheat was not employed in most of the shops. The need for preheat was eliminated by the use of steels with low carbon content and low carbon equivalence, in addition to the use of low hydrogen and high heat input processes. All of the fabricators employed robots for welding stiffeners and other plate attachments to plate girder webs and box girder webs and flanges. Wrapping around the ends of stiffeners and plate attachments was the norm in all of the countries. Robots were also used for some cutting and corner grinding where necessary. A new high speed rotating arc technique developed at NKK in Japan provided enhanced weld tracking for robotic fillet welds. One-sided high deposition welds with ceramic backing bars were used for groove welds in webs and thin flanges both in the shop and field. This method eliminates turning the plate and back gouging followed by welding. More flexible specification requirements are needed to allow the implementation of these methods.

Internal QA/QC was relied upon in the shops. Owner's inspectors normally just audited the shops at the beginning of the job and at the end. Radiographic inspection has been phased out in the U.K. Japan used ultrasonic inspection for most welds while Germany only allowed radiographic inspection. The substitution of ultrasonic inspection (with record producing capability) for radiography provides faster inspection and interpretation as well as increased worker safety. Procedure qualifications were generally accepted by all owners and had an unlimited life. The ISO 9001 accreditation was accepted by the owners as proof that the fabricators quality control procedures were adequate and did not require the owner's agent for supervision.

Extensive use of cold bending of components before and after welding was employed to maintain desired straightness. Laser cutting of plates was wide spread in Japan. All fabricators employed numerically controlled cutting equipment using laser, plasma, or oxygen-fuel torches. This equipment was often run unattended with direct programming from the CAM software. The same equipment was used for marking, cutting and fit up. The larger fabricators in the U.S. employ similar equipment. The highly automated plants in Japan were set up to make thin plate rectangular box girders with stiffened flanges and webs. These boxes were fabricated into short shipping lengths because of weight and dimensional constraints.

* Develop a workshop on gas shielded welding and new methods of welding for shop and field fabrication for fabricators and owners.

* Explore using the high speed rotating arc technique with enhanced tracking for fillet welding.

* Promote the use of ultrasonic inspection with record producing capability in lieu of radiography.

* Modify welding procedure qualification requirements to allow unlimited life provided an alternative demonstrated quality program is in place.

CERTIFICATION AND CONTRACTING

All of the fabricators visited were certified in accordance with ISO 9001. The clients accept this accreditation and do not have the expense of their own inspectors in the shops. Shop certification of U.S. fabricators may be enhanced by including appropriate parts of the ISO 9001 accreditation. In most of the countries the contract with the fabricator includes both fabrication and responsibility for the erection of the structure. This type of contract eliminates the conflict between the fabricator and erector concerning fit up and paint damage. This allows the fabricator to choose whether or not shop assembly is required and determine the most efficient method of erection. Often the erector is a sub-contractor to the fabricator. This method of contracting should be tried in the U.S. Design-build-finance-operate and transfer projects are currently favored in the U.K. Partial payment for material and progress are typically included in contracts for large projects in both Japan and Germany.

* Set up a task group, possibly through the AASHTO/NSBA Steel Bridge Collaboration, to develop a Qualification Program for fabricators which allows them to be responsible for quality.

* Explore incorporating appropriate parts of ISO 9001 in the AISC Certification Program.

* Explore incentives to enable a fabricator to accept responsibilities for both fabrication and erection so as to achieve optimum cost and delivery.

DESIGN INNOVATION

The design practice in all of the countries visited is not encumbered by the restrictions placed upon fracture critical members in the U.S. Two-girder systems, tied arches, and other structural systems which utilize two lines of support are considered acceptable throughout Europe and Japan. Tied arches are used both for road and railroad bridges where their high stiffness with orthotropic steel roadways is an advantage for high-speed rail. Welded field splices are considered an acceptable design alternative to our traditional bolted field splices throughout Europe and Japan. Erection by launching allows the field welding to be performed without interfering with traffic. Box girders, both open (i.e., tub girders) and closed, are more evident in Europe and Japan than in the U.S. Standardization of design details such as installing stiffeners and attachments on one side of plates allows for automation in fabrication for both box and plate girders. Throughout Europe, probability-based load and resistance factor design (LRFD) is the design methodology in use. Japan continues to use allowable stress design (ASD), and fatigue is not presently considered in the design of highway bridges.

* Reexamine the design practice, FHWA directives, and AASHTO specifications related to two lines of support and fracture critical members in view of modern materials, joining and quality control developments.

* Support national programs such as the AASHTO/NSBA Steel Bridge Collaboration to develop standard design details.

Updated: 07/24/2013
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000