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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: March/April 2001|
Issue No: Vol. 64 No. 5
Date: March/April 2001
In the United States, there is a need to modernize structural steel bridge fabrication and erection technologies and upgrade fabrication shops.
Recognizing the benefits that could result from an examination of international practices, a team of steel bridge experts visited leading steel fabrication facilities in Japan, Italy, Germany, and the United Kingdom in May and June 1999. The team members represented the Federal Highway Administration (FHWA), state departments of transportation, academia, associations, and the private sector. The industry representatives included fabricators, a manufacturer of welding equipment and consumables, and a steel producer.
The Steel Bridge Fabrication and Erection Technology Scan was sponsored by FHWA's Office of International Programs and was led by representatives of the Office of Bridge Technology. The team identified several practices that may have current or future value to transportation agencies in the United States.
|Computer-aided design of girder bridge and cross frames used by Fairfield-Mabey in the United Kingdom. One of the highest priority goals of U.S., European, and Japanese fabricators is the development of a computer-integrated manufacturing software package.|
All of the fabricators had computer-aided drawing (CAD) and computer-aided manufacturing (CAM) software. These systems also included three-dimensional (3-D) 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.
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 because 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 on measured section dimensions in the 3-D 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.
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 were 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. These ranged from simple digital surveying equipment to the more sophisticated computed assembling test system (CATS) to measure the structure and compare it 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, as well as the improved accuracy, reduce the need for and the cost of the owner's quality control measures. Owners in all of the countries visited are accepting work without shop assembly to varying degrees. This ranged from 10 to 20 percent in Japan and Italy to most of the fabrication in Germany and England.
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 United States, and 50 kips per square inch (ksi) (345 megapascals [MPa]) is the most common strength grade except in Germany where 36 ksi (248 MPa) is used. All of the countries have a steel grade in the 65- to 70-ksi (448- to 482-MPa) strength range 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 (482 to 689 MPa) 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 United Kingdom 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.
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, it is the only joining method permitted. Enclosed shelters are employed that allow the use of gas-shielded welding - gas metal arc welding (GMAW), flux cored arc welding-gas (FCAW?G), and electrogas welding (EGW). Field welding of webs employed automated EGW, FCAW?G or GMAW?P (pulsed). Field welding of flanges used automatic submerged arc welding (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 use 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. Preheating was not employed in most of the shops. The need for preheating 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 standard practice 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 both in the shop and in the field for groove welds in webs and thin flanges. 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 quality assurance and quality control (QA/QC) was relied on in the shops. The owner's inspectors normally audited the shops at the beginning of the job and at the end. Radiographic inspection has been phased out in the United Kingdom. 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 constant supervision.
Extensive use of cold bending of components before and after welding was employed to maintain the desired straightness. Laser cutting of plates was widespread in Japan. All fabricators employed numerically controlled cutting equipment using laser, plasma, or oxygen?fueled torches. This equipment was often allowed to 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 United States 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.
|Scan Team Members|
|Krishna K. Verma (Chair)||FHWA, Washington, D.C.|
|Ralph E. Anderson||Illinois Dept. of Transportation (DOT)|
|Hardy Campbell||American Welding Society|
|Milo Cress||FHWA, Lincoln, Neb.|
|John W. Fisher (Reporter)||Lehigh University|
|Karl H. Frank (Reporter)||University of Texas, Austin|
|James Hamilton||Utah Pacific Bridge|
|Robert Kase||High Steel Structures|
|Kathleen Linehan||FHWA, Washington, D.C.|
|Pat Loftus||High Steel Structures|
|William McEleney||National Steel Bridge Alliance, R.I.|
|Ronald Medlock||Texas DOT|
|Dennis Mertz||University of Delaware|
|Randy Sathre||PDM Bridge, Wasau, Wis.|
|Arun Shirole||National Steel Bridge Alliance, Minn.|
|Jerry Uttrachi||ESAB Welding and Cutting Products|
|Alexandar D. Wilson||Bethlehem Steel Corp.|
|William J. Wright||
FHWA, McLean, Va.
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 having 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 the erector concerning fit-up and paint damage. This allows the fabricator to choose whether or not shop assembly is required and to determine the most efficient method of erection. Often the erector is a subcontractor to the fabricator. This method of contracting should be tried in the United States. Design?build?finance?operate and transfer projects are currently favored in the United Kingdom. Partial payment for material and progress is typically included in contracts for large projects in both Japan and Germany.
The design practice in all of the countries visited is not encumbered by the restrictions placed on fracture-critical members in the United States. Two?girder systems, tied arches, and other structural systems that use two lines of support are considered acceptable throughout Europe and Japan. Tied arches are used for both road and railroad bridges. Their high stiffness with orthotropic steel decks also makes them suitable for high?speed rail. Welded field splices are considered throughout Europe and Japan to be an acceptable design alternative to the traditional bolted field splices. 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 United States. Standardization of design details such as installing stiffeners and attachments on one side of the 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 method in use. Japan continues to use allowable stress design (ASD), and fatigue is not presently considered in the design of highway bridges.
In April 2001, FHWA's Office of Bridge Technology, the American Association of State Highway and Transportation Officials (AASHTO), and the National Steel Bridge Alliance (NSBA) will sponsor a follow-up workshop on computer-integrated steel bridge design, fabrication, and construction. The workshop will be hosted by Edison Welding Institute in Ohio and will include speakers from the foreign fabricators visited on the tour.
After the tour, the team members produced a final report entitled Steel Bridge Fabrication Technologies in Europe and Japan. The report is available online at www.international.fhwa.dot.gov, and in hard copy by e-mailing the Office of International Programs at email@example.com, or calling (202) 366-9636.
Krishna K. Verma is a senior welding engineer with FHWA's Office of Bridge Technology in Washington, D.C. He is responsible for policies regarding bridge welding, fabrication, fatigue, fracture, and bridge painting. He is currently serving as a member of the AASHTO T-17 Committee on Welding, the AASHTO-AWS-D-1.5 Bridge Welding Code Committee, the TRB Committee on Fabrication and Inspection of Metal Structures (A2F07), the AASHTO-NSBA Steel Bridge Collaboration, and the International Institute of Welding's Commission V on Quality Control and Quality Assurance of Welded Products and Commission XIII on Fatigue Behavior of Welded Components and Structures. Verma has a bachelor's degree from Benares, India; a master's degree in structures from the University of Calgary, Canada; and a master's degree in materials engineering from the Rensselaer Polytechnic Institute of Troy, N.Y. Verma is a registered professional engineer in Pennsylvania.
|Recommendations for Implementation in the United States
Computer-Aided Design/Computer-Aided Manufacturing