Challenges and Opportunities of Welding Heavy Structural Steel

Welding process, manufacturing methods and industry standards are the key to the development of steel structure.

In the past ten years, the construction of the iconic steel structure has achieved tremendous growth. Architects have been using the ability of steel to meet any architectural concept and configuration, and structural engineers have been able to design steel frames to meet the challenges. Engineers, manufacturers, installers and detailed designers have strengthened collaboration on the complex, multi-faceted connections required to design, manufacture and assemble these structures.

In addition, the market requires more open space in its structure, thereby increasing the need to use giant shapes and thick plates to carry loads. The size of these structural members and the extensive use of welding to manufacture and connect them place increasing demands on materials and welding processes.

This type of work poses many challenges not only for designers but also for manufacturers, erecting equipment, welders, inspectors, and non-destructive inspection (NDE) technicians who are committed to providing and ensuring safe structures. Heavy-duty steel structures are more prone to welding-related fractures, layered tears and brittle fractures. The need to build these structures at a faster speed and in a more economical manner is greater than ever before.

These challenges should be viewed as opportunities to use new structural steel systems, higher quality steels, advanced and more effective welding processes, improved welding supervision and management, and new inspection and NDE technologies. To realize these opportunities requires not only knowledge and innovation, but also design and structural welding specifications that enable and solve these problems.
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1. Production and construction speed - The speed of construction was emphasized today. Owners and users want to use their structure as soon as possible, and time is money. In order to speed up the construction, the constructability should be considered as part of the design. In heavy-duty welded structures, the choice of welding process and technology may have a significant impact on speed and workability.​
2. Narrow gap electroslag welding (ESW-NG) - Electroslag welding (ESW) has many advantages when welding thick steel, including high deposition rate, less internal discontinuity, minimum joint preparation, minimum deformation (especially angular deformation), no process between passes Cleaning, little or no preheating, etc. . ESW can be used for butt joints, T-shaped and corner joints, and the thickness can also be as thin as 19 mm (3⁄4 inch). For a complete discussion, see Volume 1 of the AWS Welding Manual. 2. Part 1, Chapter 10. In the 1970s, when problems with ES welded joints were found in several steel bridges, electroslag welding was no longer popular. Subsequent research on ESW led to an improved method, called narrow-gap electroslag welding (ESW-NG), which has more reliable mechanical properties, including higher notch toughness in welds and heat affected zones. AASHTO / AWS D1.5M / D1.5: 2010, "Bridge Welding Code", which combines the results of this research and process development to put ESW back into industrial use, and added a section F on ESW in Article 4 "Technology"; About the procedure New clause 5.14 for qualification; and some attachments to define ESW-NG and its consumables and operating parameters. The AWS D1 Structural Welding Committee continues to improve these regulations in subsequent versions. AWS D1.1 / D1.1M (Steel Structure Welding Specification) did not include the details of ESW-NG in its 2015 or 2020 version. In order to save costs, improve speed and improve quality, manufacturers and installers should consider using ESW-NG in construction applications in accordance with the "Bridge Welding Code", and engineers should support this. The challenge for the AWS D1 Structural Welding Committee is to incorporate ESW-NG-specific provisions into the Structural Welding Code (steel) and AWS D1.8 Structural Welding Code (seismic supplement) to address static, periodic, or seismic loads. Narrow-gauge electroslag welding has been successfully used to weld gusset plates to embedded panels and box columns to connect the truss-level seismic buckling brace at the Wilshire Hotel in downtown Los Angeles, California. The gussets of the embedded board are 23⁄4 inches (70 mm) thick and 10 feet (3 m) long and are welded at a time. ESW-NG is also used for many other on-site welding connections, with plate and wide flange member connections ranging in thickness from 21⁄4 inches (57 mm) to nearly 5 inches (125 mm).
3. Automatic and mechanized processes and adaptive control - The need for more skilled welders is well known. Use crawlers, gantry frames and similar equipment to transfer welding processes such as flux cored wire (FCAW) and gas metal arc welding from semi-automatic processes (handheld) to mechanized processes (equipment requires manual adjustment by the operator). Torches, guns, wire assemblies or electrode holders fixed by mechanical equipment or automatic processes (equipment that requires occasional observation or no observation and no manual adjustment during operation) can improve productivity and quality while reducing welder fatigue. Skilled and knowledgeable welders are still required as welding operators, so no work will be lost in this process. One of the challenges in the processing workshop or on-site is that, compared with the use of manual or semi-automatic welding, the root joint of partial joint welding (PJP) and full joint welding groove welding requires stricter tolerances to achieve mechanical or automatic Welding welding. For fillet welding, if the root opening between the connected parts exceeds 1⁄16 inch (2 mm), the foot size of the foot must be increased. When welding thicker parts, the control and correction of assembly accuracy and alignment is particularly challenging. Vision systems and other forms of adaptive control can be implemented to overcome this type of assembly problem, adjust the width and position of the root bead as needed, and continuously adjust the weld from the root to the final layer.
4. SpeedCore system for steel structures - The Rainier Plaza Tower in Seattle, Washington is a 58-story, 850-foot (400 m) multi-purpose office building with high-rise residences and a new innovative technology-the core system of concrete-filled composite steel plate shear walls. . This system replaces the slower sliding-formed reinforced concrete core previously used in high-rise buildings. The installation of the steel frame took only 10 months, which is 43% less time than using a concrete core. The typical speed of steel erection is one story every three to five days, but the new system can reach two stories (four stories) per week. As this new core system, SpeedCore uses a factory-made sandwich panel that has two structural steel plates with a tie rod fixed between them. The panel is self-supporting during the construction process, and a steel structure can be erected on the surrounding steel and floor. The concrete on the panel is below the level of the steel frame, usually four to six layers behind the steel frame, which is about the time the concrete is placed on the steel floor deck. For Rainier Square, the thickness of the sandwich panel is usually 1⁄2 inch (13 mm), about 3⁄4 inch (19 mm). At the level of the band truss with buckling constraints, the plate thickness is 2 inches (51 mm). In the workshop, approximately 400,000 rods with a diameter of 1⁄2 inch (13 mm) are welded to the board to form the board, and the outside of the board is automatically welded by fillet welds. At the site, PJP groove welds are used to connect the sandwich panels horizontally and vertically, and reinforced fillet welds are used to abut against the steel bars welded to the surface of the outer panel to provide full strength connections. An internal steel backing is used, whose bent portion extends beyond the root of the weld to serve as an alignment aid during installation. Due to joint assembly tolerances, 36 miles (58 kilometers) of FCAW field welding were all done manually. Compared with the old reinforced concrete core system, the SpeedCore system can save a lot of time, thereby saving costs. The advantage of the steel structure is that it can better manage the construction tolerance of the entire structure. Welding plays an important role in workshop manufacturing and on-site installation. Adaptive welding control may further accelerate the speed of workshop welding and on-site welding. Because the system is non-proprietary, it can be designed, manufactured, and installed without restrictions.
5. Opportunity welding supervision - A well-known theme in the industry is "You build quality without checking quality". In order to take advantage of the opportunity to save welding time and money, and to perform high-quality welding at the same time, the industry needs educated welding supervisors and staff capable of job management. The supervisor is the supervisor who finds the problem before or at the same time, and can take necessary steps to solve the problem in time. AWS certified welding inspectors usually hire qualified welding supervisors, but may require other expertise in production, such as equipment and welding technology. This is an opportunity for manufacturers and assemblers to increase efficiency, reduce errors and improve quality and bottom line. AWS has released two standards for welding supervision: AWS B5.9 (Welding Supervisor Qualification Specification) and AWS QC13 (Welding Supervisor Certification Specification). The AWS report states that certified welding supervisors "can help reduce welding costs, increase productivity and profitability, and make the company more competitive, saving an average of $ 17,000 per welder per year." Manufacturers and installers certification bodies should consider Qualified welding supervisors are added to personnel descriptions and requirements. In addition to AWS standards, ISO 14731: 2019 (Welding Coordination—Tasks and Responsibilities) and CSA W47.1 (Steel Fusion Welding Company Certification) are excellent resources.
6. Take full advantage of the potential of PAUT (and other defect adjustment methods) - AWS D1.1 introduced ultrasonic testing to the Structural Welding Specification in 1972, and since then it has made many improvements. AWS D1.1 / D1.1M: 2020, by providing detailed normative Appendix H, phased array ultrasound testing (PAUT) was first introduced in the code. Advanced Ultrasound System Section 8.34 allows the use of PAUT. Although PAUT has been used successfully since its inception, the acceptance criteria in AWS D1.1 are still driven by process and detectability. Only limited to 45 degree, 60 degree and 70 degree scans; modern structural performance methods have not yet been incorporated. The use of PAUT provides an opportunity to establish acceptance or rejection decisions based on real engineering-based performance requirements. With the continuous improvement of current engineering design, the industry should strive to use PAUT to take advantage of its true potential for identifying defects and for more accurate characterization of these defects. PAUT can better ensure the structural integrity and reduce unnecessary maintenance time and costs.
7. Challenge brittle fracture - The increasing use of heavy-duty shapes and thick plates, combined with complex details, has increasingly drawn attention to the possibility of brittle fracture of steel structures. Although brittle fractures are not new, and such cracks are relatively rare, cracks in the bottom flanges of the two slab beams of the new Transbay Transit Center in San Francisco, California in 2018 have raised new concerns about this issue. As part of the Metropolitan Transportation Commission ’s peer review team report (mtc.ca.gov/sites/default/files/PRP_Final_Report.pdf), a risk assessment method was proposed for industry development that should take into account the consequences of the failure and Assessment of risks. The following factors that lead to brittle fracture: Fragile materials with low fracture toughness, especially near the medium thickness area of ​​thick materials, may be locally affected by steel composition, steel manufacturing method, low service temperature, fast loading rate or other operations (such as forming or overheating) ; Local materials formed by rapidly cooling the surface of the hot-cut edge, such as hard and brittle martensite;​ Sufficient stress may come from the applied load and / or welding shrinkage, thermal heating, residual stress from thermal cutting or forming; Geometric stress concentration, such as concave corners and transitions, whether square or radius; Initiating cracks, such as micro and macro cracks and inclusions or notches, these stresses will obviously concentrate the stress; and the inherent constraints of thick materials, but also caused by the intersection of triaxial and biaxial welds, which limits or prevents The ductile mode of operation is necessary for the redistribution of stress.
8. Welding of steel produced by new methods - Current structural welding specifications focus on carbon and low-alloy structural steels and add requirements to quenched and tempered steels, such as ASTM A514, the standard specification for high-strength, quenched and tempered alloy steel plates, suitable for welding. Quenching and self-tempering (QST) steels specified in ASTM A913 "Standard Specification for Structural Strength, High-Strength, and Low-Alloy Steel Shapes" produced by the quenching and self-tempering process (QST) provide specific rules. However, more and more steels use unique rolling and heat treatment processes and special alloy compositions to obtain better structural properties, and these steels are not directly or indirectly involved in existing welding codes and standards.

​With the development of new equipment, consumables, processes and technology, welding will continue to develop. The steel structure will also continue to evolve with the development of new steel, structural systems, connection types, and manufacturing and installation methods. The synergy and expansion of these two disciplines will continue to use welded steel to create amazing structures. It will not be without challenges and problems, but we know from history that the industry will rise to overcome these challenges and solve any problems that arise. Seizing opportunities and meeting challenges will result in better, safer, faster and more cost-effective structures.
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Reference: fabtechexpo