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Challenges and Opportunities for Welding Heavy Structural Steel
Updates to welding processes, fabrication methods, and industry standards are key to the evolution of steel construction By Robert E. Shaw Jr. Reprinted with permission: The AWS Welding Journal The last decade has seen tremendous growth in the construction of iconic steel structures. Architects have been using the ability of steel to satisfy any architectural concept and configuration, and structural engineers have been able to design the steel frames to meet the challenge. There has been increased collaboration between engineers, fabricators, erectors, and detailers to design, fabricate, and assemble the complex, multifaceted connections these structures demand.
In addition, the market is demanding more open spaces in their structures, increasing the use of jumbo shapes and thick plates to carry the load. The size of these structural members and the significant use of welding to make and join them has put increasing demands on the materials and welding process.
This type of work produces many challenges for not only the designers but also for the fabricators, erectors, welders, inspectors, and nondestructive examination (NDE) technicians working to provide and ensure safe structures. Heavy steel structures are at greater risk of weld-related cracking, lamellar tearing, and brittle fracture. There is also an increasing demand to build these structures at a faster pace and more economically than ever before.
These challenges should be seen as opportunities for using new structural steel systems, better quality steels, advanced and more efficient welding processes, improved welding supervision and management, and new inspection and NDE technologies. The implementation of these opportunities requires not only knowledge and innovation, but also design and structural welding codes that enable and address them.
Speed of Production and Construction
Speed of construction is emphasized today. Owners and users want their structures to be in use as soon as possible, and time is money. To speed up construction, constructability should be considered as part of design. In heavy welded structures, choices made in welding process and technique can have a significant effect on both speed and constructability.
Narrow Gap Electroslag Welding (ESW-NG)
Electroslag welding (ESW) offers numerous advantages when welding thick steel, including high deposition rates, fewer internal discontinuities, minimal joint preparation, minimal distortion (especially angular distortion), no in-process cleaning between passes, little or no preheat, and so on. ESW can be used for butt, T-, and corner joints, as well as in thicknesses as thin as approximately 3⁄4 in. (19 mm). A complete discussion can be found in the American Welding Society (AWS) Welding Handbook, Vol. 2, Part 1, Chapter 10.
Electroslag welding had fallen out of favor in the 1970s when problems in ES welded splices were found in several steel bridges. Subsequent research into ESW resulted in an improved method termed narrow gap electroslag welding (ESW-NG), with more reliable mechanical properties, including higher notch toughness in the weld and heat-affected zone. AASHTO/AWS D1.5M/
D1.5:2010, Bridge Welding Code, incorporated the results of this research and process development to return ESW to industry use, adding a new Part F on ESW in clause 4, Technique; a new clause 5.14 on procedure qualification; and several annexes to define ESW-NG along with its consumables and operating parameters. The AWS D1 Committee on Structural Welding has continued to improve these provisions in subsequent editions.
AWS D1.1/D1.1M, Structural Welding Code—Steel, has not incorporated the specifics of ESW-NG into either its 2015 or 2020 editions. For economy, speed, and quality, fabricators and erectors should consider using ESW-NG for building applications following the provisions of the Bridge Welding Code, and engineers should be supportive of these efforts. The challenge to the AWS D1 Committee on Structural Welding is to incorporate provisions specific to ESW-NG into the Structural Welding Code — Steel and the AWS D1.8 Structural Welding Code — Seismic Supplement that address variations, if any, for static, cyclic, or seismic loading.
Narrow gap electroslag welding was successfully used for the welding of gusset plates to embed plates and box columns for connecting seismic buckling-restrained braces at the belt truss level of the Wilshire Grand Hotel in downtown Los Angeles, Calif. Gusset plates at the embedded plates were 23⁄4 in. (70 mm) thick and 10 ft (3 m) long and welded in a single pass. ESW-NG was used on numerous other field-welded connections, with plate and wide-flange member connections ranging from 21⁄4 in. (57 mm) to nearly 5 in. (125 mm) in thickness.
Automatic and Mechanized Processes and Adaptive Controls
The need for more skilled welders is well known. The use of tracks, gantries, and similar devices to move welding processes such as flux cored arc welding (FCAW) and gas metal arc welding from a semiautomatic process (handheld) to a mechanized process (with equipment requiring manual adjustment by an operator in response to visual observation, with the torch, gun, wire guide assembly, or electrode holder held by a mechanical device) or an automatic process (with equipment requiring occasional or no observation and no manual adjustment during its operation) can improve productivity and quality while reducing welder fatigue. The skilled and knowledgeable welder is still needed to serve as the welding operator, so no job is lost in the process.
One of the challenges in a fabrication shop or on a job site is the requirement for tighter tolerances on root fitup for partial joint penetration (PJP) and complete joint penetration groove welds when using mechanized or automatic welding, compared to that permitted for manual or semiautomatic welding. For fillet welds, the weld must have leg dimensions increased should the root opening between the connected parts exceed 1⁄16 in. (2 mm). When welding thick sections, control and correction of fitup accuracy and alignment are particularly challenging.
Vision systems and other forms of adaptive controls can be implemented to overcome such fitup issues, adjusting root pass bead width and placement as needed, and continuing to adjust as the weld progresses from root to final layer.
SpeedCore System for Steel Construction
The Rainier Square Tower in Seattle, Wash., a 58-story, 850-ft (400-m) mixed-use office building with residential occupancy in the upper levels, utilized a new innovation — a concrete-filled composite steel plate shear wall core system. This system replaces the slower slip-formed reinforced concrete core previously used in tall buildings. Steel erection of the structural frame took only ten months, 43% less than the time required if using a concrete core. The typical pace for steel erection was a story every three to five days, but a pace of two tiers (four stories) per week can be achieved using the new system.
SpeedCore, as this new core system is referred to, uses shop-fabricated sandwich panels with two structural steel plates held in place with tie rods between the plates. The panels are self-supporting during construction, enabling steel erection of the surrounding steel and floor decking. The panels are filled with concrete below the steel erection level, typically four to six stories behind the steelwork, about the time the concrete is placed on the steel floor deck.
For Rainier Square, the sandwich plates were commonly 1⁄2 in. (13 mm) thick, with some 3⁄4 in. (19 mm) thick. At the belt-truss level with buckling-restrained braces, the plate thickness was 2 in. (51 mm). In the shop, approximately 400,000 cross-ties of 1⁄2-in (13-mm) diameter rod were welded to the plates to form the panels, with fillet welds made robotically to the outside of the plate. In the field, the sandwich plates were joined both horizontally and vertically using PJP groove welds, with a reinforcing fillet weld against a bar welded to the outside plate surface, providing a full-strength connection. Interior steel backing was used with a bent portion beyond the weld root to serve as an alignment aid during erection. A total of 36 miles (58 km) of FCAW field welding was completed, all by hand because of joint fitup tolerances.
The SpeedCore system offers significant time savings, hence cost savings, when compared to the old reinforced concrete core system, with the benefit of steel construction that better manages construction tolerances for the entire structure. Welding plays a significant part in both shop fabrication and field erection. There is potential for adaptive welding controls to speed up the shop and field welding operations even further. Because the system is nonproprietary, it can be designed, fabricated, and erected without restrictions.
Opportunities Welding Supervision
A well-known theme in the industry is that “you build quality in, you don’t inspect it in.” To take advantage of the opportunities to save time and money in welding, and to make high-quality welds at the same time, the industry needs educated, competent welding supervisors who manage the work and the people who do the work. Supervisors are the ones who see problems before or while they occur, and can take the necessary steps to address the issue in a timely manner. AWS Certified Welding Inspectors often make competent welding supervisors, but may need additional expertise in production aspects such as equipment and welding techniques.
This is an opportunity for fabricators and erectors to improve efficiency, reduce errors, and improve both their quality and their bottom line. AWS published two standards on welding supervision: AWS B5.9, Specification for the Qualification of Welding Supervisors, and AWS QC13, Specification for the Certification of Welding Supervisors. AWS reports that a Certified Welding Supervisor “helps reduce welding costs, helps increase productivity and profitability, and helps make a company more competitive by an average of $17,000 per welder per year.”
Fabricator and erector certification bodies should consider the addition of competent welding supervisors to their personnel descriptions and requirements. In addition to the AWS standards, ISO 14731:2019, Welding coordination—Tasks and responsibilities, and CSA W47.1, Certification of companies for fusion welding of steel, are excellent resources.
Use of PAUT (and Other Flaw-Sizing Methods) to Its Full Potential
AWS D1.1 introduced ultrasonic testing to the Structural Welding Code in 1972, and it has undergone many improvements since that time. AWS D1.1/D1.1M:2020 has introduced phased array ultrasonic testing (PAUT) to the code for the first time by providing a detailed normative Annex H. The use of PAUT is permitted under subclause 8.34, Advanced Ultrasonic Systems.
Although PAUT has been used successfully since its inception, the acceptance criteria in AWS D1.1 remain driven by workmanship and detectability; are limited to scans at 45, 60, and 70 deg; and have not incorporated modern structural performance approaches. The use of PAUT presents the opportunity to establish accept or reject decisions on true engineering-based performance requirements. With the sophistication of current engineering designs, the industry should strive to employ PAUT, using its true potential for sizing flaws and more accurate characterization of those flaws. PAUT can better ensure structural integrity, and reduce the time and expense of unnecessary repairs.
Challenges Brittle Fracture
The increasing use of heavy shapes and thick plates, combined with complex details, has increased concerns for the possibility of brittle fracture in steel structures. Although brittle fracture is not new and such fractures have been relatively rare, the 2018 fractures in the bottom flange of two plate girders in the new Transbay Transit Center in San Francisco, Calif., have brought renewed attention to the issue. As part of the Report of the Metropolitan Transportation Commission Peer Review Panel (mtc.ca.gov/sites/default/files/PRP_Final_Report.pdf), recommendations were made for industry development of a risk assessment methodology that considers the consequences of failure and the following factors that contribute to brittle fracture:
- Susceptible material with low fracture toughness, particularly near the mid-thickness region of thick materials, as may be affected by steel compositions, steel manufacturing methods, low service temperatures, fast loading rates, or local degradation from other operations such as forming or excessive heating;
- Susceptible material at a local level, such as hard, brittle martensite, that is formed by the rapid cooling of the surface of thermal cut edges;
- Sufficient stress, which may be from applied loads and/or from residual stresses generated by weld shrinkage, thermal heating, thermal cutting, or forming;
- Geometric stress concentrations such as reentrant corners and transitions, whether square or with a radius;
- Initiating flaws such as cracks, both micro and macro, and inclusions or notches that serve as significant stress concentrations; and
- Constraint, inherent with thick material but also created by triaxial and biaxial intersecting welds, that limits or prohibits the material from performing in a ductile manner, which is necessary for redistribution of stress.
