How robotic welding helped improve steel erection
March 8, 2005
|Two six-axis robot arms perform full-penetration gas metal arc welds on structural steel.|
Millions of construction companies strive every day to cut production time while maintaining, or even improving, quality.
Robert J. Simmons owns a company that has made it happen.
Over several years he found a way to cut structural erection time from seven to eight months to a matter of weeks by combining several technologies, including robotic equipment that helps reduce the amount of welding required on-site.
A 30-year veteran of the structural concrete industry, Simmons developed a concept for constructing midrise residential structures using a steel moment space frame system in the winter of 2000.
The structural system—a proprietary bolted collar system for interconnecting columns and beams—as well as the holding company for the technology are called Simmons Moment Resisting Space Frame (SMRSF). The system condenses a structural system down to its basic components: a column, a beam, and a joint comprising two interlocking plates that are self-aligning and self-positioning to enable rapid in-field assembly.
Simmons then integrated the system with other technologies he has developed and systemized to fabricate building components in-house and then erect those components at the construction site. He called this systemized approach ConXtech™.
During his years in the structural concrete industry, Simmons has focused primarily on design-build parking structures and high-rise residential projects. Before developing ConX™, the components used in the ConXtech system, he founded RJS and Associates, a structural concrete firm responsible for designing, building, and orchestrating the structural concrete components of many projects in the San Francisco Bay Area.
|ConXtech's SMRSF technology is suitable for mid- and highrise structures.|
Since his late teens Simmons wanted to create a viable method to systemize multifamily housing construction.
"Bringing change to the high-density housing industry—which has long needed a better structural solution as densities increase and wood-framed buildings are pushed to both their structural and code limits—was not going to be easy," Simmons said. His goal was to reduce time, cost, and field labor required (when compared to conventional methods) to construct wood, traditional steel, and concrete.
He envisioned a systemized approach that streamlined on-site building processes.
In the early 1990s Simmons decided to dedicate a portion of RJS's resources to that vision.
Initially Simmons worked in concrete. Although his early projects were successful from an architectural and sales standpoint, his system couldn't reach critical mass, primarily because the concrete structural system required continuous load paths all the way down to the ground.
This created challenges to accommodate parking or retail spaces efficiently below the residential levels. The first attempts were moderately successful, and Simmons built more than 250 units of the systemized concrete frame with an architecturally detailed concrete exterior panel. However, he was aware that the requirement to line up the load paths of a residential grid with parking grids below ultimately would limit the acceptance of the concrete system.
In winter 2000 Simmons came up with a way to place a lighter-weight steel frame on a concrete podium without the requirement to align the load path of the structure above with the structure below the podium.
"This would allow the architect to have infinite flexibility in the configuration of residential units," he said. "Increasing flexibility and feasibility for architects, developers, and urban planners was imperative in order for the system to gain industry acceptance."
With that, Simmons started sketching the drop-and-click joint that now is one of the core structural frame components of the ConX system. He then enlisted the help of Walid Naja, P.E., and over the next several weeks, they further developed the core joint concept of Simmons' original sketches to what is now SMRSF. Soon after Simmons and Naja hired structural engineer Constantine Shuhaibar, Ph.D., to conduct a computer-modeled structural analysis of the frame to confirm their assumption of member sizes and plate thickness.
In addition to manufacturing, structural design, and construction services for the SMRSF system, ConXtech manufactures and integrates other building components that Simmons has systemized, such as concrete-filled metal decking, stairs, and panelized walls. This fully integrated approach makes ConX suitable for high-density (50 to 300 units per acre) and mixed-use urban environments, particularly multifamily housing, retail and residential complexes, midrise hotels, nursing homes, senior housing, and student housing.
ConXtech also developed a software tool designed to streamline the flow of information from the architect to the manufacturing floor. ConX- CAD converts architectural plans to manufacturing code. The company's automated manufacturing plant can cut columns and beams to precise size and shape and weld inner- and outer-collar plates that form the joint to their respective columns and I beams without the need to create a separate set of shop drawings that then need to be converted to manufacturing code.
Components then are manufactured and delivered to the building site as needed and assembled with virtually no on-site welding. A midrise building constructed in vertical pods is ready for follow-on trades in weeks rather than months.
But with all of the initial research, development, testing, and analysis needed to prove the concept behind him, Simmons still was challenged by one thing: how to fabricate structural steel components efficiently within tolerances that were unheard of in the industry. Many industry experts he consulted said it couldn't be done.
Welding automation was the next step in Simmons' journey.
ConXtech's 25 employees manufacture a building's structural columns and beams in-house so everything plumbs and aligns automatically at the construction site. Accuracy is imperative; the company requires precision down to 0.006 inch for all of its fabrication processes.
Semiautomatic welding of one collar piece to a beam required 40 minutes. With two ends to each beam, this equaled one hour and 20 minutes of welding time per beam.
Simmons understood that his system could succeed only if those precise components could be produced cost-effectively and accurately, and assembled quickly and safely in the field.
But to get to that point, Simmons faced many challenges: peer reviews; extensive, full-scale testing while simultaneously setting up a factory; designing software tools for architects; and ramping up sales and marketing.
One of the most challenging hurdles was the peer review and testing process, which required that all automated systems needed to be up and running to produce "production" components before testing could be completed.
|Bolted collars are shop-welded—24 in. of full-penetration welds and 64 in. of fillet welds.|
Initially Simmons planned for a third party to manufacture the components. The largest manufacturing challenge was that no one in the structural steel fabricating industry was willing to match the very specific tolerances required. Other industries that could meet the specifications still worked on a batch-and-queue basis. That meant buying 10,000 of each component at a time, just to make those components economically viable.
To achieve just-in-time (JIT) delivery and the precision manufacturing demanded by Simmons' system, the only option was to bring all the manufacturing in-house. Automation was essential.
"Although many of the technologies employed at ConXtech have been proven over years in other industries, I wanted to achieve a revolutionary kind of manufacturing for the structural steel industry," Simmons said.
This included buying specialized equipment, designing custom tools and fixtures, and implementing robotic welding.
"My goal was to design a just-in-time manufacturing facility similar to Toyota's approach of lean manufacturing," Simmons said.
With lean manufacturing as the goal, Simmons set out to learn as much as possible about automation as it applies to steel fabrication. His decisions were based primarily on what equipment truly was state-of-the-art, as well as Simmons' comfort level in the vendor's ability to provide programming support and service.
As part of his research, Simmons read several books—including The Machine That Changed the World by James P. Womack and Daniel T. Jones—studied magazines, and researched on the Internet. Early on he attended an American Institute of Steel Construction (AISC) conference, which provided him with knowledge and contacts, and manufacturing tradeshows. After talking and visiting with many equipment vendors and their customers, he selected the following equipment:
The robotic equipment includes two Power Wave® 455M power sources from Lincoln Electric mated with FANUC® 120iLB six-axis robots to weld the beams and collar pieces together. This robotic system offers faster travel speeds, higher deposition rates, and higher-quality finished welds than what the company could achieve with through semiautomatic welding.
With the outside vendor-supplied systems for machining and milling, cutting and drilling, and robotic welding in sight, Simmons still faced the challenge of welding inner-collar connection components onto hollow structural steel (HSS) columns in the flat position—in a production environment. According to the company, it's the first manufacturing facility in the world to do this.
Several other obstacles also stood in the company's way. For example, the company discovered that the tolerance standards used by HSS column manufacturers were unsuitable for a precise and highly automated manufacturing environment. A typical 60-ft. HSS column can be delivered to the facility with a significant amount of sweep—as much as 1.5 in. from top to bottom. The twist on a column of the same length could be up to 2 in. from top to bottom. Still, this level of imprecision was acceptable per steel industry tolerance standards.
Simmons developed a fixturing method that transforms the column into a straighter structural member while the inner-collar components of the column are being welded. This fixturing allows the column to meet the company's tolerance requirements.
Today A992 structural I beams arrive at the factory as 12- by 9-in. or 12- by 30-in. rolled sections. They are cut to length, drilled, and stamped in the Ficep line. Those beams then move to a staging area where female dovetailed outer-collar plates are clamped temporarily to each end of the beam and readied for robotic welding.
The beams then move into a customized fixture designed to accommodate the heat-induced expansion and contraction that take place during the welding process. The result is a welded beam/outer-collar assembly that adheres to the company's strict tolerances.
"Preston-Eastin customized and reconfigured some of their more standard headstock and tailstock products into an adjustable system, to which we could mount our own fixturing and robots as the final robotic welding process was developed," Simmons said.
The cell's rotating headstock and tailstock are on linear guides and use brackets to align beams precisely, accommodating lengths from 8 to 20 ft. The end plates act as a clamp to keep beam flanges parallel. This fixturing positions the beams and outer-collar components so they are always within tolerance—if they aren't, the robot won't function.
All beam assemblies are gas metal arc welded (GMAW) with Lincoln Electric's Power Wave 455M, a digitally controlled inverter that uses Waveform Control Technology™ to control and shape the output waveform. The power source also offers arc-starting procedures that help reduce the risk of starting porosity and contribute to a flat weld bead profile.
The assembly requires full-penetration welds on the top and bottom flanges and fillet welds on the beam's web and the back side of the flanges. The 24 in. of full-penetration welds on each beam is made in four passes; the 64 in. of fillet welds is completed in a single pass. Simmons said travel speeds are 12 to 25 in. per minute (IPM), and deposition rates are 13.8 lbs. per hour of weld metal.
The company uses its power source's digital communication platform to obtain real-time production monitoring of arc current, voltage, and wire feed speed (WFS) at the robotic station. In the near future Simmons plans to tap into production monitoring via a computer connection (Ethernet) to his laptop, which is a capability Lincoln currently offers on its family of Power Wave power sources.
Once the robots were "trained," the repetitive nature of robotic welding ensured consistency in each weld. Although the welds produced by the robots are now repetitive and highly reliable, the company still has a stringent quality control process. Building owners assign an independent certified welding inspector to conduct ultrasonic testing on the welds produced in the company's factory. In conventional steel construction, much of this owner-sponsored inspection would take place in the field.
Currently the in-factory pass rate is approximately 998 for every 1,000 welds. Over time, as more welds are robotically and consistently produced, the amount of independent inspection required will be reduced.
"Because of the large amount of welding required and the tight tolerances needed, our system would be economically unfeasible without the level of automation we've achieved in our factory," Simmons said.
"To date, we have produced over 10,000 joints without a single assembly that didn't fit in the field," he said.
In addition to precision, production speed is important so the factory can meet its goal of welding 80 beams and collar assemblies during an eight-hour shift. Robotic welding and an overhead crane system enable one operator to load and unload the beam assemblies from the fixture and operate the entire cell in a continuous flow, thus meeting the company's daily production goals.
The time saved and quality realized from robotic welding are substantial, Simmons said. Not only does robotic welding contribute to a more productive factory, but it allows ConXtech to create a predictable, JIT flow of materials to the field, where erection speed, safety, and minimizing disruption and on-site disorder are advantages.
"Most important, it creates a higher-quality end product than most conventional building methods and materials," Simmons said.
For now columns and inner plates with the male dovetail still are welded semiautomatically with GMAW on Lincoln Electric DC-655 power sources and LN-10 wire feeders, but Simmons hopes to move this operation into a fully automatic robotic station in the near future.
"My goal is to stay up on new automation as it is developed and implement, adapt, or augment it to suit ConXtech's requirements," Simmons said. To date, much of the automation implemented has required customization or additional innovation on Simmons' part to make it feasible.
Eventually the company plans to finish the exterior panels completely in the factory and ship them to the building site.
"These panels will be prebuilt to the architect's specification with windows, trim, cornices, balconies, and complete exterior finish," Simmons said. "This exterior panel line will be automated to the fullest extent possible, including robotic application of the acrylic exterior finish."
Seismic Requirements and the SMRSF™ System
When Robert Simmons started developing his proprietary bolted collar system, he was on the right track—except for one thing. "I needed to reduce the overall weight of the system to more efficiently manage the seismic loads," Simmons said. "Being based in one of the most seismically demanding areas of the world, I decided the best solution would be to place a lighter-weight frame on a concrete podium, thus disassociating the frame above the podium from the parking or retail structure below the podium."
As he worked through the subsequent design details, he enlisted help to conduct a computer-modeled structural analysis of the frame to confirm the core assumptions and seismic load management performance attributes of the system. This testing was required to prequalify for the latest Federal Emergency Management Agency (FEMA) and current American Institute of Steel Construction (AISC) seismic provisions.
The first building using ConX components was erected in April 2004 in San Jose's Santana Row. This four-story residential building, with retail space and four parking decks below, is significant because it is a steel reconstruction of a wood frame structure that burned down in the largest fire in San Jose history. All welding on the project was done to the AISC Seismic Provision.
Although the ConXtech system can be used in any geographic area, it meets demanding detailing and energy-dissipation requirements for geographies with high seismic activity.
The ConX building frame uses two different connections. In demanding seismic zones, the core of the building is framed using the SMRSF moment connection. The company also has developed a gravity connection that frames the perimeter of the building. Typically, the perimeter of a building doesn't have the same demanding requirements for seismic loading as the interior or core of the building. ConXtech's gravity frame uses smaller hollow structural steel (HSS) columns (4 in. by 4 in. for gravity columns versus 8 in. by 8 in. for moment columns), as well as a connection that drops and clicks into place.
Simmons said that the bolted collar system helps buildings erected in this fashion to be structurally superior to wood frame structures and handle severe lateral loads better, such as those produced by an earthquake. This is an important factor in the San Francisco Bay Area, where seismic activity is a constant concern for architects, engineers, and builders.
Typically, a wood structure can be built only to 50 to 65 feet, but the ConXtech system allows for much taller buildings at a price competitive to wood's. Even in a Seismic Zone 4, a building that's at least eight stories and utilizes the system can be erected above a podium without conventional shear walls, brace frames, or incurring any code-related penalties. In a nonseismic zone, taller structures can be built without shear walls or brace frames.
The connection system has been subjected to 17 full-scale tests at the University of Arizona at Tucson's seismic test lab. It also has been scrutinized by two peer review panels appointed by the cities of San Francisco, San Jose, and Palo Alto, Calif., where the first six ConXtech buildings will be erected. According to Simmons, these panels determined that the SMRSF framing system qualified as an acceptable alternative to a special moment frame system and prequalified it for use in accordance with the 2002 AISC Seismic Provision.
ConXtech Inc., 24493 Clawiter Road, Hayward, CA 94545, 510-264-9111, fax 510-264-1181
FANUC Robotics, 3900 W. Hamlin Road, Rochester Hills, MI 48309-3253, 800-477-6268, www.fanucrobotics.com
Ficep SpA, Via G. Matteotti, 21. 21045 Gazzada Schianno, Italy, +39-0332-876111, fax +39-0332-462459, www.ficep.it
The Lincoln Electric Co., 22801 St. Clair Ave., Cleveland, OH 44117, 216-481-8100, fax 216-486-1751, www.lincoln electric.com
Mazak, 8025 Production Drive, Florence, KY 41042, 859-342-1700, fax 859-342-0285, www.mazakusa.com
Preston-Eastin Inc., P.O. Box 582288, Tulsa, OK 74158, 800-615-5432, fax 918-834-5595, www.prestoneastin.com