THE GATEWAY ARCH

A monumental achievement of design and craftsmanship

Dedicated in 1966, the Gateway Arch anchors Jefferson National Expansion Memorial Park on the western bank of the Mississippi River. The park itself grew out of a program to upgrade a blighted riverbank in the 1930s. With dilapidated structures demolished, the barren site served for several years chiefly as a parking lot, a victim of the Great Depression and World War II.

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In 1947, a young architect named Eero Saarinen won a park design competition with his vision of a massive arch. He would not live to see the Arch completed, but in the intervening years as money was sought for the process, he and his team refined the memorial, giving it a sleeker triangle cross-section and boosting its original planned height of 560 feet by an additional 70 feet to ensure its dominance over the St. Louis skyline

The Gateway arch soars skyward, dancing as sunlight bounces off its stainless-steel skin. Each leg consists of multiple triangular sections – 142 in all –each a sandwich of the ¼-inch stainless skin and ½-inch carbon steel. Steel tension rods traverse the gap between the two substrates, with concrete filling the gap for the first 300 feet of elevation to provide support. The gap, 3 feet wide at the base of the Arch, is only 12 inches at the top. Save for internal struts used at heights above the concrete pour, the Gateway Arch has no other means of support – much like an aircraft, the Arch’s sandwich walls carry the entire load. Is it sturdy? The Arch will sway only 17 inches at the top in a 150 mph wind, say engineers.

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The hollow interior houses the passenger transportation system, a stairway with 1,076 steps, utilities, a service elevator that rises 372 feet and an 8-foot-high, 64-foot-long observation area that accommodates 160 visitors at a time. On a clear day, those who travel to the top can see 30 miles of land through one of 16 windows on each side of the viewing area. Each window measures 7 by 27 inches – window size was constrained by the pressure of the two legs pushing together at the top. Beneath the Arch, a 100,000-square-foot visitor center contains historical exhibits.

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Challenges Met

The Arch’s unique design challenged engineers and builders from the start. MacDonald Construction Co. of St. Louis was tapped to build the Arch, with Pittsburgh-Des Moines Steel Co. of Warren, Penn., fabricating the steel sandwiches for shipment by rail to the job site. These then were unloaded from recessed tracks and welded into triangles.

From there, the construction team used ingenious methods and machinery to get the job done. Eighty-ton creeper derricks, fastened by rail to the back exterior of each Arch leg, hoisted the triangle sections into place. The derricks – in use for bridges but new to building construction – would “creep” up the Arch legs all the way to the top as construction progressed. With a new section hoisted and placed, workers would make connections to the existing leg and horizontally weld the new sections into place using devices called “Buggo units” that traveled back and forth to provide continuous, spot-free horizontal welds on the exterior stainless skin.

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The balance of welding took place in the Pittsburgh-Des Moines Steel shop back in Warren, Penn. There, each side of the triangle-shaped building block sections of the arch was carefully fabricated, tested and assembled. These pre-fabricated sections included an inner steel panel wall and a structural supporting frame that allowed room for the tensioning rods that would be installed later during the erection phase to aid in tying the sections together and holding up the entire structure.

This steel ‘sandwich’ also provided support for the outer stainless steel skin. Lincoln Electric, a supplier to Pittsburgh Des Moines Steel, provided the SMAW (stick) and SAW (Submerged Arc Welding) electrodes for all of the steel welding back at the plant and also at the construction site. It is reported there is at least six times more carbon steel in the Arch than stainless steel used for the outer skin.

Once each side of the triangle sections was transported to the site by train, each went to an on-site welding shop on the ground that welded the three sides together to form one of the 142 triangle sections that would become part of the arch. Some of the smallest sections were compact enough to be transported in a more complete form via rail from the PDM Warren plant to the construction site.

All of this pre-work in the Warren Penn shop and on the ground at the site proved to be a time- and cost-saving measure. This approach also aided in worker safety and contributed to the the high quality of welds without constraints due to awkward work angles and access difficulty for men and equipment. Later, as each section was added to the two legs of the Arch, welders welded one complete triangle section to the next one hoisted into position.

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This critical welding under difficult conditions on site called for talented welding operators. Top-notch welding technicians from St. Louis and throughout the United States contributed to the project, many bringing experience from oilfield work.

“They brought welders in for ‘tryouts’ to see how they performed on a steel test piece,” says Bob Moore, an Arch historian with the National Park Service, recalling what a former worker on the project had told him. “The welding was extraordinary on this structure.”

Separate crews were assembled, one to weld the mild steel, and the other to tackle stainless, according to Ken Kolkmeir, project manager on the Arch for Pittsburgh-Des Moines Steel.

“We would test every welder to ASME code, and if he couldn’t weld downhand, overhead and vertical, and obtain water-clear x-rays, he couldn’t weld for us,” Kolkmeir says. To test the welds, crews employed a unique procedure, according to an x-ray technician on the project who was interviewed by Moore.

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“He told me of a big lead ball containing radioactive isotopes that was used for the weld tests,” Moore explains. “The ball had a tube in it with a string attached. For an x-ray, he would tape film to the spot to be tested, place the lead ball in front of it, then shield himself behind a corner of the Arch wall. He would pull the string, which would release the tube containing the isotope from the lead ball in order to expose the film, then release the string so the tube would descend back into the ball.”

Time and again, x-rays came back clear and free of inclusions, porosity or other defects, a testament to the quality of welders on the job.

“These guys would weld all day long, and you could run your finger down there and not find a seam in it,” recalls Dean Sample, an ironworker on the project.

Work proceeded through 1965 when, on June 17, at a height of 548 feet, crews erected a stabilizing strut to support the north and south legs as the two legs began to curve inward toward a connection.

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The Final Piece

Fast forward to early morning on October 8, 1965...Bands play, crowds gather, speakers speak and all across downtown St. Louis, commuters have their eyes to the sky and office workers man every available window to view a crowning event. This morning, crews would lift and attach the final section, bringing the two legs together at the top to complete the Arch superstructure.

Each night through the two-plus years of above-ground construction, surveyors would measure the heights, locations and angles of each leg to ensure they would eventually meet at the top…misalignment of only fractions of an inch toward the bottom would multiply at the top and prevent the two legs from connecting. Night time was the right time for measurement, as the sun and heat would expand metal during the day. Cooler, dark evenings would ensure accurate readings. This attention to detail expressed itself in true alignment as workers readied the last triangle for placement. But serious work remained.

The weight of the inward-curving legs left only a 2-1/2 foot gap for the 8-foot-wide final piece. Hydraulic jacks brought 500 tons of muscle to bear, adding 6 feet to the opening. Crews maintained a sense of urgency as the morning sun threatened to expand the south leg. The fire department helped out by cooling the leg with water. Working quickly but methodically, the creeper derricks hoisted the final section – topped by an American flag – into place and workers made the final attachments…the Gateway Arch had taken its final form.

With only 30 seconds or so given to pats on the back for a job well done, crews returned to work. The creeper derricks would begin their descent, with workers following to seamlessly patch the derrick bolt holes in the stainless-steel skin.

Mission Accomplished, Safely and Superbly

Workers successfully fabricated, delivered, hoisted and placed all 142 sections of one of the most complex construction jobs in this country’s history. Hinting at the monumental scope of the project, it used 900 tons of stainless steel, more than that used in any other architectural project up to that point. Insurance actuary tables had predicted that this project could result in the loss of 13 lives. With safety a priority, the project succeeded without even so much as a major lost-time accident.

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In 1966, the Gateway Arch officially was dedicated. Saarinen’s remarkable vision was realized, thanks to the dedication and skill of engineers and constructors, and backed by materials, equipment and expertise delivered by able suppliers, such as Lincoln Electric

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