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Welding

A date with history

By FFJournal staff

July 2010 - When Allentown Metal Works Inc., Allentown, Pa., delivers a mammoth structural steel truss assembly for the World Trade Center Transportation Hub, it will become a part of history.

Designed by architect Santiago Calatrava and costing an estimated $3.2 billion to complete, the WTC Transportation Hub is under the No. 1 World Trade Center and other new WTC buildings. It will be the third-largest transportation center in New York City, providing PATH train service to New Jersey, connecting to 13 subway lines and other terminals, and linking pedestrians to the World Financial Center through an underground concourse.

A floor support
Allentown Metal Works is building the hub's west truss assembly as a subcontractor to one of the project's prime fabricators. The truss assembly is a canopy, somewhat like a bridge, that will support an entire section of the No. 1 WTC mass transit's entrance and distribute the tremendous overhead load that includes streets, traffic and the building sections above it. Allentown Metal Works' portion of the truss project is estimated to involve more than six months of production time.

Some 24 truss sections span roughly 25 ft. with diagonals tying them together with huge, 18-in.-thick plate node blocks machined and beveled, and 8-in. and 4-in. plate. The load on these structures will be tremendous, and as a result, they must be fabricated to critical-quality standardsÑthe highest quality under U.S. codes.

The production of this massive fabrication involves a boring mill, two plate rollers, various types of arc welding, heavy component assembly and the use of Tempil's, South Plainfield, N.J., Tempilstik temperature-indicating sticks or crayons, along with Estikelectronic contact-type thermometers employing thermocouple technology for temperature measurement.

Components, many weighing about 12 tons and measuring 40 ft. long, will be delivered in 2011 by truck.

Working with WTTI
The Welding Training & Testing Institute, Allentown, Pa., is an adviser, third-party expert and qualified inspection agency that will supplement Allentown's quality program for these large fabrications. WTTI also performed nondestructive inspection processes and 100 percent testing and inspection as required for the trusses by the customer.

WTTI's Bob Weissiner and other welding professionals from WTTI helped to set up and fine-tune the temperature measurements of the welded components. Allentown Metal Works' welders consistently used Tempilstik temperature-indicating devices for preheat and post-heat determination as they worked to stabilize the metal. These devices provide precisely calibrated measurements for inspection and documentation purposes.

Tempilstiks indicate a temperature when they are stroked on the workpiece like a crayon. They have calibrated melting points, and the mark turns to liquid at the stick's rated temperature. During use, the welder has one indicator stick calibrated for the lower temperature limit and one for the higher one. When the mark of the lower-rated crayon on the workpiece turns liquid, the welder knows the indicated melting point has been reached. When the mark of the higher-rated crayon turns liquid, the welder knows the piece has become too hot and must be cooled down and tested again.

For weld area preheat and post-heat of the weather-resistant, high-strength, low-alloy 709 HPS 50W steel used, it's necessary to maintain temperatures between 325 F and 375 F.

Weissiner also suggested using Tempil Estik visual calibrated measuring devices to provide a confirmation to the Tempilstiks' indication to the welder. Welders use the Tempilstiks as they heat the metal, and the foremen and inspectors use calibrated Tempil Estik instruments to check the temperature of the finished welds.

When the metal being worked is thicker than 4 in., it's heated to 350 F, but after welding, a post-heat treatment of 450 F to 650 F is used. The welder needs to reach the highest temperature required. Thick workpieces must cool for several days to be sure the molecular structure is in the final state so they can be X-rayed. FFJ

Sources

  • Allentown Metal Works Inc.
    Allentown, Pa.
    phone: 610/770-7400
    www.amwfab.com

  • Tempil, an ITW company
    South Plainfield, N.J.
    phone: 800/757- 8301
    fax: 908/757-9273
    www.tempil.com
    e-mail: tempil@tempil.com

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