NIST helps US Capitol with 'overhead' problem

March 13, 2003

Repairing a leaky roof usually doesn't require the expertise and skills of researchers at the National Institute of Standards and Technology (NIST)--unless the building in need of help is one of the nation's most treasured edifices.

When the 150-year-old dome of the U.S. Capitol in Washington, D.C., began leaking during heavy rainstorms a few years ago, the Office of the Architect of the Capitol consulted with engineers involved in welding research at NIST. The daunting task handed them: return the cast-iron supporting structure of the dome's outer shell to its original condition without replacing the cracked castings or losing any of the iron work's historical integrity.

That request sparked a search for the right weld that would integrate with 1850s technology. An initial attempt in 1998 failed when the test welds cracked. NIST engineers went back to the drawing board (and the lab) to develop and test other options. The best-working technique turned out to be oxyacetylene braze welding (a flame repair process where the filler metal melts at a temperature below that of the casting) combined with a copper-zinc alloy called low-fuming bronze. The bronze forms joints that are very similar in strength to the original castings.

NIST's experts have submitted their recommendation to the Architect's office

National Institute of Standards and Technology (NIST)

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