A section of the 
Oakland-San Francisco Bay Bridge collapsed during the 
Loma-Prieta earthquake that rocked 
San Francisco Bay Area  in 1989. The Bay Bridge was closed for several weeks while repairs were  made. Construction workers traditionally have some sort of ceremony to  mark the completion of an important project.
When the repairs to the bridge were nearly complete 
ironworkers smuggled an 18-inch metal 
troll onto the bridge and welded it permanently to a steel beam on the north side of the bridge. This unusual 
topping out surprise has become the subject of much curiosity and rumor around the 
Bay Area .
[1]  In an article about the international scope of the topping-out  tradition in the building trades, photographer, writer, and folklorist 
John V. Robinson cites the Bay Bridge troll as an example of a custom being adapted to an unusual circumstance.
[2]
The troll first came to the public's attention on January 15, 1990 when the 
San Francisco Chronicle ran a story about the small figure of a troll complete with a 
spud wrench that had been welded to the iron below the road on the north side of the bridge.
[3]  The article quotes official spokesman for the state Department of  Transportation, Greg Bayol, as saying, "If we were asked, we would have  said 'no,' I'm sure." But the iron workers didn't ask, so the state is  stuck with the troll.
The troll is placed where regular people would never see it. It is on  the north side of the outside rail, where only maintenance people and  ironworkers ever venture.
[3] The troll's creator, Bill Roan, is a 
blacksmith turned artist who lived near the Bay Bridge.
As of 2010 a new span of the Bay Bridge is under construction and  people are starting to ask what will become of the troll when the old  span is demolished. Cal-trans has not yet decided whether to retire the  troll to a museum or to attach it to the tower of the new bridge.
A little known fact about the Oakland-San Francisco Bay Bridge troll  is that it has a twin brother. When the troll was made, the contractor,  Rigging International, ordered two: one to put on the reconstructed Bay  Bridge, and one to keep on display in the companies' office in 
Alameda, California. The doppelganger is mounted on a damaged section of the old bridge and sits on display in the lobby of their Alameda office.
Found and copied from Wikipedia