OAKLAND -- On Nov. 7, 1940, a 4-month-old bridge aptly nicknamed "Galloping Gertie" twisted apart in 35-knot winds, collapsed into Puget Sound and forever changed the way engineers designed bridges.

To avert a similar catastrophe on the new Bay Bridge, ironworkers are hanging 700 square steel plates beneath the deck.

The panels will counteract the forces that brought down the Tacoma Narrows Bridge in Washington, when a combination of the wind and the structure's natural vibration frequency sent the span into a self-destructive cycle. "The panels create a vortex and disrupt the wind flow effect that could generate another Gertie," Caltrans senior construction engineer Rob Kobal said during a firsthand look at the panels' installation last week. "After the Tacoma Narrows Bridge collapse, wind vortex panels became a common feature on suspension bridges."

To show off the work, Kobal led a reporter and photographer from this newspaper to the bridge's white railing.

"You don't have a problem with heights, do you?" he asked.

They followed Kobal over the railing, and descended via a ladder onto the platform of one of the newly installed maintenance "travelers" -- a movable platform -- suspended 150 feet above San Francisco Bay.

The five travelers -- one for each direction on the two side-by-side road decks and bicycle-pedestrian path -- have their own story.

Fabricated and assembled in Santa Fe Springs in Southern California, they hang on two steel tracks from the underside of the bridge. They operate on compressed air piped from each end of the bridge and move 20 to 50 feet per minute.

The travelers provide safe exterior access to the 2,047-foot steel suspension segment, which requires frequent inspection and painting. The adjacent 1.2-mile skyway is concrete and has no traveler.

"The travelers are integral to making sure the bridge meets its 150-year life span," Kobal said.

On this day, ironworkers from Local 378 in Oakland used one of the 119-foot-wide travelers as a suspended vantage point from which to install the wind vortex plates -- roughly 3-foot square steel sheets as thick as a cellphone and 140 pounds apiece.

Image: "Vertical and torsional motion viewed from east tower of Tacoma Narrows Bridge, 7 Nov 1940," from film shot by Prof. F.B. Farquharson,
Image: "Vertical and torsional motion viewed from east tower of Tacoma Narrows Bridge, 7 Nov 1940," from film shot by Prof. F.B. Farquharson, University of Washington (Prof. F.B. Farquharson)

The four men worked together like well-rehearsed actors.

Ironworker Kevin Karber, of Concord, manned the traveler's helm and carefully engaged the throttles, one for each track.

It handles "like a bulldozer," he said. "You have to be careful you don't get the two sides out of sync."

Once in position, they attached a panel to the hoist. They started and stopped the lift to line up the panel's pre-drilled holes with those on the bracket already bolted into the deck.

They slid bolts into each of the holes and twisted on the corresponding nuts, handing off materials and tools and exchanging hand signals as they moved from one panel to the next.

The vortex panels are hung vertically in sets of 10 at alternating angles from the bottom of the deck. Only boaters will see them; most motorists will never even know they are there.

The panels were thoroughly tested on a scale model of a self-anchored suspension span placed in a wind tunnel and analyzed at West Wind Labs in Monterey, Kobal said.

"The Bay Bridge won't be doing any galloping," Kobal said.

Contact Lisa Vorderbrueggen at 925-945-4773 or Twitter.com/lvorderbrueggen.

BLOWIN' IN THE WIND
See what happened to the Tacoma Narrows Bridge, also known as "Gallopin' Gertie," on Nov. 7, 1940, just four months after it opened at http://youtu.be/j-zczJXSxnw.