Copyright 1990 The Chronicle Publishing Co.
The San Francisco Chronicle
LENGTH: 454 words

HEADLINE: Gate Bridge to Be Tested For Carrying Light Rail Tracks

BYLINE: Harre W Demoro, Chronicle Staff Writer


Engineers want to simulate a killer windstorm before deciding if the Golden Gate Bridge would still be safe with a lower deck carrying rapid transit tracks.

The tests, using a three-foot-long plastic model of a section of the bridge, will be supervised by T.Y. Lin International, a longtime bridge engineering firm.

The company already has a $ 620,000 contract to design a proposed rapid transit deck and to look for weak points that could fail in a major earthquake.

Bridge district directors are expected tomorrow to appropriate $ 39,000 for wind tunnel tests.

The bridge was not damaged by the October 17 earthquake but engineers have worried for 40 years about the landmark span's motions during big windstorms and the durability of its approach viaducts.

When the bridge opened in 1937, it was considered a wonder of the age. But three years later experts began having second thoughts when a shorter but similar suspension bridge over the windswept Narrows at Tacoma, Wah., fell down. The Tacoma bridge was less than a year old.

Then in 1951, the ''Golden Gate Bridge acted like the Tacoma Narrows Bridge'' during high winds and had to be closed for a short time, said Mark A. Ketchum of T.Y. Lin. Bracing added under the automobile deck to stiffen the structure corrected the problem.

With advances in computer design technology and wind tunnel work, engineers today can double-check the work done in 1951 and determine precisely how the bridge would perform with tracks installed below the auto deck, said Charles Seim, also of T.Y. Lin.

One thing the engineers want to do is be sure the bridge is safe in sustained winds of 110 miles per hour, the standard for modern highrise buildings, Ketchum said.

''It takes winds of 90 miles per hour to get the bridge going, but it isn't that one gust would do it,'' he said. ''It would have to take those gusts all day.''

Wind doesn't blow bridges down, but causes them to ''flutter,'' a series of motions caused by tipping, rolling and vibration, Ketchum said.

If a bridge is poorly designed, the bridge structure itself will amplify the vibration like a tuning fork, and the bridge will shake itself apart. That is what happened at Tacoma, Seim said.

Robert H. Scanlan of the Johns Hopkins University civil engineering department will look at the aerodynamic performance of the bridge, and Jon D. Raggett of the West Wind Laboratory in Carmel will conduct wind tunnel tests. Seim and Ketchum said when they look at the original design of the Golden Gate Bridge, they are awed at the work done by engineers working with pencil and paper in an era before computers and other sophisticated design techniques.

''It is all incredible,'' said Ketchum.

GRAPHIC: PHOTO,Engineer Mark Ketchum held the Golden Gate Bridge section model  that will be tested , BY JERRY TELFER, THE CHRONICLE