DAM FAILURE BLAMED ON DESIGN : ENGINEERS STUDY FOLSOM GATES TO PREVENT BREAKS AT SIMILAR FACILITIES.

Byline: Nancy Vogel Scripps-McClatchy Western Service

Eight months into an investigation of why a spillway gate at Folsom Dam broke last summer, engineers say hidden corrosion and weak design are likely culprits, not the rusted beams and bolts noted by inspectors before the accident.

A final report is months away, and tests continue. But members of the forensics team trying to figure out how and why Gate No. 3 broke after 41 years of good service say the trigger may lie in the two big pins, called trunnions, that pivot the garage-doorlike gate up and down.

Some team members theorize that the grease the bureau had been using in the pins broke down or squished out. Moisture invaded, corrosion ensued, and at 8 a.m. July 17, as an operator pushed the gate's ``up'' button, the pin might have jerked with enough force to shear bolts.

That force, and water rushing under the gate, may have set the 88-ton gate into destructive vibration, engineers say. Two of the gate's steel arms, or struts, fell off the dam. One hung on by an end. The fourth crumpled, and the gate swung open like a hinged door.

``The gate literally vibrated itself to failure,'' said Tom Aiken, area manager for the U.S. Bureau of Reclamation.

A five-story wall of water shot out of the dam, about 20 miles northeast of Sacramento. No one downstream was hurt, but the bureau had to release enough water to supply 2 million people for a year before the pressure against the dam eased enough for them to block off the gate. Since then, Gate No. 3 has been lifted out and the other seven gates have been reinforced, with more work scheduled.

Around the world, people who operate hundreds of Folsom Dam-style gates, called ``radial arm'' gates, await the bureau's forensic report. California's dam safety office has notified 58 radial gate operators to begin extensive inspections. One of them, the Santa Clara Valley Water District, already has decided to replace two corroded gates at Vasona Reservoir.

``This is why we're being so careful on this forensics report,'' said Aiken, ``because it goes way beyond Folsom Dam.''

A preliminary report is due in a couple of weeks, with a final report expected by June.

Engineers say it's clear now that the steel arms that brace the gate were designed without sufficient strength.

John White, chief of structural design for the U.S. Army Corps of Engineers, scrutinized Folsom Dam's original designs and found that the designers, working without computers, did not take into account friction in the trunnions when deciding how heavy and strong to make the arms, or struts.

``The designers met the original criteria in effect at the time,'' said White. Before the Gate No. 3 failed, Folsom's gate struts did not meet current standards. They do now, as last fall bureau contractors bolstered each of seven gates with 22,000 pounds of steel.

Gate No. 3 is blocked by steel beams and should be replaced by March 1997.

Around the country, said White, it's likely that other gates will have to be strengthened as at Folsom.

Engineers also say they believe that beam and bolt corrosion, as noted by inspectors in 1988, 1991 and 1994, played a minor role in the accident. For lack of money, those repairs were not made until after the gate broke.

``It might have had a factor of 7 percent in terms of causing the failure,'' Aiken said, noting that some of the most-corroded beams held.

In a warehouse at the dam, engineers are now using the pin from the broken gate to test different types of grease. What they're finding, said White, is that some of the brands of extreme-pressure grease the bureau used in recent years were not as effective as the lead-based grease that was phased out in the 1980s.