When it comes to driving across the Healdsburg Avenue bridge, Healdsburg Mayor Lisa Schaffner said Monday, "I drive my kids over it, but if I felt a twinge of an earthquake and I was driving over the bridge, I would gun it."

Schaffner is not the only one who admits to qualms about the narrow, structurally questionable span across the Russian River, which is rated the worst in Sonoma County.

"When you see a large van drive across, it makes you think twice," said City Manager Chet Wystepek.

On Monday, City Council members unanimously agreed to move ahead with environmental studies to replace the vintage steel-truss bridge with a concrete span. But they acknowledged that by the time the studies are done, the council may choose to simply strengthen the existing bridge.

The possibility of preserving the 1920s-era bridge, which history buffs and others have urged, was bolstered late Monday by a Healdsburg resident, an electrical engineer who told the council that a 1979 Caltrans formula that ranked the bridge the worst in the county was flawed.

"The whole thing was suspect to me. That's why I dug into it," said Mel Amato, who told the council that in essence the bridge is in much better shape than believed and can bear three times as much as the 17-ton truck limit now in place.

While council members were impressed by Amato's analysis and mathematical calculations, they still decided to go ahead with a tentative plan to replace the structure, so that federal funding can be secured if that becomes the eventual choice.

City officials have been debating off and on since at least 1990 what to do about the aging bridge, which has served as the southern gateway to Healdsburg, part of the Old Redwood Highway before Highway 101 was built to the west.

Last year's collapse of a bridge in Minneapolis that killed a dozen people lent new urgency to fixing or replacing the bridge, also known as the Memorial Beach bridge.

It has an even worse rating than the bridge in the Midwest catastrophe.

On a scale of 1 to 100, with 1 being the worst, the 398-foot-long bridge has a sufficiency rating of 2. The rating takes into account corrosion and cracking, width, alignment and functionality. But the low rating did not take into account temporary emergency structural reinforcements that were made to the bridge four years ago.

And Amato told the council that after consulting with Caltrans, he determined their formula was flawed and the actual rating on the bridge is closer to 54.

Public works officials have been unwilling to pronounce the 1921 steel-truss bridge safe, especially since trucks exceeding the weight limit frequently drive across it.

School buses and fire trucks avoid the bridge.

Consultants for the city have said their primary concern is not that the bridge will collapse immediately, but that high water in the winter, or an earthquake, could bring it down.

Winter river flows were not that high this year, but city officials say something still needs to be done.

"To not pursue something at this point, whether rehabilitation or replacement, we would be negligent," Wystepek said. "We are concerned about the condition of that bridge as we would any object that's 70 years old and has not had any major work done on it."

Actually, the bridge is more than 85 years old. And the City Council for more than two years has listed replacing or repairing the bridge as one of their top 10 goals.

But city officials say funding is a challenge, as well as deciding which project to pursue.

To simply rehabilitate and strengthen the existing bridge and retrofit it for earthquake and flood resistance would cost an estimated $13 million, according to consultants, although Amato said it can be done for less. But that would leave an overly narrow bridge and provide no room for a bicycle lane.

Replacing the bridge with a modern one would cost $23 million.

The most expensive alternative -- $40 million -- involves replacing the current bridge with a visually similar bridge.

The estimated construction costs are in 2011 dollars, the probable date a project could begin after conducting environmental studies.

You can reach Staff Writer Clark Mason at 521-5214 or clark.mason@pressdemocrat.com