The South Park Bridge, which is just as vital to South Park as the West Seattle Bridge is to West Seattle, could be damaged beyond repair in an earthquake, but there isn't enough money to replace it.

Some of the pilings that support the 75-year-old bridge over the Duwamish River were never driven deeply enough into the river bottom. As a result, the bascule piers now tilt and King County bridge crews struggle to keep the moveable span operating. The bridge is a bascule bridge, one that has a movable section hinged about a horizontal axis and counter balanced by a weight.

The South Park Bridge is seven times more vulnerable to an earthquake than the Alaskan Way Viaduct, said Tim Lane, a bridge engineer with the King County Department of Transportation. He briefed members of the Seattle City Council last week about the status of the South Park Bridge.

Nevertheless the South Park Bridge carries about 24,000 vehicles each day, and up to 10 million tons of freight a year.

The bridge connects 16th Avenue South in South Park to 14th Avenue South in Tukwila, next to Boeing Plant No. 2.

Even though most of South Park lies within the city of Seattle, there's a strip of land along the southern portion of the bridge that's part of unincorporated King County. The unincorporated portion is nicknamed "the sliver by the river."

King County built the bridge in 1930 and still maintains it. The bridge has had structural problems since 1982, Lane said.

When the bridge was built in 1930, pile drivers did not pound all of the pilings deep enough into the sediment at the bottom of the Duwamish River. Many were never driven beyond the sand and silt layer to solid footing in deeper glacial till formations.

"Some only go down so far," Lane told City Council members last week. In fact, some of the pilings are about 10 feet shy of reaching glacial till, which provides a better base.

The South Park Bridge has other problems too.

The bridge's concrete is "self-destructing," Lane said. A chemical imbalance in the concrete has been causing rocks to separate from the cement.

Another difficulty is steel in the bridge deck is undersize for today's cargo loads, Lane told City Council members.

Much of the electrical equipment on the bridge also dates to 1930.

By the way, last Sunday's bridge closure was to repave the north end of the bridge deck. No structural repairs were done.

As with most large public works projects, engineers have proposed several different ways to deal with the problem.

One idea is to rehabilitate the bridge. The supporting piers and much of the bridge would be replaced. The span's four-lane configuration would be reduced to three lanes because lane-width standards are wider today than in 1930.

This option would cost an estimated $81 million and take two to three years to complete.

Another option is to keep a four-lane bascule bridge that would be about the same size as the current bridge. It would be built a little farther downstream from the current location. This option seems to be the most popular among South Park residents and business owners.

County structural engineers proposed options that are unpopular among many South Park people. Both ideas would replace the moveable span with a high, fixed span. One proposal is for a span 65 feet above the river and another is for a 100-foot-high span. The height would allow many vessels to pass under the bridge without having to open the bascule bridge.

The problem with the fixed-span ideas is they would require longer approaches to the bridge on both sides of the river. Long ramps would have to be built at each end to carry traffic up and over the bridge. The approach to the 65-foot bridge would have to start near Cloverdale Street, a block farther south than the existing approach. That would require demolition of much of South Park's business district.

The 100-foot-high span would have even longer approaches reaching to Trenton Street, which is two blocks farther south. More businesses and more private property would have to be bought out to make way, Lane said.

"That would have a disproportionate effect on minority-owned businesses," said Tim McNeil, a South Park resident and property owner. "It would wipe out three blocks of our commercial district."

He estimated six businesses and 47 residences would be affected.

"We want to protect our small businesses," McNeil said. "We want to protect our marine industry too."

The remaining option is to close the bridge and not replace it. Removal of the structure would cost an estimated $9 million.

Without enough money by itself to replace the deteriorated bridge, King County is looking for partners. The county has pledged $10 million to the project. Tukwila committed $3 million.

The county has been trying to get the city of Seattle to contribute to the replacement project, and people have been looking for federal money too.

Efforts have been made to get the project on the to-do list of the regional transportation improvement district, but it was dropped from the regional list in the last go-round of talks.

Using an engineering "sufficiency rating" based on a top score of 100, the South Park Bridge scored an 8, because of its weakened structural integrity, narrow lanes and overall condition, Lane said.

By comparison, the upper level of the Alaskan Way Viaduct scored a 9.

Engineers say the span is likely to be damaged beyond repair if another Nisqually-size earthquake were to strike the region. They think there's a 33 percent chance that could happen within the next 10 years.

In that case, vehicles that now use the South Park Bridge would be expected to drive to the First Avenue South Bridge. Engineers estimate the additional traffic would triple delay times on the First Avenue South Bridge, Lane told City Council members.

A draft environmental impact statement on the bridge is scheduled to be released soon and a public hearing is set for Nov. 3. The King County Department of Transportation plans to select a "preferred alternative" plan in mid-December.

The final environmental impact statement is scheduled to be completed in 2007. If money is found for a new bridge, it could be finished in early 2012.

Tim St. Clair can be reached at tstclair@robinsonnews.com or 932-0300.