The 104-year-old Marsh Street bridge at Santa Rosa Street in San Luis Obispo is deteriorating.

The concrete bridge is slowly breaking into pieces. Exposed rebar can be seen at its base and underside. The railing is so worn that it can’t be saved.

The bridge, largely unseen, is driven by motorists daily and is considered vital to traffic flow in the downtown.

The San Luis Obispo City Council on Tuesday will be asked approve a $4.4 million project to remove and replace the bridge. That cost could grow to as much as $7.1 million.

Only $504,680 of that will come from city coffers. The remaining cost of the project would be paid using federal grant money.

“There is no fear of imminent collapse or anything,” said Mike McGuire, city engineer. “It is something that needs to be addressed, more so than any of the other bridges in the city.”

The new 60-foot-long and 69-foot-wide bridge as proposed would be 15 feet longer than the existing bridge and supported on tall abutments farther spaced out than the existing support and would allow for better flow of the San Luis Obispo Creek below it. Construction could take up to seven months.

The new bridge would last for 100 years and require little maintenance in outlying years, according to a report done Dokken Engineering of Folsom. The firm was hired by the city to do a structural assessment of the bridge.

A cheaper alternative replacing only the bridge deck and the top portion of its supports would cost only $2.8 million. That would extend the life of the bridge to 75 years.

Caltrans reviewed the report done by Dokken Engineering and recommended entirely replacing the bridge.

“Although it is possible to rehabilitate this bridge, it is not prudent,” wrote Robert Zezoff, a senior bridge engineer with Caltrans.

Although the Marsh Street bridge is not designated as historic, it was designed by John B. Leonard, one of the leading concrete bridge designers in California, according to city staff.

It is one of 45 bridges Leonard designed throughout California and Nevada and one of the oldest.
A Caltrans study done in 2008 rated the bridge’s efficiency at 64 out of 100. That rating was 30 points higher in 2004, signifying a rapid decline.

McGuire said that over the years, water has seeped into the bridge, causing portions of it to rust and eventually cracking the cement.

“We are now beginning to see a lot more of the internal deterioration of the bridge’s structure,” McGuire said.

Reach AnnMarie Cornejo at 781-7939. Stay updated by following @a_cornejo on Twitter.