That historic steel truss bridge suspended across the Sacramento River near downtown, a sturdy symbol of industry and a sentinel of a bygone era, celebrates a milestone this year.

No, not the flashy Tower Bridge, with its modern arches and single deck and landmark regard, which opened in 1935 to a celebration and crowd of thousands.

It's the stoic mass of dark latticework just up the river. The two-deck swing bridge, car traffic on top, trains below, that opened around the time Ford started selling the Model T.

The I Street Bridge, toiling ever quietly in the shadow of its downstream neighbor, turns 100 this year.

"I Street Bridge was always kind of the ugly stepsister," said John Snyder, former head of the architectural history unit at the state Department of Transportation. "It was the industrial bridge, where the Tower Bridge was kind of the architectural monument."

Still traversed often by foot, automotive and railroad traffic, the swing bridge sports a layer of rust and patches in its pebble cement walkways. Drive over it and the old roadway grumbles softly beneath your tires.

But both Amtrak and Union Pacific, which owns the bridge, send trains across daily. And when necessary, at the behest of a bridge operator, the 363-foot swing span slowly, deliberately executes its familiar turn to allow tall ships to pass.

"It's kind of a symbol of brute strength," Snyder said.

In a certain reverence for age, boats in the river have the right of way over bridge traffic, Union Pacific spokesman Aaron Hunt wrote in an e-mail. Because the river was there first.

The current I Street Bridge is actually the fifth bridge to stand in that location, according to Union Pacific records. The first, a wooden span completed in 1858, charged loaded wagons $1.25 apiece to cross.

Built in 1911 for Southern Pacific Railroad and opened in 1912, the current bridge is said to have been the heaviest swing span in the United States.

It opened on a late April morning to little fanfare. According to a short article buried inside the April 30, 1912, edition of The Bee, the upper deck was crowded that day with "the curious inspecting the new structure and watching the work of removing the old wooden bridge" that had stood there before.

That the bridge is still functional after a century is "exceptional," said Howard Payne, 83, of Sacramento, a retired civil engineer.

Few bridges stay in service that long because they become obsolete or are rebuilt, Payne said. The I Street span has withstood normal wear and tear along with the evolution of freight trains to carry heavier loads, he said.

Union Pacific undertook a major renovation of the bridge in the 1990s that included hydraulic upgrades and replacing the lens on which the span rotates, wrote Hunt, the UP spokesman.

The bridge is also inspected several times a year, he wrote.

But it has maintained the brawny grit of another time that appeals to Jon Affonso, winemaker at Rail Bridge Cellars on the north edge of downtown Sacramento.

When Affonso started his winery several years ago, he wanted to embrace its urban setting by using imagery of "steel and gears and beams" on his label, he said.

He chose the I Street Bridge as a kind of artistic landmark, naming the winery after it and printing "1911" in copper relief on his label.

"It came from a time in our history that had a certain metallic feel to it," Affonso said. "I just like the look of it."

Snyder, too, admits to having a "soft spot" for the bridge. His father worked for Southern Pacific for 50 years, and Snyder grew up around the railroad, he said.

In 1981, he nominated the bridge for the National Register of Historic Places. The distinction became official in 1982 – the same year that the Tower Bridge, which Snyder had also nominated, was listed.

He nominated the Tower Bridge first. But "it could've been a toss-up," Snyder said.

"You stop and think when that thing was built, those traffic lanes were still carrying horse and wagon traffic," Snyder said of the I Street Bridge. "In terms of today's traffic those are narrow lanes. But it's a symbol of its time."

© Copyright The Sacramento Bee. All rights reserved.

Call The Bee's Matt Kawahara, (916) 321-1015.

• Read more articles by Matt Kawahara