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The length of the stairs leading up to a new TriMet pedestrian bridge in Southeast Portland ultimately helped shape its design.

Initial plans for the $3.9 million Lafayette Street pedestrian and bicyclist crossing over railroad tracks to provide access to the future Rhine Street/Southeast 17th Avenue orange line light-rail station called for a taller bridge than the one being built. But the set of stairs up to it would have to zigzag with obstructed landings that could potentially attract crime, said Christopher Pitt, a structural engineer with KPFF Consulting Engineers. This concerned TriMet officials working together with KPFF on the bridge design, he said.

鈥淭hey care a lot about security and safety (and) looking at sightlines,鈥� he said. 鈥淭hat’s something that TriMet focuses on a lot in the design process.鈥�

Pitt and other KPFF staffers proposed an alternative. Instead of a precast concrete girder bridge, a truss bridge with a lower deck could be accessed via a shorter, straight set of stairs.

鈥淲e started looking at alternatives and we thought a steel truss would just bring it as low as possible,鈥� Pitt said. 鈥淲ith steel truss, you can put the walking surface to the bottom of the structure. That shortened the stair length. We can have pretty much straight stair runs. You’ll see where you’re going the whole time.鈥�

Lorentz Bruun Construction crews this past fall started building the 184-foot-long steel truss bridge. Last week, a project milestone was reached when crews used a crane to lower in place the approximately 50-ton span onto two 47-foot-tall steel-framed elevator towers that double as bridge supports on either end of the railroad tracks.

The new Lafayette Street crossing will fill the void left after demolition this past summer of a deteriorating span built in the 1940s to provide access over Union Pacific Railroad tracks. That bridge, owned by Union Pacific, was 鈥渇unctionally obsolete,鈥� Pitt said.

Union Pacific and TriMet are sharing the cost of the replacement project. TriMet will own and maintain the new bridge, which will cross between Southeast Rhine and Lafayette streets to line up with a signaled crosswalk leading to the new MAX orange line station.

The project is part of TriMet’s larger $1.49 billion effort to create the 7.3-mile Portland-to-Milwaukie orange line, which is slated to open in September 2015. The project includes $65 million in improvements to maximize access by pedestrians and bicyclists to light-rail stations.

Besides a shorter stair length, project engineers concluded that a steel truss bridge would have other advantages over a concrete girder span, said Leah Robbins, TriMet civil engineer and east segment director for the Portland-Milwaukie Light Rail Transit Project. A truss structure fit better into the tight space available for the bridge, and that type of span was easier to transport and install with minimal impacts on the active rail lines below, she said.

Safe passage across the bridge for both pedestrians and bicyclists was the top consideration in the design, Robbins said. The bridge will have clear glass elevator cabs for added visibility, closed-circuit cameras and ample lighting. The design team also reviewed bridge plans with TriMet security officials.

鈥淚t was (a) struggle to get the precise alignment of the stairway, elevator access and the span direction so there was good visibility for pedestrians at all levels 鈥� so they’re not entering an unanticipated area,鈥� Robbins said.

Bicyclists will be better served by the new bridge than the old one, Pitt said. The deck width will be 12 feet 鈥� twice the width of its predecessor. Also, the bridge stair width will be 8 feet 鈥� up from 2 feet 鈥� and have tire gutters for walking bicycles up and down. The elevators will be larger to accommodate bicycles with trailers and have doors on either side so bicyclists won’t have to turn around or back out to exit.

Both the bridge and the elevator towers are being built primarily with raw steel that will weather over time to gain a deep rust color, Pitt said. This weathering will form a protective layer around the steel, eliminating the expense of paint and paying homage to the industrial setting, he said.

鈥淲hen you can make (weathering steel) work, it’s very economical,鈥� he said. 鈥淚t kind of has a nice aesthetic to it.鈥�

The new bridge is close to Pitt’s home, and he said he’s looking forward to visiting the crossing when it’s complete next summer. It won’t be part of his regular bike commute, but will come in handy if a train happens to go by.

鈥淚’m certainly going to go down there and take my daughter,鈥� Pitt said. 鈥淭his way you can get across whether or not there’s a train.鈥�

Lafayette Street pedestrian bridge

Location: near Southeast 17th Avenue/Rhine Street station for future MAX orange line

Cost: $3.9 million

Construction start date: October 2014

Anticipated completion date: summer 2015

Owner: TriMet

Architect: Merryman Barnes Architects

Design engineer: KPFF Consulting Engineers

Mechanical engineer: Interface Engineering

Electrical engineer: LTK Engineering Services

General contractor: Stacy and Witbeck Inc.

Subcontractor: Lorentz Bruun Construction

Steel fabricator: Fought & Co.

Bridge truss erector: REFA Erection Inc.