Design Innovation: Well orchestrated

Aug. 18, 2009

Light-rail transit (LRT) may save its passengers time and aggravation, but it had the opposite effect on motorists using South Watt Avenue in Sacramento, Calif.

During peak periods, trains crossed the road every seven to 12 minutes at its intersection with Folsom Boulevard. Furthermore, they stopped at a bus-train transfer station at the southeast corner of the busy intersection, which carried 80,000 vehicles a day.

Light-rail transit (LRT) may save its passengers time and aggravation, but it had the opposite effect on motorists using South Watt Avenue in Sacramento, Calif.

During peak periods, trains crossed the road every seven to 12 minutes at its intersection with Folsom Boulevard. Furthermore, they stopped at a bus-train transfer station at the southeast corner of the busy intersection, which carried 80,000 vehicles a day.

“It’s one of the county’s most congested corridors,” said Ron Vicari, project manager with the Sacramento County Department of Transportation (SACDOT). “A blocking delay occurred every time a light-rail train came by. The arms would come down and stop traffic, tying up the intersection until the train cleared the area.”

Unyielding constraints

Building a structure to carry the two light-rail transit (LRT) tracks over South Watt Avenue would eliminate the conflicts with vehicular traffic, but designing it was not a simple matter. Not only was the traffic heavy, but trucks constituted much of the volume. Detouring traffic during construction was not an option because a nearby quarry and other area industries required truck access. Accommodating the trucks meant maintaining a minimum vertical clearance of 15 ft during construction. At the same time, the proximity of the LRT station to the at-grade crossing imposed a maximum height constraint for the structure. In particular, if the grade coming down to station level were too steep, stopping the train on wet tracks could be problematic.

Working within these track grade and clearance restrictions, and accounting for the height of the rail and its supporting plinth, designers found they had less than 2 ft of structure depth to work with below the plinth. So the cross section of the structure had to be a trough, with the deck resting near the bottom of the superstructure. Typically, that would suggest a steel-truss or plate-girder design—but concrete is more commonly used for bridges and grade separations in California. Furthermore, the falsework for a cast-in-place structure would encroach on the necessary vertical clearance during construction.

Just dropping in

“I thought of this concept that would use precast concrete box girders with a transverse slab that would just drop in,” said Tom Barnard, project manager for AECOM, the structural design firm for the project.

Each of the structure’s five spans would consist of three longitudinal girders of precast, prestressed concrete. Precast concrete deck panels would be inserted between them, near their bottom edges, forming two troughs for the LRT tracks.

“There were no standard shapes in this configuration, so it required a custom form,” Barnard said. “We simplified it as much as we could to get whatever economy was available out of the formwork. We had only two sizes of girders, and the slabs were all the same size. We made the girders all simple spans to reduce construction time. We had to keep the intersection open and minimize interference with traffic.”

Shooing challenges

SACDOT agreed to close South Watt Avenue for three weekends. During the initial closures a shoofly track was installed to detour the LRT trains around the construction site. During the third closure, the LRT traffic would be cut over to the new structure and the shoofly track would be removed. The superstructure of the grade separation would be installed during the second weekend closure. Prior to that closure, in preparation for setting the spans, workers installed cast-in-drilled-hole piles, cast-in-place columns, bent caps and bearings.

“Using a prefabricated superstructure—with more than 95% of the components built off-site, and then transported to the project location—was something that my agency had not done before,” Vicari said. “We were orchestrating many different crews and trades in a small area. It was an exciting process, but it required a lot of planning. In fact, all parties cooperated in working out an assembly schedule that was detailed down to 15-minute increments.”

The design process required careful planning as well, especially because there was no precedent for building a prefabricated rail structure of this type. To ensure project success and avoid costly problems, the design engineers remained involved throughout all stages of construction.

“It pays to think through the options and iteratively change things so that they fit together, anticipating what could go wrong and troubleshooting during the design process,” said Ahmad Abdel-Karim, who supervised design changes and reviewed the shop drawings for AECOM. For example, the design engineers had to ensure that the cambers of adjacent elements matched closely and to proportion and phase the prestressing and post-tensioning so that deflections remained small and within bounds.

Working the segments, crowd

Like a concrete Erector Set on steroids, the precast structural components were uniform in size and shape—both for economy and ease of assembly. All of the 3-ft-wide girders had the same vertical-walled cross section, although the interior girders were a foot taller than the 6-ft exterior girders. Three of the girders (for the short end span of the structure) were 75 ft long, but the other 12 were 115 ft long. The precast slab sections had two rows of hat-shaped glass fiber-reinforced polymer (GFRP) bars cast into the surface to provide the connection for the concrete plinth that supports the tracks.

“Con-Fab California, who did the precasting, did an excellent job of executing what we put on paper,” Barnard said. “The girders are almost laser straight.”

The girders were transported to the site by a 13-axle, 10-ft-wide truck system that featured a steerable dolly under the rear of the girder to enable the 175-ft-long system to turn through intersections. When the truck brought a span’s first exterior girder alongside its final resting place, two cranes were used. A 350-ton crawler crane lifted one end and walked it into position, while a stationary 300-ton hydraulic crane lifted the other end and swung it into place. Workers checked for proper alignment before the girder was set down and released to avoid double-handling the elements. They used a similar approach to set the 265,000-lb interior girder and the second exterior girder in place.

As the crew’s skill and confidence grew, girder placement time decreased from an hour and a quarter for the first girder to 45 minutes for the second and 30 minutes for each of the rest. As each girder was set in place, the crew attached a pipe brace to prevent rotation and lateral movement.

“The trains were operating within 10 ft of the south girder,” Barnard said. “So for safety, if a train was approaching or leaving the station while the crew was placing elements near that area, the workers had to stop and let the trains pass.”

The first span of girders was set on Thursday night; the second on Friday. Then the two cranes were moved out of the way and a smaller one was brought in to lift the 25,000-lb slab sections into place on temporary supports attached to the girder bottoms during fabrication. The process was repeated on Saturday and Sunday, with placement of the third and fourth spans, and on Monday with the placement of the final short span.

“It was exciting to watch those huge, 265,000-lb girders get swung into place by the two cranes,” Vicari said. “People had actually set up lawn chairs across the street just to watch. When the last girder was swung into place, 80 to 100 people stood up and clapped.”

When all of the deck panels were placed on a span, longitudinal post-tensioning was applied to the panels, followed by transverse post-tensioning through the slabs and more longitudinal post-tensioning of the girders.

“The section was transversely integrated into what I call a ‘super girder,’” Abdel-Karim said. “This opens the door for a new generation of all-precast super girders that are integrated into a single deck element.”

Finally, rails were set on temporary supports above the GFRP stirrups in the slabs, with the anchor bolts hanging down so they would be embedded in the cast-in-place plinth when it was poured.

“The Sacramento County Department of Transportation, the Sacramento Regional Transit District and Viking Construction worked closely together throughout the project,” Vicari said. “Everything had to be extremely well orchestrated.”

The through way

When the structure began operation in February 2009, it immediately relieved congestion on South Watt Avenue—even though some lane restrictions were still in place for finishing roadway work and landscaping. In addition to building the grade separation with a minimal interruption of traffic, the “structure-in-a-kit” strategy provided some bonus benefits.

“Building a through-girder structure lowers the profile,” Abdel-Karim explained. “When you lower the profile, you’re talking about approach grades that result in reduced abutment heights and reduced MSE [mechanically stabilized earth] wall lengths and heights. That lowers the cost. Another benefit of the trough is that the fascia girders [the outside girders] project above the deck, forming noise and visual barriers.”

“The precast, through-girder design is very unusual for a roadway structure, and it’s even more unusual for a railroad,” Barnard said. “But now it can become a tool in everybody’s box.”

About The Author: Hall reports on construction and civil engineering topics for national and international publications.

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