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Fix will require big crane, lots of steel and precision timing |
By THOMAS RYLL,Columbian staff writer
For thousands of motorists, the I-5 Bridge repair job will be nothing more than a monstrous inconvenience, crimping the daily commute like a semi backing over Uncle Ralph's Yugo.
For engineers, fixing the 80-year-old northbound lift span will require complex design work, manufacture of replacement parts at machine shops and foundries around the country, and what amounts to split-second timing to bring everything together. All while bridge users utter the inevitable grumbling and snorting, regardless of how well the project might go.
"It's a combination of technical challenge, very low tolerances, cataloging the pieces that are removed and working at night," said Claude Sakr, an Oregon Department of Transportation bridge engineer serving as the project manager. "You're putting a lot of expectations on the contractor."
At about $3 million, the project is hardly expensive, compared to the estimated $16-18 million cost of a badly needed paint job. Nevertheless, the crucial need to reopen the bridge to traffic as quickly as possible prompted Sakr's agency to junk the usual lowest-bid method of finding a contractor.
The routine this time was to go by qualifications and negotiated price, with a $100,000-a-day bonus/penalty arrangement for early or late completion, respectively.
Christie Constructors Inc. of Richmond, Calif., was awarded the contract for the work. The company is no stranger to big bridges and big jobs:
- The day after the Loma Prieta earthquake, Caltrans, California's state transportation agency, chose Christie to perform emergency repairs.
- Shortly after the Northridge earthquake, Christie was hired to fix the I-5 bridge just north of Los Angeles.
- Christie has done major work on the Golden Gate Bridge; Portland's Hawthorne and St. Johns bridges; the Astoria-Megler Bridge on the Columbia; and Bullards Bridge in Bandon, Ore. The company built the Snake River Bridge at Clarkston, Wash.
Christie has another specialty: retrofitting existing buildings with earthquake-resistant steel interior framing. Even more complex than the I-5 job, one such project involved threading new beams throughout a building to form a new skeleton, and then removing the pieces of an existing framework.
Carl James is Christie's local project manager, working out of temporary modular offices only a few feet from the north end of the bridge. The philosophy of company officials, he said, is to keep the Christie name low profile, despite the high-profile nature of the firm's work.
Even the company trucks are unmarked. James recently joked with company officials about the inevitability of the public's scrutiny on this project. "I told them, 'If we do a great job, we'll be famous. If something doesn't go right, we'll still be famous,'" said James, smiling.
In short, the local project involves these steps:
- 1. Attach a platform to a bridge pier and park a 250-foot-tall crane on it. (That has been done).
- 2. Use crane to erect steel-beam structure, relying in part on the existing bridge tower, to support 700-ton concrete counterweight.
- 3. Remove lift-span moving parts; replace suspect pieces and reuse others.
- 4. Disassemble support structure and crane and go home.
The work is necessary to replace a cracked steel forging called a trunnion. The axle-like part supports one of the 12-foot-diameter pulleys, or sheaves, that are part of the bridge's lift-span mechanism.
The twin spans of the bridge have a total of eight sheave-trunnion assemblies. Two, at the north end of the northbound span, will be replaced. The cracked trunnion will eventually fail, engineers say.
The counterweight must be supported with a lacework of steel threaded into the lift-span towers supporting the sheave-counterweight assemblies. The temporary support structure will be partially assembled on shore and barged to a floating platform adjacent to the crane.
Before and after the three-week closure, traffic will be stopped on the northbound bridge during activities such as erection of the support structure.
Earlier this year, highway officials juggled the issue of when to do the work. Ultimately, they decided that waiting until late summer 1998 would only increase the risks associated with possible failure of the trunnion. Scheduling the work for this summer had the drawback of needing to move fast to make it all happen.
One way was by farming out the manufacture of new parts to several suppliers: a foundry in Detroit for the trunnion forgings, a fabricator in Santa Fe Springs, Calif., for sheave assembly and finishing work. Other suppliers, included a Pennsylvania firm that provided new Swedish roller bearings - the same brand, SKF, as those installed in a late-1950s refit. The bearings enable the trunnion/sheave components to rotate.
Officials say the preparatory work has gone well, and that Christie is ahead of schedule as the Sept. 16 traffic-closure start date looms. Beating deadlines at this point doesn't necessarily mean a shorter-than-three-weeks closure, but Christie engineers have been laboring to shave days, even hours, off the process. The $100,000-a-day bonus is broken into 15-minute segments worth about $1,000 each.
The September project also will include testing of the removed parts to determine how long the two trunnions at the south end of the northbound span might be expected to last, Sakr said. They, like those at the north end, are 80 years old and have raised and lowered the bridge many thousands of times.
