By Kevin Flynn
Inside-Lane.com

The $3.8 million replacement of two structurally deficient steel truss bridges erected in Big Thompson Canyon during the Great Depression – including the lowest-rated bridge on the state system’s list of 124 poor-rated bridges – will force two 12-day closures of U.S. 34 downriver from Estes Park in February and March.

SEMA Construction is doing the work for the Colorado Department of Transportation. The full closures are schedule for Feb. 1-12 for the bridge upstream closer to Estes Park, and for March 1-12 for the second span nine-tenths of a mile downriver – dependent on the cooperation of the weather and the off-site fabrication of the new bridge girders remaining on schedule, as it currently is.

The upper bridge is rated 6 on a scale of 100 for structural deficiency, the lowest on CDOT’s list of bridges needing to be replaced and the only one rated in single digits. It’s companion downstream is rated 34. Both bridges were built in 1937 and are the last two remaining steel truss bridges in the canyon. U.S. 34 crosses 11 bridges in the canyon.

You can click here to view an interactive map, complete with pictures and descriptions including year of construction, of the poor-rated bridges on the CDOT system.

Steel truss bridges cannot be removed in stages to permit continuing traffic because their type of support system requires removal all at once. And the canyon itself is too narrow for a short detour around the work site within CDOT’s right-of-way. A work-zone detour would have required the state to acquire adjacent private property through eminent domain, a costly and unnecessary acquisition since it wouldn’t be needed after construction.

“We realize that the closures will significantly impact motorists and residents who travel US 34 on a daily basis, but both bridges are over 70 years old and are in poor condition,” said CDOT Resident Engineer Scott Ellis. “The only way to replace these steel truss bridges is to completely demolish and replace them, which is exactly what will take place during each 12-day period.”

The net difference in travel from the two detour points is 15.8 miles, with the detour being 45 miles long.

That leaves motorists with a lengthy detour, depending on their destination. CDOT is recommending than motorists use U.S. 287 south out of Loveland to CO 66 in Longmont, then up through Lyons to Estes Park. Traffic on the canyon road averages about 5,500 vehicles a day.

Here is CDOT’s suggested detour route during the two closures:

View US 34 Bridge Replacement Detour in a larger map

SEMA began work in November by drilling caissons and constructing the bridge abutments at the two sites. Completion is scheduled in May.

CDOT arranged the work schedule so that the closures and disruption occurred during the winter instead of during the summer tourist season.

Below you’ll find a Google Earth aerial view of the canyon showing the locations of the two bridges. Under that are interactive Google Street View photos of the two bridges.

Here is the upstream bridge:

View Larger Map

Here is the downstream bridge:

View Larger Map