The January 2015 collapse of bridge framework for the Highway 101 bypass around Willits was largely a product of poor oversight by a contractor, according to a newly released Caltrans report.

Three workers were seriously injured by the collapse of a 150-foot-long section of bridge falsework, an elaborate structure used for support while concrete hardens. It was a significant setback for the $275 million, 5.9-mile project aimed at alleviating traffic congestion on Highway 101, which currently runs through surface streets in downtown Willits.

The contractor, Benicia-based Flatiron West, was responsible for designing and inspecting the falsework, according to Caltrans spokesman Phil Frisbie. Caltrans designed the bridge itself, which was being poured when the falsework collapsed.

“The falsework was not installed as shown on the approved falsework drawings,” the Caltrans report states.

The report also indicates the collapsed structure’s design had shortcomings and the inspection conducted by a person designated by Flatiron’s engineer was “ineffective.” Frisbie said he cannot elaborate on the findings.

Flatiron representatives did not respond to a phone call and email Thursday seeking comment.

The report echoes the conclusions of a California Occupational Safety and Health Administration report released in July 2015.

The OSHA report found: “the falsework was not property designed, was not erected as per the design plans, was missing components, (and) deficiencies were not identified when inspected and signed off on by the project engineer for the company erecting it.”

The falsework failed at less than 50 percent of its design capacity while concrete was being poured, Cal-OSHA found.

OSHA assigned Flatiron most of the blame, fining it $93,900. But it also cited another contractor on the job, Dublin-based DeSilva Gates Construction, for safety violations and fined it $49,500. Caltrans was fined $21,600.

Flatiron remains on the job at the Willits bypass and is a regular contractor on Caltrans projects.

Frisbie said he does not know whether the person who inspected the failed structure is still employed by Flatiron.

Frisbie said there had been no issues with falsework for the other seven viaducts constructed for the bypass and that the reconstruction of the failed section also was problem-free.

Caltrans’ report is aimed at preventing future problems, not laying blame, Frisbie said.

You can reach Staff Writer Glenda Anderson at 462-6473 or glenda.anderson@pressdemocrat.com. On Twitter @MendoReporter.