Scott Blair Piers tumbled into Central Arizona Project canal.

A mysterious Aug. 9 bridge collapse in Mesa, Ariz. caused by "lateral instability" is prompting the Arizona Dept. of Transportation to update its construction specifications. A newly released 122-page report by CTL Group, a Skokie, Ill.-based investigative consultant, blames "a number of factors adding together in a critical combination" as the reason for a 114-ft-long bridge section under construction to fall for no apparent reason. No one was injured. Around 8:30 a.m., nine of 11 precast, pre-stressed girders of a future bridge on the Loop 202 Red Mountain Highway, just east of Power Road, dropped 25 ft from their pier caps. The girders on the westbound lanes of the 87.8-ft-wide bridge – each 2.5-ft-deep and 105,000-lb – had been placed atop the pier caps three weeks prior. They fell into the Central Arizona Project’s (CAP) nearby canal, cracking upon impact. The girders are being replaced at the contractor's expense – estimated at $185,000. Pier caps sustained only minor scraping.

ADOT Girders tumbled like dominoes.

Results from the four-month investigation cite "bearing eccentricity" and un-removed girder protective material as contributing to the incident. Other potential factors include thermal expansion, wind load, and a lack of temporary lateral bracing.

No work had occurred on the unsecured girders while awaiting the cast-in-place diaphragms, says Doug Nintzel, Arizona Dept. of Transportation spokesman. Temporary bracing, although a common practice, isn’t required per ADOT contract specifications, he adds.

Related Links: CTL Group Final Report ( PDF, 10 MB)

But that will soon change. ADOT is now updating its specifications to require temporary bracing for all girders after they are placed and before permanent braces are added. It was one of the report’s many recommendations for future construction, including checking for girder centering and clearing surfaces before placement.

"Specifying temporary bracing in all cases is another step we can take to enhance safety," says ADOT State Engineer Sam Elters. "While this was a rare event and there are always risks in construction projects, we are prepared to work with our contractors to prevent any such accidents in the future."

The 4.83-mile, $195-million project, which completes the final Loop 202 segment between Power Road and University Drive, isn’t expected to be delayed by the incident. Phoenix-based Pulice Construction Inc., the general constructor, has a 650-working-day schedule. In February, the firm replaced a defective bridge pier that lacked a sufficient amount of reinforcing rebar. The contractor discovered the problem, removed and rebuilt the pier at its expense, which ran roughly $100,000, Ninztel says. Royden Construction Co. is the Phoenix-based subcontractor responsible for girder manufacturing and erection.