AMESBURY — A long, rusty metal beam has fallen from the Interstate 95 bridge, and a review of the structure shows at least three other spots where beams on the 57-year-old span are missing and may have fallen into the Merrimack River.

Rusted and bent metal plates on the underside of the bridge indicate where the T-shaped beam — about a half-inch thick, 6 inches wide and 30 feet long — was once attached to the bridge. The beam now sits on a grassy patch alongside the river, about 80 feet below where it once was.

State officials were unaware the beam had fallen until North of Boston Media Group photographed it and sent electronic copies to the Highway Department.

After viewing the photos, officials said the bridge is safe for motorists — although it may be a hazard for boaters.

With more than 75,000 trips across it a day, the John Greenleaf Whittier Bridge is one of the most heavily traveled in the region. It shares many of the same design elements as the Interstate 35 bridge that collapsed last summer in Minneapolis. In the wake of the Minnesota bridge collapse, where 13 people were killed and 100 injured, the Whittier Bridge and hundreds of similar bridges around the nation were inspected.

In August, the Whittier Bridge was deemed safe though "functionally obsolete" and "structurally deficient." It's slated to be replaced in 2011, but not until after the 1st Lt. Derek Hines Bridge is fixed to minimize the impact on traffic.

According to a MassHighway spokesman, a senior engineer reviewed the photographs of the bridge and said the bar appears to be "a steel angle sway bracing," a secondary component meant to give the bridge rigidity. If those are missing, it does not affect the safety of the bridge or its structural integrity, he wrote in an e-mail.

But the loose pieces are a safety concern for the many boaters who pass under the bridge each day.

"We are concerned about these loose pieces falling on boaters. The district has acted promptly to remove a piece as soon as it was reported before it could fall. We have had instances of bracing reported loose and hanging in the past, and the district has responded by removing the loose pieces," the MassHighway spokesman said in an e-mail to the newspaper.

"Since the deck is reinforced concrete and it is very stiff, it acts like a diaphragm to distribute the sway loads, so these members really would not see that much load anyway. In fact, several years ago, District 4 spent a considerable amount of money to strengthen the wire rope hangers that suspend the bridge at the arch span," he continued.

Following the collapse of the bridge in Minneapolis last summer, Massachusetts released a list of 558 "structurally deficient" bridges — bridges in some form of disrepair, but functional — 14 of which were in the Merrimack Valley, the Whittier Bridge among them. Structurally deficient bridges are inspected at least every year.

State officials declined to provide a full copy of the inspection report, saying the Whittier is a "security sensitive bridge." It offered to produce a redacted copy in a week.

"If there was an issue with the bridge, it would be closed," a MassHighway official said.

North of Boston Media Group sent copies of the bridge photos to several local bridge experts, but none responded by press time. Sen. Steven Baddour, D-Methuen, chairman of the Legislature's Transportation Committee, whose district includes Greater Newburyport, did not return several phone calls to his office seeking comment.

'Functionally obsolete'

The beam likely fell recently, based on the growth of marsh grass around the beam, which was examined by North of Boston Media Group staff last week. In three other spots, the distinctive "X" pattern formed by the sway braces have missing beams. Some 80 feet above the river, looking up toward the bridge's understructure, observers can clearly see where the four missing beams once joined the bridge. Two are missing from the Newburyport end, and two from the Amesbury end.

Additionally, near the middle of the bridge, cars and trucks pass over an uneven surface, perhaps an expansion joint, causing a loud banging noise. The vibrations can be faintly felt through the hull of a boat 80 feet below.

The Whittier Bridge, which connects Amesbury and Newburyport and spans the Merrimack River, is 1,346 feet long and 100 feet wide, and is modeled after the Sagamore and Bourne bridges on Cape Cod.

Most of the bridge uses a common engineering technique called a Warren deck truss, which was the same concept used on the I-35 bridge that collapsed last year. The collapse of the I-35 bridge was tied to a design flaw in "gusset plates," which tie the main beams together. It was not caused by problems with beams like those that have been falling from the Whittier Bridge.

Built in 1951, the Whittier is on an expedited list of bridges throughout the state that need critical repairs and replacements. Work is set to start in the winter of 2011 on the estimated $200 million project. It will follow the replacement of the 1st Lt. Derek Hines Bridge in Amesbury, an $11 million project set to begin next year.

According to MassHighway, the massive project will involve replacing and widening the Whittier Bridge from six lanes to eight. The state says the project will be done in a manner that will allow it to stay open throughout the construction.

"When we undertake these projects, we work closely with local communities to minimize traffic impacts and other disruptions," MassHighway spokesman Adam Hurtubise said. "We would certainly take into account other bridge projects in the area as part of our outreach to local communities."

The Whittier Bridge has been categorized as "functionally obsolete," meaning it is unable to meet demands of an increase in traffic. It was built as part of a rerouting of Route 1. When Interstate 95 was built years later, the bridge became the highway's passageway across the Merrimack River.