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Posted: March 14, 2013

Palm Beach's Flagler bridge damage discovery detailed in FDOT documents

A gap between the double-leaf bascule section of the Flagler Memorial Bridge shows the settling caused by construction. (Jeff Langlois / Palm Beach Daily News)
A gap between the double-leaf bascule section of the Flagler Memorial Bridge shows the settling caused by construction. (Jeff Langlois / Palm Beach Daily News)

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            Palm Beach's Flagler bridge damage discovery detailed in FDOT documents
This FDOT photo shows cracking at the support structure just west of the movable spans of the Flagler Memorial Bridge.

            Palm Beach's Flagler bridge damage discovery detailed in FDOT documents
This FDOT photo, looking toward the east, shows a long crack along part of the span.

By David Rogers

Palm Beach Daily News

Engineer: ‘Condition of the bascule is critical’

Florida Department of Transportation Secretary Ananth Prasad is set to visit Palm Beach Tuesday to present an engineering assessment on whether the Flagler Memorial Bridge can be stabilized and remain open while its replacement is constructed.

The DOT chief may also present other alternatives, such as the feasibility of constructing a temporary bridge, in his meeting with the Town Council. The assessment is by engineering firm Parsons Brinckerhoff.

A Nov. 6 letter from Timothy J. Noles, principal of Hardesty & Hanover, the engineering firm on the design/build team for the replacement bridge, suggests that fortifying the existing, damaged bridge could be challenging, at the least. The bridge has experienced settling due to construction vibration levels at or below 0.2 inches per second, Noles said in a letter to PCL Civil Constructors project manager Ryan Hamrick. “It’s our opinion that the condition of the bascule is critical.”

Noles writes:

“The following potential mitigation measures were evaluated and found not to address the settlement issue:

Removal of Truck Traffic: Although vibrations due to truck traffic are larger in measured magnitude than those due to the drilled shaft operations, the duration is much shorter and frequency of occurrence is much longer than the vibrations caused by drilled shaft casing embedment. We do not believe removal of trucks will prevent the settlement from occurring …

“Crutch Bents: Crutch bents (lateral structures) can be installed to support the east flanking span to ensure this fracture-critical span is supported if severe distress causes the connection to the rack frame to fail. However, there is no practical method of supporting the existing bascule pier. There is a concern of an increased eccentric load on the timber piles. Access to existing timber piles would require drilling through the adjacent rock layer and further damage to the existing foundation. Also, there is a possibility construction of the new drilled shafts near the existing bascule piers would be compromised by the grout.

Alternative Construction Methods: Drilled shafts can be installed using an oscillator, which has proven to cause less vibration than the current method. However, using an oscillator requires a reaction frame supported on steel pipe piles to counteract the forces necessary to install the casing for the drilled shaft. This reaction frame would require approximately 6 pipe piles (per shaft) that would need to be installed into a firm rock layer. The installation of the pipe piles would result in the similar vibrations in magnitude and duration as the measured vibrations” (of work done by PCL before it stopped drilling shafts near the old bridge this year).”

— DAVID ROGERS

Correspondence among Florida Department of Transportation officers, the engineers and the contractor building the replacement for the 75-year-old Flagler Memorial Bridge pinpoint when engineers realized the extent that construction preparation for the new bridge was damaging the existing span.

Those records, released to the Palm Beach Daily News this week, also note that slight settling of the east half of the bridge into the Intracoastal Waterway occurred even when crews working weren’t drilling shafts for the new bridge’s foundation.

Construction on the new $94 million bridge began in mid-October. Settlement of the south side of the east bascule pier, first recorded Oct. 19, spurred misalignment of the locking mechanism on the current span a number of times. The 1938 bridge was built on wood pilings.

The design/build team consists of Tampa-based PCL Civil Constructors and the infrastructure engineering firm Hardesty & Hanover. At contractor PCL’s recommendation, the Flagler bridge was closed to vehicular traffic from Nov. 5-12.

On Nov. 11-12, the design/build team conducted tests to determine whether the bridge could handle trucks of the maximum legal weight limit. Before the settlement issue arose, trucks of all legal weights were allowed on the bridge. Commerical trucks can easily weigh 10 tons or more. PCL determined the tests caused no additional settlement at the east bascule pier.

At engineer Hardesty & Hanover’s recommendation, however, the DOT soon thereafter re-opened two of the bridge’s four lanes to vehicular traffic, but excluded vehicles weighing more than 5 tons.

On Nov. 12, Michael Sileno, principal associate with Hardesty & Hanover, directed PCL to continually monitor the structure for additional signs of settlement and distress, and to bore into the soil around two drill shafts for the new bridge that are close to the existing bridge. The soil borings would detect additional settlement.

PLC reported the old bridge settled an additional 1/16 inch on Nov. 14 — when no major construction was occurring, according to a memo by Sileno.

That put the total settling at 1 5/8 inches, according to DOT officials, the documents show.

Sileno recommended to PCL project manager Ryan Hamrick on Nov. 14 that the bridge be closed to traffic if additional settlement of 1/16 inch — or more — occurred, so the machinery of the east movable span and the span locks could be evaluated.

The next day, Nov. 15, Peter Nissen, a senior project engineer with DOT consulting firm New Millennium Engineering, told Hamrick that the situation required more than ongoing observation.

“We agree that, based on your engineer’s assessment, there is no immediate concern with public safety due to yesterday’s settlement of 1/16,” he wrote.

“Additionally, the Design-Build Team previously determined that there is significant concern with total settlement exceeding 2” due to potential operating difficulties with the mechanical system,” Nissen wrote. “We have repeatedly indicated in a number of meetings over the last several weeks the need for the Design-Build Team to complete its engineering evaluation/analysis as well as (provide) identification of a means to stabilize the existing structure.”

Hamrick replied that his team did not exceed the construction vibration level allowed specified by the DOT contract.

“After being awarded the contract, the existing structure started settling at times when PCL was not working and at times when PCL was working, but at vibration levels not exceeding the FDOT allowed threshold,” Hamrick wrote in a Nov. 19 to Nissen. “Either the existing structure has a latent defect or the specifications provided by FDOT are defective.”

In mid-February, after the DOT announced the closing of the bridge was “imminent,” district secretary James Wolfe said the ability to adjust the old bridge’s alignment is finite and that construction of the new bridge would continue to damage the existing bridge and pose an increased risk to the public.

“If one corner of the bridge abutment settles and the other doesn’t, it puts stresses on the concrete and results in cracking. It could result in spalling (flaking and chipping off of concrete) and ultimately result in major failing,” Wolfe said at the time.

Imminent closing of the bridge was pushed to April 1 following a DOT discussion with Mayor Gail Coniglio. DOT Secretary Ananth Prasad is set to present a report by international engineering firm Parsons Brinckerhoff on potential ways to lessen the impact of closing the north bridge for 18 months, including repairing the existing bridge, at a council meeting set for 2:30 p.m. Tuesday at Town Hall. He is scheduled to speak to the West Palm Beach City Commission at 4 p.m. the same day.

Gov Rick Scott has said the state is open to exploring that scenario or constructing a temporary bridge if either is feasible.


Engineer: ‘Condition of the bascule is critical’

Florida Department of Transportation Secretary Ananth Prasad is set to visit Palm Beach Tuesday to present an engineering assessment on whether the Flagler Memorial Bridge can be stabilized and remain open while its replacement is constructed.

The DOT chief may also present other alternatives, such as the feasibility of constructing a temporary bridge, in his meeting with the Town Council. The assessment is by engineering firm Parsons Brinckerhoff.

A Nov. 6 letter from Timothy J. Noles, principal of Hardesty & Hanover, the engineering firm on the design/build team for the replacement bridge, suggests that fortifying the existing, damaged bridge could be challenging, at the least. The bridge has experienced settling due to construction vibration levels at or below 0.2 inches per second, Noles said in a letter to PCL Civil Constructors project manager Ryan Hamrick. “It’s our opinion that the condition of the bascule is critical.”

Noles writes:

“The following potential mitigation measures were evaluated and found not to address the settlement issue:

Removal of Truck Traffic: Although vibrations due to truck traffic are larger in measured magnitude than those due to the drilled shaft operations, the duration is much shorter and frequency of occurrence is much longer than the vibrations caused by drilled shaft casing embedment. We do not believe removal of trucks will prevent the settlement from occurring …

“Crutch Bents: Crutch bents (lateral structures) can be installed to support the east flanking span to ensure this fracture-critical span is supported if severe distress causes the connection to the rack frame to fail. However, there is no practical method of supporting the existing bascule pier. There is a concern of an increased eccentric load on the timber piles. Access to existing timber piles would require drilling through the adjacent rock layer and further damage to the existing foundation. Also, there is a possibility construction of the new drilled shafts near the existing bascule piers would be compromised by the grout.

Alternative Construction Methods: Drilled shafts can be installed using an oscillator, which has proven to cause less vibration than the current method. However, using an oscillator requires a reaction frame supported on steel pipe piles to counteract the forces necessary to install the casing for the drilled shaft. This reaction frame would require approximately 6 pipe piles (per shaft) that would need to be installed into a firm rock layer. The installation of the pipe piles would result in the similar vibrations in magnitude and duration as the measured vibrations” (of work done by PCL before it stopped drilling shafts near the old bridge this year).”

— DAVID ROGERS

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