Posted: 7:22 a.m. Thursday, March 14, 2013
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.”
“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