By: Bill Wilson
Since when is a tugboat denied the opportunity to see things through?
It happened frequently on the Piscataqua River between Kittery, Maine, and Portsmouth, N.H. With a bridge sitting at a 25° skew, tugboat operators had no choice but to release vessels right before the span, chug to the other side and hope for the best. The Piscataqua is not friendly, producing current as fast as four knots.
“It was a span of just over 200 ft, and with the 25° skew you really ended up with about 175 ft you could use,” Jeff Folsom, assistant bridge program manager for the Maine DOT, told Roads & Bridges. “To make matters worse there was not any fendering system.”
The challenge was to reduce the severity of the skew, and adding to the complexity was the fact the new vertical lift bridge, named the Sarah Mildred Long Bridge, had to accommodate both vehicular and rail traffic. After looking at several different combinations and alignments the project team was able to reduce the skew to 15° and increase the span length to 300 ft.
“From a safety standpoint that is really a pretty big deal,” said Folsom.
The new skew, however, also had to be gentle to the existing rail line, which had to turn right before the bridge to get onto the structure and turn again to continue along the river bank. The project team was able to match the degree of curvature on the New Hampshire side and even made it flatter on the Maine side.
The Sarah Mildred Long Bridge has three different types of piers: two tower piers in the water, six standalone piers, and three shared piers which supports both the vehicular span and the rail span. There are four approach piers, and the three shared piers are on the approach spans. In total there are 15 piers. The bearings below where the railroad bridge rests allow for sliding and rotating, while the bearings underneath the vehicular portion are monolithically tied in.
“You are trying to accommodate those various movements, displacements and stiffness criteria for the load cases, which are vehicular and rail,” Jay Rohleder, senior vice president at FIGG Bridge Engineers Inc., told Roads & Bridges.
Due to the fast currents, crews constructed a fixed work trestle that was pile-supported, which provided a stable platform for cranes and drilled shaft equipment.
There are a total of 29 drilled shafts socketed into 20 to 35 ft of rock, with the overall shaft length varying from 50 to 120 ft deep. Four piers were placed on spread footings that bear directly on bedrock.
At roadway level the bridge offers 65 ft of clearance. All of the mechanical and electrical elements for the vertical lift are located in a room at the base of one of the towers.
Project: Sarah Mildred Long Bridge
Location: Kittery, Maine, and Portsmouth, N.H.
Owners: Maine and New Hampshire DOTs
Designer: FIGG/Hardesty & Hanover joint venture
Contractor: Cianbro Corp.
Cost: $163 million
Length: Vehicular: 2,803 ft; Rail: 1,795 ft
Completion Date: March 30, 2018
About The Author: Wilson is editorial director of Roads & Bridges.
