After careful historical and engineering review during preliminary and final design of the Longfellow Bridge Rehabilitation Project, MassDOT developed a plan to dismantle, clean, restore and re-set the granite stones of the iconic “salt and pepper” towers of the Longfellow Bridge. The plan will safely maintain the stones’ integrity during the process. Each 58-foot tower has 529 granite stones, which vary in size and can weigh as much as three tons. The towers have settled over the past century, requiring the dismantling and re-setting.
The first step in dismantling the towers is to erect scaffolding around them. Concurrent with removing the stones, the old mortar is cut out from between the stones. Additionally, the concrete liner is demolished, and this work takes place inside the towers.
The original bridge plans and numbering system were used to create a series of stone coursing drawings. The coursing drawings, or two dimensional models, show the individual stones level by level, or course by course. These drawings are important as the current stone numbering system is based on this historic information. Also, these numbers and plans ensure that the proper stones are put in the correct locations during reassembly of the towers. Using this combination of historic and current information, each stone is documented, its condition noted, and then tracked throughout the removal, restoration and reassembly process. Prior to removal, the stones are coded with the corresponding stone coursing drawing number and photographs taken of their condition and location. A temporary marking number is attached to the stone using painter’s tape, which is non-marring. To mark the stones permanently after removal, a number is written on a non-exposed face of the stone, typically the top of the stone, and a photo taken to confirm the number. An arrow showing the direction of installation is also put on the non-exposed face.
Once the stones are documented, they are lifted via a barge-mounted crane on the Charles River using straps or pins and chains. Cracked or damaged stones are lifted onto the barge using additional support and protection. Any stones that are broken or in danger of breaking may be removed in parts, which will be carefully documented and marked to ensure that the pieces are kept together throughout the process.
The stones will be moved via barge to the contractor’s staging area where they will be cleaned, repaired and stored on wood pallets covered with tarp for protection until it’s time to reassemble them. The upstream towers are currently being dismantled and will be reassembled later this year. The downstream towers will be dismantled in fall 2015 during the final phase of construction.
The towers will be reassembled on level platforms that are structurally and seismically reinforced to ensure longterm durability. During reinstallation, stainless steel dowels will be installed at the granite stones for additional seismic reinforcing and a new concrete liner will be installed. The granite stones will be removed to the sidewalk level only. The tower piers will be cleaned and restored in place.
For more information on the project, visit the website at www.mass.gov/massdot/longfellowbridge. For questions or to report issues related to construction, please call the project hotline at 617-519-9892 or email email@example.com.
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