Unique Repair Strategies Ensure Lighthouse Longevity

After more than a century of extreme storms, ice floes and controlling waves, the foundation on which Maryland's Thomas Point Shoal Lighthouse sits required extensive restoration due to corrosion and deterioration.
August 11, 2010

About one mile out from the shore of the Chesapeake Bay stands the nation's last operational screw-pile foundation lighthouse in its original location — Maryland's Thomas Point Shoal Lighthouse. The 43-foot-tall lighthouse has withstood the most tremendous weather conditions of the Bay and South River since 1875. Located about four miles from Annapolis, Md., the lighthouse is a beacon of safety for vessels traveling the bay. It was constructed offshore to alert watercraft operators of the dangerous shoal near the coastline to the west. The Victorian cottage-style lighthouse, nested atop a hexagonal foundation, has also served as a weather center for many boats that have serviced the Chesapeake Bay waterways. Before 1986, when it became fully automated, Maryland's Thomas Point Shoal Lighthouse was the last of its kind to operate manually. The significance of this lighthouse is so great that it was awarded National Historic Landmark status in 1999.

Determining The Extent Of The Damage

After more than a century of extreme storms, ice floes and controlling waves, the foundation on which the lighthouse sits had seen better days. A structural assessment conducted by the United States Coast Guard in 2001 determined galvanic corrosion was occurring at the splash zone (waterline) of the lighthouse's foundation. Anne Arundel County of Maryland, the Annapolis Maritime Museum (Annapolis, Md.), the United States Lighthouse Society (San Francisco, Calif.) and its Chesapeake chapter based in Annandale, Va., teamed to help with the first phase of the much-needed restoration project. At an estimated cost of about $300,000, the group was able to obtain $182,385 in grants to devote towards funding the first phase. Among the grantors were the Maryland Historical Trust, the National Park Service and the Getty Foundation, a program of the J. Paul Getty Trust (Los Angeles, Calif.), which awarded a $60,000 grant.

To begin the first phase of restoration, a second assessment, carried out by Kann & Associates (Baltimore, Md.) and Silman Associates (Washington, D.C.), confirmed the U.S. Coast Guard's initial discovery of corrosion at the splash zone. A section loss of about 10 percent was reported for steel WF-beams and cast iron knuckles, as well as a loss of about 30 percent of tierods and turnbuckles.

The group turned to Structural Preservation Systems, Inc. (SPS), a national leader in marine facilities repair, to aid in the preservation of the lighthouse's foundation. SPS began its portion of the project by removing the existing coating and rust scale from the splash zone. This process gave Keast & Hood Co., a Washington, D.C. firm that specializes in historic preservation engineering projects, the opportunity to inspect and fully assess the magnitude of the corrosion.

Challenges To Restoration

Although the lighthouse's foundation originally consisted mostly of wrought iron and cast iron, it had been replaced at some point with standard A36 carbon steel as part of a repair attempt. Unfortunately though, the introduction of these dissimilar metals to the marine environment had resulted in galvanic corrosion. However, recoating the entire foundation was deemed to be too costly of a solution.

Another factor that impacted the restoration approach was that the project had to follow the guidelines of the Secretary of the Interior's Standards for the Treatment of Historic Properties and the Maryland Historical Trust (Crownsville, Md.). Furthermore, some of the grant-providing organizations involved in the project placed restrictions on the manner in which work must be performed, including a time limitation. These challenges required a creative restoration approach.

"We sought a long-term yet cost-effective solution to these challenges," said Mark Howell, project development manager of Structural Preservation Systems. "We wanted to break the costly cycle of a quick fix and ongoing repairs. As such, SPS devised unique treatments for the screw pile foundation that will extend the useful life of the structure. As a trusted marine facilities repair specialist, SPS was able to offer time-sensitive, affordable and safe solutions to aid in the preservation of this National Historic Landmark."

Repairing, Replacing The Steel, Iron And Wood

Duties performed by SPS included the repair and partial replacement of units within the existing steel and iron substructure, along with the assessment and repair or replacement of deteriorated structural wood components. Wooden replacement beams were mortised in order to accommodate existing tenoned floor joists at the perimeter edge of the cottage level balcony. The new edge beam between piles four and five was notched, instead of mortised, to receive the joists (full mortising was not possible without removal of the privy structure). The ends of the beams were then mitered and mortised, or mitered with cut tenons.

Commensurate with the Secretary of the Interior's Standards for Historic Preservation, liquid wood consolidant was applied to the tenoned ends of six balcony floor joists between piles five and six. Consolidation was determined to be the appropriate treatment to stabilize the existing deteriorated wood and retain the original fabric. Because the fascia boards were extremely rotted and unable to be removed intact, they were replaced at all locations where edge beam replacement occurred. Finally, to complete the replacement of the edge beams between piles five and six, as well as four and five, the floor joists had to be jacked up to match the likely original elevation before it began to sag due to deterioration of the perimeter beams.

At the end of the first phase of the project, corroded connections within the foundation's splash zone had been exposed by removing existing coatings and rust scale, inspected by engineers for structural integrity, and coated with a polyamide epoxy. Significantly deteriorated tierods, clevis ends and turnbuckles that were not original to the structure were removed and replaced. When these replacements took place, isolator shims were added where possible to lessen the galvanic effect of dissimilar metals.

The Thomas Point Shoal Lighthouse's location in the middle of the Chesapeake Bay required SPS craftsmen to reach the job site by boat. Shallow draft areas surrounding the lighthouse and limited docking space restricted the size of boat workers could use. Weather conditions on the Bay and the South River had to be closely monitored to ensure worker safety. At times, work was prevented because of small craft advisories issued by the National Weather Service. Because the majority of the work being performed took place near the splash zone of the foundation, work areas were often submerged by the tide and powerful wave conditions.

Working in line with fellow contractors and the preservation group, SPS was able to successfully implement its repairs and revitalize the splash zone portions of the Thomas Point Shoal Lighthouse. In July 2007, spectators, tourists and historic preservation enthusiasts gathered to witness the reopening of the lighthouse, and public tours have been ongoing ever since.

         
 

RELATED ARTICLES FROM BD+C

Comments on: "Unique Repair Strategies Ensure Lighthouse Longevity "