B09_Amendment_3_Structural_Investigaton_Fort_Washington_Lighthouse_021319_0003.pdf

Fort Washington Lighthouse Structural Investigation

Date: February 13, 2019 Project Name: Fort Washington

Lighthouse

Attention: Diana Bramble Silman Project #: W3655

Company: NPS RE: Structural Investigation

From: Nathan Hicks cc: Emily Garrison

Silman has been retained by the National Park Service (NPS) to investigate the current condition of the lighthouse at Fort Washington and provide recommendations for stabilization. On Friday, December 18, 2018 a representative from Silman visited the site to perform the structural assessment. In addition to the site observations, Silman was also provided with a 2017 “Lighthouse 80; Inspection & Recommendations” report and a 2009

Historic American Building Survey (HABS) “Fort Washington Light” report.

The lighthouse, or fog bell tower, was built in 1882. In 1903 the lower section of the tower received flooring and was encased with siding as the bell tower was turned into a lighthouse. In 2005 the tower was turned over to the NPS, and in 2009 the NPS Historic

Preservation Training Center (HPTC) performed a major restoration. The lighthouse is on a peninsula, where the north, south and west elevations all face the river (see Figure 1).

Figure 1 1944 Plot Plan Showing Tower Highlighted in Red (National Archives)

The lighthouse structure is wood post and beam construction with a hipped roof. Past reports have indicated that the wood is typically southern yellow pine, with white pine used for the floor sheathing. The tower is around 35-foot-tall from grade to the roof peak. The wood-framed portion sits about 2 feet above grade on brick masonry piers. The tower is about 13-foot square at the base and tapers to about 5-foot square near the roof (see

Figure 2).

Figure 2 Lighthouse Elevation from HABS Report

The HABS report included drawings for a contemporary fog bell tower, which is of similar construction to the Fort Washington Lighthouse (see Figure 3). Some rough measurements were made in the field but will need to be confirmed as part of future repair work.

The wood framing consists of 8”x8” corner posts that sit on the 8”x8” wood perimeter beams at the base of the tower. These perimeter beams bear on 2-inch-thick wood sill plates above the roughly 2-foot square brick masonry piers. The beams appear to be anchored into the piers with ¾” diameter steel rods.

The first and second floor levels are sheathed with wood siding boards at the exterior, while the third-floor level is open with the bell exposed. The wall construction consists of 2”x6” studs with 6”x6” X-diagonal bracing between the first and second floor levels. There are ¾” diameter steel rods just above the first floor and below the second-floor framing to better tie the structure together.

The first-floor framing consists of 3”x8” joists with cross bridging at mid-span and 1” thick floor sheathing. The second-floor framing consists of perimeter 6”x6” beams with 3”x6” joists and 1” thick floor sheathing above. Observations for the third floor and roof framing could not be made.

Figure 3 1886 Drawings for Fog Bell Towers at Lazaretto & Blackstone Island (HABS Report)

Investigation

In general, there are areas where the wood framing at the lighthouse is in poor condition.

The north elevation of the lighthouse, along with the north ends of the east and west elevations, show the highest degree of damage and deterioration. Damage to the wood framing is typically more severe at the base. The deteriorated regions of wood are likely due to increased exposure to the elements given the prevailing winds and flood events. Figure 4 shows the north elevation with significant damage, while Figure 5 shows east elevation on the left and south elevation on the right.

Figure 4 North Face of the Lighthouse

Figure 5 East (Left) & South (Right) Elevation of the Lighthouse

The most significant damage is concentrated at the connections between the masonry piers and the wood framing at the northeast and northwest corners. The piers themselves appear to be in relatively good condition, although a concrete cap was added to these piers at some point in the past. The wood sill plates on top of these piers have significant section loss and the beams above are crushing what wood remains. The sill plates are tied together with steel rods, which are doing little given the extent of deterioration (see Figure 6 & Figure 7).

Figure 6 Wood Plate Deformation and Deterioration at Northeast Support

Figure 7 Deterioration of Northeast Corner

Given the prolonged exposure to moisture significant deterioration is also evident at the base perimeter beams and the posts that frame into the northwest and northeast piers.

Figure 8 shows the wood of the base beams crushing and the associated deflection at the northwest pier support. Figure 9 shows the longitudinal splitting at the bottom of the north beam, and the associated deflection of the structure due to the section loss.

Figure 8 Crushing of West Base Beam at Northwest Support

Figure 9 Splitting of North Base Beam

At the posts above these two piers there is similar splitting and deterioration evident, which is concentrated near the bottom of the posts. Figure 10 shows the deterioration evident at the bottom of the northeast corner post. Deterioration is also evident in the wood siding on the north elevation and at the north ends of the east and west elevations.

Figure 10 Splitting and Deterioration of Northeast Corner Post

Recommendations

Silman has been asked to look at temporary stabilization measures to address the observed wood deterioration. Below we have outlined two options for this temporary work. It should be noted that long-term permanent repairs will eventually be required and will likely require replacement in kind of the affected wood posts, sills and beams.

Option 1

Cribbing would be added below the existing wood structure to support the deteriorating north beam and the north ends of the east and west beams (see Figure

11). The benefit of this approach is that it could be used to help facilitate permanent repairs. Given the taper of the structure the cribbing beams could be used to pick up the north posts and transfer much of the load into this temporary cribbing to allow for wood replacement/repairs above the two piers. The downside with this approach is that it could be more susceptible to damage during a flood event.

Figure 11 Schematic Design for Option 1

Option 2

New wood sisters would be added to the exterior of the existing wood perimeter beams at the north, east and west elevations. The sisters would be installed along the full length of the members to bear on the wood sill plate (see Figure 12). The benefit of this approach is that it would less susceptible to damage during flood events. The downside of this approach is that it could accelerate deterioration of the perimeter beams if left in place for a prolonged period by creating a surface for water to pool.

Figure 12 Schematic Design for Option 2