Watson Mill Bridge, Spanning South Fork Broad River, Watson Mill Road, Watson Mill Bridge State Park, Comer

Source document Quoted word for word from HAER GA-140. Not written, edited or summarised by this site.

The Watson Mill Bridge carries Watson Mill Road across the South Fork of the Broad River, Watson Mill Bridge State Park, Comer, Madison and Oglethorpe Counties, Georgia. The center of the bridge is located at latitude: longitude: These coordinates were obtained in August using Google Earth There is no restriction on their release to the public. Date of Construction: Designer: Washington W. King

Source document Quoted word for word from HAER GA-140. Not written, edited or summarised by this site.

The Watson Mill Bridge is an wood covered bridge that spans the South Fork of the Broad River and links Madison and Olgethorpe counties. Featuring a Town lattice truss, it is long and approximately 19' wide and 19' high from the deck to the roof ridge, with a single-lane passage. It is located upstream of a nineteenth century mill dam and an industrial complex that served Watson s gristmill and other facilities. Watson Mill Bridge is the longest of sixteen surviving covered bridges within the state of Georgia and one of four remaining covered bridges designed and constructed by Washington W. King.

King was the son of the distinguished bridge builder and former enslaved person Horace King The bridge now serves as the focal point of the Watson Mill Bridge State Park and continues to serve as a vehicular and pedestrian bridge. WATSON MILL BRIDGE HAER No. GA-140 (Page 11) Description: The Watson Mill Bridge is a three-span Town lattice truss covered bridge on two abutments, one mortared rubble stone and one concrete, and two mortared rubble stone piers. Both abutments were rebuilt, one with the original stone and the other made from concrete in the It was originally long, but after restoration efforts in the ground was built out, shortening the span to There is a short stringer approach at both ends of the bridge seated on the abutments.

Overall, the bridge is wide with a 16' roadway, a height restriction of 9', and a weight restriction of three tons. On site, these regulations are identified by a height guard at both entrances of the bridge and a load sign on the bridge. The lattice truss design in the Watson Mill Bridge is based on Town s updated patent, which added a secondary upper and lower chord and doubled the number of treenails at each intersection of the members. The height of the lattice system from primary lower chord to primary upper chord is 14' and spans the entire length of the bridge. The upper and lower chords are two pairs of 2- " x 12" planks that hold the diagonal members of the truss together. The truss s diagonal members are composed of 3" x 10" planks set 57 degrees off the horizontal.

Where the planks intersect, they are fastened with two 2" diameter treenails (modified trunnels, wood pins). Two treenails of the same dimension fasten the diagonal members to the chords at every intersection. There are two windows on the east side of the bridge and one window on the west side of the bridge; however, the lattice is not cut or interrupted by these openings. The openings were included primarily to counteract the darkness that resulted from the length of the bridge, but they also allow for a draft into the bridge that dries the interior members, reducing rot. The primary lower chords rest on 7" x 13" timber bolster beams on the mortared rubble stone piers and 12" x 12" transverse steel support beams on the piers and abutments.

There are two 6" x 6" chin braces attached to the top of the inside transverse steel support beam on each pier and abutment. The steel beam and chin braces are connected by a chin brace anchor assembly and go through the floor decking to support the lattice truss. There are sixty-five 5" x 14" transverse floor beams on top of the primary lower chords. On top of the transverse floor beams are eleven lines of longitudinal joists spaced apart. Laid atop the longitudinal joists is decking of 3" x 8" planks, and then above are timber treadways, two sets of four-plank-wide strips running the span. The overhead bracing includes 6" x 9" notched tie beams that rest transversely on the primary upper chords. In between the tie beams is WATSON MILL BRIDGE HAER No.

GA-140 (Page 12) lateral 4" x 4" cross bracing. There are knee braces and sway braces connecting the lattice and tie beams to the principal rafters on the gable roof, respectively. The principal and common rafters form the gable from the primary upper chords to the ridge beam at the upper portion of the ridge. The common rafters are approximately 2" x 6" whereas the primary rafters, found next to the tie beams, are approximately 4" x 4". Longitudinal purlins run along the top span of the rafters, spaced 8" apart. The roofing is comprised of cedar shingles nailed to the purlins. The roof of the bridge overhangs 12" at the eaves.

The bridge is clad in board-and-batten siding fastened to the outside of the chords and reinforced by a 2" x 5" longitudinal nailer at the mid- height of the truss on each side. The center span between the two piers has one window on the eastern side of the bridge and two windows on the western side. The lattice continues behind these three openings without interruption. The portals to the bridge are straight with hipped openings. There is a cedar sign over each portal that has the painted inscription: Watson Mill Bridge, Est. Watson Mill Bridge is in good, functioning condition and is still in use as a vehicular and pedestrian bridge.

The field team verified that there is significant deflection in members throughout the structure, but, due to the Town lattice truss structural redundancy, the bridge remains active. Simple thermal imaging did not indicate significant water intrusion damage to the lumber. History: Design and Construction On May 5, the Oglethorpe County commissioners recorded that the Watson Mill Bridge was completed in accordance with the original plan and specifications. The construction contract was for including materials. The specifications were exceeded in some respects, for example, the bridge was longer than contracted, and there was additional rock and masonry supporting both ends of the bridge. Upgraded siding on the exterior also surpassed expectations.

The bridge was designed and built by Washington W. King, eldest son of former slave and bridge builder, Horace King (Figure 1). Horace King learned the trade of covered bridge design and construction from his former enslaver, John Godwin, an Alabama bridge builder who utilized the Town lattice truss. King established his own career as a WATSON MILL BRIDGE HAER No. GA-140 (Page 13) bridge contractor following his manumission.1 His five children were also engaged in this business, but his eldest, Washington W. King, brought this trade to Georgia and helped revolutionize transportation in the late nineteenth century. Covered bridges became common in the late eighteenth century due to their durability and longevity.

The Town lattice truss featured in the Watson Mill Bridge was an early effort to update timber bridge design. It replaced traditional joinery with a more structurally efficient and easily assembled system of layered diagonal members pinned with wooden treenails between upper and lower horizontal chords.2 The truss namesake, noted architect Ithiel Town, patented the design in and updated it in to increase the allowable load.3 Its many small, closely spaced, diagonally gridded lumber, provide structural redundancy and promise a long period between initial signs of stress and failure.4 Thousands of bridges built around the world featured the Town lattice truss.

By only about 110 survived in the United States.5 Evolution and Alterations The Watson Mill Covered Bridge has demonstrated considerable durability, remaining in service for well over a century despite environmental changes and periodic structural interventions. Its construction of primarily rough-sawn heart pine enables estimates of how the timbers were manufactured and when they were added to the structure. The original roof was also made from wood but was first replaced with a metal sheet for ease of maintenance and then replaced a second time with wooden shingles to evoke the original design. In a photograph taken by the Soil Conservation Service showed the surrounding landscape, including terracing southwest of the bridge, under cultivation.

Although no structural adjustments were made to the bridge within this timeframe, changes in land use may have modulated runoff and sediment patterns that impacted the channel of the river. In the bridge was restored by the State Highway 1 Joseph D. Conwill, Builders and Practices, in Covered Bridges and the Birth of American Engineering, eds. Justine Christianson and Christopher H. Marston (Historic American Engineering Record, National Park Service, 2 Lola Bennett, History of Covered Bridges in the United States, in Covered Bridges and the Birth of American Engineering, eds. Justine Christianson and Christopher H. Marston (Historic American Engineering Record, National Park Service, 57. 3 Ithiel Town, U.S. Patent No. January 28, 4 Brenda Krekeler, Covered Bridges Today (Daring Books, 19.

5 Bennett, History of Covered Bridges, 58. WATSON MILL BRIDGE HAER No. GA-140 (Page 14) Department at a cost of The restoration work included the replacement of weak or rotted timber members with treated pine, installation of new siding, and installation of a new roof. The old metal roof was taken out and replaced by a wooden shingle roof, returning the bridge to a historically appropriate roofing material. In preservation and stabilization work was carried out under contract for Work included applying wood preservative to the exterior siding (for protection from weathering, adding steel rod cross-bracing to the underside of the floor system, and installing horizontal guard rails at the chin braces, safeguarding the truss against vehicular impact.

Structural improvements included steel beams to stabilize jack posts under the bottom chords, heel plate modifications to ensure proper bearing of the bottom truss members, and excavation with drainage improvements to alleviate hydrostatic pressure on the south approach fill.7 In the bridge was extensively reconstructed and rehabilitated. One of the original piers was replaced with reinforced concrete capped with timber members. Timber caps in pier two through four were replaced with steel tube caps. These modifications drastically changed the original substructure system, moving major load-bearing parts from masonry and timber to reinforced concrete and steel components.

Estimated to post the lower bottom chord of the left truss in span two was retrofitted to correct sagging, indicating prior tensile deflection in the member. The retrofit appears to consist of supplemental timber reinforcement mechanically fastened to the existing chord. This intervention retained the original load path while augmenting the tension capacity of the member. New chin braces were installed at pier three, and new cross-bracing was added to span one. By the time of the inspection, the bridge was posted for a three- ton load limit and restricted vertical clearance. These adjustments were made in response to impact damage, long-term degradation of material, and concerns about lateral stability.

Watson Mill State Park Watson Mill Bridge State Park is approximately five miles south of Comer, Georgia. This area began to grow when it was purchased by Gabriel Watson, who developed the site into a place for tourism. Watson began development by commissioning the construction of the 6 Lisa Raflo, Watson Mill Covered Bridge and Mill Historic District, National Register of Historic Places Nomination Form (National Park Service, 7 Lisa M. Russell, Georgia Covered Bridges (History Press, 44. WATSON MILL BRIDGE HAER No. GA-140 (Page 15) Watson Mill bridge which connected Oglethorpe and Madison counties. Shortly after construction of the bridge, there was a fire in that destroyed the Watson family home and a local community store. Between and the site was purchased by L.

Frank Edwards, who developed the hydroelectric mill, powerplant, and other ancillary buildings downstream of the bridge. This site heavily influenced agricultural and industrial development in both neighboring counties. While the bridge itself was not modified at the time, the river s hydrological conditions were altered by the construction of the dam and mill race.8 Plant operations ended in and the state acquired the property twenty-five years later to restore the bridge and safeguard remaining archaeological data. The site still serves as a gathering place for travelers and a reminder of the technological development characterized rural industrialization in North Georgia. Today, the Watson Mill Bridge is the longest covered wooden bridge still standing in Georgia.

It is one of only fourteen covered bridges surviving of the more than 200 built from the mid-nineteenth to early twentieth century in Georgia.9

Source document Quoted word for word from HAER GA-140. Not written, edited or summarised by this site.

AASHTO Guide for Commonly Recognized (Core) Structural Elements. Commonly Recognized (Core) Structural Elements. American Association of State Highway and Transportation Officials, Bian, H., S. E. Chen, and W. Liu. "Error Sources in Processing Lidar- Based Bridge Inspection." International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. XLII-2/W7, no. 2 455 59. Bennett, Lola. History of Covered Bridges in the United States. In Covered Bridges and the Birth of American Engineering, edited by Justine Christianson and Christopher H. Marston. Historic American Engineering Record, National Park Service, Cha, Gichun, Sung-Han Sim, Seunghee Park, and Taekeun Oh. "LiDAR-Based Bridge Displacement Estimation Using 3D Spatial Optimization." Sensors 20, no. 24 19.

Conwill, Joseph D. Builders and Practices. In Covered Bridges and the Birth of American Engineering, edited by Justine Christianson and 8 Georgia Department of Transportation, Bridge Inspection and Load Rating Report: Watson Mill Road over South Fork Broad River (Structure No. inspection date October 1, 9 Thomas L. French and Edward L. French, Covered Bridges of Georgia (Frenco Company, 35. WATSON MILL BRIDGE HAER No. GA-140 (Page 16) Christopher H. Marston. Historic American Engineering Record, National Park Service, De Proft, K., C. A. Brebbia, and J. Connor. Timber Structures and Engineering. WIT Press, DeLony, Eric. "HAER's Historic Bridge Program." IA. The Journal of the Society for Industrial Archeology 15, no. 2 57 71. French, Thomas L., and Edward L. French.

Covered Bridges of Georgia. Frenco Company, Gasparini, D.A., and Caterina Provost. "Early Nineteenth-Century Developments in Truss Design in Britain, France and the United States." Construction History 5 21 33. Jones, Harvie P. "The Town Lattice Truss in Building Construction." APT Bulletin: The Journal of Preservation Technology 15, no. 3 39 41. Kemp, Emory L., and Jet Lowe. "The Fabric of Historic Bridges." IA. Journal of the Society for Industrial Archeology 15, no. 2 3 22. Knapp, Ronald G. "In Search of America s Old Covered Bridges." Built Heritage 5, no. 1 9 23. Krekeler, Brenda. Covered Bridges Today. Daring Books, Liscombe, R. W. "A New Era in My Life : Ithiel Town Abroad." Journal of the Society of Architectural Historians 50, no. 1 5 17. Lupold, John S., and Thomas L. French.

Bridging Deep South Rivers: The Life and Legend of Horace King. University of Georgia Press, Miller, Terry E., and Ronald G. Knapp. Watson Mill Bridge. Tuttle Publishing, Raflo, Lisa. "Watson Mill Covered Bridge and Mill Historic District, National Register of Historic Places Nomination Form, National Park Service, WATSON MILL BRIDGE HAER No. GA-140 (Page 17) "Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation s Bridges." U.S. Department of Transportation, Federal Highway Administration, Russell, Lisa M. Georgia Covered Bridges (History Press, 44. Sutherland, R. J. M. "Early Nineteenth Century Developments in Truss Design in Britain, France and the United States." In Studies in the History of Civil Engineering, edited by R. J. M. Sutherland.

Routledge, Historians: Joseph Holevinski, Grace Littler, Jaiya Dua, and Cody Johnson; February March

Source document Quoted word for word from HAER GA-140. Not written, edited or summarised by this site.

This report was researched and produced by students at the Georgia Institute of Technology as part of their senior capstone design studio led by Professor Danielle S. Willkens. Field work was completed February by Abdulrahman Alsaeed, Anna Carter, Violet Cerbone, Jaiya Dua, Maya Giorgianni, Matthew Godat, Cynthia Hamlett, Joseph Holevinski, Cody Johnson, Jiayi Li, Grace Littler, Ella Newell, Julia Raczkowski, Brian Roberts, Keaton Tsepas Bucciero, Sydney Wetterhan, and faculty members, Danielle S. Willkens and T. Russell Gentry. This historical report was submitted along with a set of measured drawings to the American Institute of Architects for the HABS Charles E. Peterson Prize. Supplemental Material: WATSON MILL BRIDGE HAER No. GA-140 (Page 18) Figure 1.

An illustration of bridge builder Horace King and his sons. Washington W. King is on the upper right, from John William Gibson and W. H. Crogman, Progress of a Race (J. L. Nichols & Co.,