Knowlton Covered Bridge Rehabilitation

Background

Ohio is home to more than 115 historic covered bridges, most of which were built during the 19th century. These attractive timber structures are a charming reminder of our country’s history, and they remain popular tourist destinations. Covered bridges are also the subject of many long-time traditions and stories, such as holding your breath as you cross to make a wish come true or the nickname of “kissing bridges” due to the seclusion they provide couples as they cross. Woolpert—along with Ohio’s counties and the Ohio Department of Transportation (ODOT)—understands the importance of preserving and restoring these cultural resources.

Built in 1887, the Knowlton Covered Bridge in Monroe County is one such historical treasure that draws visitors and locals throughout the year. It was known historically as the Long Bridge, and at 196 feet, it is still Ohio’s second longest covered bridge. In 1980, it earned a spot on the National Historic Places list, giving it national recognition. Unfortunately, age, decay, and flooding left the structure in poor condition and led to eventual collapse of the center span in 2019, followed by an end span in 2020. Woolpert’s reconstruction efforts were able to restore this landmark—which is a part of the Wayne National Forest Covered Bridge Scenic Byway—and bring back a part of state history.

Challenge

The Knowlton Covered Bridge project faced several challenges that added to the effort’s complexity. Restoration planning began in 2015, when Monroe County contracted Woolpert to inspect the bridge and rehabilitate the existing, damaged members. Inspection of a covered bridge requires accurate observations, detailed analysis, and knowledge of timber types and characteristics. Because no existing plans of the Knowlton Bridge were available, thorough field measurements and hands-on inspection procedures were especially important. In addition to the condition rating of the existing members, the team developed a finite element model using Bentley STAAD to analyze the truss system and determine which members required replacement to meet current pedestrian capacity requirements. By 2018, the project was ready for bid, but after these came in higher than estimated for this unique work, rehabilitation requirements and materials were reevaluated.

In 2019, the rebid project encountered a major setback when the almost 100-foot-long center span—including the prominent timber Burr Arches—collapsed into the river below only a few weeks before bids were due. The dramatic event was captured on video by locals. The following year, one of the end spans also fell, further complicating the project. The team pivoted their goal to reconstruction of the two spans and rehabilitation of the remaining end span.

Solution

The innovation in this project is rooted in bringing back the beauty and character of a historic covered bridge built more than 130 years ago. The Knowlton Covered Bridge’s truss system utilizes Burr Arches, a creative and visually appealing style that features multiple timber arches integrated with a multiple kingpost truss design. While most modern bridges are built using materials such as concrete, steel, and fiber reinforcement, covered bridges are special due to their use of timber and stone local to the area at the time of construction. The Knowlton Bridge was constructed using large white oak timber members, some more than 50 feet long. Initially, the project team had planned to source timber for the Knowlton reconstruction through standard bidding and commercial supply, which at that time substantially increased the cost of the project due to the bourbon industry’s expansion and demand for white oak. However, in a unique agreement with Monroe County, ODOT allowed the county to provide all of the project timber for their share of project costs. In support of the restoration efforts, more than 115 white oak trees were harvested from within the county, and the milling and grading labor was performed by a local ODOT-certified Amish builder and Mennonite-owned mill. Using locally sourced materials elevates the historic and special nature of the bridge for the public, and the contribution of home-grown timber provided the county’s funding match to ODOT’s $1 million, reducing the project’s overall cost to the county.

By nature, covered bridge projects are complex, as the team must restore the structure to meet current standards while maintaining the original construction’s craftmanship and authenticity. Construction of these bridges requires the services of experienced timber framers who understand timber selection, orientation, fabrication, and installation. To recreate the historic detail and match the beauty and character of the original bridge, it was a project requirement that traditional means and methods were used in construction in addition to using historic materials. As such, the selected contractor was obligated to have an experienced timber framer on site at all times during performance of timber framing work. Additionally, these bridges often incorporate historic metalwork, which poses other challenges. In the case of the Knowlton Bridge, the floor beams and decking are suspended from the lower chords by continuous looped iron hangers. To match the original, hand-forged hangers, the team specified replacement hangers that were also hand forged by blacksmiths experienced in traditional metalworking methods like forging, forge welding, smithing, and finishing. Hanger fabrication notes created for the project included tension tests of samples to verify design strength.

Outcome

Aesthetics are vital to a covered bridge’s popularity. Therefore, a significant focus of the Knowlton Covered Bridge project involved improving the structure’s appearance while recreating its historic character. Even prior to collapse, the existing bridge had been in poor condition, with rotting, buckling, and broken timber members, many missing siding panels, and a weathered roof. The existing coating of paint had also been weathered away and was no longer effective. For a covered bridge, the roof and siding not only provide a unique feature, they also protect the structure from weather to prevent early decay and rot. This project replaced the roof with a new standing-seam metal roof and replaced the siding with poplar timber to match the original. The center-span arches were restored using layered timber members framed into the lower chords to match the construction of the original arch. All exposed wood surfaces were coated with fire retardant, and a new Woodscapes color stain was used to give the bridge the classic red look popular to many historic covered bridges. Now rehabilitated to its original prominence, the Knowlton Covered Bridge will continue to provide enjoyment to visitors for generations to come.

Benefits

Now restored to its former glory, the Knowlton bridge serves as a destination for tourism that benefits the local economy. The bridge is conveniently located adjacent to a park, and pedestrians can once again use the bridge to cross the Little Muskingum River. During the 2024 winter holidays, the county strung solar-powered lights inside and outside the bridge to provide a festive atmosphere, further highlighting the structure as an area showcase. Plans are in the making to establish an annual festival celebrating the bridge to share the experience with visitors.