Hoovers Mill Bridge closed for repairs

(2/24) In early January, the county announced the temporary closing of the bridge on Hoovers Mill Rd. that spans Owens Creek. Google Maps showed an image of a spindly iron one-way bridge. Intrigued, I drove out to see the bridge. It’s a bit off the beaten path. Both roads leading to it, Hoovers Mill and Frushour, are narrow roads with a few farms along them.

The bridge was blocked by concrete barriers placed across the road on both sides of the bridge. The bridge looked fine so I walked out on it. There I saw the damage. An intricate vertical truss looked as if a giant had swung a telephone pole like a baseball bat into it. The metal was bent and cracked with smaller braces ripped away from their rivets. Yes, rivets.

That got me thinking, "How old is this bridge?" As I looked further, I realized the roadway was made of wood planking. Most of the metal work was connected by pins and rivets. Some of the damaged metal was cracked in addition to being bent. That made me wonder if the metal was wrought iron rather than modern steel. Wrought iron can be less ductile and crack rather than bend when overloaded. The construction reminded me of my brother’s old Erector Set with hundreds of little braces riveted together to make up the bridge.

The metal of the bridge was covered with flaking paint which, up close, wasn’t very pretty. With the ancient riveted construction and the rough state of the paint, I began to worry about the future of the bridge so I contacted Frederick County’s Mike Ramsburg. He is the Maintenance Section Supervisor, Division of Public Works, Office of Highway Operations (and I thought the Federal government had long titles!). Mike and Jason Stitt, P.E., Division Chief, Office of Transportation Engineering, answered my questions and provided interesting additional information.

First of all, the Hoovers Mill Bridge is one of five historic bridges crossing Owens Creek. All five are county-maintained. I counted a total of 14 bridges (or at least road crossings, including the five historic bridges) over Owens Creek as it falls from the Catoctins on its way to supply the Monocacy. Just think how many bridges there must be in Frederick County if just one creek has so many!

The damaged Hoovers Mill Bridge was built in 1887. It is a single span pony Pratt truss bridge. The single lane bridge is set upon random stone abutments with a span length of 68 feet. The clear roadway width is 11’-0". Joints of the bridge are secured with pinned connections. My perception of the bridge as "spindly" is a key trait of truss bridges. Truss bridges are usually what engineers call "statically determinate". This means the stresses on the bridge can be accurately calculated allowing designers to use a minimum amount of metal. This makes truss bridges easier to design efficiently and economical to build.

The Hoovers Mill Bridge is right in the middle of a 5.5 mile stretch (as the creek flows or 4 miles "as the crow flies") of Owens Creek that features 5 historic, county-maintained bridges.

The first crossing up stream has a similar truss bridge on Apples Church Road. It was built in 1917 and is a simple span steel pony truss bridge. The structure is 62’-0" long measured between centerlines of pins and has an overall length of 65'-7" measured back-to-back from the backwalls. The superstructure carries one lane of traffic with a clear roadway width of 16’.

Further upstream is one of three wooden, covered bridges in Frederick County. It carries Roddy Road over the creek, just outside of Thurmont. This structure, originally built in 1892 and rehabilitated in 1995, is a simple span combination steel beam and timber deck covered bridge that is 39’ long. The superstructure carries one lane of traffic with a clear roadway width of 12.7’. It is befuddling that the short road that parallels Owens Creek as it travels from Route 15 to then pass under the Roddy Road is named Roddy Creek Road. A search for Roddy Creek was unsuccessful, but I’m sure it is nearby.

Down stream of the Hoovers Mill Bridge, is the modern, SHA-maintained, Route 77 Bridge. Next is another covered bridge on Old Frederick Road. This bridge is a two span, single lane, timber covered bridge with timber deck planks supported by steel stringers originally erected at this site in 1889 and reconstructed in 1994 due to arson. The bridge was built as a single span king post truss acting as the principal support. Subsequently, a middle pier was constructed, and steel stringers were introduced as the deck support system. The abutments consist of concrete caps over stone masonry. The center pier is a solid concrete shaft. Wingwalls are made of stone masonry.

The final historic bridge on Owens Creek is on Old Mill Road. It is a 68'-9" single span wrought iron through truss. The bridge was constructed by the Pittsburgh Bridge Company in 1882. The bridge was rehabilitated in 2008 and carries one lane of traffic with a clear roadway width of 12'-0".

Mike and Jason allayed my concerns for the bridge by describing their plan of action, "Many of the connections on the existing bridge were made using rivets, which is typical bridge construction for that era. The repair will be made utilizing bolts in place of rivets. The bridge is scheduled for rehabilitation in the current Capital Improvement Program (design in Fiscal Year 2018, construction in Fiscal Year 2020). During the design of the rehabilitation, consideration will be given to restore the riveted connections.

"We don’t anticipate any difficulties in executing the repairs. The materials for the repair have been ordered, and the repair will be executed as soon as the materials arrive and weather permits."

I asked how much use the bridge gets, "This roadway has a very low traffic volume (less than100 vehicles per day). Notifications have been made to alert roadway users, including the Board of Education, of the closure so adjustments to regular routes can be made."

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