August 27September 3, 1998
A Bridge Too Old
City officials race to save Philadelphia's only covered bridge.
by Amy Choi
Set in a rugged landscape and framed by an overgrowth of towering trees, the faded red exterior of the Thomas Mill Covered Bridge is starting to show its age.
More than 130 years old, much of the outer structure of the bridge has decayed and split, indicating deterioration of the only standing covered bridge remaining within Philadelphia city limits.
Registered as an historical place on both city and national records, the Thomas Mill Bridge in the Wissahickon extension of Fairmount Park is hidden away from corporate Philadelphia. It spans the Wissahickon Creek and marks the site of the Thomas Mill, which later during the Industrial Age became one of the first mills in the nation to use wood pulp for paper manufacturing.
Now, more than a century after the bridge's original construction, the Capital Program Office of Philadelphia is advertising for and accepting bids for the renovation of the bridge. The project entails an overhaul of the structural members, siding, decking and roofing of the bridge as required by an investigation that will determine the necessary changes.
"This is going to be a very tricky operation," said Jack Eckenrode, the project design engineer of the Fairmount Park team, a unit within the Capital Program Office. "We have to uncover things to see if they need to be repaired and to the extent that they need to be repaired. We can put a definite value on only the visible things, like the roof, the siding and the decking."
The project necessitates bringing in a wood pathologist to determine the amount of decay in the decking and actual structure, even though little decay of the structure is expected due to its protection from the roof. Actually repairing the deterioration of the bridge is also going to require a bit of specialized attention, according to Eckenrode.
"We're hoping to get a contractor who has some specialized training and who has had experience doing historical renovation," said Eckenrode. "With the sort of delicate work required combined with the whole project being over the water, it is going to be very difficult."
The water complicates the project greatly, as it endangers the workers and creates a unique environmental concern.
"A lot of cost is going to go into scaffolding all around the bridge and below the deck to access the bridge and protect the workers," said David R. Slocum, architect and consultant for the project. "We have to also maintain a pristine work environment to try and make sure that nothing falls into the creek, and also provide for catching whatever might fall into the creek."
The bridge is situated on rocky terrain on the north end, while an equestrian trail lines the south.
"It's generally a pedestrian bridge, although people do take their horses across it," said Slocum. "Sometimes park vehicles use it to do work in the park, but it is not a true vehicular bridge."
Of course, it can't be expected to support two-ton vehicles. The Thomas Mill Bridge, like most other covered bridges, was originally designed for inland farmers who needed overland transport. But why covered bridges?
With the Northeast being forest country with many short rivers and creeks, wood made an easy building material for smaller bridges such as the Thomas Mill Bridge, which is 19 feet wide and spans about 87 feet across the creek. Covering and roofing the bridge was essential to protect the wood from the weather.
The Thomas Mill Bridge sports unusual sawtooth decorations along the roofline. Its truss, the framework of beams supporting the bridge and its roof, is a Howe truss, considered the most complex of six common bridge designs. It is the only bridge in the Northeast with this particular truss.
"The Thomas Mill is a simply designed bridge but it has lots of different plates and tension rods," said Slocum. "It was definitely exceptionally designed and it was very well done."
The builder of the Thomas Mill Bridge is unknown, as is the exact construction date. Some records of the Philadelphia Historical Commission point to 1855 as the birth of the bridge, while others indicate that it was built in 1859.
Often, the builder of the bridge can be determined by the style, since most builders use a specific style for all of their bridges. For example, Elias McMellan was known for his many Burr-truss bridges in Lancaster County, which has 29 covered bridges, the most of any county in Pennsylvania.
The Thomas Mill Bridge was rebuilt by the Works Progress Administration in 1939, but those working on the bridge are assuming that the standing bridge maintains the original structure.
"We are keeping very loyal to the original design," said Eckenrode.
However, the renovation does entail a change in the roof of the bridge. Planners intend to remove the existing shingle roof and install a standing metal seam roof.
"We had to do this very carefully," said Slocum. "We went through the Historical Commission to get approval for this project. We're still trying to investigate and we are going to test the fiber stress of some of the structural members to find out if it's all right, see what we can keep.
"After all, this is the only one left in the city, one of the very few left at all," he continued. "It was very well designed and just to be able to preserve it over the Wissahickon would be the best thing that could happen."