Bridges exist to span gaps, to connect two points on the physical landscape separated by some barrier, be it a river, a ravine or something else.
Not so the Inwood Iron Bridge. Not anymore, anyway.
It certainly served that purpose originally. Built in 1899 by Nelson & Buchanan of Chambersburg, agents of the Pittsburgh Bridge Co., the 160-foot span carried Iron Bridge Road across Swatara Creek for more than 120 years, until closing to traffic in 2005.
But these days, it’s a time machine of sorts, carrying walkers between two eras in transportation history.
Inwood Bridge is a Pennsylvania, or Petit, metal truss bridge, a style that debuted in the U.S. around the 1840s, said Veronica Martin, architectural historian and acting historic bridge program manager in the state Department of Transportation Bureau of Design and Delivery.
Their heyday came a few decades later, between the 1880s and early 1900s, when they got bigger and stronger to serve an increasingly industrialized nation.
“They’re really heavily influenced by the railroads,” Martin said. “So they’re really, really heavy, really, really thick because they needed to handle locomotives.”
Earlier bridges made of wood, she said, not only couldn’t support those loads, but were prone to catching fire from sparks thrown by coal-burning engines.
Pennsylvania wasn’t the only place where metal truss bridges were used. But it likely had as many as anywhere, Martin said, given the state’s natural resources and location. Trains crisscrossed the state, carrying coal to mills all across Pennsylvania, as well as people and goods traveling north and south.
“So you have a massive need for bridges. You need to be able to cross all the roadways, you need to be able to cross the Susquehanna and the Juniata and all these big rivers,” Martin said. “So they just kind of boomed.”
In time, those metal truss bridges were put to use on highways as vehicular traffic increased and the use of trailer trucks to move goods grew.
That’s when metal truss bridges revealed their limitations, though.
“When these bridges were first being built, according to the transportation demands of the day, they were designed to handle 8 to 12 tons,” said Jeremy Ammerman, environmental supervisor in PennDOT’s Engineering District 8. “With modern traffic today, the legal minimum bridges must be able to handle is 40 tons.”
Old metal truss bridges generally can’t accommodate today’s semi-trucks, giant fire engines and the like, he added.
They typically don’t meet modern safety standards either, Martin noted. She said they’re “non-redundant,” meaning each piece of a metal bridge does one job and one job only, with no other piece backing it up in the event of failure, and “fracture critical,” meaning that a vehicle hitting any one part of the bridge makes the whole thing more likely to fail.
Modern concrete bridges, by comparison, are more standardized, faster to install, more cost effective and full of redundant pieces.
“Counties may still be able to build new metal truss bridges. Their rules are a little different sometimes,” Martin said. “But we’ll never again see them on state or federal highways.”
But that’s not to say existing metal truss bridges don’t have value. More and more people believe just the opposite, in fact.
“Across our country, we devote millions of dollars to the preservation of buildings, schools and other cultural and historic landmarks,” wrote Kitty Henderson, executive director of the Historic Bridge Foundation, which formed in Texas in 2007 but has since worked on bridge projects around the country.
“Often, however, when a community takes inventory of their historic properties, the local historic bridge fails to make the list. Somehow, we must elevate the importance of our historic bridges in the stories that identify the communities of our nation and say, ‘this bridge is part of who we are and it must be saved.’”
Pennsylvania’s taken that to heart and is in many ways a leader in bridge preservation, Ammerman said.
“I think the big driver is the connection to the steel industry in Pennsylvania,” he said. “The number and the importance of the bridge fabricators that we had in Pennsylvania, compared to things on a nationwide scale, is one of the big impetuses for our finding value in these structures besides looking at them as antiquated artifacts from our past.”
But time is of the essence; these bridges are disappearing fast. According to PennDOT, Pennsylvania had 851 metal truss bridges in 2001. By April 2018, only 414 remained, some of them the final examples of their kind. There have been more losses since, Martin said.
So PennDOT – in conjunction with the Federal Highway Administration and the state State Historic Preservation Office – developed a Historic Truss Bridge Management Program to preserve what are “by many accounts, the earliest, most diverse and most significant population of metal truss bridges in the United States.”
“Generations of Pennsylvanians have heard their tires sing on the metal decks, seen a river or railroad pass below, fished over the railings, or watched the sun descend behind an old truss,” a summary of the program reads. “Some of these bridges are iconic parts of Pennsylvania’s historic communities and are fondly recalled symbols of many of our hometowns and communities.”
The program’s aim is to identify historic bridges around the state and preserve them when possible. A metal truss bridge marketing program that’s part of the program links historic bridges with people or sites that can use them.
Preservation, though, can be tricky, as the case of the Inwood Iron Bridge reveals.
PennDOT first discussed saving it in 2010, said Ammerman, who was the project’s architectural historian of record at the time. That worked continued through 2012.
Then, assorted snags that shelved the effort for almost three years to the day.
Even when things got back on track, it took another year or two to fully investigate the health of the bridge and consider alternative uses. One thought was to use it along the Lebanon Valley Rail Trail, another to put it in a state park. Ultimately, the bridge found a potential home when Charles and Charlotte Allwein donated ground to house it, less than a mile downstream from the original crossing.
The next step was actually moving the bridge. That’s even harder than it sounds.
These bridges, though tied to steel mills, are not made of modern steel, Martin said. They’re cast iron, contain cast iron pieces, or even feature wrought iron, which no one uses for bridges anymore. So the pool of people who can rebuild or refurbish component parts is small.
Then, Martin said, while some bridges are like erector sets – “you pull out the pins holding it together, you take it to a new location, you put the pins back in and, boom, bridge” – others are trickier, or potentially fragile.
The Inwood Bridge was lifted in pieces, moved on flatbed trucks and put back together, though only after new concrete abutments and other preparation work was completed. Ammerman remembers how nervous the move made him.
Read More: Historic Inwood Bridge off the Swatara, being readied for new placement
“There were essentially straps attached to a crane,” he recalled. “So when they lift it up, you’re just hoping it’s not putting too much strain on 150-year-old metal. It can be a little nerve-wracking until it’s actually free and up on the air.
“But, fortunately, there were no problems on the pick or the reassemble.”
Today, the bridge sits completely on one side of the creek, so that visitors can not only walk on its deck, but poke around underneath the bridge, too, getting the kind of up-close look at its unique structure that’s rarely available. Visitors can see, for example, its “Z-plate” floorbeam hangers. They’re a design feature unique to the Pittsburgh Bridge Co. The Inwood Bridge was just one of five left in the state, as of 2017, to have them.
Educational panels on site explain the history of the bridge, its relocation and more, too.
For those reasons and more, Inwood Iron Bridge is a piece of local history well worth exploring.
“I think a lot of times, when people think of historic bridges, they think of the Brooklyn Bridge or the Golden Gate Bridge, the big suspension bridges,” Martin said. “People sometimes forget that Pennsylvania has some of the oldest bridges in the country. We have some of the top 10 oldest, for sure.
“But even among metal truss bridges, we have some of the oldest surviving examples, and in some cases the only surviving example of certain types. They’re really special.”
Getting there
To reach Inwood Iron Bridge, follow Route 72 north from its intersection with U.S. Route 22 for 4.3 miles. Turn right to cross Swatara Creek on Iron Bridge Road (former site of the Inwood Bridge), then turn right again. Inwood Bridge is 0.3 miles away, on the right.
Learn more
People were saving metal truss bridges in one sense long before this.
“We have documentation of bridges that are two different types, where maybe a flood wiped out one half of a bridge and a new half was garnered from somebody else that didn’t need it,” said PennDOT’s Jeremy Ammerman. “It was shipped out and the two were cobbled into one.”
A good example of that, he noted, is the Hall Drive bridge that crosses Bermudian Creek in Kralltown, York County. It’s a combination of two different structure types.
Modern bridge preservation is more formalized.
PennDOT moved a metal truss bridge – one of just three of its kind left – to Pymatuning State Park in Crawford County, for use on its Spillway Trail, in late 2022. It’s relocating two others currently, one in Berks County, another in York.
Thirty-six others available through the marketing program are either already disassembled and in storage, awaiting a taker, or still in use somewhere, said PennDOT’s Victoria Martin. That those and others aren’t moved faster is attributable to two things.
“We always say there are two issues with metal truss bridge relocation. One is the financial, the other is the timing,” Martin said.
“Sometimes people want a bridge that’s not available yet, and in even more cases we have bridges that are ready to go, and no one is ready to take them yet.”
Cost is often the issue in those latter cases. But she said PennDOT is hoping to develop a dedicated fund to help with relocation expenses in the future.
In the meantime, PennDOT has a map of historic bridges around the state and a listing of those available for purchase, typically for little or no fee.
Background in historic bridges is available at PennDOT’s website.
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