safety, accessibility, and aesthetics shape pedestrian suspension bridges

There is something wonderful about a walk in the woods. The nearness of nature, the wind rustling through the trees, the magical light almost any time of day, and never knowing what’s around the next bend. For Gus Smithhisler, PE, it’s an adventure that never gets old. Smithhisler is a Natural Resources Engineer for the Ohio Department of Natural Resources (ODNR), where he has worked since graduating from Ohio State University in 1994 with a degree in Civil Engineering. ODNR’s mission is to ensure a balance between the wise use and protection of natural resources for the benefit of all; an ambition Smithhisler and his colleagues live every day.

“I love connecting people with nature. My role with ODNR gives me firsthand experience with the joy and beauty of the natural world surrounding us. There is no greater reward in my work than opening that up to others,” says Smithhisler of his career. Within natural resources, ODNR segments its services into several divisions: the Division of Natural Areas & Preserves, Division of Wildlife, Division of Forestry, and the Ohio State Parks & Watercraft. A licensed Professional Engineer with a civil concentration, Smithhisler is ODNR’s road, bridge, and trail expert, and he has made getting the public closer to nature his life’s work. Over his 28-year tenure with ODNR, Smithhisler has led the development and maintenance of many miles of trails through the state’s parks and preserves and overseen the construction or rehabilitation of dozens of different bridges along the way.

“In the realm of the physical infrastructure we introduce into natural environments, there are a few musts in everything we do,” says Smithhisler. “The first is sensitivity to context; it has to fit in. Public safety, of course, is critical. Next is low-maintenance durability; it must last. And certainly, it has to positively enhance the experience of nature in a meaningful way.”

Of all the assets Smithhisler and ODNR are responsible for maintaining within the state’s expansive parks and preserves, the most dramatic have to be Ohio’s pedestrian suspension bridges. Generally located deep within a less-than-accessible wilderness, suspension bridges are making a resurgence in parks across the country. Smithhisler is delighted to have a hand in bringing these one-of-a-kind engineering marvels to the public’s attention.

“ODNR has built several different suspension bridges throughout our state parks system in the last few years,” he says. “A bridge suspended over a river in the middle of the woods embodies the adventure and untamed spirit of nature in a way that is accessible both literally and figuratively.”

ODNR’s first new pedestrian suspension bridge was built in the Hocking Hills State Park, located about 60 miles southeast of Columbus, Ohio. Recipient of the 2018 Outstanding Special Purpose Bridge Award from the Association for Bridge Construction and Design, the Hemlock Pedestrian Bridge has a suspension length of 100 feet and a walkway length of 64 feet. Suspended 10 feet above the bottom of a stream, the bridge features a deep V configuration that is 42 inches wide at the handrails and 16 inches wide at the walking path. The thrill is two-fold. Located deep within the park, getting to it is a challenge in itself, and once there, the swinging sensation while crossing is unmistakable.

“There is a cave on the far side of the site that we wanted to reopen access to. When we conceived the idea for building a bridge at the Hocking Hills site, we originally considered suspending it from some very large, live trees,” says Smithhisler. “That proved unworkable for a variety of reasons, but the sense of adventure in that idea never left us.”

While considering more traditional options for building a bridge, ODNR quickly realized standard solutions simply were not going to work. The site’s remote location and the inability to get any heavy equipment made off-the-shelf solutions an impossibility. That’s when ODNR put out a Request for Qualifications for engineering service. They selected Woolpert, an architecture, engineering, geospatial (AEG), and strategic consulting firm specializing in solving infrastructure’s most compelling challenges.

Tom Less, PE, SE, is a Senior Associate and Discipline Leader at Woolpert who manages Transportation, Aviation, and Water Resources Design teams. Less played a crucial role in designing the Hocking Hills bridge and several other pedestrian suspension bridges that the firm has recently developed in remote locations.

 “One of the things I enjoy most about my work is Woolpert’s willingness to take on challenges that are outside of the norm,” says Less, who earned both a B.S. and an M.S. in Structural Engineering from Ohio State University and teaches part of the University’s Senior Engineering Capstone program. “These suspension bridges are an excellent example of a non-conventional situation that pushed us to go back to the fundamentals of engineering: sound analysis, following the load path, and proper material application.”

On the Hocking Hills bridge, the difficulty of site accessibility exposed a series of situational challenges that are rarely considered on a vehicular bridge. The planned crossing was in a relatively shallow flood plain. Heavy rains routinely flood the site. The fear of a flood’s impact on a pedestrian bridge made of cable and wood members was not necessarily damage from the rising water but damage from debris floating downstream. If water levels are high enough, a small tree or a clump of branches floating downstream could snag on the bridge and damage it. A large tree could destroy it.

“Scrutiny of the site and the potential for damage in a storm event led us to develop a unique floating foundation system,” continues Less. “When the water rises, the lower deck attachments can rise as well, allowing the walkway to lift out of the way of debris being swept downstream. It can then be reset by park staff after the flood subsides.”

Accessibility and the delicate nature of the bridge were not the only particulars at Hocking Hills. Once the bridge was designed and ready to build, ODNR and Woolpert were not able to find contractors interested in building it despite their best efforts.

“Typically, on bridge projects, we get between 3 and 10 competitive bids from general contractors who are eager to do the work,” says Less. “On the Hocking Hills bridge, we got none – literally zero bids.”

Less explains there are various reasons contractors won’t bid on a project, all of which boil down to perceived risk. When the design is non-standard and in a relatively inaccessible area, contractors see the cost, effort, and profitability equation as disadvantageous, which generally increases the price. Furthering matters, given the unconventional nature of a bridge built in the woods that is subjected to routine flooding and potential damage, long-term safety concerns can put the project beyond insurability for some carriers. Without qualified bidders, ODNR was left to their own devices on Hocking Hills and ended up building the bridge themselves through the skill and ingenuity of parks department employees.

With the first pedestrian suspension bridge receiving a very enthusiastic reception from the visiting public, ODNR decided to expand the program and commissioned a second suspension bridge for Mohican State Park in Loudonville, Ohio. Building on the lessons learned from the no-bid environment of the Hocking Hills bridge, this time, ODNR decided to solicit Design-Build services rather than Design-Bid-Build, as attempted previously.

“The Mohican bridge is a 120-foot suspension bridge that traverses the Clear Fork River,” says Smithhisler. “Up until the late 1960s, there was a bridge there that the oil and gas industry had put in. It used to be sort of a pilgrimage for local teenagers and adventurers. However, that bridge was destroyed by a flood in July 1969. ODNR wanted to build a new bridge because it makes the existing trail system a loop and opens up access to some more challenging hiking terrain.”

For the Mohican bridge, general contractor, The Righter Company, Inc. elected to team with Woolpert as their engineering partner for the Design-Build execution. Completed for roughly $840,000 in 2020, the bridge was designed and built in less than a year. From a foundation of experience, Less shares some design insights that illuminate the behind-the-scenes thinking that shapes a buildable solution.

“Understanding site-specifics goes beyond accessibility,” shares Less. “Geotechnical conditions at the bridge’s connections points and a detailed hydraulic survey of the waterway below are imperative pieces of information from which design begins. Those relate to structural stability, but as with any infrastructure, the asset’s anticipated use is also quite significant.”

Though the need to accommodate foot-traffic on a hiking trail is obvious, Less points out that in some cases, that could mean people walking exclusively single file, in others a person pushing a stroller, a wheelchair, or even the need to account for something as wide and heavy as the ATVs used by park maintenance staff. The wider the bridge, the more loading it is subjected to and the stronger all the members must be. Less shares that the three pedestrian suspension bridges designed by Woolpert in recent years have all accounted for pedestrian loading of between 65 and 90 pounds per foot. After the Hocking Hills bridge was open, the local high school football team reportedly tried to see if they could overwhelm it and found they could not.

“Choosing contextually-sensitive materials is also essential to a finished product,” Less says of the considerations shaping specifications. “Generally, we are specifying weathering steel that will turn a rust-brown for the tower members or built-up timber sections. In addition, the cables and concrete used are coated with matte, earth-tone finish sealers, and we specify low-shine, galvanized steel connectors.”

Sensitivity to the site in material selection can’t overlook environmental factors that cause materials to degrade over time. Long-lasting, low-maintenance choices are even more important than aesthetics.

“For the wood, we specify white oak,” says Less. “This is the same kind of wood that was used to build the historic covered bridges in this area that have been standing for generations. White oak is unique. It is a closed-pore timber with self-sealing properties that resists the elements in exposed conditions.”

Since the successes at Hocking Hills and Mohican, Less and the Woolpert team have completed a third pedestrian suspension bridge in Ohio, the Stillwater Prairie Connector Bridge, which recently won an American Council of Engineering Companies (ACEC) 2022 Engineering Excellence Award. At 170-feet-long with a deck width of 32 inches, the Stillwater bridge is the longest of the three. Built for the Miami County Park District, the Stillwater bridge connects the Stillwater Prairie Reserve and Maple Ridge. What stands out to Less as unique about this project wasn’t the application or materials, but the mathematics required to deliver it.

“We had an interesting situation on the Stillwater bridge during construction,” he shares. In this case, after the primary cables were affixed to the towers and the contractor began adding the decking across the 170-foot span, the bridge seemed to drop dangerously low as the first few feet of flooring were installed. “We went to the site, took a lot of measurements, and went back to work in our modeling software. We did a full construction stage camber analysis to reveal that the uneven load of starting the decking on one end gave an impression of excessive deflection. Once the decking passed the center point, the bridge began pulling back up toward the other end and straightened itself out.”

Unlike bridges designed to accept the live loads of vehicular traffic moving across them, pedestrian suspension bridges don’t fall under the National Bridge Inspection Standards, which requires they be formally inspected every two years.

“We recommend clients like ODNR conduct an inspection of the entire bridge using their staff at least once a year. In addition, they should schedule a professional inspection and analysis every two to five years,” says Less of the need to always keep public safety front of mind. Meanwhile, Smithhisler says the public’s response to the suspension bridges ODNR is responsible for could not be more enthusiastic.

“Both the Hocking Hills Bridge and the bridge in Mohican State Park have become destination amenities for the public,” he finishes. “People are coming from near and far to see these signature structures in Ohio’s parks, and they are enjoying a wonderful walk in the woods along the way.”

Bio:

Sean O’Keefe is an architecture and construction writer who crafts stories and content based on 20 years of experience and a keen interest in the people who make projects happen. You can reach him at www.sokpr.com.