CATEGORY: Small Projects

PROJECT FIRM: Wiss, Janney, Elstner Associates, Inc.

OTHER CONSULTANTS: Penn Trails, LLC, S&J Construction Co., Inc., Chase Engineering, LLC, John Keno and Company, Inc.

OWNER: Lake Forest Open Lands Association

DESCRIPTION: Wiss, Janney, Elstner Associates, Inc. (WJE), Penn Trails, LLC, and Chase Engineering, LLC, teamed together to design a pedestrian suspension bridge spanning 120 feet over the No-Name Ravine in the Jean and John Greene Nature Preserve in Lake Forest, Illinois. WJE served as the engineer of record for the structure, now called the Deromedi Family Bridge, which lets visitors explore a previously inaccessible area in the nature preserve. The engineering process was initiated by Chase Engineering, LLC, who provided all design services and drawings for the bridge structure. Led by Douglas Crampton, the WJE team reviewed the design documents for the proposed pedestrian bridge structure. The AASHTOLRFD Guide Specifications for the Design of Pedestrian Bridges (2nd Edition) was used as the design standard. In addition, WJE performed a peer review of calculations by others, compiled independent calculations, and created finite element analysis models to evaluate the behavior of the suspension bridge to validate the bridge design. As Peter Chase of Chase Engineering noted, “WJE provided welcome and useful confirmation and checking of the design calculations.” Significant obstacles were overcome during planning and design. “The challenges were numerous,” said Ryan London, president of the Lake Forest Open Lands Association. Not only was the site three-quarters of a mile from the edge of the pavement through a pristine natural area, there were many wetlands that needed to be avoided. “Aesthetically, the structure also needed to appropriately complement the natural beauty of the rare ecological conditions as well as function through the unique weather patterns experienced along the Lake Michigan coast,” London said. Out-of-the-ordinary technology and ingenuity were used to achieve the project’s goals in the 61-acre preserve, which includes some of the highest-quality ravines in Illinois. The construction methods in the remote, environmentally sensitive spot (more than 100,000 plants were added to the slopes over the years) needed to be innovative and overcome challenges. WJE selected tails to expedite fabrication and construction, which subsequently lowered costs. Bridge pieces were created to be small, so they could be moved without large cranes in a way that did not disturb the natural world. Off-the-shelf cable connections were used so the team could build an erector set that workers could put together in the field. The project team’s approach provided society with social, economic, and sustainable development benefits. The sensitive nature of the preserve area precluded the use of deicing chemicals and required a solution that was compatible with its natural beauty. Using weathering steel, galvanized steel, and stainless steel creates a sustainable, visually-appropriate structure–one that will not require much maintenance in the future and fits into its environment seamlessly. Overall, the main elements of the suspension bridge (aside from the weathering steel components) include galvanized wire rope main cables and suspenders, galvanized helical soil anchors, and discrete reinforced concrete abutments, which should all provide many years of efficient crossings. And the social impact of the structure is profound. “With the implementation of the Deromedi Family Bridge, we now have an amenity that offers a life-changing experience in nature for preserve visitors young and old,” London said. “The accessible bridge allowed us to implement a new trail system.” The overall project also incorporated the 2013 ABA Outdoor Developed Areas Accessibility Guidelines as best management practices in design and construction. The project involved a complex balance between ensuring the strength of the bridge and maintaining its serviceability. For example, if the bridge ended up bouncing too much, visitors could feel uneasy on it. But Lake Forest Open Lands and London agreed the bridge should have a little bounce, which adds to its rustic nature today. The team made sure that structural safety was the first priority. The materials the team chose to use–such as weathering steel–helps the bridge blend into its environment. “The bridge appears on the landscape differently each time I approach it,” London said. “Like a sculpture, I connect with it differently depending on the light, weather, and the season. I like to say it has a heartbeat that you can feel as you walk across it.” The bridge design was performed under agreement with the owner’s primary designer of the larger park system. Coordination and cooperation were paramount in the bridge project, especially as it relates to the overall success of the larger project and meeting the owner’s needs. It is hoped the successful execution of this bridge project will encourage others to pursue this type of work, including the specific engineering solutions that permitted construction of this noteworthy structure in an unconventional situation. The engineers successfully engaged the client in the overall project development process. “I like to use the example of the fasteners for the hand railing,” London said. “We had at least two lengthy calls about how they should be oriented and machined to feel just right on your hand as you walk across the bridge.” The project was completed on schedule and on budget, which impressed London because the original budget was crafted in 2017 and implementation took place during the pandemic, among rising inflation and mounting supply-chain pressures. He noted the total design, study and engineering costs were $89,000, construction management was $40,000, and construction costs came in at $716,000, which includes everything from earthwork to soil anchors. He said the solution met 100 percent of Lake Forest Open Lands’ goals. “This project is a great example for the public to see how something as substantial as a 120-footbridge can be implemented in sensitive environments,” London said. “I think any suspension bridge successfully erected is deserving of recognition for engineering excellence because of how complicated they are.” London said the relationship with all of the consultants on this project was “amazing.” Aside from WJE and Chase, other key participants included John Keno and Company, Inc., the construction contractor, and S & J Construction Co., Inc., the steel erector. Several other local and out-of-state consultants were also used for the overall project that included installation of a trail system and other improvements throughout the nature preserve. In summary, this project is worthy of special recognition. The type of structure installed was both unique and complex. The weathering steel, galvanized steel, and stainless steel were chosen to ensure a durable structure that will not require much maintenance in the future. There was a complex balance between ensuring the strength of the bridge and maintaining its serviceability. This project identified an elegant, safe solution in a rustic environment. The construction methods in the remote, environmentally sensitive spot needed to be innovative and overcome challenges. Bridge pieces were designed to be easily handled by the ironworkers, so they could be erected in a way that did not disturb the surrounding environment. It was a larger-scale erector set that could be put together in the field with standard hand tools and small equipment. The economy of the work saved money while increasing the value of the user’s experience. The overall social impact is seen as scores of people daily enjoy the preserve, in part because they can access all parts of it thanks to the bridge. The outcome demonstrates the suitability of this type of structure to accomplish the necessary crossing as part of a larger social good (the creation of a user-friendly park). The work also provides an aesthetic element to the overall experience of the community, as it blends into its surroundings and offers an old-style way of getting to the other side.