Construction Update: Phase III, the bridge

by Eliana Brown, RORG Director

I’m sure many of you have the seen the sign at the Red Oak Rain Garden letting you know that a new boardwalk/bridge was coming in Spring 2020 and, perhaps, you’re probably wondering where it is. A combination of the spread and mitigation of COVID-19 and other extenuating circumstances has meant that we’ll have to wait until the spring of 2021 for the final installation to be finished. But, trust me: it will be worth the wait.

The story of the boardwalk goes back to the very beginning of the renovation design process. The garden sits along a major pedestrian and bicycle route, serving as a gateway to the south campus area for Urbana residents. On normal days when school is in session, it’s a busy hub (and popular Pokémon gym). The sidewalk path going east-west is straight, but for those people traveling north-south, going around the rain garden is a bit of a detour. Prior to the redesign, the shortest path for people traveling this way was to cut right through the garden – and so they did! Enough foot traffic can lead to pretty severe compaction of the soil over time, so these “desire paths” were a major area of concern.

While some people don’t mind traveling a little extra way around the garden, as with most things, convenience wins out for most. Therefore, it’s up to designers to provide a solution. The redesign needed to include a way to get across the garden that didn’t put additional stress on the trees or impede its function as a rainwater management practice. The original idea for this pathway was a simple wooden bridge, but the design concept had issues with safety and accessibility that made campus stakeholders wary. The bridge idea was then replaced by a concrete path with PVC pipes underneath to allow for water to flow throughout the garden. However, when project engineer Tony Heath joined the team, he had some concerns about this proposed design. An at-grade sidewalk would require significant earth moving and compaction within the rain garden, which could significantly damage the tree roots. Also, by adding more impervious surfaces, we would be reducing the infiltrative and storage capacities of the garden. With these issues in mind, Tony developed three new boardwalk concepts: a custom steel bridge, a pre-cast concrete boardwalk, and a timber-frame bridge. The design team presented these alternatives to Facilities and Services for review and they agreed a timber-frame bridge would be the best alternative, if we could ensure its stability and resiliency.

To ensure that our bridge was resilient enough to handle the daily pedestrian and maintenance vehicle traffic on campus, we partnered with Allerton Park, who recently had installed two timber boardwalks. Tony and I visited Allerton and met with Director Derek Peterson who gave us great information about their boardwalks which are made from Osage Orange trees removed to make way for native trees. He even offered to gift our project Osage Orange trees. Osage Orange is an exceptionally hard wood that has been used in the Midwest as hedge, fence posts, and other outdoor uses for hundreds of years. Park staff normally cull these trees over the winter and were willing to donate the pick of this winter cull for our use. In addition to its exceptional strength, Osage Orange is rot resistant and doesn’t require chemical treatment, making it a great locally sourced, sustainable lumber choice.

To mill the wood, Allerton put us in touch with Mike Stevenson, owner of The Wood Shop, a local mill in Monticello, who they’ve worked with before on their boardwalks. He’s an expert when it comes to shaping Osage Orange. Although Tony is a professional engineer, he isn’t licensed for structures, and so in addition to the expertise that Mike was able to provide, we partnered with Bill Gamble, a structural engineer and professor emeritus in Civil and Environmental Engineering. Professor Gamble graciously offered to volunteer his time to work with Tony on the bridge design and, together, they adapted the US Forestry Service’s boardwalk design to suit the Red Oak Rain Garden’s purposes to ensure it would be structurally sound with Osage Orange. We are grateful to Professor Gamble for sharing his vast knowledge and experience with structures to help us. Here he is in 2008 working on a volunteer bridge building project:

Once we’d found the wood and finalized the design, the Facilities and Services Architectural Review Committee gave the go-ahead to move forward with the wooden bridge.

Accessibility was another design aspect which the RORG design team wanted to address. Our goal for this project was to achieve a universal design that goes beyond minimum ADA compliance. For this, the design team met with ACES Doctoral Student and Paralympian Susannah Scaroni. She met the team onsite to discuss what elements would be important to users in a wheelchair and how we could make sure that we were addressing their needs. One item that Susannah noted was that typical boardwalk designs have wooden boards at eye level for someone in a chair, which then blocks their view. To address that, we included solid planks at the top and bottom of the railing for safety reasons, and used metal cabling in-between to ensure that someone at that height could look out over the garden and enjoy the view. To see what this will look like, see the photo above of the Allerton boardwalk, which has this same design feature.

Once the design was finalized, the team began the first stage of bridge construction in fall of 2019. The concrete footings and abutments for the bridge were poured before the plants were installed to avoid accidental trampling. With the foundations poured, everything was ready to go. Allerton culled the trees over the winter and sent them off to Mike Stevenson to be milled. At that time, back in January, the RORG Team visited Monticello to see the trees and the mill shop and meet with Allerton’s Assistant Park Manager Micah Putman and Mike. The next step was for Mike to begin milling the wood into lumber per the design.

Unfortunately, things weren’t going to be that easy. Once Mike started working with the wood, he realized that it wasn’t going to be enough to meet our needs. The downside to Osage Orange, compared to pine, is that it can be knotty and bends rather than growing straight and clean. This meant that out of all the wood they culled, Mike would only be able to mill a few pieces of usable lumber.

Since we’d already gotten the best Osage Orange that Allerton had, Mike recommended that we consider using Black Locust lumber instead. It’s not quite as hard as Osage Orange but is still harder and more rot resistant than most conventional hardwoods. Mike had reached out to Micah at Allerton already about a grove of Black Locust he was aware of and they agreed to give it to us, with the downside being that due to COVID-19 and their typical work schedule, they wouldn’t be able to harvest until winter 2020. In addition to the wood from Allerton, the Urbana Park District has offered to donate Black Locust logs, which will give us a supplemental source to ensure we can build in 2021. We are grateful for these wonderful partnerships and their staffs’ willingness to help.

So, although it’s hard for everyone to wait — including us — we’re excited about using the Black Locust lumber for the bridge. We’re glad that there is another great locally sourced and sustainably harvested alternative wood available.

Soon, everyone, soon.

%d bloggers like this: