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Canopy Walk at the Atlanta Botanical Gardens

Canopy Walk at the Atlanta Botanical Gardens

The Canopy Walk makes itself known by crossing over one of the roadways entering Piedmont Park, bridging (literally) two areas of the Atlanta Botanical Gardens. The bridge makes a striking first impression to those visiting the park and the Gardens, and is even more interesting to me as an engineer for its unique shape.   

Figure 1: The Canopy Walk [1]

Structural Information

The Canopy Walk is a steel pedestrian bridge that snakes through the trees at the Atlanta Botanical Gardens in Piedmont Park. The bridge was designed by architects Jova/Daniels/Busby and structural engineers Halvorson and Partners, P.C. [2]. The bridge began construction under Hardin Construction Company during 2008, and was opened to the public in May 2010. The project was ordered and developed by the Atlanta Botanical Gardens.

Historical Significance

Just by looking at this bridge, one can tell it is unique – it’s sweeping pathway form and unconventional support structure make it stand out as different and intriguing. Its form speaks true – the Canopy Walk is the only winding tree canopy walkway of its kind in the United States [2]. The form is inspired by the Spanish architect and structural engineer Santiago Calatrava, who is known for sweeping, dynamic forms and bridges supported by single pylons. This bridge is a graceful imitation of his style, snaking between trees while appearing light and open through its suspensions. The bridge is the sole example of this suspended type bridge through tree canopies, and could serve as an example for future bridges to take inspiration from.

Figure 2: Canopy Walk under construction, prior to collapse. [3]

Cultural Significance

This bridge was added as a part of a large expansion project by the Atlanta Botanical Gardens that nearly doubled the size of the Gardens. Given the scope of this expansion, the bridge was inevitably going to be a gateway for heightened attention for the Gardens, and attention it brought – although, not in the best of ways at first. At the end of 2008, the park hosted a news event to commemorate concrete being poured into the deck to excite the public about the upcoming bridge. Unfortunately, during the pouring, the Canopy Walk collapsed due to failure in its shoring systems during construction, leading to the injury of 18 construction worker and the death of one more [4]. The failure led to the opening to be delayed until 2010, when structural issues were fixed and it was finally opened to the public. Despite the rough start, the Canopy Walk has become a beloved addition to the Atlanta Botanical Gardens, leading the park to expand to further attractions and allowing guests to gain an unparalleled view of the forests and Gardens from above. To this day, it remains a popular attraction.

Structural Art

Now that the Canopy Walk is staying up, it can definitely be seen as an example of structural art in its lightness of form and its open design. Structural art, according to Professor David Billington’s works, is characterized by three dimensions: scientific, social, and symbolic.

Scientifically, this bridge succeeded, eventually. The pedestrian walkway is a cable-stayed bridge supported by 4 masts nearby the bridge, but also includes two inclined columns at a fixed span halfway through the bridge. The bridge is composed of steel, supporting a concrete walkway for foot traffic only. Since its opening, it has remained steady.

Socially, this bridge exists as the first of its kind in the United States. As a canopy bridge, it allowed the Atlanta Botanical Gardens to expand over roadways and improve the experiences of visiting guests. Since opening it has been popular with the public, drawing in more visitors with its clear presence. [5] Additionally, the cable-stayed structure uses minimal materials to keep the bridge up, increasing its economy through efficiency of materials.

Symbolically, this bridge rises over the ground giving visitors a bird’s eye view of the park and its expanded grounds with its unobtrusive cable suspension. However, aside from that flavor text, it isn’t really considered widely impactful outside of the Gardens themselves.

Billington was clear in another aspect of structural art that is apparent with the Canopy Walk: openness of form. The bridge is without much ornament, with the main visual impact coming from the walkway itself and the steel cables and towers supporting it. The structural pathways and reasons for each part of the bridge are clear and make sense. From all these factors, I would consider this first-of-its-kind bridge structural art.

Structural Analysis

The Canopy Walk uses two different methods of support along its 575-foot length to suspend the walkway above the ground. The more prominent and impactful support structure is in its cable-stayed supports, with four masts connecting support cables to the structure of the bridge. These cables hold the bridge in place through tension with the 24”, ½” thick structural pipe columns off to the side. The second support structure is in two columns simply supporting a 70-foot fixed span near the center.

The support systems carried a 30” diameter, ¾” thick structural pipe supporting cantilevered framing members holding up the steel and concrete deck. Three support cables per column connected via gusset plates to this main pipe – one connecting perpendicularly, two at angles. These cables connected at the top of the mast and then connected angled back to be anchored in the ground below the columns. The cantilevered steel tube framing members were welded above the structural pipe. The deck above consisted 2” steel and 6” concrete. During construction, the steel pipe frame was supported using 21 temporary shoring towers used until the masts and cables were set in place. It was structural failure in these showing towers that resulted in the bridge collapse during 2008.

Figure 3: Underside of the bridge [6]

The three cables act in tension to suspend the bridge in the air from the angled support columns. The cables typically are connected to the bridge 25’ from the top of the support columns. The bridge is also designed to take 85 psf live load. After this however, information gets more tricky: the incident occurred before the cables and columns seemed to be installed, so I cannot find information on cable thickness or column height or exact figures on dead load. It is given that the average bridge load over each of the 21 temporary shores was 22 tons, so by that figure, the dead load can be calculated to 803 psf, resulting in a total load of 888 psf.

Figure 4: Detail of cables [7]

As stated, the cables serve to both hold the bridge in its curvy form and to keep the bridge raised. The total distributed load over the bridge from dead load and live load is 510.875 kips. The ends of the bridge connect to the ground via concrete abutments. Each cable supports its own tributary area, meaning that cable supports on the ends carry greater load than the central cable supports. The bridge can be simplified to a 2-dimensional system to determine cable reactions carrying the load.

Tributary areas are assumed to be split between support structures. For cable supports at the ends of the first and last structures, it is assumed they take the entire load of the bridge ends without aid from the concrete abutments. From there, vertical load requirements can be simply calculated using the load per foot over each tributary area. The loads of the deck then travel up the cables which are attached to the offset masts, which then transfer the load from each cable down to the ground.

OSHA documentation from after the accident show architectural and engineering details of walk. The detail sheets point out cross sections of the bridge, along with locations of the support systems. Since this project was completed, both the architecture and construction firm have been disbanded or absorbed, so finding more drawings and models was difficult beyond those provided to the accident report.

 Fig. 5: Engineering details of the Canopy Walk, prior to collapse in 2008. [3]

 

Personal Response

This bridge was one of the first moments of awe I felt when visiting the Atlanta Botanical Gardens, as I had never seen a bridge quite like it before. Looking at it a second time, now through the armed with a stronger knowledge of structural engineering, I can better appreciate the cable towers and their purpose in keeping up the bridge. Before, the bridge caught my eye because it was interesting to look at. I now know more about the history of its construction, and how the bridge succeeds now in carrying these wondering guests through the air.

References

[1] http://journeyleaf.typepad.com/journeyleaf/2014/07/atlanta-botanical-garden.html

[2] http://atlantabg.org/about-us/news-blogs/canopy-walk

[3] https://www.osha.gov/doc/engineering/2008_12_19.html

[4] https://www.nytimes.com/2008/12/20/us/20collapse.html

[5] https://www.myajc.com/entertainment/atlanta-botanical-garden-expands-into-woods/ClIsngYS4p5lApfzKvGS6O/

[6] http://www.wanderlustatlanta.com/2010/08/atlanta-botanical-garden-oasis-in-city.html

[7] http://www.wanderlustatlanta.com/2011/01/atlantapix-600-foot-canopy-walk.html

Comments

  1. hmurray8 says

    I really liked how detailed your blog post was in the beginning. It made me feel like I was actually there. How do you think engineers can alter the construction process so that an accident like the one here doesn’t happen again? Also, I would have liked to see a little bit more analysis of the system with load paths and calculations. Great job!

    • jhartwell3 says

      According to accident reports I analyzed, problems came about due to the shoring towers: the contractors involved in the installation and construction of the towers did not follow plans developed by the actual engineers and created faulty towers which could not support the weight of the structure. Additionally, the engineer failed to specify important components of the shoring towers, such as the minimum distances between support towers and strength requirements of members. Going forward, contractors must follow the direct instructions of the engineers, who also must be more specific in their drawings to ensure safety.