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Canopy Bridge at the Botanical Gardens

Canopy Bridge at the Botanical Gardens

Canopy Bridge at the Botanical Gardens

The Canopy Bridge in the Atlanta Botanical Gardens is a structure that has always held a special place in my heart. I have been to the Botanical Gardens and walked across this bridge several times over my years at Tech, one of the most memorable times being when my girlfriend and I talked about our thoughts on the load paths of the bridge. And that conversation happened before this class started! So if you weren’t sure a nerdy Tech student was writing this blog you can now put any doubts you had to rest.

Structure Information

Canopy Bridge [1]

This bridge was constructed as part of an expansion of the Botanical Gardens that was completed in 2010 [2]. The purpose of this bridge is to provide guests with a way to see the gardens and flowers from a different perspective than they might get while on the ground. It also allows you to get from one part of the gardens to the other more quickly than walking on the ground. Jova/Daniels/Busby Architects of Atlanta were the architects that designed the bridge [2]. Halvorson and Partners (now a part of WSP) were the structural engineers that ran the analysis of the bridge design [3]. The design was inspired by the works of Spanish architect Santiago Calatrava. The bridge was part of a $55 million expansion project funded by a variety of private donors, the Kendeda fund, and the Botanical Garden itself [4].

Historical Significance

The design of the structure itself is not innovative, as the architects designing it were specifically modeling it after Santiago Calatrava’s works. Santiago’s bridge designs all featured cable stays. Instead of the bridge being supported from above by cables (such as the Golden Gate Bridge), the cables are anchored into the ground. In the figures below, you can see the comparison between the two styles.

Golden Gate Bridge, Suspension Bridge [5]

Canopy Bridge, Reverse Suspension Bridge [6]

The best existing example of Santiago’s work that influenced the design can be shown in the figure below. The style of using an anchored member with cables attached to it support the main structure is a signature style of Santiago Calatrava. That style can also be seen throughout the Canopy Bridge in the Botanical Gardens.

Reverse Suspension Structure at Quadracci Pavilion [7]

There was no special construction technique that was used for this bridge, but there was an incident regarding its construction that will be mentioned later.

Cultural Significance

This bridge was built as part of a large expansion of the Botanical Gardens in 2003 and finished in 2010 [2]. The expansion was able to take place because of a special and unique addition to the garden. In 2002, the Chihuly exhibit was presented for the first time in the gardens, and its impact was dramatic [8]. Numbers of attending people more than doubled from 200,000 to 425,000 [8]. Memberships to the garden increased from 12,000 to 19,000 [8]. Mary Pat Matheson, executive director for the gardens, said ‘The Chihuly exhibit was our coming out party. It was very deliberate. I knew what the impact would be: tremendous’ [8]. Matheson was purposely trying to make people want to visit the gardens, for more than economic reasons. She wanted people to see the garden as ‘more than just a pretty place. I want the people of Atlanta to see it as a cultural asset’ [8]. The spike in interest for the gardens made investments easier to come by when the expansion was announced in 2003.

Chihuly Exhibit [9]

Part of that expansion was making use of the previously undeveloped Storza woods, which included the Canopy Bridge. However, the building of the bridge did not come without cost. During construction of the bridge, a section collapsed killing one worker and injuring several others [3]. The metal frame of the bridge had been constructed with the shoring beneath the bridge, each column spaced out 30’ each [3]. When concrete was being poured into the top of the bridge for people to walk on, the bridge collapsed. The shoring contractor made several mistakes (or cut certain corners) which led to the collapse. The column was spaced at a distance greater than 30’ from another column, the steel beams in some towers were discovered to be W10x12 instead of W10x19, and the contractor failed to provide required lateral bracing between anchors, which were embedded at insufficient depths (anchor embedment distances on seven different towers ranged anywhere form 43” to 17”) [3]. This accident was a black mark on the expansion, but the overall public opinion of the opened section was overwhelmingly positive. Visiting numbers increased further and the gardens now featured a ‘Monet piece’ in the canopy bridge as Matheson describes it [8]. It is used today as it always has been as both a way to transport people across the gardens, or to be viewed and admired with everything else the gardens have to offer.

Structural Art

Structural art can be defined using three principles: efficiency, economy, and elegance. Efficiency describes a structure’s ability to carry the maximum amount of load with the smallest amount of material. Economy describes a structure’s cost versus utility. Ideally, a structure has minimal cost and maximum utility. Elegance is the aesthetic choices that a designer makes.

In terms of efficiency, this structure seemingly fits that role. The bridge is cantilevered at the ends and suspended by thin metal rods along the span. With its unique shape that curves around the Storza Woods, the design allows the bridge to withstand loads while keeping materials to a minimum. Solid rectangular supports could have been used as well, but it would have used much more material and not increased the capacity of the bridge. The horizontal profile of the bridge is also very small, so the bridge as a whole uses minimum materials considering its shape and carries the necessary capacities.

Speaking to its economy is a bit more difficult. The bridge costs were included in the total expansion which was $55 million [2]. The expansion was mostly privately funded by organizations and people that were interested in making the Botanical Gardens a more beautiful place. The money came in quickly so there was a high public interest in this project being undertaken [2]. The expansion brought in an increase in visitors and was beloved by all. Considering the quick funding that came in and was quickly repaid by the increase in visitors and acclaim of the gardens, I would say this structure fulfills the economy portion of structural art.

The elegance of this structure is undisputed. The bridge as a whole is very thin and and does not obstruct views in the garden, but rather becomes a part of it. The inspiration for the design came from Santiago Calatrava, but pop culture had an influence on the design as well. The bridge had a connection to the performers Fred Astaire and Ginger Rogers. The cable stay design looks like the two dancing. This attention to the appearance of the bridge checks the elegance box for me.

Figure 6, Canopy Walk Concept Art

Canopy Walk Concept Art [10]

Overall, I would definitely qualify this bridge as structural art. The design itself is not innovative but the way it is implemented shows off its beauty and strength.

Structural Analysis

The Canopy Bridge has a fairly simplistic design to it despite looking complex. The main member that carries the deck is the HSS30″ diameter by .25″ thickness tube that is cantilevered into concrete abutments at each end [3]. In the middle of the bridge is a straight span that is 70′ long and 11′ wide that is supported by two sloping HSS 16″ diameter by .625″ tubes in a V-form [3]. There are four HSS 24″ diameter by .5″ thickness pipes that have cables attached to them that support the deck of the bridge in various locations along it [3]. The framing of the bridge mainly consisted of HSS 8″x8″ members that were welded slightly above the center of the main HSS pipe [3]. These pipes were spaced at a 10′ interval from each other [3]. These pipes were diagonally braced with 6″ diameter pipes [3].

The construction of this bridge started with the concrete abutments being poured and the main HSS tube being installed [3]. The rest of the framework was constructed after that. Shoring towers were constructed to support the bridge while the concrete was being poured. As mentioned prior, the shoring towers were not constructed properly and the bridge collapsed during the first attempted construction effort [3].

The type of structural system employed for this system was a cable stay bridge [8]. The dead load of the bridge is supported by the cantilevered ends and the four HSS members with cables attached to the deck. Each HSS member had three cables connected to the deck and two cables attached behind them as backstays [3]. The pipes beneath the deck take a linear dead load but their main purpose is to provide an area for the deck to sit on. The cables in the HSS members are what keeps the entire bridge suspended.

Canopy Bridge Components [11]

Cable Tension Load Paths [12]

An analysis can be done on the bridge itself to see how much capacity is needed in the cables to support the structure. We can simplify the bridge into a straight segment to analyze a part of it via methods we have learned in class. We can take an 80′ span of the bridge and one of the sets of the cable connections to determine the strength needed in the cables. I am assuming a density of 145lb/ft^3 for the concrete and ignoring all but the main HSS member that has a density of .284lb/in^3 for the self-weight of the bridge. The calculation and analysis of the bridge is as shown below.

Simplified problem set up

The cables are assumed to be at 45 degree angles from the deck. All other assumptions that were made in the problem are listed in the calculations.

Cable tension forces and deformation

The tension in the second cable resulted in an almost zero force because of its orientation in this simplified example. In reality, the curvature of the bridge would result in non-zero tensions in each cable.

The design drawings successfully communicated the idea of the beauty of the bridge to stakeholders as many people were very eager to invest in the building of this bridge and the gardens. The technical drawings were less successful in communicating ideas between different parts of the job. The shoring consultant left out details regarding the type of steel that should be used and the shoring contractor ignored parts of the instructions from the structural engineers. Communication breakdowns between the companies ultimately led to its collapse during construction.

Personal Response

I have always admired the Canopy Bridge from my visits to the Botanical Gardens in the past, but it has taken on a whole new meaning now that my civil engineering background keeps developing. The first time I noticed that change was that first conversation I had about its function as a structure.

Researching this project and seeing the passion behind the bridge’s design process and construction has made this an even more special part of the gardens to me. I am definitely going back to the gardens in the future and dragging along any friends that will listen to me talk about its bridge.

References

1. Shell, B. (2018). Atlanta Botanical Garden Canopy Walk Images. [email].

2. https://www.myajc.com/blog/arts-culture/atlanta-botanical-grows-support-for-nourish-and-flourish-enhancements/xknBqvbdeSFd1AvTSj8HoK/

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

4. https://kendedafund.org/grantee/inspiring-the-public-through-beauty/

5. https://wall.alphacoders.com/by_sub_category.php?id=160347&name=Golden+Gate+Wallpapers

6. Shell, B. (2018). Atlanta Botanical Garden Canopy Walk Images. [email].

7. https://www.bluffton.edu/homepages/facstaff/sullivanm/wisconsin/milwaukee/calatrava/calatrava.html

8. https://www.accessatlanta.com/entertainment/calendar/atlanta-botanical-garden-grows-phase-opens-saturday/HGedrkVTonq00qRMImLbIN/

9. https://www.pinterest.com/pin/182958803590374050/

10. Shell, B. (2018). Atlanta Botanical Garden Canopy Walk Images. [email].

11. Shell, B. (2018). Atlanta Botanical Garden Canopy Walk Images. [email].

12. Shell, B. (2018). Atlanta Botanical Garden Canopy Walk Images. [email].

 

Comments

  1. slowentritt3 says

    I really liked how you were able to find such specific specifications on the bridge and use that information in your structural analysis. I also liked that you went so in-depth on the construction problems, as I’ve always heard there was a failure but did not know why. As far as your structural analysis goes, I think it may have been better to use tributary areas rather than a point load to find the forces at the cables, and it would have been interesting to find the forces on the mast poles.

    • jgarrett39 says

      I was very lucky to find some very detailed plans with the layout of the bridge. And yeah, the construction had to be talked about because of that unfortunate accident. I wanted to be sure I didn’t leave any details out! And thanks for the tributary area comment, I’ll be sure to do that next time I have a cable stay bridge!

  2. skyrazis3 says

    I loved your analysis on the Canopy Bridge as structural art. When we talk about elegant forms in class, elegance usually seems to stem from function of the structure as a load bearing entity, and the ability of the structure to communicate that. We have never talked about how structures could be elegant because they were based on human forms as you mentioned in your comparison of the shape of the cable stay to the silhouette of Fred Astaire and Ginger Rogers dancing. I know that this is a tradition dating back to ancient architecture and engineering, and it would be interesting to analyze a structure to see if there is structural advantage in designing with human-inspired forms.

    • jgarrett39 says

      Yeah I was very surprised when the Gardens reached out to me with that photo! It absolutely showed the beauty of the bridge and the inspired form. And that last project you mentioned would be a very cool project for a CE and BME student!