Emirates Stadium

Quite possibly my favorite part of visiting Emirates Stadium was being able to read the stories written on the outside walls. Some of them were about inspired players practicing all their lives to play for Arsenal. Some were historical, regaling the tales of its founding members or defining play off games. But my favorite one happened to be one tucked away on the end of a series of stories. It was from a random fan of Arsenal talking about one night in a bar, with the time approaching 1:30. He looked to his left and saw Arsenal’s goalkeeper. He stammered out a hello and began talking to him. Around 3:00 the two are quite drunk and the fan suddenly remembers an important fact and asks the goalkeeper ‘Say, don’t you have a match in 12 hours, when are you calling it a night?’ The goalkeeper responded ‘When the sun starts to come up, kid’. The fan attends the Arsenal match the next and saw the players run out of the tunnel. Sure enough, the goalkeeper also ran out. The fan noticed the goalkeeper looked like he had been awake for a week straight (that’s the polite version of how he looked). Anyways, the match started and Arsenal ended up winning the game in a shutout. The very hungover Arsenal goalkeeper allowed no goals. Being in Europe and not having as cranky of people is great, and one reason is that a story about a goalkeeper staying up all night to drink before a match can be put on a stadium alongside all the other important and famous moments in this football club’s storied history.

Structure Information

Emirates Stadium was first opened on July 22, 2006 [1]. Construction began in July of 2003 [1]. The building is home to the Arsenal Football Club. The architect for this project was Populous (formerly known as HOK Sport) [1]. The structural engineers for this project were from Buro Happold [1]. The funding for the project was all private. Arsenal secured a total of 260 million pounds from loans from various banks [1]. However, one of the banks pulled out as construction started so the stadium’s building was delayed. Middle Eastern airline, Emirates, jumped at the opportunity to help with funding. They lent 100 million pounds in exchange for a 15 year shirt deal with the club and naming rights to the stadium [1].

Figure 1, Emirates Stadium

Historical Significance

The structural design of the roof was very unique. Its four trusses that support the roof is something that is not seen very often. It is presented here because of the type of events taking place at the stadium. The roof only needs to cover the fans in the stands, so there is a rectangular portion taken out of the ceiling about the size of the pitch. The cut out in the roof is not unique, but the four trusses supporting them is.

A special technique used during the construction of the stadium was the attention that was given to the seats. The concrete that makes up the stands was tested repeatedly for dynamic loading that would occur when fans jump/move around during the games [1]. The seating for the stadium was also outfitted with Ferco seats to make the fan experience even more enjoyable [1]. The thought was that if the stadium treated both fans and rivals with respect that they would respect the club in return. You can go to a game yourself to see if that holds true.

Regardless of how that strategy worked out, Emirates Stadium has gained a reputation as the most comfortable seating on the market. Another very cool thing that the stadium has going for it is the lighting for the pitch that the designers employed. They used computer modeling to make sure the lighting and air circulation was adequate for the level of quality they wanted in the grass [1]. The moving air also increased spectator comfort.

The best existing example of this stadium’s design is Estádio da Luz. This stadium is in Lisbon, Portugal and is primarily home to the Portuguese club S.L. Benfica. The stadium also features the same four truss design seen in Emirates Stadium. The view for fans in the stadium is also completely unobstructed. In addition to the unobstructed view, fans are mostly covered by a roof that extends out from the trusses. This roof structure can be a model for future buildings as it accomplishes one important task and one essential task: cover the fans from rain/sun and provide them with a clear view of the pitch.

Figure 2, Estádio da Luz [2]

Cultural Significance

No one died at the stadium during construction, but one worker was badly injured. Michael O’Donovan was kneeling to clean steel shuttering that is used to form reinforced concrete structures when a dump truck ran over his right leg [3]. His pelvis was fractured from the accident and his leg had to be amputated above the knee because the injury was so severe [3]. The City of London Magistrate’s Court determined that the site did not properly separate pedestrians and vehicles [3]. The companies in charge of construction were fined a total of 66,000 pounds in damages [3].

The beginning of the stadium’s tenure as the home of Arsenal was not good. Arsenal is a historically great team and their fans have very high expectations. However, they drastically under-performed in their first seven years. But once the 2013 season finished, Arsenal went on to win three out of the next four FA Cup titles, marking Arsenal as the most successful club in the history of the competition.

The stadium was loved by fans once it was opened. There had been some grumblings about the length of construction but once the stadium was open fans were more than at ease. The new stadium more than doubled the old stadium’s capacity, now allowing 59,867 people to attend games [4]. The land was undeveloped when it was bought and the surrounding area has had a major face lift as a result of the new stadium. The stadium is still used as the home of the Arsenal Football Club.

Figure 3, Arsenal’s Asia Cup Championship in 2015 [5]

Structural Art

To evaluate this stadium on whether it qualifies as structural art we can use the three fundamental principles of structural art: efficiency, economy, and elegance. The main concept behind each of these is to create a structure that utilizes the least amount of material, the least amount of cost both in design and social aspects to the people that will use it, and the most pleasing aesthetics possible.

Looking first at efficiency, this stadium does not seem to utilize the least amount of material from a first glance. The stadium was built to be ‘dramatic venue that highlights their ambition to become a global force in football’ and they used a large amount of materials to do that. Some materials were for the structure, others were used for fan comfort like the roof that extends over the seats. While it is something that was put into place for the fans, it is technically a waste of material. Other stadiums do not have a covering like Emirates. The load path of the building itself is also unclear. Much of the concrete that supports the stadium is covered up by facades making it hard to determine what is happening structurally without building plans. However, the truss structure that composes the roof does have a very light appearance to it. In this aspect, the roof does succeed in a light form with less materials. But, overall as a structure, I would say that this building does not check off on efficiency.

Figure 4, Emirates Stadium’s Dramatic Size

Economy was a problem for this stadium, with construction even being delayed because of it. However, the stadium’s cost can be looked at in a different light when the people funding the stadium are revealed. This stadium, unlike many in America, was entirely privately funded. Arsenal secured loans from various banks and companies to fund the building of this stadium. In terms of the social cost of this stadium for people, there was almost none. They did not have to chip in for this stadium unless they wanted to invest in bonds that Arsenal was selling (one of their other fundraising efforts). The fact that the stadium was privately funded does not entirely qualify it as hitting the economic checkbox though. David Billington, creator and major proponent of the three fundamental principles of structural art, notes that an unlimited budget is a hindrance to economy because designers will add unnecessary things because they have the budget for it. However, Arsenal privately funding this project (and requiring loans because they did not have the money outright for the stadium) means they wanted the stadium as good and cheap as possible. Taking these thoughts into consideration, the stadium does meet the economy criteria.

Finally, we look at elegance. Elegance as mentioned above, refers to the structure’s ability to create the maximum aesthetic possible. I, along with most everyone that goes to this stadium, would agree that the stadium is beautiful to look at. It stands tall and massive when you walk up to it, but it does not stick out in the skyline. When I got off at the Arsenal tube stop, I had to get directions to the stadium because I could not see it. The stadium emphasizes its massiveness for those that want to experience it, but does not do so in an ostentatious way. The stadium serves as an important symbol for the tradition of Arsenal as a powerhouse football club. The increased revenue from everyone attending this stadium has allowed them to sign better players and continue their dominance on the pitch. The stadium is a symbol for the powerful Arsenal team and is a meaningful place for every fan that attends it. With everything considered, I would argue that the stadium does meet the elegance standard.

Taking the three principles into consideration, the stadium almost passes the test of structural art. It meets two of the three requirements, but does not meet the efficiency criteria. This of course, was intentional. As mentioned previously, the goal of Arsenal was to make their new stadium a dramatic venue to symbolize their prominence in the world of football. Thinness and light form do not meet that idea. Architectural elements added to the stadium like facades out front and decorations adorning the structure serve that purpose. While the stadium is impressive from an architectural art perspective, it does not meet structural art standards.

Structural Analysis

Reinforced concrete was used for the flooring and framing of the first three levels of the stadium [1]. The lower, club, and box tier are supported by reinforced concrete rakers while the frame and structural steel rakers support the upper tier and level 4 [1]. The angle of a stand is known as the rake and the members used to support the stand are called rakers. The main stadium structure was able to be built at the same time as off site steel and precast concrete members [1]. Eight concrete cores are just inside the elliptical perimeter of the stadium to support it and transfer loads into the ground [1]. The stadium’s roof has 3,000 tons of steel and the entire stadium has 10,000 tons of steel throughout [6]. The stadium used 60,000 m^3 of concrete throughout the stadium [6].

The main concept of the structural system that is employed for the roof is actually a complicated version of a simple column and beam set up. This allowed the roof to cover fans while also keeping the view of the pitch unobstructed. The roof consists of three trusses: a primary, a secondary, and a tertiary set. A fourth perimeter truss also encompasses the entirety of the stadium. The length of the primary truss has a span of 204 m [1]. There are eight ‘tripods’ that transfer all of the vertical load to the columns.

Figure 5, Structural Components [1]

The load path of the roof is fairly simple once you visualize it as a beam and column structure. The tertiary trusses are in place all along the perimeter to help support the load of the roof and brace the primary trusses. The secondary trusses also help to support the roof and transfer load to the perimeter truss. The primary truss takes load from the tertiary and secondary trusses, as well as the roof itself and transfers that load to the tripods placed along the perimeter. There are also four additional tripods along the perimeter for stability reasons to make eight total tripods. Once the load has reached the tripods, it is transferred into the concrete cores and then into the ground.

Figure 6, Tripod

In addition to the tripods, there are props around the perimeter that help to transfer load down into the rakers. These props are responsible for getting the load from the perimeter truss to the ground.

Figure 7, Load Path of Primary, Secondary, and Tertiary Trusses on One Side

To analyze the structure and its load carrying capacity, we can model this structure as a truss. The analysis will be primarily looking at the primary truss. The simplified version of the truss is shown in Figure 8.

Figure 8, Truss FBD

The distance between each bottom connection is 14.5 m. The total length of the span is 203 m. This is not the exact length of the actual span but this allows a simpler analysis with the numbers involved. The load at each bottom joint signified by a blue arrow is equal to 10.4 tons [6]. The reaction at each end can be calculated as shown in Figure 9.

Figure 9, Solving for Reactions on Each Tripod

Another important aspect to consider for the analysis of this truss is the bearing stress that is induced on the tripod. The following calculations show the stress on the tripod. The radius of the pipe the truss connects to is 18 inches and the thickness of the pipe is 3 inches [6].

Figure 10, Bearing Stress

This means that the tripods need to be able to handle that amount of stress on the contact area it has with the primary truss.

A final set of calculations can be made for this truss structure, but it involves simplifying it even farther. If the truss, which only has loads on the bottom span, was reduced to a beam we can calculate the maximum moment occurring. To do this, we assume the same loads and reaction values that were calculated prior. From there we can solve for the shear diagram to get the maximum moment.

Figure 11, Shear Diagram

Using Figure 11, we can calculate the area of the first half of the graph that is positive to get the maximum moment. This value is equal to 3166.8 ton-ft. In calculating this number, it is obvious why the truss structure is braced so much. The bracing that occurs throughout this structure helps to prevent moment from deforming the truss. This extremely high moment value is definitely a large factor into the design conditions of the roof truss structure.

The design drawings were expressed well to Arsenal, even though there was not a firm need to. Populous had completed projects for Arsenal prior to this, so they were picked as the architects before a design was even in place. Of course, they wanted to do as best a job as possible so that they continued to receive work, but the initial design was not important. In terms of executing their vision, the break in construction as a result of the bank pulling their funding for the stadium was actually a blessing in disguise. With construction halted, Populous and Buro Happold were able to rethink parts of their building process and refine small details in their plans so that when the project started up again, everything would go smoothly. This was the case, as construction went on without any hitches when it started back up again.

Personal Response

It was a very humbling experience to visit the site of such a legendary and revered (or reviled if you are a fan of any other club in Europe) team. As a giant sports fan myself, I admittedly did not know much about the history of Arsenal before writing this blog. However, after talking to friends of mine it is clear why this stadium and team are so prominent in London and Europe as a whole. After reading about their accomplishments (and hearing from many salty friends) I better understand the connection this stadium has to Arsenal’s long history.

References

  1. https://issuu.com/jmurphy93/docs/dissertation_final-_emiratesstadium
  2. https://en.wikipedia.org/wiki/Est%C3%A1dio_da_Luz
  3. https://www.healthandsafetyatwork.com/arsenal-stadium-dump-truck
  4. https://www.boomtownbingo.com/history-emirates-stadium/
  5. https://www.unilad.co.uk/sport/internet-reacts-to-arsenal-winning-the-asia-trophy/
  6. https://www.designbuild-network.com/projects/ashburton/

Blackfriars Bridge

The first time I was able to visit this bridge was on my second day in London while our class was on a bike tour. The bridge stood out to me because of its name and certain features. One of the cooler features of the bridge are the small statues of birds on each side of the bridge. These birds represent the saltwater of the sea and freshwater of the river meeting under this bridge. Fresh water birds like swans face west while seabirds like gulls face the east. Regarding its name, the Blackfriars Bridge actually gets its name from a monastery that used to be by the river that had friars that wore black robes. That’s not a very complex background to the name but it still sounds very cool. I think the friars that play for the San Diego Padres need to look into re-branding!

Figure 1, San Diego Padres Baseball Team [1]

Structure Information

In 1756, the Mayor of the City of London received permission from Parliament to build a bridge at Blackfriars, the third bridge to cross the Thames in the London area [2]. The young Scotsmen Robert Mylne was the designer. Construction on this bridge started in 1760 and was opened to traffic in 1769 [2]. A toll was installed to help the British government fund the bridge [2]. It was removed in 1785 [2]. This first bridge lasted for over 100 years [2]. The bridge that you see today was designed by Joseph Cubitt and commenced in 1869 [2]. It was funded in a similar manner as the first, and no longer has a toll on its road as well [2]. The main purpose of this bridge, when it was initially constructed, was to direct more traffic away from the overwhelmed London Bridge [2].

Figure 2, The Old Blackfriars Bridge [3]

Figure 3, Current Blackfriars Bridge [4]

Historical Significance

The design of the current bridge is not an innovative one. The bridge is supported by five arches which was a standard material and design used in 1800’s [5]. However, there is an innovative part to its foundation. The piers themselves are pointed to help direct water flow, and the caissons are made out of iron to prevent scouring [5]. Scouring is the erosion of soil surrounding a bridge foundation. This was the beginning of a period where pointed piers began to be used as a construction technique.

The best existing example of this bridge had been the previous Blackfriars Bridge. Much of the design had not changed but instead updated. The same columns remain along the bridge for people to sit on and look down the river, and the ornamental designs of the original were kept in mind for the second bridge. This bridge is not a model for future bridges as it did not innovate a significant way. The caisson design would be used in future bridges, but the visual component of the bridge did not innovate.

Figure 4, Bridge Scour [6]

Cultural Significance

There was a great bit of drama surrounding the first Blackfriars Bridge. The City of London had not made many civic changes since the Great Fire of 1666, but needed another bridge that led into the city [7]. At the time, London Bridge was overburdened with traffic and was always crowded. The city was against creating a new bridge that would harm the business of the Thames watermen, and buildings would need to be bought and demolished to create a new approach road [7]. However, an engineer, John Smeeton, suggested the creation of a bridge at the western extreme of the city as to not require the demolition of any buildings. The area was also known for being impoverished and was stricken with criminal activity, so the bridge provided the opportunity for improvement. A bridge design competition was held, with a decent amount of propaganda being spread around the final fourteen candidates. 25 year old Robert Mylne won the competition with his elliptical arch design [7]. Mylne had just arrived in London the previous year after excelling in architecture courses at St. Luke’s Academy in Rome.

The bridge was found defective in the 1832, and the city demolished the old bridge in 1865 and finished building the new one in 1869 [5]. The new bridge was not as well received as the first bridge. The new bridge was rotated and moved slightly to make construction easier, to make access to the bridge easier, and to provide pedestrians of the bridge a better view while on it [8]. To do this, several buildings were demolished to create the new approach road [8]. The people that used to live in those buildings were understandably, extremely unhappy. They were so unhappy that when Queen Victoria came for the Royal opening of the bridge, she was booed and hissed at during the ceremony [8]. The fact that she had not been seen in public for over five years also did not help her case.

Figure 5, A Cold Reception for the Queen [9]

The second Blackfriars Bridge is primarily used to provide cars and pedestrians a way to cross the Thames River into London. However, it was used for a more gruesome purpose in 1982. The body of one of Italy’s most prominent bankers Roberto Calvi was found hanging from the bridge with his pockets stuffed with $14,000 and 5 bricks [8]. The Metropolitan police initially treated this as suicide [8]. However, in 2002 evidence arose that Roberto had actually been murdered by the mafia [8].

Structural Art

There are three components that are used to determine if a structure can qualify as structural art. They are efficiency, economy, and elegance. The ultimate goal of each of these three is to be found in a structure. This means that the structure minimizes the amount of materials used to make it, does not provide unnecessary costs and expenses, and provides the maximum amount of aesthetics as possible.

Looking at this bridge initially, the first thing a person can see is the very simple design of the bridge and its arches. This paints a very clear picture as to what is going on in terms of its load path. If we were to not go any closer to the bridge, it would pass on this aspect. However, when you approach the bridge on foot, you can see all the extra ornamental decorations that the bridge contains. There are small architectural flairs all along the bridge, a requirement by the City of London [7]. There are also large columns along the outside of the bridge that serve no structural purpose. They only exist to give people a place to sit while walking and to provide more opportunities for ornamental design. Taking in the sum of all its components, I would argue that the bridge does not check the efficiency box.

Figure 6, Example of Ornamentation

From an economy perspective, the bridge actually was not too strenuous on the City of London. The bridge only needed a toll to help pay for expenses for less than 10 years each time it had been built. The social gain that citizens were provided was also a tremendous boon. The area started as a place of thieves and low class people, but the bridge provided economic growth and improvement to the area. Today it is the scene of many large businesses and serves as a business powerhouse in the City of London. In terms of economy, I would say that this bridge did in fact meet and exceed this standard.

Finally looking at elegance, this bridge appears to be balancing between a light and gaudy form. The arches are very simplistic and tasteful, but the columns that are placed along the bridge break up the light form. There are also very complex lattice structures that break the flow of the load paths on the bridge. This bridge took a solid design base and added extra things for no reason. It actually reminds me of my brother and his teeth brushing habits when he was a kid. Instead of doing the simple act of brushing his teeth for a minute, he would close and lock all the bathroom doors, run the water and put some in his mouth to act like he brushed, squeeze some toothpaste out into a tissue… well you get the idea. Instead of keeping it simple, he tried to do all these complicated things in attempt to give the appearance of clean teeth. But in reality, they were not. This bridge could have kept the beautiful and simple arch design but chose to do complicated things for no reason. Well, the government telling them to is the reason. But this is a great example of why the engineer is the most capable of creating structural art.

Basing my opinion on the three aspects that were discussed, I would argue that this bridge is not structural art. It comes very close but the ornamentation adorning the bridge takes it away from structural art. However, the basis for a simplistic and elegant design is hiding under the surface.

Structural Analysis

Because there is little information on the bridge’s method of construction, assumptions have to be made regarding this part of the bridge. There are a few pictures that shed some light on the construction process. Figure 7 shows men in diving gear. The piers would be the first part of the bridge to be constructed and the caissons that were sunk into the river would need workers in the water to help with that process. Figure 8 shows workers fabricating metal on site. In doing so, the material did not need to be transported very far. This saved time and money for the project. While the arches are different sizes, they are close enough for the on-site fabrication to be feasible. This would also be consistent with the type of material the bridge’s arches are made of.

Figure 7, Bridge Workers [10]

Figure 8, Making of the Material [10]

The main feature of the Blackfriars Bridge is its five wrought iron arches [5]. These arches sit on piers that have granite stonework [5]. The central arch of the bridge is 56.4m long [5]. The next two arches are 53.3m and 47.25m respectively. The bridge’s total length is 281m [5]. The width of the bridge was originally 22.9m, but it was widened by 9m on the west side to accommodate tramways [5]. The arch is 3.3m high [5]. The main structural system employed for this bridge is the arch form.

The system carries load using its arch form to its benefit. The bridge experiences loads on top of it in various forms. There is the dead load of the bridge itself, live load of people walking across it, live load of cars driving over it, and possible rain/snow live loads. Arches are fantastic at managing compression forces, which is exactly what this bridge does. All of the loads mentioned are transferred through the arch down to its connections at the bases where it connects to either an abutment or a wall that has another arch on the other side. That is important because the thrust forces of two arches next to each other will cancel out. This is why repeated arch forms are so efficient.

Figure 9, Load Path

Figure 10, Repeated Arch Form

Now that the load path of the structure is understood, we can do an analysis of it to see what kind of forces this bridge is withstanding. The main loads were mentioned above, but some math is needed to get numbers to those. We will assume the worst possible scenario which would mean maximum traffic, maximum people, maximum amount of rain, and the weight of the bridge itself. Below is a very simplified version of the arch.

Figure 11, Simplified Arch

Figure 11 is a symmetric arch with linear loads going across it. We can solve for the reactions at the ends using the following math.

Figure 12, Solving for Reactions & Maximum Force on Abutment

The design drawings were able to communicate the idea of the bridge to the government very well. There was little in the way of disagreement amongst them, but the drawings were changed slightly. The government wanted more architectural flairs than what the original design called for, so the giant non-load bearing columns and ornamental designs were added to make the bridge ‘more appealing’.

Personal Response

After studying and researching the Blackfriars Bridge, I think it is a very important and historic bridge that could use a little less ornamentation. I enjoyed being able to bike across it so the expansion for the top of the bridge greatly enhanced my first experience with it. My first impression of it was that this was a very pretty bridge, but after having studied it I can now see the more unnecessary parts of it that I can do without. While the Blackfriars Bridge may not be the most famous bridge in London, it certainly stands out as one of its most important bridges.

References:

  1. http://www.peoplequiz.com/trivia-quizzes-5536-San_Diego_Padres_Baseball_History__Facts.html
  2. http://www.british-history.ac.uk/survey-london/vol22/pp115-121
  3. https://londonhistorians.wordpress.com/tag/blackfriars-bridge/
  4. https://www.mrsmithworldphotography.com/photograph-of-Blackfriars-Bridge-5-London-England/WOF_G043_0033
  5. http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=693
  6. https://blogs.scientificamerican.com/but-not-simpler/scour-why-most-bridges-fail/
  7. http://www.thehistoryoflondon.co.uk/the-original-blackfriars-bridge/
  8. https://lookup.london/the-old-blackfriars-bridge/
  9. https://www.gettyimages.co.uk/detail/news-photo/queen-victoria-opening-blackfriars-bridge-london-1869-news-photo/463951667
  10. https://www.theguardian.com/cities/gallery/2016/aug/02/how-london-was-built-tower-bridge-southbank-in-pictures
  11. https://www.quora.com/How-much-does-an-average-car-weigh
  12. https://www.ag.ndsu.edu/archive/dickinso/research/2004/range04c.htm
  13. https://hypertextbook.com/facts/2004/KarenSutherland.shtml

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].