The BT Tower

Walking through Fitzrovia in London, it’s hard to miss this tower of steel, glass and radio disks jutting vertically into the sky. At first glance it appears to be a case of wild and modern architecture, but this structure is older than you think, and has a deep history in the city. Known previously as the General Post Office Tower and the Telecom Tower, it now goes by BT Tower, and remains one of London’s signature structures.
Related image

Fig. 1 The iconic BT Tower. [11]

Structure Information

The BT Tower was commissioned by the General Post Office in 1961 to support the growing use of microwave aerial transitions in London. The tower replaced an older, shorter steel truss tower that could no longer transmit and receive effectively in Fitzrovia London. With buildings rising up all around, it was about time for the tower to get a boost too, lest the airways would get blocked by progress. The solution was a 620-foot tall, 66-foot wide complex tower made of concrete clad in glass, 13000 tons worth of a building, costing £2.5 million to construct. The first 16 floors consist of utility platforms for communication; the next 35 meters open up to house 57 antennae and satellite dishes; 6 more floors are used for utility and functional space; the 34th floor is a revolving room for restaurant use; and finally a 40-foot weather radar caps the tower off. Construction took three years and the GPO Tower was opened for use in 1964. The building was designed by the Ministry of Public Building and Works, the chief architect being Eric Bedford, who was most famous for this tower.

Historical Significance

The design of this building was very tricky to get right, because in order for the communication satellites to function properly, the building could not sway more than 25 centimeters under wind loading. Wind force is very strong for a structure of this size and for the area it stands in. Designers took inspiration from circularly designed buildings that survived in Japan after nuclear bomb strikes in World War 2 to design a building that would resist high wind loads. The end result was a narrow cylindrical tower built around a tapered concrete core. The concrete core runs through the building and is anchored into the ground via a 6.7-meter deep concrete foundation, consisting of a 27.4 square meter prestressed concrete base and a concrete pyramid to support the core, which held the record for deepest foundation in London until 2004 [1]. It was also the tallest building in London until 1980. This structure is designed to bend only 10 inches at its top under wind speeds of 150 km/hr. If this seems like over-designing, that’s because it is – a sturdy structure is necessary due to the important need of accurate and steady microwave links. As such, it was a glowing example of structural stability through its core and its cylindrical design proved beneficial in reducing wind deformation. Its structure also makes the tower able to withstand nuclear blasts landing as close to a mile away, which during the cold war was an important consideration. To prove the initial structural analysis of this building, a model was tested using wind tunnels to confirm minimal swaying and stability.

Cultural Significance

Despite changing technologies and its growing age, the BT Tower remains one of London’s most important telecommunications tower, proving that old dogs can indeed learn new tricks. Originally used for microwave and broadband linkages, the tower has expanded its use in becoming a hub for fiber-optic cables, and currently about 95% of television content is channeled through the tower in its journey to people’s screens [2]. This became possible when Margaret Thatcher opened the tower to privatization in 1984 [4].

The tower has also acted as more than just a hub for UK communications – it has also been a tourist attraction. Upon its completion, the tower featured a revolving restaurant only 3.35 meters wide on the top floor. The tower and restaurant opening attracted huge celebrities, including the Prime Minister Harold Wilson, entrepreneur Sir Billy Butlin, and politician/write Tony Benn [3]. The tower proved to be hugely popular, attracting 1.5 million visitors during its first year alone! The tower was proving to be an enormous success both in utility and public popularity. Unfortunately, the restaurant did not have a peaceful history. During 1971 the restaurant was bombed, blowing the windowed walls straight out, injuring none but requiring 2 years of maintenance and tons of cause for alarm. Indeed, the restaurant was closed in 1980 for security reasons, and hasn’t been reopened since.

While I cannot find examples of persons dying at this site, it wouldn’t be inaccurate to state this tower as the location of many disappearances. This is because until 1993, this tower was considered a secret by the UK government. Yes, you read that right – a 620 foot tall tower in the middle of London was top secret, with its location missing from any city maps and its address in no books. This seems a bit odd considering how obvious a tower of that size is to spot, but the reasoning behind this is because of the military importance it held in transmitting signals. Its secret location was “revealed” in 1993 by Parliament when a member noted how dumb this was to keep under wraps. The tower has been seen elsewhere in a number works, including Harry Potter, The Fog, and V is for Vendetta (where the tower was blown up, so perhaps some fictional carnage has existed) [5].

Structural Art

Fig. 2 The BT Tower as it stands today.

This tower truly stands as a landmark in London, but the question of its place as structural art requires a closer look. Structural art must satisfy the three E’s – efficiency, economy, and elegance – and otherwise must be clear in the way it structurally works. I’m going to break down this structure based on the requirements.

 

Efficiency and economy are all about minimizing material and monetary use during construction and design. This tower tackled several huge problems for stable tall buildings in pretty effective ways. The slender form of the building and its cylindrical shape both contribute to the reduction of wind loading and the effects of dynamic loads without resorting to bulky framework. The substructure of the building was also efficient. The deepest suitable bedrock at this site was 174 feet below ground level, so a concrete large concrete base with a concrete pyramid supporting the concrete core shaft was constructed only 26 feet below the ground. This was a stable and economical solution. Additionally, the building cost only $2.5 million to construct. Elegance-wise, the tower is relatively simple and doesn’t feature unnecessary features. I even believe it to be pretty good looking, despite architects like Christopher Woodward calling it “poorly proportioned” [7], and it remains popular in opinion to the masses.

The one drawback of this tower is that its primary support, the concrete core and substructure, is completely obscured. However, I think the thin shape speaks for itself in protecting against exterior forces, and taking a look inside the building, the pyramid providing lateral support to the core is definitely visible in the bowels of the basement [6]. Based on the innovation in design and the satisfaction of the 3 E’s I conclude this building to be considered structural art, and an example for future buildings where stillness is key.

Structural Analysis

As stated several times previously, the tower’s circular cross section was chosen in the design because of its resistance to wind loads and other external forces. To explain why this is requires more knowledge on aerodynamics than I have ever been taught or ever wish to be taught. However, basic diagrams showing flow of air around a cylinder, such as the one below, do help in understanding the concept. When moving around a curved surface, air tends to bend around smoothly instead of impacting the surface dead-on. This would imply that cylindrical surfaces feel less wind load than those with rectangular surfaces. The building is built to withstand force of up to 150 kmph, and is able to do so with minimal deformation due to its cylindrical shape. During construction, a model 1:67 scale was tested in wind chambers at the National Physics Laboratory to prove this design could minimize deformation to the GPO and designers [1].

Fig. 3 Streamlines around a cylinder. [6]

The building is composed of 13,000 tons of concrete, 790 tons of structural steel, and 50,000 square feet of glass glazing, specialized to reduce heat variations from the sun. Assuming architectural glass is .5″ thick, the total dead load of the building due to material is 14,119.2 kips, giving the vertical reaction of the building supports. The building is cantilevered into the ground, and 26 feet below the surface, a 3-foot thick, 88-square foot square base anchors the building, sometimes referred to as a “raft”. The foundations are sunk 173 feet down into London blue clay. This base is connected to a 23-foot tall concrete pyramid which provides support to a hollow concrete core. This core shaft is 34.4 feet in diameter and 2 feet thick, and reduces to 24 feet in diameter and 1 feet thick after the 205 foot elevation mark. Each floor is a cantilevered concrete disk radiating out from the core. At its widest (near the top) the tower is 64 feet in diameter, however the main width of the tower in its lower levels is 51.8 feet diameter. The contractor used for construction was Peter Lind and Co. The main tower was constructed using slip-forming techniques and cranes to reduce the use dangerous scaffolding for such a high elevations [1]. Additionally, the pyramid structure in the foundation allowed contractors to avoid digging all the way to bedrock to support the building. Below is a diagram of the tower’s vertical exterior plan.

Fig. 4 Tower vertical plan, showing the varying cylinder widths and the pyramid foundation. [8]

The tower acts as a vertical cantilever structure, and each floor acts as a rotated 3-D horizontal cantilever. Each cantilevered floor transfers the dead and live loads they carry to the central core. The self-weight of the structure is carried purely by the central concrete core and is transferred down into the ground. The structure’s stability relies in part on this core and in part on the concrete pyramid below ground, photographed below. The pyramid and plate base help distribute the load of the tower above and increase its surface area to the ground. A stable foundation was critical not only in supporting the load above, but also in reducing movement of the core.

Fig. 5 Pyramid substructure to the BT Tower [9]

Personal Response

This structure is an example of smart engineering decisions made to address problems greater than just keeping the tower standing. The BT Tower was built in order to reduce deformation to the greatest degree possible, due to its importance in telecommunications. I know unfortunately little about aerodynamics and wind loads on cylinder surfaces, however learning about this structure provides me a better understanding about how surface geometry is an important consideration in design. This building is a striking landmark in the city, deeply ingrained in the communications infrastructure of London. Learning about its history and design give me a greater appreciation for its form, and make me look critically at building design going forward.

References

[1] http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=46

[2] http://home.bt.com/tech-gadgets/behind-the-scenes-at-the-bt-tower-11364182741212

[3] http://www.urban75.org/london/telecom.html

[4] https://www.designingbuildings.co.uk/wiki/BT_Tower

[5] http://www.cabbieblog.com/londons-top-secret-tower/

[6] http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/poten/node37.html

[7] https://www.theguardian.com/culture/2001/nov/10/artsfeatures1

[8] http://home.bt.com/pictures/history-of-bt/the-rise-of-the-bt-tower-in-pictures-41363857137460

[9] https://www.theregister.co.uk/2013/05/21/geeks_guide_bt_tower/

[11] https://www.europadigital.tv/single-post/2017/09/06/BT-ahead-of-the-game-receives-Emmy-Award

Grosvenor Bridge

A favorite pastime of mine when visiting other countries is to just walk in a direction and get lost. Cities are composed of so many different communities, landmarks, and secrets that you never know what you’ll stumble across on an afternoon stroll. And that’s especially true for London, in part because of how diverse and bustling it is, but also because it’s so huge I get lost constantly. Grosvenor bridge was one such discovery I made after wandering across the Thames, where I found this bridge among a blooming new community.

Structure Information

Grosvenor Bridge is a multi-track railway bridge crossing the River Thames between Abbot’s Manor and Battersea Park, just adjacent to Chelsea Bridge. Initially known as the “Victoria Railway Bridge”, this bridge was designed by engineer John Fowler and opened for the London Brighton and South Coast Railway in 1860 [1]. At the time, the bridge’s scope was much smaller, with only two railways running across it, built at a cost of 84,000 pounds (funded entirely by the railway industry). Since that time, the bridge has undergone many expansions and redesigns – the current crossing is actually a series of ten parallel bridges carrying ten railways across the river. Grosvenor Bridge is the widest bridge on the Thames, and one of the busiest rail bridges in the city, connecting up Victoria Station with the rest of the nation [2].

Historical Significance

At a first glance, this bridge is easy to look past. It lacks fancy decoration and seems simple in form, which doesn’t betray its deep historical impact on the city of London. This bridge was the first rail bridge constructed over the River Thames in London, which was a huge milestone in improving the rail connectivity of the city. The bridge was initially constructed of wrought iron arches, and its designer John Fowler was bound by tight design constraints. The bridge had to have a clearance of 22 feet for shipping traffic by water below. It had to feature 4 spans, with mid-river piers lining up with Chelsea Bridge to the west [3]. By this time, construction with wrought iron was not a new practice – the bridge was built without issue over the course of a year. However, the designer, John Fowler, would go on to be one of the most influential rail engineers in London, growing the early London Metropolitan Railway, developing tunneling technology, and working with Benjamin Baker on the Firth of Forth Bridge in Scotland [4]. This bridge really was a starting point for his revolutionary career with trains!

Additionally, this bridge design was used as a template for further expansions of rail crossings at this spot on the Thames. As capacity increased, additional adjacent river bridges were constructed in similar design, and simply connected via deck to the original. It wasn’t until 1963 that the bridge underwent major renovations, mainly due to increased train loads and deterioration. The bridge continued on with its four-arch design for aesthetics and practicality [5], and kept consistent with history in that the bridge is really designed as 10 separate bridges able to carry 10 tracks. During reconstruction, it was necessary to keep 8 tracks open for use along this high-volume bridge – so each bridge and track was replaced one at a time and added to the expanding new Grosvenor Bridge, which is why the tracks and arches were kept relatively separate.

Cultural Significance

As previously mentioned, this bridge was one of John Fowler’s first landmark projects during his career in bridge design and railway work. As the first bridge in London to allow train passage across the Thames, it truly was a pioneering structure, if not in form than in purpose. By looking at a history of its widening, the history of railway expansion in general comes to light. As rail gained speed in London, so did this bridge gain width. Especially today, this bridge remains busy transporting people around the country – so busy that not even structurally integral construction to renovate the bridge in 1963 could halt its traffic.

Nowadays, the bridge serves a further social purpose to compound on its economic providence to the area. The southern end of the bridge meets with the Battersea Power Station. This region of London is an up-and-coming neighborhood situated around a large power station reaching up along the riverside. This structure was what allured me to the area to begin with – my curiosity of the construction zone led me to finding a hip residential area with plenty of new restaurants and hang-out spots. Grosvenor Bridge’s southern end meets the boundary of this place, and the community has begun to integrate this old bridge into new life. Below the bridge runs the Thames Path, a popular walking route connecting up neighborhoods and parks along the River. This tunnel running beneath has turned into quite the social scene: pop-up restaurants, ping-pong tables, and relaxing residents can all be found here. Grosvenor Bridge is becoming popular part of this new scene, adding to its importance here in London.

Structural Art

Now comes the difficult task of assessing the Grosvenor Bridge and its place as structural art. Based on the current structure of the bridge, I have decided it to be a good example of structural art, especially when examining it as a renovation of previous form.

The 1963 reconstruction of the bridge constructed the arches as steel-box girders connected to an orthotropically stiffened steel deck through slender spandrels [6]. Orthotropic steel decks, a relatively new concept at the time, strengthen the deck against the vehicular loads above by resisting bending. This results in lighter forms allowed in supporting the bridge. Despite efficiency increases in the 100 years between construction and renovation, essentially the same bridge design was used. This could be seen in two lights – one that conserves cost by utilizing existing piers, or one that fails to innovate given an opportunity.

Economically, the bridge’s continual expansion over the years became very costly. The most recent renovation from 1963 to 1967 was long-winded and expensive due to the continued use of railway above. However, the new integration of stiffened steel decks and hollow steel box arch supports required relatively little material to be used. Additionally, the fact that this is 10 separate bridges in essence reduces the material needed to support each segment of the bridge.

Elegance is where the bridge seems to lack the most. The bridge doesn’t come across as a complete structure, and in reality, it isn’t – this is a crossing consisting of 10 bridges joined at the same piers. Additionally, despite being a fairly modern bridge, it doesn’t look it. The bridge relies on the same design as the original back in 1860, failing to show off too much new in the ways of bridge forms. However, the simplicity of design and openness of the structure really do shine.

Structural Analysis

This structure is interesting to analyze because the deck is disconnected in a way such that each individual track is supported by an individual bridge structure yet also connected by the same concrete piers. Each of the 10 possible tracks is kept separate, rather than acting as one individual structure. This really simplifies analysis on the bridge, since only one live load can exist on each structure at any time, and each structure is essentially a copy of the next. Also fortunate is that dimensions of the truss structure itself are known, and other loads such as that of the deck and trains can be reasonably assumed.

As stated before, the structure is composed of 4 arches spanning over the river, connected to concrete piers above. Each rail bridge is connected to two arch spans supporting it on the sides. These arches are hollow steel box girders measuring 1.13 (3.7 ft) meters tall by 0.61 (2 ft) meters wide, approximately 0.1 feet thick, pin connected to the concrete piers and supporting the orthotropically stiffened steel deck above via tubular steel spandrels (which appear to be approximately 0.5 ft in diameter). The arches each have the same clear span between piers of 50 (164 ft) meters. Each arch connects to the bridge deck with eight slender spandrel columns symmetrically spaced and at the apex of each arch. The rise of each arch is 5.4 (17.6 ft) meters. Each rail bridge deck segment is approximately 5.4 (17.7 ft) meters wide, and plan views show the deck thickness to be 5/8″ ft. These measures give a good idea as to the dimensions and can help determine forces felt in arch members under load.

This bridge acts essentially the same way any arch bridge would: the dead load of the bridge deck and live load from crossing trains is transferred downward via the steel spandrels and center connection to the arch tube. Each spandrel is designed to transfer load from its individual tributary area and does so through compression forces. These loads are transferred to the arch through compression forces, moving down to the ends of the arch and finally into the concrete piers supporting the bridges. There are two arches, so the overall load to be carried is split in half.

Idealizing the bridge as a distributed load including deck weight  and both arches acting as support, the bridge can be described as below:

Because of even areas, the load distributed to each arch is similar. For the center three concrete piers, the horizontal forces of the archways cancel out, resulting in only a vertical resultant force. The end piers do have to support horizontal loads produced by the arches. When a live load (train) is introduced on the deck, more horizontal loading occurs. I will be analyzing the reaction and member forces in one arch span. Construction documents specify the deck to be of mild steel, which has a density of 0.284 lb/cu. in, or 490.75 lb/cu. ft. Using a single span length this results in the deck weight being 890,343 lb. Assume a passenger train load weight of 200,000 lb, resulting in a total vertical load of 1,090,343 lb. Due to symmetry, the vertical reactions on each side of the arch are the same, so each end has a vertical reaction force of 545,172 lb. To find the horizontal reactions at each end, cut at the arch center and take the moment about the center of the arch. (Distributed load [-6648.43lb/ft]*82ft*41ft) + (Vertical reaction [545,172lb]*82ft) + (Horizontal reaction *17.6ft) = 0. This equation results in the horizontal force being 1,270,003 lb, acting inward towards the arch. This exists on both sides. The force in the steel box member then would be 1,382,071 lb, or since in the actual bridge there are two arches supporting each deck, a member force of 691,035 lb. Further, the member stress can be calculated by dividing this force by its area (found to be 0.56 square feet), which gives the result of 8,569 psi.

Personal Response

Along the River Thames, communities are growing, and Grosvenor Bridge has not been left out. Below the bridge, families take walks and friends play ping-pong. Above the bridge, trains cross the river carrying passengers away. This structure plays a large role integrating into the community today, and it holds great importance in the history of London as the city’s first rail bridge to cross the Thames. There is more to Grosvenor bridge than meets the eye as I discovered through my research. Even its simple form goes further than surface level, being truly one bridge composed of 10, increasing the efficiency of its design and paying homage to its long history of expansion and heavy use. When first approaching this bridge I did not expect it to be significant. It felt basic from a distance. Only when examining it closer did I come to appreciate this simple arch bridge more for what it was and is.

References

[1] http://thames.me.uk/s00160.htm

[2] https://londonist.com/london/transport/9-secrets-of-the-grosvenor-railway-bridge

[3] http://www.semgonline.com/structures/struct_34.html

[4] https://www.britannica.com/biography/Sir-John-Fowler-1st-Baronet

[5] http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=697

[6] http://happypontist.blogspot.co.uk/2011/06/london-bridges-8-grosvenor-railway.html

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