Bankhead Highway Bridge

I saw this dilapidated bridge from Tech Parkway, and walked by later along Northside Drive to get a closer look. Although somewhat nondescript, it caught my attention because I was surprised to see an abandoned structure of its size in the middle of the city. The simplicity and eccentricity of the bridge led me to delve further into its history and decide that it would make for an excellent blog post.

Figure 1 – Bridge Against Atlanta Skyline

Structure Information

The Bankhead Highway Bridge was built in 1912 to carry Bankhead Highway (roughly modern day US 29) over the Norfolk Southern and CSX railroads. It was most likely funded by GDOT and designed by a contractor they hired [1]. At the time of its construction, both the railroad and the highway saw very heavy usage, but eventually highway reroutes caused the bridge to become extraneous. Along with high maintenance costs this led to its abandonment and ultimately the destruction of one of the approach ramps. [2]

Figure 2 – Bridge Location

Historical Significance

This bridge is not at all innovative in either construction or design, but is an excellent example of a trussed steel bridge from the time period. It can be viewed as the typical quick and easy solution for land based spans that needed to carry only the load of cars during the early 1900s [1].

Cultural Significance

During active usage, the bridge provided a major causeway for access to the center of Atlanta which otherwise would have been obstructed by the railway. This railway was not for passengers, but instead long commodity filled trains ran along it. In the early 1900s these lines were an important artery for goods transportation to and from Atlanta [3]. Today the bridge is banned from public access (both vehicle and foot traffic) due to extreme structural integrity problems, and the deck, superstructure, and substructure have all been rated “Imminent Failure” in inspections since 1991 [2]. There is also a missing approach ramp and the bridge terminates abruptly at that side with no guard railing or warnings to keep people from falling. However this does not stop graffiti artists and homeless people from climbing onto it, and these are the only people who currently utilize the structure for anything other than the background of grungy Instagram pics.

Structural Art

The three ideals of Structural Art are efficiency, economy, and elegance, and I would argue that the Bankhead Highway Bridge accomplishes the first two, and closely approaches the third. The steel truss structure itself is composed of smaller trusses, creating a light but very strong superstructure and using a minimum of materials. The trusses support a span made of concrete that rests on reinforced concrete pillars, both span and pillars using a reasonable amount of materials. The bridge is therefore quite efficient, and uses materials that were cheap and commonly produced during the time period. It was also built using fairly quick and easy construction processes, as the land based nature of the span eliminates many of the challenges seen in bridges over bodies of water. Both of these factors lead to the conclusion that the bridge also fills out the economic ideal. When it comes to elegance however the bridge is weakest, and I’m sure Billington would hate it, but I personally appreciate its appearance. The bridge is skewed, meaning that the side trusses are the same length, but displaced a single truss length so that from above the bridge is parallelogram shaped. This is an interesting aspect and drew my eye initially as it can create a subtle optical illusion. I also believe that the truss structure connects solidly with the concrete base and together look simple, but strong. Along with the clearly visible load paths from truss to concrete span to pillars (and laterally through the top truss) I believe that the Bankhead Highway Bridge is Structural Art, although Billington may have disagreed based upon its lack of innovation and heavy looking form.

Figure 3 – View of Bridge from Below Missing Approach Ramp

Structural Analysis

The bridge approaches were simply supported cast in place concrete slabs on concrete pillars, and the truss structure is made of steel and supports the concrete middle span of 99.7 feet. This concrete span is 47.9 feet wide and approximately 2 feet thick and the trusses have a vertical clearance of 13.1 feet [2]. The truss structure is a Warren Truss (equilateral triangles) with added verticals and the trusses themselves are also smaller Warren Trusses but without verticals. The top chords and non vertical horizontal trusses have a hollow rectangular cross section with two sides consisting of trusses and all the other members are just single trussed beams. The trusses are riveted together and it is a through truss, so motor vehicles would pass between the upper and lower chords [1]. The truss is skewed as it crosses the railroad at diagonal angle, and from an elevation view looks like a long parallelogram. The top cord is also trussed in the same manner to provide lateral stability, although due to the small span, stiff base, and minimal footprint from an elevation view, the lateral stiffening is somewhat redundant. There is also a concrete sidewalk cantilevered off both sides of the bridge deck. The building techniques of the time were pretty similar to current bridge building techniques (other than the new automatic bridge building machines), and involved wooden form and scaffolding to pour the concrete and the steel was Bessemer mass produced [1].

As the bridge is not currently in use, the only important load is dead load from the self weight of the concrete span. This load is carried by the truss structure which supports the weight of the large concrete slab through members in both compression and members in tension. The weight is ultimately carried by two thick concrete pillars on either side of the span. In the single remaining approach span, there is no truss structure, so its entirely supported by large columns. Below is a simplified truss structure that represents the sides of the bridge to show in a basic way which members take compressive or tensile forces. The green members are experiencing compressive force and the red members are taking tensile force, while the white member has forces that balance out to zero. When the self weight (simplified in the picture [4]) is applied across the bottom chord,  the max stress in any member is in the outside diagonals and is approximately 0.75x the total force. The total force on the bridge due to the self weight of the concrete using a density of 145 lbs/ft^3 and previously stated measurements is 1,384,932.7 lbs which means that the max force in a member is 1,038,699.5 lbs (the total multiplied by 0.75). The compressive strength of old steel is about 36,000 psi (from an internet database), and the end diagonals on the bridge are the only non-trussed members, I had to assume a cross sectional area of about 50 in^2 from the photos. Using these values and the formula (F/A) the normal compressive stress on the outside diagonals is 27,698.7 lbs/in^2 which is less than the compressive strength of steel but only barely. If any live loads from vehicles were added to the bridge failure would rapidly occur, making it abundantly clear that closing the bridge was the correct choice.

Figure 4 – Simplified Warren Truss with Verticals

Another area of possible failure is in the concrete support columns, which could buckle or crumble from compressive bearing stress as shown in the photo below as green arrows. The four columns each have a symmetrical tributary area of 1/4 of the bridge and so must each support 346,233.2 lbs (total weight/4), and are 20 ft tall (previously stated). The columns are slightly tapered squares, and from pictures I will assume that the area at the top of the beam is 3×3 ft and is about 3.5×3.5 ft at the middle. Using these values (and F/A) the bearing stress on each column is 267.2 lbs/in^2, and the compressive strength of concrete is higher than that by at least a factor of ten, so there is no danger of failure from crumbling at the supports. The critical buckling load would be at the red line halfway down the column in the picture below. Using a modulus of elasticity of 2.9 x 10^6 psi (from internet database) and an estimated moment of inertia of 139,968 in^4 (I=b(h^3)/12 using 3×3 ft approximation) comes out to be 6,9551,102.2 lbs (Pcr = (pi^2)EI/(L^2)), a simply massive number that the bridge would never reach. The Bankhead Highway Bridge therefore is in no danger of failure due to its columns, but instead its trussed superstructure and concrete span.

Figure 5 – Compressive Bearing Stress and Hypothetical Buckling Location

Personal Response

It’s somewhat difficult to see from the pictures, but when I was actually walking around the bridge I was fascinated by the skewed truss. It hadn’t occurred to me that such a design was an option, maybe I had seen some in the past but never really took notice, but it was an exciting departure from the normal truss bridge I’ve always seen. I’ve always had some difficulties with trusses (ever since statics) and it was very interesting to try and trace the load paths in person, and then check my answers through equations during the analysis, and my understanding of the joint and section methods has definitely improved. I would love to go up on the bridge, but it’s kind of hard to get to, probably dangerous, and had some homeless people camped out on it, so I may not actually go for it.

References

  1. http://historicbridges.org/bridges/browser/?bridgebrowser=georgia/bankhead/
  2. https://bridgereports.com/1096608
  3. https://bridgehunter.com/ga/fulton/12151210/
  4. http://ivanmarkov.com/truss-simulator.html

Mercedes-Benz Stadium

Structure Information

Mercedes-Benz Stadium is an 80,000+ capacity stadium in downtown Atlanta, used for football and soccer games and other large events. The design team was led by HOK on the architecture side and BuroHappold on the structural engineering side. HOK won the design for the unprecedented pinwheel-style retractable roof, which is the defining feature of the stadium. The building is owned by the Georgia World Congress Center and the Atlanta Falcons.

Figure 1 – Mercedes-Benz Stadium

Historical Significance

The stadium has all of the expected features for an NFL venue—a large reinforced concrete shell, huge overhanging seating areas, towering columns, a Chick-fil-a even though games are on Sundays—but what sets Mercedes Benz stadium apart is its roof. Its “Ocular Roof” uses eight “petals” that slide past each other simultaneously on steel trusses. These petals retract in the same way a flower or camera aperture would. The roof can open or close in around ten minutes, and roof’s lightweight ETFE membrane allows natural light to inside. Also, under the roof is the first ever 360° display screen. The other important features include the building’s ‘Window to the City’, a floor-to-ceiling glass curtain wall that allows views of Downtown Atlanta, and eight triangular steel and glass sections whose angular sides make up the building’s façade. The roof is the most important and impactful engineering challenge of the stadium, as its design is literally the first of its kind. Owner’s did not want another “vanilla” stadium—they wanted to be “game-changers” in the world of stadiums. Mercedes-Benz Stadium is an unprecedented stadium design, and its influence on other stadiums is difficult to measure since it’s brand-new. My feeling is that its design elements will be used in future building for years to come, as the example set by the stadium will challenge other stadium architects and engineers to push the limits of structures.

 

Figure 2 – The Stadium from the Inside

Cultural Significance

The stadium is arguably the coolest stadium in the world, and it’s a huge source of pride for Atlanta natives. Some did believe that it was wasteful to build a $1.6 billion stadium when the former stadium, the Georgia Dome, was fully functional. Though there was some pushback, most see the stadium as a welcome addition to the city, with its attractive appearance and ground-breaking features allowing Atlanta sports fans to actually have some self-respect for once. It’s also worth mentioning that we would have never had the joy of seeing a Marta bus block the live stream of the Georgia Dome implosion if the Dome would have been kept. In a city that basically worships sports, this new stadium, primarily used for professional football and soccer, is basically a monument to its beloved sports teams. For the time being, Atlanta has the most impressive stadium in the country, and those who live in the city are pleased to be the best at something in the world of sports and have an amazing addition to the skyline. Besides within the city itself, the stadium has even larger impacts. The SEC football championships will be held there every year, and it will be the venue for the Super Bowl in 2019. Additionally, Wallpaper named it one of the top buildings that shaped the world’s culture in 2017.

Structural Art

Even though many of the design choices made for the stadium were architectural, many aspects of the stadium are great examples of structural art. This is an example of a structure where the architectural pursuits governed the engineering side, but the ingenuity required to solve these engineering challenges allowed for elements of structural art to shine through. For the purposes of analyzing the structure as structural art, I will focus on the steel roof system, as this feature of the stadium informed the design more than any other component and is the first of its kind in the world.

The first ideal of structural art, efficiency, is definitely accomplished by this structure. The system has to carry the typical wind and gravity loads of a roof in addition to eight 500-ton petal-shaped retractable roof pieces. Oh, by the way—the 500-ton steel flower petals move! Not only does this design succeed in carrying these (seemingly) impossible loads, but it was actually the first design to ever attempt to carry this type of load. The second ideal of economy is when things become a little less definitive. The project went $600 million over budget, winding up at about $1.6 billion—largely due to the roof features. Although this is true, this type of roof had never been attempted before, and it could be argued that the costs were difficult to predict. Perhaps the cost was actually the lowest possible to achieve the structure’s lofty goals, but there is no definitive evidence. Lastly, the ideal of elegance is certainly achieved by the structure. The structure was able to provide a complex roof opening system that is amazing to view, and even the components that support the moving parts are aesthetically elegant. The mosaic of interlocking trusses is impressive in its complex shapes, incredible in its size and scope, and it creates a sense of confidence in the structure without appearing too bulky. Overall, the roof system demonstrates structural elegance and efficiency, but it is difficult to argue that it succeeds in terms of economy.

Structural Analysis

On the most basic level, the stadium can be divided into two main structural systems—the massive reinforced concrete shell (Figure 3) and the steel roof (Figure 4) made up of a vast truss system. The reinforced concrete shell, which supports and includes all of the spaces occupied by people and also supports the roof system, was constructed first. Most of the smaller members of concrete were precast and brought to the site, but the larger pieces (such as the “mega-columns” that support the roof) were cast in place. The roof system, which was designed to support the intricate retractable roof portion, was constructed on top of the concrete shell.

The closing roof is supported by a massive cambered steel roof underneath it. Giant steel trusses make up most of the roof, and the world’s largest movable crane was used to put it all together.

Figure 3 – Concrete Shell

Figure 4 – Roof Structure

The roof required over 21,000 tons of steel. Each of the four main trusses (which support the petals above it and are depicted in figure 4) are 72 feet above the floor and are about 720 feet long. Each of the eight petals (Figure 5) on the roof weigh 500 tons, and they must move in sync—cantilevering over the field 200 feet when closed. The petals are 128′ wide.

Figure 5 – Retractable Petal

Figure 6 – Petal Cantilever Calculations

These massive cantilevers use a fixed reaction system, and, based on my calculations (shown above), the reaction has to supply 1,000 k in the vertical direction. Additionally, the reaction must supply a counter-clockwise moment of 6.66×107 ft*lb. Since the petal is fixed to the support across a line instead of a point, these reactions will be distributed as a line load across the 128′ width of the petal.

Moving on to the concrete shell, a very typical stadium approach was used. Deep foundations were used for the cast in place concrete bowl, while shallow foundations were used for the precast inner dome. After the foundations, the main bowl structure was built, with three separate seating areas. Where the concrete shell becomes atypical is with the 19 “mega-columns” that dominate the structure. These columns were built to support the concrete structure and are also the 19 connection points for the roof system.

 

Figure 7 – Mega-Column Connection To Roof

Figure 8 – Mega-Column Calculations

The load of the roof system is transferred through the mega-columns, and (simplifying the loads as point loads) I calculated that the average load carried by these columns 2,210.5 k. The combination of this large amount of load from the roof system and the loads from the concrete shell structure led to these columns being designed to be so large. These columns look extremely big in person, and when I visited the stadium I thought that they might have been the largest columns I had ever seen.

The overall design for the stadium was selected by the owners, with HOK winning the design mainly due to the retractable roof. After this design was selected, HOK and BurroHappold continued to communicate their design ideas through presentations, sketches, models, and especially through detailed computer models.

Personal Response

I first visited the stadium for Georgia Tech Football’s opening 2017 game against the Tennessee Volunteers (maybe the most scarring sporting event I have ever had to endure). While I don’t have the emotional strength to describe how badly that game went, the experience of the stadium itself was incredible. Years of hype about the stadium had my expectations through the (retractable) roof, but I have to say the stadium went even beyond what I expected. Being inside the building justifies the structural backbends that the design team had to go through to complete this stadium, as the massiveness of the space, the jaw-dropping roof design, and the 360° screen are well worth the construction and design pains. Before I visited I suspected that the complex design and construction process was all overkill, but standing inside the stadium convinced me that the entire process was justified.

References 

[1] https://www.burohappold.com/projects/mercedes-benz-stadium/

[2] https://www.designboom.com/wp-content/uploads/2017/08/mercedes-benz-stadium-atlanta-falcons-hok-designboom-1800.jpg

[3] https://www.stadiumsofprofootball.com/wp-content/uploads/2016/08/mbs17950.jpg

[4] http://mercedesbenzstadium.com/the-stadium/

[5] http://www.seaog.org/Presentations/MBS/MBS%20Presentation_SEAOG.pdf

 

 

 

 

 

Bank of America Plaza

Bank of America Plaza

There’s no way you could have been to Atlanta and not have seen this building. The Bank of America Plaza, or more popularly known as “The Pencil Building” is a prominent feature of Atlanta’s skyline. It boasts not only being the tallest building in Atlanta, but also in the entire southeast region!

When I came to Georgia Tech’s freshman orientation almost three years ago, I remember a student proclaiming, “If you are ever lost, look for ‘The Pencil Building’ and that’s where home is.” As I began adventuring off campus during my freshman year, I always kept that statement in mind; I looked for the pencil building wherever I was and I always found it’s glowing point. When I was returning to Georgia Tech for my sophomore year after a summer at my house in New York, I vividly remembering driving to Georgia Tech from the airport and seeing The Pencil Building in sight. I may have shed a tear… for me The Pencil Building symbolized my return to my new home in Atlanta, Georgia.

Structure Information

Pencil Building in Atlanta, GA [1]

The Bank of America Plaza was built in 1992 and is located on North Avenue in Atlanta; it stands between Midtown and Downtown, representing the dividing line. The building currently serves as office space and has a few restaurants, geared towards the tenants. The architecture firm of Kevin Roche, John Dinkeloo and Associates had designed it, Beers Construction built it, and Shorenstein Properties LLC was funded the construction[2].

Historical Significance

Left to Right: Bank of America Plaza [3], Messeturm [4], Empire State Building [5], Chrysler Building [6]

Designed in the 1990s, the building architecturally demonstrates an Art Deco style. Similar styles to this include the Empire State building and the Chrysler Building, both in New York City and the Messeturm in Frankfurt, Germany shown in the above images. Structurally, the long columns are exaggerated, fabricating a visual effect of leanness. Additionally, all the columns are on the outside perimeter,  creating an open floor plan throughout the offices. The entire tower was built in only 14 months, which is one of the fastest construction schedules for any 1,000 ft building. While there are no specifics on how they constructed the building so quickly, from experience working in the field, I can say this is impressive.

Cultural Significance

During the time when the Bank of America Plaza was built, Atlanta was undergoing a huge transformation. The city was booming, unemployment was low, businesses were prospering, and the 1996 Olympic Games were coming to town. The Olympics gave Atlanta the chance for world recognition and many companies took this as their chance to “tap into Atlanta’s latest potential.” The owners wanted to build a structure to symbolize the new reputation of Atlanta: the city of growth.

While the building was supposed to serve as a symbol of affluence, there were some critics. Urban planners dubbed the name “tower in a park” because they felt that it separates itself from the already built surroundings. There are no street level pedestrian entrance ways and all the retail space can only be entered from the inside, leaving the tenants to be isolated from the outside. One critic called the building’s shaft “unconventional,” while noting that its apex “looks peculiarly like a pencil” [7]. While this critic deemed that the building looking like a pencil was a negative remark, here at Georgia Tech, I think that it is the building’s most notable quality, thus explaining the nickname “The Pencil Building.”

Structural Art

Pencil Building in Atlanta, GA

In order to decide if this building embodies structural art, I first need to define exactly what qualifies as structural art. According to Professor Billington of Princeton University, structural art should be interpreted in terms of the ‘Three S’s’; the scientific, social, and symbolic meaning [8].

The scientific detail will be explained in more detail below, but the Bank of America has eight super columns at each corner of the building. The columns get all the loads from the slabs and from the truss system on top and carry the load down to the foundation. The building is composite meaning it has a mixture of both concrete and steel like many other buildings in Atlanta.

The social role this building plays into society is a limited one. This building was designated as an office space, and with that role it doesn’t separate itself from many of the other downtown buildings in Atlanta. However, it was built during a prosperous time in Atlanta during the 1990s right before the Olympic Games when Atlanta wanted to look good as the whole world would be watching the city. Unfortunately, the building is currently only half occupied which clearly shows that this structure plays a minimal role in the functioning of society.

In a symbolic sense, to me and many others at Georgia Tech, the Bank of America Building symbolizes “home.” Only two blocks away from campus, the building is a literal thumbtack into the map for where Georgia Tech is. Because you can see it from anywhere in Atlanta due to it being the tallest building in Atlanta, it’s a sure way to locate where home is.  Although many people don’t like the design of the building (which I will blame on the architect, not the civil engineer), it is clearly a symbol as in every movie/ TV show that is located in Atlanta always has a scene panning around this building.

As to Billington’s criteria, the structure is not “transparent.” This means that the it isn’t apparent how the loads are being transferred. Starting from the top of the structure, the trusses that create the obelisk shape are very crowded and thin, clearly meant to be an architectural detail. There are so many bars that it was even hard when I was looking at a picture to try and trace how the load was being carried. Side note: the spire at the top is mostly covered in 24 karat gold leaf!! [9] Talk about ornamentation… Once the load reaches the building, it seems as though the load is carried straight down due to the eight super columns. While this is mostly true, I do not think this represents structural art because it seems that the purpose of this façade was to make the building appear taller and slimmer, not to depict it’s function. Due to the fact that the Bank of America Plaza was not built to serve a specific function, but mostly for architectural components, I determined that the Bank of America Plaza is not structural art.

Structural Analysis

The Bank of America building is consistently referred to defeating an extraordinary construction feat as it was only built in 14 months, making this one of the fastest construction schedules for any building over 1,000 feet. To my surprise, despite this feat, neither the structural engineer, CBM Engineers Inc., nor the main contractor, Beers Construction, have any information on this project. The structural material is composite: the core is reinforced concrete, the columns are concrete encased steel, and the floor spans are steel [10]. The Bank of America Plaza was constructed with a composite frame. It was also an early example of the use of super columns, which are actually quite super because the 2 large eight foot square columns at each of the edges of the tower are the only columns that take loads, creating a completely structural column free interior. The super columns are set into the facades of the tower, adding an exterior texture through 8 granite clad points that extend from the base to the crown.

 

Shows how as the height increases, the load decreases.

Loads shown on Bank of America Plaza.

From the figure to the left, it can be assumed that the gold part of the building puts its load onto the rest of the building through the corners. Additionally, from the figure, you can see how the forces flow. Keep in mind the horizontal lines represent the concrete floor slabs and the drawing is not to scale; there are 55 floor slabs in the Bank of America Plaza. From the corners of the structure, the forces flow down through the columns all the way to the bottom concrete footings. Each of the concrete slabs also has their own load. These forces travel to the closest column and join in on the way down to the footings. The Bank of America Plaza was designed with all the load bearing columns to be on the perimeter of the building to provide for open floor plans for office, hence the picture’s two distinct columns in the cross section shown. Shown in the figure on the left, as you go down the building (get closer to the ground) the loads increase. This is because all the columns on the bottom are carrying the self-weight and the weight of everything above it, but at the top, there is less weight to carry.

In addition to acting as a column, the Bank of America building also works as a cantilever. Due to the building being a tall structure (over 1000 feet), wind loads play a role in how the forces affect the structure. Illustrated in the picture below are the forces that are applied to the Bank of America Plaza due to the wind.

 

Illustration showing how the winds loads transfer to the building and can be converted into a cantilever beam.

 

The force of the wind was found in the Georgia State International Building Code and I chose to design for a Risk Category III Hurricane and EF2 Tornado due to the building’s location in Atlanta. Under these conditions, the minimum wind speed to design for is 145 mph [11]. From there I used a wind velocity chart to determine that the pressure is 52.5 psf [12]. Since this pressure was in pounds per square foot, I found the width of the building to be 252.5 feet. Then, by multiplying the width of the building with the pressure per square foot, I found the pressure of the distributed load, 13,258 lbs/ foot. In order to move from the first figure to the second, the building was turned 90 degrees to be symbolized as a cantilevered beam. I then expressed the distributed load as a point load in order to calculate the reactions and moment at the base of the tower.

∑Fy = AY – 13606k = 0

AY = 13606K

∑MA = MA + 136060k (511.5 ft) = 0

MA = -6,959,469 kips∙foot

After making these calculations, I checked my theories on Mastan2. Shown below is the moment diagram and deflected shape of my “beam” aka The Bank of America Plaza. The structural program proves my calculations are accurate and helps illustrate how the largest moment is at the bottom of the tower while the largest deflection is at the top due to wind load.

Mastan2 illustrating the moment diagram of the load.

 

Mastan2 illustrating the deflection of the load.

By stating that the moment at A is a negative moment, it describes that the bean has developed tension in the upper portion because it gets elongated and compression in the lower portion because it gets shortened. This makes sense if we think it through; if there is a giant force of wind on the left side of the building, the deflection will be shown the building bending to the right, describing the scenario above perfectly. Of course, this is all in theory because wind comes from all sides. Also, there are numerous factors to consider including how close the other buildings are to the Bank of America building. If the buildings are close together then there are other buildings that take an extent of the wind load on their own structure.

Strangely enough, I had found no references to this building from the architect, structural engineers, or contractors. It seems as though none of them wanted credit for it. The closest thing I got to a description was from the structural engineer’s project page that states it was built in Atlanta, California…someone needs to fire their intern [13].

Since none of this information was readily available online, I do not think this information was used to communicate the design to the stakeholders. In current times, most stakeholders are only concerned with their cost revenue and the ratio between the cost of building and the cost earned from tenants; their only concern about the structure would then be the cost and to make sure the building doesn’t collapse.

Personal Response

After visiting the Bank of America Plaza, I understand how isolating it could be. There were no doors for me to get inside from the street and the exterior was just a plain brick façade. The main door was through the parking garage, but I felt uncomfortable entering the building since I did not work there. For being my symbol of Atlanta, it did not feel very “homey” in person.

I also never realized how controversial the Bank of America Plaza was until I start researching it. The building was foreclosed in 2012 due to it’s vacancy demonstrating how unwanted it was for an office space. Many people also think that the glowing obelisk at the top is destructive to the skyline and sticks out like a sore thumb. Also, numerous people thought that the building represented either an ugly pencil or a cigarette (yes, cigarette. I never thought of that one! It gets worse at night when the top starts to smoke too…).

Despite all this, I will maintain my positive image of The Pencil Building because whenever I spot the building, it reminds me that Atlanta is home.

References

  1. https://www.atlantadowntown.com/go/bank-of-america-plaza
  2. http://www.skyscrapercenter.com/building/bank-of-america-plaza/429
  3. https://www.tripadvisor.com/Attraction_Review-g60898-d527262-Reviews-Bank_of_America_Plaza-Atlanta_Georgia.html
  4. https://commons.wikimedia.org/wiki/File:01-01-2014_-_Messeturm_-_trade_fair_tower_-_Frankfurt-_Germany_-_05.jpg
  5. https://www.embusyliving.com/2016/06/empirestatebuilding-topoftherock.html
  6. https://www.askideas.com/27-very-amazing-night-view-pictures-of-chrysler-building-manhattan
  7. http://www.atlantamagazine.com/90s/nationsbank-plaza-now-bank-of-america-plaza-rises-up/
  8. http://www.curee.org/CASCE-5/Lectures/L0009.html
  9. https://www.ajc.com/business/bank-america-plaza-becomes-atlanta-priciest-repo/yKEFrl8ElBb2oAiPguIjfJ/
  10. http://www.skyscrapercenter.com/building/bank-of-america-plaza/429
  11. https://dca.ga.gov/sites/default/files/2013_drbc_ibc_appendixn.pdf
  12. http://www.nctlinc.com/velocity-chart/
  13. http://www.cbmengineers.com/project_bank_of_america_plaza.html

Park Drive Bridge

I found this bridge while walking in Piedmont Park this weekend to break in my hiking boots and immediately thought of how great it would be to write a blog post about. I took some pictures (shown below) and explained to my friend how we could see the load paths (which went right over his head).

Figure 1: Park Drive Bridge


Structure Information

Figure 2: Location of Park Drive Bridge [2]

Upon further research (pulling up Google Maps), I figured out the bridge was on Park Drive. The bridge is called the Park Drive Bridge, previously known as the Piedmont Park Boulevard Bridge. The structure was built in 1916 and designed by O.F. Kauffman, who was a city engineer working at the Department of Bridges and Estimates. The purpose of building this bridge was to connect neighborhoods to the park without having to walk over the railroad tracks that ran along the park, which is now the Atlanta Beltline (see map to the left). The bridge was funded by four sources: City of Atlanta, Fulton County, Southern Railway, and Northern Boulevard Park Corporation. [1]

 


Historical Significance

Since the bridge was built long after the invention of reinforced concrete and the arch bridge, there was nothing significant about the Park Drive Bridge’s design or how it was built. In fact, since the bridge is over land rather than water, it was easier to build than many of the bridges we have learned about in class so far.


Cultural Significance

Figure 3: Mural under the Park Drive Bridge [2]

This bridge previously served as a connection from the developing Druid Hills neighborhoods into the park over the railroad [1]. After the Piedmont Park parking lot was build, the bridge was used for parking lot access until the parking area for the park was moved. Now, the bridge is not open to the public for vehicles, but is still open for pedestrians and bicyclists, although there is no railroad to worry about crossing over anymore. Additionally, the Park Drive Bridge has been incorporated into the Art of the Atlanta Beltline project and features a mural.


Figure 4: Close-Up View of the Middle of Park Drive Bridge

Structural Art

 

From far away, this bridge can very well show structural art. The load paths seem clear, there doesn’t seem to be any extra beams or columns, and you can see through the bridge, as Billington commonly uses as a requirement.

Once closer to the bridge, however, you notice the tiles for decoration on the entire bridge and the heavy-looking deck with thick, ornamental bricks as a railing. You can also see small beams connecting the spandrels, which do not seem to be load-bearing given the large girders underneath the deck. These beams are most likely for decorative purposes or because the engineer was worried that the girders could not hold the entire load. This extra material and the cost to add these decorations go against the values of economy and efficiency needed for a structure to be structural art.

 

 


Structural Analysis

This bridge was built as an arch bridge in the middle and simply supported sections on the sides out of reinforced concrete. The construction was contracted to Case & Cothran and cost $28,904.75. The brick section at the top was laid and plastered after the completion of the bridge and the 7 foot deep slab. [1]

In the middle section, the longer span, an arch is in compression against two abutments. The spandrels above the arch are also in compression. There are beams connecting both arches across the bridge as well as beams running between the spandrels in line with the arches. The girders lie perpendicular to the arches and connect to the spandrels that are above the arch. These components can be seen in Figure 4.

Figure 5: Load Path on Park Drive Bridge

The load goes from the slab to the girders. These loads are then transferred to the spandrels down to the arches. The arches transfer the load to the large abutments, which finally take the load to the ground.

The load comes from both the dead load of the railings and the very small live load from the pedestrians walking across the bridge; however, there used to be a larger live load when vehicles were allowed on the bridge.

The braces connecting the arches are there for stability to make sure they do not start leaning. The beams between the spandrels are either for decoration or to provide extra support for the variable side of the girders at the edges of the deck of the bridge.

Figure 6: Full Arch Diagram

 

If the span of the bridge between the abutments is 500 ft, and the load

Figure 7: Free Body Diagram of the Left Side of the Arch

on the deck is 1000 lb/ft, the reaction forces on the abutments can be calculated, neglecting the self-weight of the structure, seen in figure 6.  The reactions in the y-directions can be found to be 250 kips.

 

 

Figure 8: Free Body Diagram of Left-Most Point of Arch

To find the reaction in the x-direction, you must split the arch in half, and find the sum of the moments about L/2 (see figure 7).

The reactions in the x-direction are towards the arch and equal 625 kips if the maximum height of the arch is assumed to be 50 ft.

To find the maximum stress of the arch, the maximum force must first be found. This is done by finding the internal compressive force at the end of the arch. A free body diagram of the edge of the arch can be seen in Figure 8. The maximum force is equal to 673.15 kips. Assuming an area of 8 square feet, the maximum stress in the arch is 84.14 kips/square foot.


Personal Response

I have walked by this bridge many times and never thought it was anything special. I think that the ornamental tiles and brick wall on top make it look like it belongs in another time period. If the bridge were cleaned up and these parts were removed, I think I would like it a lot more. Overall, I think it was very cool to see a bridge that looks similar to the sort of designs we’ve been learning about in class. Seeing this in-person, and realizing on-the-spot that I could follow the load paths, was very cool.