Blog 1
Mercedes-Benz Stadium

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

 

 

 

 

 

Comments

  1. zcollinson3 says

    Great stuff Dane! You had some stats and diagrams I would have loved to put in my 3000 project. As the stadium has open air functionality, did the designers account for expansion and compression of the concrete mega columns due to weather conditions at all? I bet in such a massive structure there might be some large deflections.