Walkie Talkie

Structure Information

Figure 1 – Picture by Me

Figure 2 – Picture by Someone Else

20 Fenchurch Street, known as the “Walkie Talkie,” was designed by Rafael Viñoly Architects and CH2M Hill structural engineers for Land Securities plc and Canary Wharf Group plc. This skyscraper, used as office space, residential space, and park space, has a really unique form—it curves outward on the front and back like a lens with an angled curve on the top (Figure 2). It’s pretty cool, especially when the building is 38 stories and 177 meters tall. This massive landmark began construction in January 2009 and opened in January 2015, being one of the newest additions to London’s skyscrapers. The Walkie Talkie is part of the booming skyscraper trend in the City of London (not “city of London,” but the confusing name for the smaller jurisdiction within the city), in the financial hub of the city. Even though it laser-beams cars (we’ll get to that later), this structure is a great addition to the skyline.

Historical Significance

This specific shape was unprecedented in skyscrapers when it was designed, and the shape is so unique that it has earned both praise and harsh criticism. With such a new building, the impact of the structure cannot be measured yet, but my guess is that it will inspire many architects and engineers to think outside the box in terms of form. I also think that the criticism earned by the building will motivate these designers be a little more conservative in design, still displaying new forms, but in a less exaggerated way. Outside of form and more on the engineering side, the unique cantilever approach (see structural analysis) used was basically custom-made for this building. Setting the core off center and cantilevering part of the curved façade in suck a specific way was extremely innovative, and we will have to wait and see if this technique gets adopted in other skyscraper designs.

Cultural Significance

The building’s culture impact is mainly through its divisiveness. The reception for the building has been very mixed. Many admire its innovative design and unique visual style, but the building is also hated by many for some pretty hilarious reasons. Besides some not enjoying the aesthetics of the skyscraper, many Brits are mad because it melts cars and causes wind tunnels, sometimes adding that it functions like “Bond villain tower.” If a building blew wind in my face and melted my car with a solar beam I wouldn’t like it either. The Walkie Talkie also affects the culture of the city through its environmental style. Along with various awards due to sustainable design, the building’s three-level park is an important fixture of the city. Although the park’s lack of complete public access is another point of criticism directed at the Walkie Talkie, this feature is still a positive.

Structural Art

In term of efficiency, this building’s structure carries complex loads extremely well. The architectural planning can be blamed for the poor performance in some areas (melting cars, channeling wind), but there have been no significant structural issues. Though the building is very new, and still must face years of trial, the efficiency required out of the structure

The whole point of this building is that it goes against what physics demands out of skyscraper form, and the result is poor economy. A tower of the same height could have been built with far less materials, so there is high cost is due to this over-the top design. Although this problem is due to the architectural design, it is impossible to say that this skyscraper minimizes cost for its particular function.

In the ideal of elegance, the Walkie Talkie building is difficult to measure. It succeeds in looking awesome, but using Billington’s concept of structural elegance shows that it does not succeed in this area. The visual aspects of the building were guided by architectural ideas, with the engineering just being done to ensure safety, and the load path is not on display.

Structural Analysis

One of the most challenging aspects of designing this skyscraper was that its architecture demands that its design is the opposite of a typical column. Columns carry gravity loads to the grounds, so a tapered shape toward the top is optimal, while increasing width toward the top is not preferred. The solution the team used was to set the concrete core of the building off center, since the building is heavier on one side, and also use perimeter columns of steel plate box sections. The loads travel downward through four main paths: the two outer columns of the inner core, and the two steel columns near the perimeter (Figure 3).

Figure 3 – Load Path

The distance from the core to the edge of the building range from 11 meters to 22 meters, and the solution was to have a max 18-meter span from core to perimeter column. Any beam distance above 18 meters is cantilevered. To get an idea of the forces present in these cantilevered, I calculated the forces in the beam with the longest cantilever, assuming simple supports (Figure 4). I named the weight per meter of steel beam w, the load transferred to the inner core column L1, and the load transferred to the outer column L2. Since max cantilever occurs just below the top floor, these loads are due to the slab and roof above.

Figure 4 – Cantilever Calculations

The calculations show that By is always supplying a positive force, but Ay only supplies a positive force if L1 > 52.6w. As the cantilever length decreases going down the building, L1 increases while w decreases, showing that the support in the inner core column increases significantly moving toward the base of the tower, due to both the weight above and the cantilever length.

The geometric changes at each floor demanded advanced technology to communicate the design and construction of the building to those outside of the design team. To tackle these challenges, advanced 4D-BIM modelling—3D building information modelling with time added as a fourth dimension—was used, allowing the team to anticipate challenges and maximize safety. This information was mainly used to communicate the building’s program with the clients and mitigate communication issues with the construction team.

Personal Response

The Walkie Talkie Building was only ever slightly impressive to me until I got up close. The first time I visited London I didn’t really give it a second thought. I thought it was a slightly cool building that was probably very difficult to build. When I stood under the amazing vertical curvature of the building, though, I was blown away. It was pouring rain, but standing in front of the building kept us out of the rain. Looking up, it looked as if though the building should be falling on top of me, and the curvature makes the building appear even taller than it really is. As a bonus, I also learned that the top of the building has a full park, which I would love to check out. Overall, I think this is one of the most visually impressive structures in the world with some very impressive engineering that allows a really unique form. 

References

[1] https://www.steelconstruction.org/design-awards/2014/commendation/20-fenchurch-street-london/

[2] https://www.telegraph.co.uk/business/2017/07/27/walkie-talkie-building-sold-hong-kong-company-record-128bn-deal/

[3] https://www.steelconstruction.info/20_Fenchurch_Street,_London

Westminster Bridge

Structure Information

The current Westminster Bridge was constructed from 1854 to 1862 to replace the old Westminster Bridge after the old bridge started showing signs of decay. The bridge spans the River Thames to connect Westminster and Lambeth, and it directly connects to the Houses of Parliament. Thomas Page designed the bridge with consulting from Sir Charles Barry, costing £400,000 for the Westminster Bridge Commissioners and a parliamentary grant. 

Figure 1 – Picture by Me

 

Historical Significance

There were a couple of features of the bridge that were significant in the world of bridge engineering. First, the construction process was very significant, as the technique of using caissons was used for the first time on a large scale. Also, at the time it opened, the bridge was one of the first to use Robert Mallet’s buckled plates as decking material. The plates had just been patented in 1852, and at the time they were the best combination of maximum strength and minimum weight. Later on, the plates were replaced by the more efficient reinforced concrete. Besides moving the caisson technique forward for bridge building, the bridge itself did not have much impact on future bridges, as its design did not necessarily push the boundaries of engineering.

Cultural Significance

The bridge was created during the Victoria Era and officially opened on May 24th, 1862 at four in the morning—Queen Victoria’s birthday and time off birth. Since the Brits are so into their royalty, this is probably a big deal to them, but I couldn’t care less. Moving on to actually important things, the biggest cultural impact of the bridge is that it connects the South Bank area of London to the political center of the city—Westminster. The city of London was rapidly expanding at the time, so the South Bank area was becoming more populated, and the city desperately needed a connection between these two areas. Even though this problem was solved by the original bridge, the current bridge continues to symbolize the unity and integration of these two important cultural centers. I think of the importance of the bridge the same way as I see the function of the bridges that cross I 75/85 in Atlanta. There is already a big contrast in the two “halves” of the city with the bridges in place. Without those bridges, the city would suffer in the areas of cultural unity, transportation, and business.

Structural Art

If I’m being honest here, the bridge is really just okay. There’s not really anything special about it structurally or visually, but it does have some elements of structural art. In terms of efficiency, the bridge has been used for over 150 years, so it is doing its job well. Though it has been standing for so long, I believe there could have been a more efficient design. There are seven spans for only 820 feet of length, so if a more intelligent design was applied the bridge could have performed equally well with less structural members. This observation also affects economy, which would have also improved if less materials were used. A positive note on economy is that money was saved when the first half of the new bridge was built upstream and put into use before the second half was built on the site of the old bridge. This construction technique was a great creative solution to cut cost. When analyzing elegance, it is fairly easy to visualize the load path in the design. The arches and piers are all very visible, and load can be traced from the deck to the foundations of the piers. There is some decoration between the arches and deck, which looks like convoluted spandrels, but it really just serves aesthetic appeal. Billington would believe that the elegance of the bridge is compromised by this design choice, but he would most likely applaud the overall simplicity. Overall, the design is really not innovative or creative enough to be considered elegant, and the bridge does not excel enough in economy or efficiency to be considered structural art.

Structural Analysis

Many of the construction processes used for the bridge were very creative. The foundations were laid in caissons, cavities were dug in the bed of the river for the reception of the caissons, and the piers were built directly on to the soil and not on piles. As mentioned before, this was the first time caissons had been used on a project of this scale, so it took a lot of creativity on the part of Page and Barry. The construction of the bridge itself included building half of the bridge upstream, putting it to use, and building the second half on the site of the old bridge. This process was used to save money by not building a temporary bridge. Money was also saved by using Portland stone from the remains of the old bridge.

The bridge has seven semi-elliptical spans—the central is 130 feet wide, the next two spans are 125 and 115 feet, and the ones adjacent to the abutments are 100 feet wide. The roadway is 58 feet wide and 13 foot footpaths run along each side. The abutments provide the horizontal reactions that counteract the thrust of the outer arches. Speaking of thrust, the bridge uses a repeated arch design, which allows all the thrust forces in each arch to be canceled by the adjacent arch, except for the arches interacting with the abutments. This feature of thrust cancellation can be visualized in the load path of the bridge (Figure 2), and the vertical forces of the bridge are transferred through the piers and into the foundations in the river.

Figure 2 – Load Path

 

To calculate the reactions from the piers and the abutments I had to come up with a model load. If an average car is assumed to be 4,000 lbs and a car takes up an area of 5×10 feet, then I assumed this load is distributed across the entire bridge at 80 lb/ft2. Using this distributed load, I calculated the reactions in the piers and abutments (Figure 3).

Figure 3 – Calculations

 

What was most interesting to me was the comparison of the horizontal reactions to the vertical reactions. While the vertical forces are distributed across eight reactions instead of two, it is still surprising to see how much force must be provided by the abutments.

Personal Response

I like this bridge more for what it symbolizes than for the structure itself. The first time I visited London, my senior year in high school, my hotel was just behind the bridge and across the river from the Houses of Parliament. The bridge allowed us direct access to Big Ben and Westminster Abbey from the hotel, and I loved being able to view these buildings from across the bridge. When you actually visit the bridge, it is surprising how long it actually is, and the walk across takes longer than expected. What’s also surprising is how much use it gets from pedestrians, as the bridge is almost always crowed. Vendors and street performers line the bridge and add to its exciting atmosphere. While I was never blown away by the structural features of Westminster Bridge, I love the overall experience and symbolism of this famous structure. 

References

[1] http://www.british-history.ac.uk/survey-london/vol23/pp66-68

[2] http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=703

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