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The BT Tower

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