Blog 2
The London Bridge (the real one)

The London Bridge (the real one)

I think I can speak for everyone when I say that the current London Bridge is a big disappointment to whoever sees it. It does not appear to be special and looks awfully plain. No one would imagine that this is the bridge whose name conjures images of beauty and wonder and who, for most of my childhood, embodied the idea of London. I was disappointed, I will not lie, but through this blog, I will try to capture the magic surrounding this structure and show that this unassuming piece of concrete may deserve its fame.

 

1.  Structure Information

The London Bridge is very different from the Tower Bridge contrary to what Google seems to think. I have provided a picture of London Bridge in Figure 2 so that we may all be on the same page. I promise you, figure 2 really is the London Bridge.

Image result for london bridge

Figure 1: Tower Bridge, NOT LONDON BRIDGE

 

 

 

 

 

 

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Figure 2: London Bridge (yeah I know :/)

 

 

 

 

 

 

The current London Bridge is actually the 3rd bridge officially of that name. It is referred to as the Modern London Bridge (1). It is located over the Thames River, between the city of London and Southwark and located between 2 other bridges that cross the Thames River: Cannon Street Railway Bridge and the famous Tower Bridge (see figure 1).

Construction for the bridge began in 1968 and ended in 1972 (1). The Modern London Bridge was opened in 1973. It was inaugurated by Queen Elizabeth II on March 17th of the same year (5).

The purpose of the Modern London Bridge was to replace the New London Bridge which had begun to sag. The bridge was sinking at a rate of an inch every eight years starting in 1896. By 1962, the problem was so prominent that the bridge had to be replaced (6); it was struggling to adapt to the higher traffic volumes of the 20th and upcoming 21st century.

The Modern London Bridge was designed by architect Lord Holdford, the engineers were Mott, Hay, and Anderson. The contractor was John Mowlem and Co (7). It cost £4 million at the time which equals to about £51.9million today (There was crazy inflation!). The costs were covered by the Bridge House Estate Charity.

Figure 3: London Bridge

 

2. Historical Significance

By the time the Modern London Bridge was constructed and opened to the public in 1973, many innovations had already been made in bridge design, especially in prestressed concrete. This bridge sadly did not contribute to any and this bridge did not serve as a model for any other bridges. The innovative part of this bridge is found during its construction. The bride was constructed using the cantilever method (1). They completed the span of the bridge by placing a concrete beam between the two cantilevered parts of the bridge. It represented a major innovation in bridge engineering post World War II. Yet this was not the first time someone had used this method. David P Billington himself said that the bridge itself is not of great historical significance(1). It mainly used existing innovative construction methods. It also used the hollow box girder previously developed by Maillard. The best existing example for that would be the Salginatoble bridge of 1929 and prestressing concrete, an innovation inspired by Freyssinet’s work.

 

3. Cultural Significance

The London Bridge has a long history. In fact, London Bridge is the name of any bridge that was constructed in the area. The first bridge had been built by the Romans around 55 AD. The majority of the first London bridges were made of wood. It was not until 1176 that the first stone London’s construction started thanks to the work of a priest of St Mary’s of Colechurch (1). This bridge, referred to as the Old London Bridge opened in 1209 and was the site of calamities.

It became an important commercial site and over time it also became a business and residential area (It still amazes me how they were able to fit houses on this bridge). Anyway, there were shops, houses on the shops (138 premises were recorded in 1358), walkways and rooms extending about everywhere. It got so bad that the bridge started to resemble a tunnel. At some point, watermills were added to the mix (in the 1580s) (1). The roadway was narrowed down to 12ft – this was used by carriages, travelers, merchants, and commuters. Somehow this chaos survived one fire in 1212 that killed 3000people (1), the collapse of 5 of its arches in 1282 and it wasn’t until 1762 that plans were made to remodel this bridge. This is the bridge that inspired the nursery rhyme ” London Bridge”.

Old London Bridge, lithograph after a manuscript illumination of c. 1500 in the British Library (Royal M.S.S.16.F.ii.XV.) .

Figure 4: Old London Bridge

By 1722, all the houses were removed to and the city decided to try and improve the bridge. This proved to be too much of a burden and they commisioned a renowned engineer, John Rennie to construct a new structure a few feet upstream (1).  The old one was demolished in 1831 after 622 years of use (Holy Moly!). It had seen most of London’s historic events and occupied an important place in the city of London. The new design would be known as the New London Bridge. The new London Bridge only lasted 140 years. The most interesting fact about this bridge is that is was dismantled and shipped to the United States Lake Havasu City in Arizona for a tourist attraction. It was sold to the millionnaire Robert McCulloch for 2.460 million dollars. He spent an additional seven million to build it (10). The Modern London Bridge was the site of a terrorist attack that injured 48 people and killing three on June 3rd, 2017. Three terrorists used a rented van to run over pedestrians on the bridge.

 

4. Structural Art

In order to determine if this is structural art, I will be looking at the three E’s economy, efficiency, and elegance and applying them to the structure.

This structure is elegant enough in its shape.  It is aesthetically pleasing and easily integrates itself with the surrounding environment. The concrete works well with the Thames River and it easily merges with the other buildings. Looking at it, it is hard to distinguish where exactly it begins and where it ends. The design is light and fluid, the deck is very thin and the arches are very wide. The abutments too are very narrow and integrate themselves well in the design  Nothing looks excessively bulky or out of place in this bridge; it is rather plain for my taste but I must admit that the elements work well together. The main issue with this bridge is that it is not exciting and does not inspire awe because of how plain it is, however, this bridge does have elegance.

The bridge cost £4 million at the time which equals to about £51.9million today with inflation. Comparing it with the Waterloo bridge that has a somewhat similar style, that cost around 1.3million in 1877 which equals to about 145 million today(8), we can conclude that the bridge’s price was reasonable. It was built using methods to reduce the price: hollow box girder, prestressed concrete, and thin overall bridge to reduce material.

The Modern London Bridge is a very efficient bridge. It was built with efficiency in mind to some extent. It does not have any superfluous unnecessary elements such as excessive decorations or facades and was built to be as slender and simple as possible. The main criterion for this bridge was for it to be simple and functional. They were looking for something to replace the old bridge and did not care much for anything else besides functionality. The bridge is very thin, due to its wide span arches as well as the use of a hollow box girder. The abutments are very thin and long, allowing for less use of material than if the arches were to continue all the way down. The bridge is made out of concrete which is a relatively cheap material, and it is prestressed allowing for the use of even less material.

I will qualify this bridge as structural art. It is very efficient, it possesses some elegance and was constructed at a reasonable price.  The three E’s are met.

 

5. Structural Analysis

The London Bridge is made of three prestressed-concrete box girders. It is 882.5ft long and 105ft wide. It has a minimum vertical clearance of 26.9ft. The bridge was constructed using the cantilever method. Construction started on both sides. Starting from the piers, segments were built and connect to the previous using high strength steel tendons. The two halves of the bridge were connected by a beam of concrete. The bridge had a very interesting construction process. The old bridge was used partly throughout the construction of the new one. They built the new bridge next to the old one and demolished the old one as they were constructing the new one. Construction began with workers excavating giant shafts under the existing bridge. At that point, the balustrades of the old bridge were taken down and a truss structure was built no the side of the bridge.  On this structure were assembled twin celled precast concrete box units (11). This was repeated on the other side of the bridge. The system is illustrated in Figure 5.  The old bridge was then demolished with spandrel and infills removed first. The gantry truss was used for the final two girder boxes on the new design.

 

Figure 5: Construction of twin-celled precast concrete box units over the river

 

The weight of the bridge, as well as the live loads acting on the bridge, are supported by the arches. The load travels down the arches and into the abutments then the foundation and then into the ground as can be seen in figure 4. It is important to note that arches generate thrust forces due to the tendency of arches to want to spread. This design has three repeating arches which cancel the thrust forces with the bridge. the remaining thrust forces are carried horizontally into the ground at the point where the bridge deck connects with the ground.

 

Figure 4: Load Paths

In order to analyze this bridge, we will be only using the dead weight of the bridge and we will be assuming an area load of 480lb/ft^2 (11). We will base our calculations on the central arch which has the largest span.  We will also be using the density of concrete: 150lb/ft^3. The span of the central arch is 109 ft and vertical clearance is 29.6ft (ref fig.5) and a width of 105ft. We will calculate the maximum force on the abutment at A.

Figure 6: Simplified load distribution

 

I was not able to find the thickness of the hollow box girder so I will be assuming a thickness of 1 foot.

w = (150lb/ft3 * 1ft+480lb/ft2) *105ft =66,150lb/ft

Sum of the forces in Y direction = 0

RAy = RBy = w*109/2 = 7,210.4k/2=3,605.2k

Sum of the forces in the y-direction

Make a cut at the center where h is max

Figure 7: Cut at the middle where h is max

 

Resultant = d*w= w*109/2 = 7,210.4k/2=3,605.2k

Sum of the moments about O = 0

0= RAy (109/2) – RAx(26.9) – Res (109/4)

0= 3605.2k(54.5) – RAx(26.9) – 3605.2k(27.25)

RAx = 3,652.1k = RBx

Force A = (3652.1^2 + 3605.2^2)^1/2 = 5,131.8k

By finding the maximum force on A we now know the maximum load that the abutments can take and help prevent the collapse of the arch. It is important to determine the maximum force as it will also allow to determine the maximum weight of trucks crossing the bridge.

 

The design of the bridge was chosen by a competition. The judges were mainly looking for a simple, functional bridge that was easy to maintain and that did not cost too much. The designers communicated their design to the judges by sending the schematics and plans. Upon examining these schematics, here is what some of the judges said: “The designers selected concrete for its low- maintenance characteristics and visual sympathy to its surroundings” (7) and “the bridge is a testament to both careful design and construction”(7). They appreciated the simplicity of the design.

Figure 5: Schematic of London Bridge Design [8]

6. Personal Response

Looking at the London Bridge for the first time can be a bit ( a lot) of a disappointment. It is beyond simple with its monochromatic facade, its lack of decoration and plain arches. It is all GREY. I believe that says it all. We have been blessed by so many marvelous pieces of structural art, that we sometimes fail to appreciate the simplicity of certain designs. The London Bridge may not be the prettiest (by far) or the most intricate design for a bridge but it carries its beauty in its history. With such an amazing background. it does not need much pump or circumstances to be important. Its importance goes much deeper then its aesthetics and if people took the time to learn more about it, I think they would be impressed. When you see a billionaire walking across the street, you will not see him in colorful, boisterous clothing with shiny and obviously expensive watches. He or she will easily blend in the crowd, he or she might even seem underdressed because they have reached a point where they have nothing more to prove. They are rich beyond comprehension and do not approval from anyone to know that they have achieved something wonderful in life.  I believe the London Bridge is making the same statement. Seeing it was very anticlimactic, but the more I researched it the more I realized that it did not need much to be fantastic.

 

References

[1] https://www.britannica.com/topic/Old-London-Bridge

[2] https://www.londonpass.com/attractions-london-bridge.html

[3] http://www.geograph.org.uk/photo/2630157

[4] http://projectbritain.com/london/bridges.ht

[5] https://uk.news.yahoo.com/on-this-day-queen-elizabeth-ii-opens-london-bridge-1973-121915868.html?guccounter=1

[6] https://www.citylab.com/design/2013/10/how-original-london-bridge-ended-arizona/7200/

[7] https://web.archive.org/web/20120420230132/http://www.buildingtalk.com/news/cyl/cyl123.html

[8] http://happypontist.blogspot.co.uk/2013/09/

[9] https://www.officialdata.org/1877-GBP-in-2018?amount=1300000

[10] https://www.citylab.com/design/2013/10/how-original-london-bridge-ended-arizona/7200/

[11] https://www.inti.gob.ar/cirsoc/pdf/puentes_hormigon/25-Lecture06-Design%20loads.pdf

Comments

  1. kunangst3 says

    I agree that the bridge is impressive because of its history. My research on the Southwark Bridge showed that London Bridge is heavily used and congested. How do you think the live loads from all the traffic would affect the structure, especially compared to the dead loads?

    • adiabre3 says

      It’s funny that you point this out because the bridge before the modern bridge was put out of commission because of the high dead loads. It caused sagging of the bridge to such an extent that they could no longer repair the bridge. When designing this bridge they took into consideration the heavy use of the bridge and made calculations accordingly. For my calculations, I used the live load of a large truck just to make sure I covered all live loads. Over the years though the loads still might cause some sag on the bridge but not too much.