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The Southwark Bridge

The Southwark Bridge

Walking along the Thames River during the Walking Bridge Tour in London, I encountered so many bridges–10 to be exact–that I was slightly overwhelmed. Although I love bridges and I encountered many different types, I choose to research a bridge that is less known and even less used–the Southwark Bridge.

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Figure 1: Southwark Bridge

Structure Information

The Southwark Bridge lies on the Thames River like the many other bridges in London. This bridge can be seen as one of the older ones and even then, another bridge existed before the Southwark Bridge was constructed. The older existing bridge was known as the Iron Bridge and it was constructed to alleviate congestion as London and the existing London Bridge was getting extremely congested. This bridge had its construction commence in 1813 and finish in 1819. It was a toll bridge. Just the mention of a toll bridge made the residents not want to use this bridge and return to using the London Bridge near by. After demolition of this old bridge, the new bridge, now known as the Southwark Bridge, was rebuilt from 1913 to 1921. This new Southwark Bridge, which will be referenced as just the Southwark Bridge, was built to be stronger and wider than its predecessor without the use of tolls later on. It still fulfills the purpose the bridge was originally designed to do: alleviate the traffic and congestion due to growth in the city of London while getting ride of the negative aspects of the old, Iron Bridge. It linked the Upper Thames Street to the other side of the Thames River, or the city part. Specifically, the south end sits near Museums and tourist areas while the north end sits near the Cannon Street Station. The bridge was designed by a combine effort of architects, Ernest George and Alfred Yeates and engineer, Basil Mott. The contractor was Sir William Arrol but the bridge is owned by City Bridge Trust, a charitable trust run by the City of London Corporation.

Historical Significance

The Southwark Bridge was built in place of the old Iron Bridge that was demolished at the time of World War I. Although it does not have many innovative and striking characteristics, it has adapted to attain the best characteristics of the old bridge and has achieved a few standards for its time. The Iron Bridge was made to be the longest spanning cast iron bridge in London with 3 separate arches placed across the river. Innovation can be seen in this old bridge as the material of that time, cast iron, was used to attain an engineering structure that was large and vast. On the other hand, Southwark Bridge was not built along those lines or even as an innovative idea. Steel arches were constructed and steel plate girder ribs were used. It was built to fulfill the purpose of the old bridge and used materials and construction methods that were expected of its time. Nonetheless, 2 facts can be noted. Firstly, in order to assist with the painting process, 1,000 tonnes of expandable abrasives were put on the bridge to attain the original metal framework. Secondly, 13,000 liters of paint was used to maintain the color of the bridge: the yellow and the green that can be seen from far away. This emphasizes the metal work and the importance of it. Again, the emphasis here is the lack of innovation in the engineering design and construction method used as this bridge was added as a replacement of a previously functional bridge.

Contrary to being an example or model for future buildings, the Southwark Building was in competition with the surrounding bridges–one of them being a similar bridge, the Blackfriars Bridge. It was built after this bridge and has a very similar appearance and form. Also, it should be noted that the piers of the Southwark Bridge aligned with those of the Blackfriars Bridge for ships and water vehicles to better flow through. In conclusion, the Southwark Bridge does not stand out as being a breakthrough structural engineered structure for the materials and processes it used but also because it followed other modeled bridges than becoming a model itself.

Southwark Bridge 1829

Figure 2: The old, Iron Bridge

Cultural Significance

There are less details behind the construction process and time of when the bridge was built so there are no facts to state the human cost or the number of deaths both while the bridge was being constructed and after it was built.

At the south end, there is the city of Southwark that is both quiet and not as well-known as some of the other areas. Therefore, there are no historical stories or events that characterize this bridge. However, popular culture has some link to this bridge. Some films have either been filmed at the site or have made referenced to it. For example, Charles Dickens references the Southwark Bridge in Little Dorritt and Our Mutual Friend not once but many times. The bridge has also appeared in films like Harry Potter and The Order of the Phoenix or Mary Poppins. It seems as though the location has been chosen due to the area being connected to London and the lack of population on the streets rather than the actual marvel of the bridge.

The Southwark bridge stands as being the least used bridge along the Thames River. It is not because the bridge is disliked but rather, it was a tolled bridge with narrow and steep lanes that made it inconvenient to use. The residents used the London Bridge that was close by. Also, it should be noted that the bridge runs through a quiet area while the London Bridge ran through a major road within the city. These reasons made the bridge less popular than it was intended to be but it should not be characterized as a bridge that is disliked. Now, the bridge is one of the quietest bridges connecting the two areas at the ends of the Thames River with only one-fortieth of the traffic from the other bridges but still allowing people and cars to cross it.

Structural Art

The decision of the Southwark Bridge being structural art will be made by me using the 3 E criterion.

Firstly, the bridge is efficient to a small extent. The load path can be seen clearly. However, there are some members that are either too heavy-looking or have no structural purpose. I am making reference to parallel ribs where it does seem a bit discontinuous. Less material could have been used to have an as efficient structure but I do realize that indeterminate structures are usually safer and preferred. In conclusion, the bridge is not as efficient due to the complexity that can been seen in the structural members.

Secondly, the cost of the project was about 2.5 million pounds. Other than the process of repainting, no major rehabilitation efforts were put into place. One more point should be noted: even after facing material shortages, the bridge was finished. Therefore, the economy of this structure is reasonable and can be considered an advantage.

Lastly, the elegance of this bridge is more of a personal opinion. I do not think that the bridge is very aesthetically pleasing of this time and generation and has a very heavy-structural feeling associated with it. It could have been considered to be aesthetic and elegant during its time. However, I do not classify this bridge as being elegant in comparison to the other bridges surrounding it.

In conclusion, this bridge does not classify as structural art. Nonetheless, I do not want to undermine its functionality and the purpose it fulfills.Image result for southwark ugly bridge

Figure 3: A very dull image of the bridge

Structural Analysis

Although there is lack of information about both the construction process and structural systems implemented, the Southwark Bridge will be analyzed below using engineering techniques.

Starting at the foundation, there are granite piers and turrets on each side of the arches that sit in the water. The piers can be seen to be slightly heavy and elaborate due to the time they were built: before the start of the War. They were designed by Sir Ernest George. 5 steel arches span over the Thames River: 2 spans of each being either 45 meters or 48 meters rest on the side and the 3 middle spans that are known as the central spans with length of 73 meters. These particular dimensions were designed for the span to match and align with the Blackfriars Bridge and The London Bridge. This would allow vehicular traffic on the waters to flow easily and smoothly. The arch system has specifically 7 arches layered underneath and attached by metal fixtures to make sure it would not bend. Then, each arch has parallel and vertical steel rods to allow for the loads to freely flow.

The bridge has two particularly complex and slightly hidden structural systems other than the ones that can be seen such as the deck, piers, etc listed above. Firstly, it is the layer of 7 arches underneath the deck. They are spaced out so that the inner 5 get the most load through them (based on their rectangular tributary area) while the arches on the outer sides get the least loads due to their smaller tributary area. Secondly, trusses within the system of arches are used for bracing and greater stiffness of the bridge. Although not visible clearly, the arches are connected through these rectangular and crossing sections.

Image result for southwark bridge deckImage result for southwark bridge

 

Figure 4: The 7 arches underneath the deck and the vertical rods

The load flows through the bridge from first the deck where all the loads are typically acted upon. The loads are then transferred to the vertical rods down to the arch. From there, it flows from each side of the arch to the nearest pier. As can be seen in the figure below, half of the loads flow to the left pier and half flow to the right pier. This load then flows into the piers and then into the foundation and the ground. And, that is how a bridge will transfer loads acting upon it into the ground.

Figure 5: Load Path on Southwark Bridge

The middle arch of the Southwark Bridge will be analyzed in the section below. I am looking at just one arch because when looking at the entire bridge, it acts like an indeterminate structure. Also, once one arch is analyzed, the others have a similar behavior.

Figure 6: Arch Free Body Diagram

I assumed that a uniform load of 5 Kilonewtons/millimeter squared or 105 pounds/feet squared was acting uniformly across the bridge deck. This was retrieved from an European engineering website which suggests typical loads on a short-medium span bridge. This value was multiplied by the thickness or in this case, the width of the bridge to get 5880 pound/feet which is the value as can be seen in the figure above (measurement per linear foot). From here, the vertical reactions at the base of the arch were calculated solely from statics. As can be seen from the symmetry of the Arch, the reactions at both points are equal to each other.

Figure 7: Calculations

Now, to calculate the horizontal forces, or the thrust enforced by the Arch, a cut was made at the middle. This cut allows an internal tension force to appear as seen in the figure below.

 Figure 8: FBD of Cut Arch

The horizontal reactions are calculated using a Moment equation.

Figure 9: Calculations of the Arch

Although the values do not reflect this, it can be seen that all the arches are in compression. The vertical forces are carried or held down by the piers while the horizontal forces, or the thrusts are cancelled out by each arch next the one being looked at except the arches at the ends. These arches at the end have their thrust force taken by abutments which are placed at the ends of the bridges. In all, the forces are calculated for the arch to see how it behaves and the overall behavior can be understood too. One more note should be made: to simplify the calculations and the results, a uniform load was considered. However, bridges are acted upon by combinations of dead, live, point, uniform and dynamic loads.

Bridge House Estates operates the City Bridge Trust which was used to build the Southwark Bridge. This Estate as well as all the stakeholders (engineers, architects, construction team, etc.) that were involved in the construction process of the bridge used drawings–newly rendered ones as well as inspiration from the old bridge to communicate the design on the bridge.

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Figure 10: The Iron Bridge vs. The Southwark Bridge

Personal Opinion

While standing near the bridge and taking pictures of it, I realized that the bridge does its job extremely well. It links two sides of the Thames River to allow for greater accessibility. Sure, it’s not the most eye-pleasing bridge like the 9 other bridges in its vicinity. Nonetheless, it’s a cool bridge that has peace due to the lack of people walking or driving across it. I would also like to admit that I felt particularly sad or empathetic towards it due to how it is seen by others. We should definitely give credit to this Bridge– after all, it still glows at night!

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Figure 11: The Southwark Bridge at night

 

References:

http://eurocodes.jrc.ec.europa.eu/doc/WS2008/EN1991_8_Calgaro.pdf

https://web.archive.org/web/20131215153238/http://www.southwarkbridge.co.uk/project

https://www.bristol.ac.uk/civilengineering/bridges/Pages/NotableBridges/Southwark.html

http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=667

http://www.southwarkbridge.co.uk/history/old-southwark-bridge.htm

https://historicengland.org.uk/listing/the-list/list-entry/1385913

http://www.towerbridge.org.uk/about/bhe/

 

Comments

  1. ndzanic3 says

    I thought about doing a blog about this bridge too but the I wasn’t able to find information about it. I think you were able to find good information and assume about significances you weren’t able to find directly. I think this bridge is aesthetically pleasing and the colors of it immediately stood out to me as I was walking around London. I like that you said that the values do not match what is supposed to happen in the bridge as a sort of sanity check for the calculations.

  2. jhartwell3 says

    I appreciate the research that went into this piece. It’s unfortunate that a bridge like this isn’t more utilized by the public, given its stability and it’s pleasing design. There are several assumptions about this bridge however they seem to be reasonable. How do you think this bridge compares to others along the Thames so far as structural achievement and art? This seems like a pleasing bridge overall.