Blog 2
Southwark Bridge

Southwark Bridge

Structure Information

Southwark Bridge in London crosses over the River Thames and was built in 1921.

Figure 1. Southwark Bridge

The Southwark Bridge was constructed to provide an additional Thames River crossing with the goal of alleviating traffic on the London and Blackfriars Bridges [1]. The bridge that stands today was the second bridge to be built over this span. The 1921 bridge was designed to reduce the effects of tidal scour and cross currents [2]. This bridge was designed by architect Sir Ernest George with Basil Mott of Mott, Hay, and Anderson (now known as Mott MacDonald) as the engineer [3]. Sir William Arrol and Co. were the contractors [3]. It is owned by Bridge House Estates [2].

Historical Significance

Basil Mott worked with Benjamin Baker, designer of the Forth Rail Bridge in Edinburgh [4]. Besides the architect, this same group of engineers constructed the Forth Rail Bridge. While I haven’t found any thing stating Southwark Bridge is innovative or a model for future structures, the fact that it was designed by engineers who designed other remarkable bridges makes it noteworthy. Southwark Bridge is similar in appearance to Blackfriars Railway Bridge, Blackfriars Bridge, Westminster Bridge, Vauxhall Bridge, and Grosvenor Bridge, just to name a few. All of these bridges cross the Thames and were completed within the 50 years prior to the design of the Southwark Bridge.

As mentioned earlier, this is not the first bridge to be called the Southwark Bridge. The original bridge was completed in 1819 and designed by architect John Rennie [1]. It was a three span arch bridge made of cast iron and masonry [3]. It ended up being the longest cast-iron arch span ever built [5]. The original bridge was innovative because its centers were formed, reducing the disruption to the river during construction [3]. This was a new and unique way to construct bridges at the time.

Figure 2. Old Southwark Bridge Construction [3]

The Old Southwark Bridge form was also innovative because the cast-iron was formed into ribbed arches made up of blocks of iron, similar to how a stone arch would work [1]. This can be seen in Figure 2. This eliminated the need for bolts and the blocks were, instead, tied together [1].

In 1856, the old London Bridge was removed, changing the currents that flowed around the Southwark Bridge [3]. This increase in current caused the bridge to be subject to scour as seen in Figure 3 [3]. Scour is the erosion of the parts of the structure around the waterline due to rapid currents. To reduce this problem, the new Southwark Bridge had five arches that aligned with the piers of the Blackfriars and London Bridges [3]. It also has thinner piers to allow water to flow more easily around the structure. This was a method developed in the earlier similar bridges mentioned previously.

Figure X. Scour along the Thames

Cultural Significance

The Southwark Bridge was built into old steps that had been used by watermen waiting for customers [6]. Because of its low traffic volume, it is a popular spot for filming. The bridge was used in the broomstick flight scene in Harry Potter and the Order of the Phoenix [7]. The bridge was mentioned in Mary Poppins when the Banks family thinks that Mr. Banks had jumped off the bridge after losing his job at the bank [8]. In real life, a pleasure boat collided with a barge near Southwark Bridge and killed 51 people in 1989 [9].

Figure 4. Scene from Harry Potter featuring the Southwark Bridge [10]

The Southwark Bridge was built as another crossing over the Thames to alleviate traffic on the London and Blackfriars Bridges. Several factors resulted in the bridge being unsuccessful in relieving traffic. The Old Southwark Bridge charged a toll, unlike its neighboring bridges, causing people to avoid using the bridge [1]. A major problem was the bridge’s connection to the other roads on the North and South sides. The Old Southwark Bridge was also steep and narrow, qualities that made drivers feel less safe. Because of its lack of use and the fact that some coach drivers park their cars on it, some people refer to it as the “car park bridge” [11]. Today, it is said that if you’re on the bridge, you are either lost or will be lost soon. It is often used as a way to get around the traffic of its neighboring bridges, but even then its traffic is minimal as it is the least used bridge over the Thames.

In 2009, the bridge underwent restoration that included repainting the bridge in its original green and yellow colors [2]. The reason for the green and yellow coloring is unknown, but I think most people would agree that it is aesthetically pleasing.

Structural Art

The structure is comprised of 5 steel arches with 4 stone piers [2]. The steel members are relatively thin. It is easy to see how the load transfers from the deck to the vertical steel members to the large steel arches. There are 7 arches in each span that are connected by truss members. This contributes to the transparency of the structure. Especially when compared to other London bridges like the Vauxhall Bridge, the Southwark Bridge has little ornamentation, unlike Vauxhall’s large statues atop each pier. However, Southwark has some components that are nonstructural. The towers on each pier were originally designed to provide a space for sculptures on the bridges. It was later decided to not include sculptures so now the extra height of the towers serve no use. However, these towers are not much taller compared to the overall structure. Another nonstructural element is the lampposts. There are 30 lampposts on the bridge that each have 3 lamps. This is clearly an architectural choice. Looking back at the structure, the extensive truss stiffening of the arches makes the structure less efficient and requires more material, making it less economical. While the overall arch structure is aesthetic, the addition of material in the trusses and the towers make the structure less elegant. Overall, I would say that while the bridge has some successful components and was designed by engineers who have been involved in structural art, the Southwark Bridge is not structural art.

Structural Analysis

When designing Southwark Bridge, the engineers were very aware of the damage that scour was causing to the bridges over the Thames as mentioned previously. Scour ending up being a major design factor in this bridge. The bridge features a 5 arch span, very similar to the nearby Blackfriars Bridge. This was done to allow for a smoother flow of currents and boats through the bridge [2]. The arches of the bridge are made out of steel and are supported by stone piers [2].

While information about construction of the current Southwark Bridge could not be found, I was able to find information on the Old Southwark Bridge construction and on similar construction projects in the early 1900s. The construction of the Old Southwark Bridge began with the foundations, using a cofferdam for the necessary excavation [5]. The central arch was made of segments of iron connected by dovetails and sockets to form the 8 ribs that made up the arch [5]. The formwork used to construct the arch can be seen back in Figure 3. Like the Old Southwark Bridge, the construction for the new Southwark Bridge would have started with the piers. In the 1900s, it was common for caissons to be used for foundation work, so it is likely that the current bridge used caissons instead of cofferdams. In 1903 when the proposed concrete Vauxhall Bridge was being constructed, it was found that the clay soil would not be able to withstand the weight of the concrete [12]. After realizing this, the design was changed to steel [12]. While their relation is unknown, it is possible that the problems of Vauxhall may have affected the engineer’s decision to use thin steel members for the Southwark Bridge.

The structural system of Southwark Bridge is a three-hinged arch made up of six smaller arches connected by trusses for stiffening. Each arch is supported by pier towers.

Figure 5. Loads on the arches

Figure 6. Load Path of the arch

The deck and vertical members transfer traffic and dead loads to the arch. The arch then transfers the load to the supporting piers. The truss members that connect the arches together and connect the deck to the arches are primarily there for stiffening of the structure.

The following assumptions can be made to analyze this system: the arch is pin connected to the piers, the road paving has a density of 145 pcf with a 3 inch thickness, steel with a density of 489 pcf and a 6 inch thickness, and a live traffic load of 200 psf [13]. The resulting distributed load is 480.75 psf. This is then multiplied by the width of the bridge, 55 feet, to get the load over one arch. The linear distributed load is 26.4 k/ft. The length of each arch is approximately 160 feet [3]. Because the load is uniformly distributed, the vertical reactions would be equivalent to half of the distributed load. After summing the vertical forces, the vertical reactions are (26.4 k/ft)*(160 ft)*(0.5) = 2,112 kips. The horizontal reactions from the piers can be found by taking the moment about either end. The distance between the top and bottom of the arch is assumed to be 50 feet. The horizontal reactions, which are equivalent by sum of the horizontal forces, is 3,368 kips. Figure 7 shows the math behind this.

Figure 7. Calculations

The internal force of the arch can be found by using Pythagorean Theorem with the reactions. The internal force is found to be 3975 kips. As the resultant of the reactions, the force would be acting towards the center of the arch making the arch in compression, as it should be.

To communicate to the stakeholders, the Bridge House Estates, renderings such as the one in Figure 8 were developed. The Bridge House Estates also owns nearby Blackfriars Bridge which has a similar design and was built earlier. Since Southwark Bridge has a simple and common style, it was probably easier for the engineers to communicate with the stakeholders about the bridge because the design wasn’t innovative; there was no need to prove that the bridge would work.

Figure 8. Rendering of Southwark Bridge

Personal Response

After visiting this bridge, I can definitely say it is the bridge less traveled. The reduced traffic on the bridge was an interesting change of pace in a city as busy as London. I found it to be the calm in the middle of a city in addition to providing a good view of the city’s skyscrapers. Seeing the bridge in person gave me a better idea of the many components that make up the truss system under the bridge. But when looking at the bridge from a distance in person, it is hard to see just how many members make up the structure.

Figure 9. Underneath Southwark Bridge