Blog 2
Queen Elizabeth II Bridge

Queen Elizabeth II Bridge

Structure Information

The Queen Elizabeth II Bridge spans the River Thames and joins Dartford and Thurrock. It is a part of the M25 London Orbital Motorway. In Figure 1, you can see where the bridge is in relation to the city of London.

Figure 1. Location of Queen Elizabeth II Bridge relative to the city of London.

Construction began in 1988, and the bridge was completed in 1991. This bridge was designed to allow for more roadway traffic using the M25, which circles London, to cross the River Thames. It consists of four southbound lanes, and this was important to match the needs of those using the M25. Instead of using the two  preexisting tunnels, southbound traffic moved to the bridge. You can see the four lanes in Figure 2.

Figure 2. Looking at Queen Elizabeth II Bridge from above. [2]

The bridge was designed by Dr. Hellmut Homberg, a German engineer, and other companies like Kvaerner Technology Limited helped in the design process. [2] In order to pay for the bridge, a Private Finance Initiative was created through the Dartford-Thurrock Crossing Act of 1988. This allowed for the toll revenue to pay off the debt from building the bridge. The Department for Transport manages the tolls. [1]

 

Historical Significance

When built, this bridge was the largest cable-supported bridge in Europe. Since the Pool of London is very important for large ships to access, this area was new territory for the bridge. Even though a cable-supported bridge did not use any new techniques, the design had to account for a height of 65 meters to allow ships to pass underneath, which is high. It is the only bridge east of the Tower Bridge on the Thames, so it was the first time a bridge was built high enough in this area to allow for large ships to pass through as well as support four lanes of traffic. [2] The cables are made of steel and the bottom piers are made of concrete and the pylons are made of steel, so no new materials were used. This bridge is relatively new, so it hasn’t been a model for future bridges yet, but since most of the other paths around this bridge that cross the Thames are tunnels, more people may start thinking of building more bridges or expanding this one. With the population of the London area growing, there will be more traffic, and the Department of Transport will have to determine how to increase traffic flow. What was cool about constructing the concrete pylons was that they were slip formed. Concrete was continuously powered for 24 hours a day for 10 days for each column. Also, no scaffold was used, so abseilers on ropes patched up holes as necessary. [4]

 

Cultural Significance

In an article I read, Dennis McNally, supervising the construction of the four piers, talked about how the area has become so industrialized since the construction of the bridge. In 1991, he could just see fields all around, but now it has changed significantly. McNally also expressed that there was controversy about what to name the bridge. On the Essex side, people wanted to name the bridge the Tilbury Bridge, not the Dartford Bridge. Its name is the Queen Elizabeth II Bridge because she opened the bridge in 1991, but many still refer to it as the Dartford Bridge. [4] One interesting thing that occurred earlier this year in February was the closure of the bridge for a brief time when a WW II bomb was found nearby. Luckily, it contained no explosives. [3] The present human cost of the bridge is a toll, which is used to help pay for the construction of the bridge and upkeep of the bridge. Today, the bridge is used for four lanes of traffic southbound for the M25.

 

Structural Art

Using Billington’s 3 E’s method to determine if the bridge is structural art, I will first determine if the bridge fulfills the efficiency component, or minimum materials. This type of cable-stayed bridge requires less material than a cantilever bridge and needs less cable than a suspension bridge. I believe that this bridge fulfills the efficiency component of structural art because it uses minimum materials with this design of the bridge considering the central span is 450 meters. Looking at the economy aspect of structural art, the bridge was estimated to be 120 million British pounds (about $160 million). The average cost of a cable stayed bridge is between $4500 to $5000 per square meter. [5] If I assume a width of 14.6 meters (4 lanes) and complete length of 2872 meters, the estimated cost would be about $187 million. The actual cost was less than that and compared to the cost of a suspension bridge ($8000-$9000 per square meter), is considerably less. [6] Therefore, I believe that this bridge fulfills the second requirement, which is economy or minimum cost. The third aspect I will address is elegance. This bridge is stunning and you can see the load path, but there is a disconnect between the piers and the steel pylons and cables. The color is different and the different material for the pylons creates a disjoint. The concrete piers also look bulky compares to the steel pylons and the thin cables. In Figure 3 you can see how it doesn’t look elegant. It’s not continuous. If the structure had been more cohesive, I would have given this bridge a point for elegance. Since this bridge satisfies two out of the three components for structural art, Billington would say it isn’t structural art because you need all three.

Figure 3. Noncontinuous aspects of the bridge. [7]

Structural Analysis

The design process consisted of a highway scheme under the “Department of Transport’s design, finance, build, operate, and transfer (DFBOT) principle.” [8] Once designed, construction took place. The four main pylons are made of steel and rest on top of concrete piers. The deck is made of reinforced concrete over steel. 112 cables support the bridge. Viaduct sections connect both sides of the bridge to the roadway. Construction lasted for only about three years, but like I mentioned before, there were some cool construction techniques used for this bridge. The concrete pylons were slip formed in which concrete was continuously poured. Also, abseilers on ropes checked everything after construction was completed to make the finishing touches because no scaffolding was used. Reinforced concrete caissons were used to support the piers, and these were constructed in the Netherlands. [8] When building the deck, they built the deck away from the pylons, which acted as a cantilever. The cables were installed as the deck was built. The structural system is a cable stayed bridge in which cables are attached to pylons and these cables hold up the deck. The bridge supports its own self weight, a dead load, and a live load from cars moving across. All the cables are in tension and the pylons and piers are in compression. The deck is also in compression. The dead load from the weight of the bridge (I assumed a concrete bridge with a certain cross-section for the box) and the live load from cars and trucks driving across are transferred to the deck and then the piers. This can all be seen in Figure 4.

Figure 4. Depiction of the load path for the QEII Bridge. [11]

Assuming the deck is a hollow box, I made assumptions based on research about the size of the deck’s cross section considering it’s four lanes across. I found the total cross-sectional area of the deck and then divided it by two because each section of the bridge has two adjacent piers, so the load on one pier only covers half the cross-sectional area. Assuming the deck is concrete with a density of 145 lb/ft^3, I found the dead load to be 9177.31 lb/ft. I also used AASHTO’s specifications for H20-44 and HS20-44 trucks (640 lb/ft) to ensure that the maximum load was used for safety. [10] Since traffic is very congested on this bridge, I assumed that the load was applied over the entire bridge. As seen in Figure 5, I calculated the entire load for the bridge.

Figure 5. Calculations for the loads.

Once I found the load I assumed lengths in between each cable. Each pylon had 14 cables on each side. With my distances assumed based on the length of the main span, I calculated the angles of each of the 14 cables. I only needed to do calculations on one side because the cables are symmetrical to the other side of the mast. As seen in Figure 6, I calculated the angles. I labeled cable 1 as the inner cable and cable 14 as the outer cable.

Figure 6. Calculation of cable angles.

Considering that cable 1 includes the area between the pylon and the cable and the half the area in between cable 1 and 2, I found the weight force, tension force, and force in the deck. All of these calculations can be seen in Figure 7.

Figure 7. Calculations for the tension in cable 1 and bridge force.

I found the forces for the other 13 cables, which can be seen in Figure 8.

Figure 8. Calculations for all the cables.

Once I found this for all the cables, I found the force in the mast. Since there are identical cables on the other side of the mast, I multiplied all the weight forces by 2. This can be seen in Figure 9.

Figure 9. Calculation of the mast force.

By finding the tension forces and the force in the mast, designers can determine how large the cables need to be and how large the pylons need to be to support the bridge based on the materials used. To communicate the design to stakeholders, plans were printed so they could follow them. Many supervisors like McNally oversaw different aspects of construction of the bridge. Models weren’t used for this type of bridge, but since it was built close to the end of the twentieth century, plan sets and standards were used to guide builders. Since a Private Finance Initiative was used for this bridge, it was important that engineers were clear with the stakeholders about all aspects during construction of the bridge with plan sheets. Construction of this bridge was relatively quick.

Personal Response

On the way to the Cliffs of Dover, this striking bridge caught my eye. In the main area of London, the height of many bridges is realistic. Sitting on the train, the bridge caught my attention almost immediately because of how tall and different is was. Being there, I never realized how different this bridge was than the other bridges along the Thames west of this bridge. The size and style of the bridge aren’t like any of the bridges down the river. Looking at this bridge shows how far we’ve come in the bridge design process since the other London bridges were built.

 

References

[1] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_

data/file/604529/59263_Dartford_unnum_cov_and_text_A4_v2.pdf

[2] http://www.transporttrust.com/heritage-sites/heritage-detail/queen-elizabeth-ii-bridge-dartford

[3] http://www.echo-news.co.uk/news/15990727.Dartford_Crossing_bomb_contains_no_explosives/

[4] http://www.bbc.co.uk/news/uk-england-kent-15467112

[5] http://www.partnershipborderstudy.com/pdf/Cable%20Stay%20Bridge_2.pdf

[6] http://www.partnershipborderstudy.com/pdf/Suspension%20Bridge_2.pdf

[7] http://www.geograph.org.uk/photo/1515684

[8] http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=105

[9] https://www.newscientist.com/article/mg13217924-900/

[10] http://www.ce.memphis.edu/3121/notes/notes_06c.pdf

[11] https://www.flickr.com/photos/dgeezer/13179342704

Comments

  1. nzukerman3 says

    I agree that the disjointed look throws everything off. However, I think it still counts as structua art because that judge of elegance is subjective and not enough to outweigh the other factors.