Blog 1
Westminster Bridge

Westminster Bridge

Structure Information

The current Westminster Bridge was constructed from 1854 to 1862 to replace the old Westminster Bridge after the old bridge started showing signs of decay. The bridge spans the River Thames to connect Westminster and Lambeth, and it directly connects to the Houses of Parliament. Thomas Page designed the bridge with consulting from Sir Charles Barry, costing £400,000 for the Westminster Bridge Commissioners and a parliamentary grant. 

Figure 1 – Picture by Me

 

Historical Significance

There were a couple of features of the bridge that were significant in the world of bridge engineering. First, the construction process was very significant, as the technique of using caissons was used for the first time on a large scale. Also, at the time it opened, the bridge was one of the first to use Robert Mallet’s buckled plates as decking material. The plates had just been patented in 1852, and at the time they were the best combination of maximum strength and minimum weight. Later on, the plates were replaced by the more efficient reinforced concrete. Besides moving the caisson technique forward for bridge building, the bridge itself did not have much impact on future bridges, as its design did not necessarily push the boundaries of engineering.

Cultural Significance

The bridge was created during the Victoria Era and officially opened on May 24th, 1862 at four in the morning—Queen Victoria’s birthday and time off birth. Since the Brits are so into their royalty, this is probably a big deal to them, but I couldn’t care less. Moving on to actually important things, the biggest cultural impact of the bridge is that it connects the South Bank area of London to the political center of the city—Westminster. The city of London was rapidly expanding at the time, so the South Bank area was becoming more populated, and the city desperately needed a connection between these two areas. Even though this problem was solved by the original bridge, the current bridge continues to symbolize the unity and integration of these two important cultural centers. I think of the importance of the bridge the same way as I see the function of the bridges that cross I 75/85 in Atlanta. There is already a big contrast in the two “halves” of the city with the bridges in place. Without those bridges, the city would suffer in the areas of cultural unity, transportation, and business.

Structural Art

If I’m being honest here, the bridge is really just okay. There’s not really anything special about it structurally or visually, but it does have some elements of structural art. In terms of efficiency, the bridge has been used for over 150 years, so it is doing its job well. Though it has been standing for so long, I believe there could have been a more efficient design. There are seven spans for only 820 feet of length, so if a more intelligent design was applied the bridge could have performed equally well with less structural members. This observation also affects economy, which would have also improved if less materials were used. A positive note on economy is that money was saved when the first half of the new bridge was built upstream and put into use before the second half was built on the site of the old bridge. This construction technique was a great creative solution to cut cost. When analyzing elegance, it is fairly easy to visualize the load path in the design. The arches and piers are all very visible, and load can be traced from the deck to the foundations of the piers. There is some decoration between the arches and deck, which looks like convoluted spandrels, but it really just serves aesthetic appeal. Billington would believe that the elegance of the bridge is compromised by this design choice, but he would most likely applaud the overall simplicity. Overall, the design is really not innovative or creative enough to be considered elegant, and the bridge does not excel enough in economy or efficiency to be considered structural art.

Structural Analysis

Many of the construction processes used for the bridge were very creative. The foundations were laid in caissons, cavities were dug in the bed of the river for the reception of the caissons, and the piers were built directly on to the soil and not on piles. As mentioned before, this was the first time caissons had been used on a project of this scale, so it took a lot of creativity on the part of Page and Barry. The construction of the bridge itself included building half of the bridge upstream, putting it to use, and building the second half on the site of the old bridge. This process was used to save money by not building a temporary bridge. Money was also saved by using Portland stone from the remains of the old bridge.

The bridge has seven semi-elliptical spans—the central is 130 feet wide, the next two spans are 125 and 115 feet, and the ones adjacent to the abutments are 100 feet wide. The roadway is 58 feet wide and 13 foot footpaths run along each side. The abutments provide the horizontal reactions that counteract the thrust of the outer arches. Speaking of thrust, the bridge uses a repeated arch design, which allows all the thrust forces in each arch to be canceled by the adjacent arch, except for the arches interacting with the abutments. This feature of thrust cancellation can be visualized in the load path of the bridge (Figure 2), and the vertical forces of the bridge are transferred through the piers and into the foundations in the river.

Figure 2 – Load Path

 

To calculate the reactions from the piers and the abutments I had to come up with a model load. If an average car is assumed to be 4,000 lbs and a car takes up an area of 5×10 feet, then I assumed this load is distributed across the entire bridge at 80 lb/ft2. Using this distributed load, I calculated the reactions in the piers and abutments (Figure 3).

Figure 3 – Calculations

 

What was most interesting to me was the comparison of the horizontal reactions to the vertical reactions. While the vertical forces are distributed across eight reactions instead of two, it is still surprising to see how much force must be provided by the abutments.

Personal Response

I like this bridge more for what it symbolizes than for the structure itself. The first time I visited London, my senior year in high school, my hotel was just behind the bridge and across the river from the Houses of Parliament. The bridge allowed us direct access to Big Ben and Westminster Abbey from the hotel, and I loved being able to view these buildings from across the bridge. When you actually visit the bridge, it is surprising how long it actually is, and the walk across takes longer than expected. What’s also surprising is how much use it gets from pedestrians, as the bridge is almost always crowed. Vendors and street performers line the bridge and add to its exciting atmosphere. While I was never blown away by the structural features of Westminster Bridge, I love the overall experience and symbolism of this famous structure. 

References

[1] http://www.british-history.ac.uk/survey-london/vol23/pp66-68

[2] http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=703

Comments

  1. kkpetsu3 says

    There’s no doubt this bridge is more meaningful than the general information we all know about it! It’s an interesting concept with the use of arches. However, as you mentioned it in your structural analysis the spans are of various length according to the data but during your analysis you’ve assumed that all spans are equal to simplify the craziness. Do you think the results would have been of a much bigger difference with the original case scenario? Also, in terms of design/elegance would you consider a trade-in of number of spans for much longer ones?