Blog 2
Westminster Bridge

Westminster Bridge

Structure Information

The Westminster bridge is in central London. If that area is hard to visualize, it is the bridge that helps pedestrians get from the London Eye across the river to see Big Ben. When I rode the London Eye, his bridge caught my eye. The original Westminster bridge was designed by Charles Barry to connect the east of Westminster to the west next to the Houses of Parliament in 1750 [1]. There were only two bridges in London at the time, and the London bridge was closed. Over time the bridge was subsiding badly and expensive to maintain, so a new bridge was put into the works [1]. The new bridge, designed by Thomas Page and opened in 1862, consists of seven bold elliptical arches, with three flood arches on the Lambeth ride [2]. The bridge has two footways, two tramways at the sides, and two roadways for traffic. This bridge was funded by an Act from parliament. The Act appointed that £625,000 to be raised by a lottery by the sale of £5 tickets from which £100,000 was to be paid to the commissioners, another lottery that raised £197,500, and £380,500, was granted by Parliament [3]. The total cost of the bridge was £1,103,000.

Figure 1:Westminster Bridge from London Eye

Figure 2:Westminster Bridge

 

Historical Significance 

This bridge was the second bridge in London when it was originally built, so the designer of the present bridge Thomas Page modeled the old bridge which had 13 spans. The new bridge is simple in detail and has seven spans of which are all assumed to be symmetrical. It has a style of Gothic design, which matches with the Houses of Parliament [2]. The downstream parapet coincides on plan with the equivalent parapet of the old bridge, though with a 58 foot roadway and 13 foot footpaths at each side, the present bridge is of almost twice the width [2]. Another difference from the old bridge to the new was the materials that were used, which I will discuss in the structural analysis.

 

Cultural Significance 

Previously stated above, this bridge was built to connect the east of Westminster bridge to the west. There are a lot of tourists, such as myself who visit the London Eye and want to explore more. I used to bridge to get closer to the house of Parliament, where Big Ben is housed. Although, the House of Parliament was under construction, I could view the clock in front of Big Ben a little. From the London eye, this bridge made it easier to go visit may historical places in London with well-known landmarks around it. I thought it was funny how London scheduled the grand opening of the new bridge on Queen Victoria’s 43rd birthday, 24 May 1862 and she did not show [5]. I later learned that she was still grieving from the loss of Prince Albert, who died the previous December [5]. A huge event that happen was the terrorist attack on March 22, 2017. A man drove a grey Hyundai across the south side of Westminster Bridge and Bridge Street, injuring more than 50 people [4]. He then crashed the car into the perimeter fence of the Palace grounds and ran into New Palace Yard, where he fatally stabbed an unarmed police officer and was then shot by an armed police officer and died at the scene [4]. The UK thought the guy had ties with the Islamic military, but found no connections.

 

Structural Art

David Billington stated that structural art can be defined using three E’ principles: efficiency, economy, and elegance. The bridge is very efficient when it comes to holding its weight, and getting the traffic from the east to the west.  In comparison to the old bridge, the new bridge had less spans, which less materials were used. Using less materials would have lowered the bridge cost and made I more economical. The elegance of this structure is simple. The bridge is very thin and does not obstruct views in the river, but enhances it. The bridge reveals its most elegant secret when the sun shines at around 1pm on certain days. The beautiful trefoil cut-outs do a little reverse shadow play: the two lower ‘leaves’ keep their shape, while the top ones stretch out a little into one of the best (unintentional?) Architectural jokes the city has ever known [5]. I thought this was hilarious, because it is no way the designer knew this would happen and be made a joke of. Being able to view the load path and from the three E’s principals, I think this bridge is structural art.

Figure 4:Westminster Bridge Trefoil Cut-outs

Figure 5:Westminster Bridge Trefoil Cut-outs at 1 p.m.

Structural Analysis

The cast iron spans are symmetrical in shape and spring from the piers which face the water.  The spans are 117’ in length and are separated by the 8’ pillars that are built in grey Cornish granite. The caisson method was used to get the base of the bridge.  This helped a portion of the bridge sink o its proper place to create the flat bottomed barged.A horse-powered pile driver and a sinking caisson was used to build the piers.

 

Since the spans are symmetrical, I analyzed one span and assumed that the depth of the bridge was about 71 feet based on the given lengths of the roadway and foot path.  I assumed the height of the load was 8 feet, and the weight of cast iron is 442 lbs/ ft^3.

 

Figure 6:Load path of span

Figure 7:Calculation of weight of cast iron & vertical reaction forces

When I got a number for the load distribution, I calculated the live load at different parts of the main span since the traffic is a live load. I assumed the traffic load as a uniform load. In London, the average weight of a woman is 150 lbs, man: 180 lbs, and child:70 lb. The average of those weights was 133 lbs, in which I assume 400 pedestrians could fit on the footpath giving me a weighted total of 53,200 lbs. The average weight of a passenger car in London is 4,000 lbs and 13 tons for a bus. I assumed 8 passenger cars and 2 busses could fit on the roadway. The total weight for 10 vehicles are 84,000 lbs. The uniform load is 137.200 kips existing on top of the dead load of the cast iron. Using this information, I calculated the vertical reaction forces.

Figure 8:Calculation for vertical reaction forces

Next, I made a cut in the middle of the arch. I was then able to solve for the horizontal reaction force and the maximum force at point A, as seen in.

 

Figure 9:Calculation of the maximum force

 

Personal Response

From research, I did not understand why the original bridge did not hold up and needed so much up keeping. By visiting the bridge, of course I realize that times had changed. Meaning, when the old bridge was built it was built to withhold houses and buggies, no cars, busses and people. Also, I felt a little uneasy being where the terrorist attack took place, because pedestrians on the footpath still are not protected. I think bollards should be put in place for the safety of the pedestrians.

Figure 10:Me on the Westminster Bridge

References

[1] http://www.victorianlondon.org/thames/westminsterbridge.htm

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

[3]https://www.bristol.ac.uk/civilengineering/bridges/Pages/NotableBridges/Westminster.html

[4] http://www.bbc.co.uk/news/uk-39365569

[5] https://londonist.com/2016/08/se