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Blackfriars Bridge

Blackfriars Bridge

Structure Information

Figure1: Location of Blackfriars Bridge [1]

I’ve came across Blackfriars Bridge during my last bicycle ride around Jubilee Gardens. Blackfriars Bridge is an arch designed structure that crosses the River Thames about a mile away from the famous London Eye and located between Waterloo Bridge and the Millennium Bridge.

Opened  for public circulation in November 1869, the 5 spans bridge is 923 feet long and, was designed by the English Civil Engineer Joseph Cubitt. Due to an increasing traffic, the initial 70 feet wide structure had been widened to the actual 105 feet. Funding came from a charitable trust “Bridge House Estates” on behalf of the City of London

 

Figure 2: Blackfriars bridge in 2018

Historical Significance

The original Blackfriars Bridge was the third to be constructed in central London after London Bridge and Westminster Bridge. The present Blackfriars Bridge consists of five wrought iron arches and was built in replacement for the first crossing toll bridge “William Pitt Bridge” designed by Robert Mylne with nine semi-elliptical arches made out of Portland stone and was poorly executed. The use of wrought iron was unique at the time of the inauguration, however not the first time to be used. Joseph Cubitt was conjointly working on another similar project: the Blackfriars railway Bridge adjacent to this structure. Both structures look a lot alike with the use of the material iron which made construction much less time consuming (five years) versus the nine years of construction for the original stone designed by Mylne. The structure was inspired and also inspire tourists across the world. Here is an old image of the Original Blackfriars Bridge

Figure 3 : Original Blackfriars Bridge in 1775 [2]

Cultural Significance

Figure 4: Blackfriars at night

Blackfriars Bridge was inaugurated by Her Majesty the Queen Victoria herself on the 6 th November  1869. The international notoriety of the structure came into play thirteen years after the opening when a former chairman of a renowned Italian bank was found dead on one the arches. What seemed to be at first a suicide was later proven to be a murder by the Mafia to whom he was related and indebted. Sounds like a movie isn’t? Decorations on the structure are historically meaningful. It is said that this bridge marks the boundary between sea water and salt water in the Thames and the choice of bird subjects reflects this idea: sea birds on the downriver side, fresh water birds on the other. For instance, the pulpit-like shape at the ends of the bridge is purposely designed in reference to Black Friars while the stone carving on the piers of the bridge was in respect for the marine life and seabirds. The dedication of the bridge to Queen Victoria was represented by her statue which was by the way funded by a certain Alfred Seale Haslam. Used as road and pedestrian bridge, the Blackfriars’ structure boosted the pride of the Londonians, especially of the merchants crossing the bridge for their daily activities leading to donations from the wealthy ones for the funding of the House Estate Trust which is an organization in charge of four other Thames bridges. Guess who is the trustee? The City of London! Yes, the City relies on these funds to insure repairs and maintenance of the infrastructures within the limits of the city.

 

Structural Art

The Bridge has to qualify for the rule of the three E’s (Economic, Efficiency and Elegance) in order to be called a structural art. As shown in the following section, the load path for the structure is clearly represented despite the multiples decorative elements in place which consolidate the aesthetic-elegance aspect of the design. The load path could be seen from the deck trough the multiple trusses and transmitted to the piers. It has been relayed that the City of London had been clear during the design phase of their expectations of an ornamental structure which justify the decorative semi-circular columns on both side of the bridge. The bridge costed  £151,000 at the time; worth a roughly  amount of £17M in 2017 [3] , which make me consider the costs to be within the limits of reasonable during that period. Don’t get me wrong, I’m not saying that the same structure -if it had to be designed today- couldn’t be evaluated more cost-efficiently! Anyway, with the three E’s checked out, I could affirm that the Blackfriars Bridge is definitely a structural art.

Structural Analysis

Figure 5: Load path on Blackfriars Bridge

The bridge is a 5 spans structure made of cast-iron arches assembled on site with the deck made of reinforced concrete built on site as well. As the Thames was well-known to be a fast flowing river causing damage from scouring, iron caissons were used to help deep into the clay riverbed. These caissons were half filled with concrete and surmounted by the granite-faced piers. Thorn and Co. the builder of the structure had to deal with the use of caisson for the first time on this project for the piers which was a challenge that was won. In terms of materials, wrought iron was used for the ornamental elements. Portland stone, well-known for its strength and polished red granite was used for the piers. The 5 spans structure bridge has been designed to cover 922 feet over the river and in between we have the 3.3m high columns, said to weigh over 30 tonnes each carrying loads from the 56.4 m central arch of the bridge followed by the next two 53.3m span. As expected for the load path, the use of repeated circular arches helps reduce the lateral loads at connections (columns/piers) with the exception of the exteriors ones which are contained by the abutments on both banks of Thames. The piers collect both the dead load and live load according to the respective tributary area. The deck, by the way provides a uniformly distributed load on the top the structure over the entire length. The reaction of the soil below the piers prevent the entire structure from failure. In order to design this structure, I’ll assume the extreme cases scenario in terms of live loads especially; meaning I’m considering the following at an instant t on the bridge: 10 trucks of 10,000 lbs each, 100 pedestrian of 120lbs each. These assumptions are computed in the following as:

Evaluating for example the lateral and horizontal forces for the central span requires the following:

ΣFx= 0 => Cx = Dx

ΣFy = 0 => Cy + Dy – W (185’) = 0 but since there’s symmetry between both reactions,

We have Cy = Dy = (WL/2) = 7,832/2 = 3,916 K

The lateral load equals to Cx = Dx = (wl2/8h) = 42,333*1852/ (8*10.8 ft.) = 16,769 K

Resultant Force to be cancelled by neighboring arch Rx = Sqrt. (Cx2 +Cy2) = 17,220 K.

From here, with the same increment method we could determine the force coming from the immediate left’s span until the left side abutment. Due to symmetry, the value on the left side is more likely to be the one on the right side as well.

Furthermore, Shear and Buckling could also be checked out to ensure the appropriate section for the piers in order to avoid failure.

Personal Response

Maintenance on this structure has been said to occur on a yearly basis. However, my impression is that the maintenance is exclusively focusing on the structural aspect of the Bridge. The sidings looked rusty and off paint which, not only affect the aesthetic but could be a trigger for much bigger structural issues in the future. There’s always this strong feeling being in physical contact with structures constructed centuries earlier. I’ve never realized the need for a more thorough maintenance schedule for public structures until my eyes captivated the rusty trusses on Blackfriars.

Figure 6: Structural concerns, Please HELP!

 

References

[1] https://www.google.com/maps/d/viewer?msa=0&mid=1Yv2q2kSjL-2fqJczikCqxQGiJy8&ll=51.51021609855225%2C-0.10235587337115248&z=16

[2] http://www.thehistoryoflondon.co.uk/the-original-blackfriars-bridge/

[3] https://www.bankofengland.co.uk/monetary-policy/inflation

http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=693