Ponts des Arts Bridge

The Ponts des Arts Bridge is known by most people, especially tourists, as the bridge of Loveeee. Ever since the bridge was built, couples in love would write their names on a lock and lock it to the bridge gates, declaring their love to the world and making it everlasting in history. Sorry, I’m trying not to gag at the cheesiness. Talk about PDA. Anyway, the locks were removed in 2014, but the bridge still stands as the bridge of love to those who visited it. What most people don’t realize is that this bridge carries far more cultural significance than the locks. Technically this bridge is the second Ponts des Arts bridge, but since it is meant to replicate the original as closely as possible, the importance still stands. The original bridge was a symbol of France’s competition with Britain, and Napoleon ordered the original bridges creating personally. So, can we please move away from the love and focus on the interesting stuff? Because the both the past and present Ponts des Arts bridges are nothing if not interesting.

Figure 1: Ponts des Arts Bridge [1]

Structure Information

Okay, now we can get down to the basics, so stick with me.

Location: River Seine, Paris, France

Purpose: Connects the Louvre to the Insitut de France

Type: Deck arch pedestrian bridge

Main Material: Steel

Abutment and Piers Materials: Reinforced Concrete dressed in stone

Deck Material: Timber

Elements:

  • 7 arches
  • 6 piers
  • 2 abutments
  • Deck
  • Deck gates

Total Length: 509’

Arch Span Length: 72’

Width: 36’

Figure 2: Ponts des Arts Close Up

 

As I mentioned in my introduction, this is the second Ponts des Arts bridge. The first bridge in some ways is more historically and structurally significant because it was the first iron bridge in France, and Napoleon wanted it to compete with the Iron Bridge, the first bridge made of iron in the world. However, the bridge that currently stands is modeled to resemble the original, but has two main differences that must not be confused:

  1. The CURRENT bridge is made of STEEL. The original bridge was famous for being made of cast iron in the early 19th century, but since steel is much more efficient and widely used now and when the current bridge was built in the 1980s, it is made of steel, NOT IRON.
  2. There are SEVEN arches on the CURRENT bridge. The original bridge had nine arches, but when remaking it, they decided that it would fit in more with its neighboring bridges over the River Seine if it only had 7 arches.

Historical Significance

To properly explain the significance in terms of technology, I have to address both the original bridge and the current one. The original bridge was much more innovative for its time being the first even iron bridge in France and one of the first few in the world. It was far more lightweight and elegant when compared to the Iron Bridge

or other stone arches at the time.

Figure 3: Original Ponts des Arts Bridge circa 1804 [2]

Figure 4: Iron Bridge [3]

Every member of the Ponts des Arts held a purpose and it efficiently used material, whereas the Iron Bridge was designed to show off its designer’s iron working skills, not its efficiency as a structure. So, the original Ponts des Arts Bridge is extremely historically significant and could be considered structural art in its own right.

Now moving on to the current bridge and the subject of this blog, this bridge is not as historically significant in terms of technology. However, since it does make adjustments like using steel instead of iron and forming fewer arches, it can still be seen as historically significant in regards to a better version of the original. It is not however the first of its time to use steel or anything like that.

 

Cultural Significance

The Ponts des Arts bridge is massively culturally significant for three reasons. The first being that it is a replica of the original bridge which was a statement of French power and engineering competition. So, while the first bridge was destroyed, its symbolism is renewed and everlasting in the current bridge. So Yay.

Secondly, this bridge is hugely popularized as the bridge of love, and is known worldwide through movies, popular culture, writers’ and painters’ depictions, and celebrity social media to people. So, not surprisingly, the decision to remove all of the locks and place glass in their place so that no additional locks could be added was extremely controversial. Being the dork that I am, I think the glass better highlights the structural prowess (and yes, I just used the word prowess) of the bridge which was obscured and came in second to the wall of locks that stood before. But I by no means represent to universal person; I don’t even really represent the normal person considering I like to look at and analyze structures for fun, so I can see why this is a big deal to people. But, as a structural engineer, any additional and unnecessary weight (especially one that equals 50 tons) should be added if it takes away the structure’s ability to fulfill its primary function: to resist natural forces. If part of the bridge collapses, because of the locks, the locks have got to go. But like I said, many don’t agree with me, so this has been somewhat of a hot topic since the bridge gate collapsed in 2014. In fact, when you search the bridge on Google, headlines after headlines come up that look like the one below.

Figure 5: Lock Gate Collapse Headline in Independent Co [4]

Despite all of this, the bridge still is popularized by tourists, locals, and artists alike because it stands at the core of Paris’ draw and art center. And to mitigate the anger, Paris auctioned off the love locks and donated all of the proceeds to a charity for migrant workers and refugees, which I have to say is pretty fricken awesome! So don’t feel bad if your lock was removed, because it was for a good cause and we can now happily move on.

So, lastly, this bridge is popular simply because of what it connects: the Louvre and the Insitut de France. Tourists and students can be seen not only walking on it (which, duh because it’s a pedestrian bridge), but Ponts des Arts is also a common picnicking location, the site of different art exhibitions, and a hub for local artists to make money off of tourists (which can be quite annoying having just been harassed by street vendors in Paris). Nonetheless, the bridge is still widely loved both around the world and locally.

 

Structural Art

Now, let’s take a look at Billington’s Three Es (probably dreaded by those of you who have read of these over and over again, so sorry): efficiency, economy, and elegance.

Efficiency

This bridge is extremely efficient. It uses very little material, reduced the number of arches to seven instead of the original nine, and is very transparent.

Figure 6: Structural Transparency

 

When passing under it, you can see all of its members, which are not that many, especially when compared to the other bridges around it made of stone. However, stone is used to hide the reinforced concrete on the abutments and piers, which is inefficient. Nevertheless, this bridge is efficient in its design and utilization of steel as a material in its own right. The form fits the material.

Economy

For some reason, the exact cost of construction of the Ponts des Arts is extremely difficult to find. But not to worry, we can use look at its use of material to gauge its economy. As stated above, the bridge uses very little steel in the superstructure, but uses extra material in the abutments and piers. Since stone is unnecessarily added to the reinforced concrete, that choice is not economical, but the superstructure is economical. So, I’d say that this bridge achieves half credit in the economy category.

Elegance

This bridge is extremely elegant. For one, it is just plain pleasant to look at. But in terms of Billington’s criteria, the load path is clear in how it travels from the deck to its foundations, it is transparent and uniform in its design throughout the bridge. The stone abutments are a bit bulky, but in my opinion do not take away from the overall aesthetic of the bridge. So, without a doubt, I would say this bridge is elegant.

When taking all of the aforementioned factors into consideration, I would say that this bridge is structural art, although not the best example of it due to the piers and abutments. While casing the reinforced concrete abutments in stone was purely architectural, the elegance seen in the actual superstructure of the bridge overpowers that and wins for structural art in the end.

 

Structural Analysis

As stated above, this bridge functions like a typical arch bridge, with redundant arches next to each other to help with horizontal thrust. The spandrels also form semi-arches with diagonal bracing to help stiffen the deck and distribute the load throughout the arches rather than all at one point. This prevents the arches from buckling, almost as a flying buttress would. The load path, as demonstrated in the diagram below, starts at the deck as a combined uniform live and dead load, moves down into the arches either at the top of the arch or first into the spandrels and then into the arches. The load then transfers to the piers, which send the load down below water level and into the river bed.

Figure 7: Load Path

 

As any other typical arch, all of these arches are in compression, that is an arch’s defining characteristic. The spandrels are in tension, connecting the deck to the arches, each side pulling on the spandrel. The piers are in compression because they act like columns, with the load from the bridge pushing down and the normal force from the river floor pushing back up against the pier.

Figure 8: Members in Tension and Compression

 

We can use the principles of equilibrium to determine the forces acting on each member. To simplify the model, the following assumptions are being made:

  • Assume Deck is one solid surface
  • Assume pedestrian live load = 85 psf
  • tdeck = 1” = .083’
  • ρdeck= 35 lb/ft^3
  • ρsteel=490 lb/ft^3
  • Assume piers are made only of reinforced concrete with no added stone dressing
  • ρconc=115 lb/ft^3
  • Width of pier = 2’
  • Depth of pier = 31’

There are 5 arches in each span and 7 spans in the length of the bridge. So, for each arch:

The arches are made of steel which also have a dead weight. The free body diagram for each arch is the same and can be seen below. Note that the horizontal force that counteracts the arch’s thrust is either the thrust from the juxtaposed arch or from the abutment. Either way, the value should stay the same in order to achieve equilibrium.

Figure 9: Arch Free Body Diagram

The horizontal reactions caused by the thrust of each arch cancel each other out, and therefore do not require any extra calculations.

Moving onto the piers, each of the 6 piers in the middle hold a reaction from the arches on either side of it, and extend the width of 5 arches. So, each pier must support 10 times the point load coming from the arches. The free body diagram for the pier is shown below.

Figure 10: Pier Free Boody Diagram

There are 6 piers, so the bed of River Seine is actually supporting 6 distinct surface loads all equaling 5,714.64 psf.

The reason the reaction at the ground has units of force per area is because it is a surface load acting over the bottom of the pier, which has a length of 36’ and a width of 2’.

 

Personal Response

Okay, so this is probably more of a side effect to the program and having the three E’s drilled into my head continuously, but I can honestly say that my reaction when taking the boat tour and passing under the Ponts des Arts Bridge was “damn, that’s a piece of structural art”. While I ended up doubting that more when I took the time to analyze it as I did above, I just really admired how I could see the load path so clearly and it was so transparent. I didn’t even realize it was the love lock bridge I had seen so many times in movies, but clearly taking away the locks brought out the real character of the bridge rather than distracting from it.

 

References

[1] https://structurae.net/structures/pont-des-arts-1984

[2] https://en.wikipedia.org/wiki/Pont_des_Arts#/media/File:Paris,_Pont_des_Arts_by_Neurdein,_c1885-90.jpg

[3] https://www.shropshirestar.com/entertainment/telford-entertainment/2018/04/16/let-there-be-light-restored-iron-bridge-to-open-with-permanent-illumination/

[4] https://www.independent.co.uk/news/world/europe/part-of-paris-bridge-collapses-under-weight-of-love-locks-left-by-tourists-9512594.html

[5] http://www.eutouring.com/pont_des_arts_history.html

[6] https://www.napoleon.org/en/magazine/places/pont-des-arts-bridge/

[7] https://www.cometoparis.com/paris-guide/paris-monuments/pont-des-arts-s959

Chelsea Bridge

Of the 30-some odd bridges that cross the River Thames, many people, including me are probably wondering why the Chelsea Bridge is at all different or special. Most of the bridges have been there a long time and some look similar. There haven’t been any huge scandals involving the bridge since it was the original bridge that stood in the same place a long time ago. However, this bridge is unique, at least to the structural engineers out there. It’s especially unique to the structural engineering Brits, as the Chelsea Bridge was the first of its kind in the U.K. and still is one of the only ones in the country like this.

As someone new to the country and only being here for a short while, I don’t have any fun anecdotes to share about the Chelsea Bridge; pity. But, I think the Chelsea Bridge speaks for itself and anyone who cares about bridges would be interested in this bridge’s history and I don’t know about you, but looking at any bridge I can’t help but ask myself: “how the hell does it stand up like that?” I guess we’re going to find out.

 

Structure Information

 

Name: Chelsea Bridge

Location: London, United Kingdom

Figure 1: The Chelsea Bridge at Night [1]

Figure 2: Satellite Image of Bridge Location

Construction Start Date: 1935

Bridge Opening Date: May 6, 1937

Main Span: 352’

Side Span (2 of them): 173’

Width: 64’

Total Length: 698’

Budget: £365,000 (approximately $486,300 with 1937 conversion rate)

Budget (2017 equivalent): £ 22.4 million (approximately $29.8 million with 2017 conversion rate)

Designers: E.J. Buckton and H.J. Fereday of Rendel, Palmer & Tritton

General Contractor: Holloway Brothers

Steelwork Subcontractor: Furness Shipbuilding

Cables Subcontractor: Wright’s Ropes Ltd.

Fun fact (maybe not fun, but relevant): The construction finished 5 months ahead of schedule

[3]

 

Structural Elements and Materials

  • Towers – steel
    • Sit on rocker bearings

Figure 3: Chelsea Bridge Pylons

  • Cables – steel
    • Comprised of 37 strands of high tensile strength wires
    • Each wire has an 1 7/8 ” diameter
    • Hexagonal
    • Self-Anchored

Figure 4: Cables and Self-Anchors

  • Foundations – steel sheets
    • Cofferdams

Figure 5: Rocker Bearings Under Piers

 

  • Piers – concrete encased in granite

The Chelsea Bridge I am discussing in this blog is actually the second of the Chelsea Bridges. The first one was built to connect the densely populated north side of River Thames to the new (at the time) open green space, Battersea Park. The current bridge replaced that bridge to deal with the high traffic volumes that doubled in the time from the beginning of the 1900s to 1929.

 

Historical Significance

The Chelsea Bridge was the most significant suspension bridge built in Great Britain in between World War I and II. Being the first ever self-anchored suspension bridge (meaning it balances itself) in the country, its erection signified a new era of British bridge engineering. Despite the U.S. having already achieved this bridge form multiple times, Britain was recovering from the Great War and by this time had fallen a bit behind other engineering world leaders. Nonetheless, this type of bridge was perfect for its location because soil in London is more of a clay, and not very easy to stabilize a bridge with. As a self-anchoring suspension bridge, the cables anchored to the deck, rather than the clay which made the whole structure more stable and strong while also being innovative. Even cooler, the old Chelsea Bridge’s abutments were left and strengthened when the rest of the bridge was demolished, and the new bridge rests upon them bringing the past into the future (sorry for the cheesiness but, come on. That’s cool.) However, it is important to note that while the abutments were the major stabilizing and strengthening support for the Old Chelsea Bridge, they are entirely secondary to the new.

 

Despite being innovative in its location and in its time, only one other bridge in the U.K., built in 2011 is a self-anchored suspension bridge. Chelsea Bridge may have been significant in regards to the past, but historically it did not serve as a trendsetter when looking ahead all the way into 2018. Furthermore, the United States had already started mastering the form of self-anchoring by the time the Chelsea Bridge was built, so the Chelsea Bridge may be the best example of its kind in the country, it is not in the world. Perhaps this is due to the difficulty in constructing self-anchored suspension bridges because normally the suspenders would be able to serve as temporary supports for the deck during assembly. Since the cables have to connect to the deck, this method doesn’t work, meaning a lot more money must be spent on constructing temporary falsework until the system can work as a whole.

 

Cultural Significance

 

The Chelsea Bridge holds massive amounts of cultural significance. The original bridge was meant to connect Battersea to the rest of London, but also serve as a bridge for the lower class to connect to the upper classes. But, the bridge’s history predates even the original bridge: during construction in the mid-1850’s, many Roman artifacts were unearthed (literally). Researchers now believe that Julius Caesar crossed River Thames at this same point in 54BC. A Celtic Battersea Shield, one of their most famous and coveted pieces of military equipment was also found at the site. To be honest, I don’t know much about either of these things and I might be nerdy, but not so much for history. But, crossing over the River Thames at the exact same place Julius Caesar once did is almost unfathomable to me; I honestly can’t describe what that even represents, so I’m not even going to try and describe it. I will just move on and let that sit.

 

Much more recently (of course relatively) Queen Victoria opened the Victoria bridge in her name and walked across it for the first time in 1858. Of course, once the bridge started showing significant failures and people worried it might collapse, she ordered for the name to be changed to Chelsea Bridge so as not to associate any negative connotation to her name. I mean, she named the bridge after herself and then un-named it because she wanted to only be seen as perfect. I am legitimately rolling my eyes right now. Anyway, the original Bridge was taken down. Adding even more to controversy, the bridge was meant to better connect the lower and upper classes, but crossers had to pay tolls to pass over it. Finally, along with all the other bridges in London, the tolls were removed by an act of Parliament and people started using the bridge again.

 

Moving forward to the new bridge, the current Chelsea Bridge, many cultural events demonstrated not only its local significance but also its national and international importance. The bridge opening was in the news (you can watch the original news reel below attached below) and the Prime Minister of Canada was the one who cut the ribbon ceremonially to cross over the bridge with King George VI. The bridge is made of materials all sourced from within the U.K., or at least its territories, to stimulate the economy.

The bridge, though originally hated for its tolls and basic inadequacy (it couldn’t even be crossed once the sun went down because it didn’t have any street lamps), Chelsea Bridge is well received today. It still stands as a road and pedestrian bridge. Chelsea Bridge has also passed with no drama since the original bridge – Queen Elizabeth II hasn’t tried to name it after herself since its success – and has represented positive engineering progress, especially in the time during the two World Wars when London clung to everything that could be positively symbolic.

 

Structural Art

 

When considering the 3 E’s, efficiency, economy, and elegance, we have to consider both what the bridge serves as and stands for now and in the past, but also what the bridge was meant to represent when in the design and construction phases. However, since we are now into the 20th century, we are moving more towards company collaborations between architect and structural artist, and the exact intentions become more obscure in all the data provided for these bridges. Chelsea Bridge is decades old, but when comparing to bridges built a century or two ago, even when comparing to some of the other bridges crossing River Thames, Chelsea Bridge is young. Consequently, not as much time or analysis has been done in regards to its building or its overall impact. Whereas I can say Roebling was known for the Brooklyn Bridge and his cables as well as his overall understanding of structures as an art, or Telford cared immensely for all the E’s, not enough has been unearthed about the intentions or techniques of the specific designer, but rather to the technology as a whole. As a result, I will analyze and compare this bridge as structural art from my own lens, looking at what it is and represents now that it is built. This is valid analysis because thinking back to the Amann and the George Washington Bridge, the final product of the Iron Skeleton that the bridge is notorious for now was never the intention of Amann in the first place, yet the bridge still stands as one of the most beautiful and ideal examples of structural art in my opinion. So, with that in my mind, and being the perfectionist (or so I like to think) that I am, I have provided a pros and cons list to really simplify my analysis as much as possible, despite my realizing that it just takes me one step further on the nerd scale.

 

Efficiency

Pros:

Chelsea Bridge as a self-anchoring suspension bridge saved material and effort.

  • By not trying to tear down or restoring the old abutments that were used unsuccessfully for the first Chelsea Bridge, but rather letting the deck rest on them as an added precaution, material and effort was saved.
  • Workers did not have to demolish the old abutments and then start from scratch and use extra material to stabilize the bridge in the unstable clay soil.
  • A brand new bridge was made with less hassle and was innovative in its design so that it could essentially hold itself up.

Cons:

  • The concrete, was covered in granite for aesthetic effect, which was entirely unnecessary to the structure.
  • Construction required more materials and effort including an independent pedestrian bridge to allow crossing during construction.

Considering all factors in terms of efficiency, the overall finished product of Chelsea Bridge is efficient, the construction was not necessarily. I am deciding this because even though Granite is used inefficiently, the load path is still clear to the viewer and there are very few ornamental elements besides some lamp posts that are there for architectural rather than structural purpose. To learn more about the old bridge and the construction of the new, see the newsreel by News in a Nutshell on the topic:

Economy

Pros:

  • The bridge has had no failures since its erection, meaning relatively low maintenance costs.
  • Saved money by not completely erasing the old bridge and starting from scratch.
  • Only used materials from within the commonwealth of Great Britain.
  • Generated work within the commonwealth during the time in between the wars.

Cons:

  • An entire temporary bridge was built to accommodate the construction of the Chelsea Bridge.
  • Granite cost was unnecessary.
  • Since the bridge is toll-free, it doesn’t generate any revenue itself.

When weighing the pros and cons, I conclude that since economy is inherently tied into efficiency, and since the bridge has lasted so long without significant damages or issues, the Chelsea Bridge is economical. Furthermore, the bridge helped Britain’s economy in general by only using materials from within its jurisdiction, which sort of counterweighs the negative effect of additional material and lack of direct revenue generation.

Elegance

Pros:

  • Small pylons
  • Self-anchored allows for no large abutments
  • Clear load path
  • Relatively few ornamental and unnecessary flourishes

Cons:

  • Painted to represent national pride, but in my opinion takes away from the look of the steel (again, I love the GW Bridge so I’m a bit biased).
  • Granite obscures the steel piers above the water line
  • Lamp posts are ornamental in their composition and take away from the sight of the bridge as a system

Overall, the new Chelsea Bridge in my opinion achieves the label of structural art. I cannot say much about the intentions, but I believe the bridge is in fact elegant when compared especially with some of the other bridges across River Thames such as Tower Bridge which is mainly ornamental. This bridge stands for the purpose of standing while also fulfilling social and symbolic missions such as connection the classes in the neighborhoods surrounding the bridge and aiming for elegance, efficiency, and economy.

Structural Analysis

As previously stated, though probably not adeptly explained, Chelsea Bridge is a self-anchored suspension bridge constructed of steel. It has two towers, each containing two pylons, that transfer the load from the cables to the foundations. The cables connect the main span of the bridge to the deck of the side spans, right above the abutments from the last bridge. This means that the deck counteracts the tensile forces of the cables that point towards the center of the bridge rather than the abutments. The load path and free body diagrams are shown below in figures 5 and 6 respectively.

Figure 6: Load Path

Figure 7: Simplified Free Body Diagram of Entire Structure

 

Load calculations:

Material weights:

Steel: 490 lb/ft^3

Reinforced Concrete: 150 lb/ft^3

Assume Live Load factor is .8: LL = .8*DL

Structural Element Weights:

Pylons (x4):

Hp= 69.2’

Assume square cross section with side length of pylons, Sp = 1.5’

So, weight of pylons, Wp=4*((1.5ft)^2)*69.2ft*.490k/ft^3=305.2kips

Cables:

Based on common values for high tensile strength cable properties, WC =3.97 kips/698’ = .006k/ft

Deck:

L = 698’

wD = 68’

Assume cross sectional area AD=.75’*68’=61sqft.

WD = 150lb/ft^3*51ft^2/1000lb=7.65k/ft

DL = ((7.65+.006)k/ft * 698’ + 305.2k)/698’ = 8.09k/ft

Therefore, the total load W = DL + .8DL = 14.57k/ft

Using equilibrium to solve for the reactions we can take the following steps:

Figure 8: Solving for Vertical Reactions

Figure 9: Continue Calculations

Figure 10: Calculations Continued

Now try and picture this: if you make a new cut and look at the bridge from one of the piers to the anchor on its respective side, then you find that Ax = Cx =Bx = Dx = 17325.16kip. Otherwise, the pylons and anchors would not be in equilibrium and everyone would be watching the pylons tumble down all dramatically.

Figure 11: Continued Calculations

To find the maximum and minimum tensile forces in the main and side spans, we can use Pythagorean theorem and sum of moments:

Tmain,max= sqrt(“(8662.58kip)^2 + (2564.32 kips)^2) = 9034 kips”

Tside,max = sqrt(“(8662.58kip)^2 + (5142.26 kip)^2) “=10074 kips

Figure 12: Final Calculations

Unfortunately, there is not much information on the original drawings and plans of the Chelsea Bridge or how they were disseminated to the public to increase engagement. My guess would be that the public did not have access to the bridge plans, but that they were passed around among the team, which was quite large. Similarly, since there is not much information on the funder of the bridge, it’s unclear whether or not they would’ve been able to understand and use the drawings to inform their decisions. However, considering there were so many subcontractors and parties involved, the drawings would have to be pretty legible and easy to read, otherwise everyone would’ve have fought and there is no way (I mean seriously, imagine if they couldn’t communicate because of bad handwriting) Chelsea Bridge could have been finished 5 months ahead of schedule. I mean, when my family tries to do a group gift at Hanukkah time, we have trouble communicating and that’s just looking at emails and different sites, never mind that my older siblings complain that I text in “hip” lingo that they can’t understand. So, imagine that on a huge scale with absolutely no room for error; I’d say the drawings were both made and utilized pretty successfully.

Personal Response

At first glance, the Chelsea Bridge didn’t seem all that special to me. However, the history of it, the fact that while self-anchored bridges are uncommon in the area attracted me to it. Plus, I can’t help but laugh about the fact that the Queen changed the name of the bridge so she wouldn’t have any negative connotations. Like, I’m pretty sure that the British Monarchy has been battling many haters, especially Queen Victoria who was surrounded by scandal. And yet, an unsuccessful bridge can’t be associated with the royal family. Anyway, the colors catch my eye and the idea that the bridge balances itself really amazes me.

References

  1. http://www.job32v8.com/_2/?p=293
  2. https://londonist.com/london/features/secrets-of-chelsea-bridge
  3. https://historicengland.org.uk/listing/the-list/list-entry/1393009
  4. https://www.greatlondonlandmarks.com/place/chelsea-bridge/
  5. http://web.engr.uky.edu/~gebland/CE%20382/CE%20382%20Four%20Slides%20per%20Page/L2%20-%20%20Loads.pdf
  6. https://www.youtube.com/watch?v=uHEtLvHeaOE
  7. https://www.youtube.com/watch?v=mmAo9Ft8xHw
  8. https://www.britishpathe.com/video/chelsea-bridge-news-in-a-nutshell/query/Bridge
  9. https://www.bristol.ac.uk/civilengineering/bridges/Pages/NotableBridges/Chelsea.html
  10. http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=53

 

Peachtree Street North Bridge

At the beginning of my sophomore year at Georgia Tech (end of 2016), I started making the weekly drive on I-85 from campus to Buckhead and back on Fridays. For those of you who don’t know, rush hour in Atlanta on Friday afternoons can be a slow journey, leaving lots of time to look at the scenery. I started realizing towards April and then in the following months that the bridge by the Peachtree St. exit kept catching my eye. There hadn’t been any major construction, so I was positive the bridge wasn’t new, however, a bridge that had never crossed my mind started stealing my attention every Friday. At first, the only difference was the signage, where Peachtree Street was written in large letter signs over the bridge, which I started to use to mark how much further I had. I eventually grew used to the sign and it slept out of my mind again until the first semester of my junior year, many arches appeared above me as I drove past the bridge. In the blink of an eye, the unmemorable bridge became the lasting impression of my Friday afternoon drives.

Figure 1: Finished – Gateway to Atlanta [1]

Structure Information

Figure 2: Rendering of Bridge [1]

Name: Peachtree Street North Bridge

Location: Peachtree Street exit, I-85, Atlanta, Georgia, USA

Date finished: November 2017

Owner: Midtown Alliance

Implementation Partners: Midtown Alliance, Central Atlanta Progress, Silverman Construction Program Management, Kimley Horn, CW Matthews, and Henry Incorporated

Funded by: Midtown Improvement District, Atlanta Downtown Improvement District, Georgia Department of Transportation, the State Roadway and Tollway Authority, and the Woodruff Foundation [1]

The Peachtree Street North Bridge was a renovation of the already existing utilitarian bridge that functioned as a path over I-85 for pedestrians and vehicles. The new additions serve a more symbolic purpose as the gateway to metropolitan Atlanta.

 

Historical Significance

Historically, in building connector bridges, Atlanta has always leaned towards a fully utilitarian approach in design. When looking at the three E’s, economy, efficiency, and elegance, Atlanta designers and public agencies completely ignored elegance in the formation of the metropolitan area. This renovation signals not only the massive beautification of Atlanta for its travelers, but also a significant shift in mindset from function to global effect of structures in all capacities of serving society.

In terms of the structure itself, the design does not employ any new or innovative techniques. At its most bare essence, this structure is still a simple highway connector bridge, just with added dead weight from the arches and signs. Moving forward though, this bridge renovation may trigger a brand-new approach to design for Atlanta that integrates the symbolic purpose into the structural design. If this happens, this bridge project will become historically significant as the catalyst for this change despite not being all that technologically innovative in its own design when compared to other historically significant structures. I am very excited to see if designers and structural engineers start using the Peachtree Street North Bridge as inspiration. I mean, as a New Yorker, adding arches to a highway connector doesn’t seem all that special. But, as a civil engineer in training whose main prospect for life after college is in Atlanta, I can’t help but be my dorky, typical Georgia Tech self, and get giddy at the idea of participating in the beautification of Atlanta.

 

Cultural Significance

While this bridge didn’t have anybody die or have some massive political proclamation made upon it to add drama to this post, the Peachtree Street North Bridge does reflect a larger cultural shift in attitude towards the infrastructure of our city. While I am the typical New Yorker who brags about being from a “real city,” – and yes, you should imagine me doing the obnoxious air quotes when I say that because, unfortunately, I do – I still can’t help but look at Atlanta when I flyover it and feel slightly dissatisfied, and I think this is more universal than my bias. So, I am excited to report to you all that this bridge renovation is not a unique exception to the utilitarian approach that I mentioned earlier. In fact, this bridge is one of two for this specific Midtown Alliance project to “beautify the city” and make everyone aware when they are entering Atlanta.

Here are some quick notes to briefly sum up how the Peachtree Street North Bridge is part of a larger cultural mission:

  • Midtown Alliance (the driving force behind this project) has embarked upon a $6 million beautification project for Metro Atlanta – this bridge is the first step.
  • This massive endeavor included the cooperation of multiple public and private agencies, signaling its cultural importance to everyone, not just one sector of society.
  • While currently Peachtree Street North Bridge is one of only two bridges either in construction or completed, this project has garnered enough interest that Midtown Alliance is already in the process of procuring more.
  • Like many of the famous bridges that signified new eras in structural design, the designs for all the bridges encompassed within this the confines of this project were procured through a competition demonstrating this bridge’s cultural relevance.

 

Just to demonstrate how much value Midtown Alliance is placing upon the addition of these arches to the city landscape, here is a slightly dramatic or theatrical quote from the organization itself describing it as a “sweeping, 35-foot tall gateway arches and illuminated ‘Peachtree’ signage, providing bold visual impact from the interstate that creates a sense of arrival into Atlanta’s urban core” [2]. I don’t know if I would put it exactly that way, but nevertheless, this bridge is supposed to make an impact, and to those who pay attention, I believe it does.

After painting this bridge in such a beautiful light, I must remind all readers that this was in fact a construction project in the middle of I-85, and as Oscar Wilde put it so delicately, “No good deed goes unpunished.” Okay, I might be overstating the level of issues faced in construction, but, as I’m sure many Atlantans remember not-so-fondly, the construction of this project mid-way through during the I-85 bridge collapse. While it was not this specific bridge that collapsed, Atlantans do tend to value something over the sightseeing on their ways to work: getting to work in a timely manner. Traveling on I-85 after the bridge collapse was a nightmare. My usual 20-minute drive to Buckhead, if that, turned into at least a 45-minute one. And in my experience, Atlanta drivers are not known for their patience and sound judgements in traffic-heavy circumstances. Just to add onto all of the chaos, and right when it seemed like the project was getting back on track, Hurricane Irma hit. Construction, which was originally set to end in April 2017, did not finish until November of 2017. Yikes! So, while the appearance of these arches and signage did convey a message, it was somewhat muddled by the not-so-great circumstances. I just hope people moving forward will still put in the effort to read between the lines of chaos and through to the potential beauty that could put Atlanta on the map in a new way.

 

Structural Art

Okay, evaluating the bridge in its entirety as a piece of structural art is difficult. The original bridge built in the 1980s and pictured below in Figure 3, was purely utilitarian and is not a sight I want to set my eyes on all that much.

Figure 3: Peachtree Street Connector Before Renovations [1]

At best, it fits into the landscape of the highway and doesn’t really catch my eye; at worst, I might call it ugly. However, when looking at the three E’s, it does seem to cover at least one of them: efficiency and even possibly economy although not much information on the original bridge is available given that it was supposed to be monotonous and fit in with the surrounding concrete. At the bare minimum it succeeded in its purpose of getting people from point A to point B. However, it is most definitely not elegant, in fact its appearance was very consciously dismissed or ignored in its design.

On the other hand, the renovations, mainly the added 22 arches pictured in Figure 4, had very little if any structural purpose in terms of helping the bridge resist the natural forces acting upon it, mainly gravity as well as a live load of the foot and vehicular traffic.

Figure 4: First Arches Added on Southbound Side [1]

Really, it is using extra, unnecessary material to add dead weight to the already existing structure. So, the arches, which function as the key part of the renovations, have a large symbolic purpose which would achieve elegance, but most definitely on its own did not endeavor for good economy and efficiency. I would say that they serve an architectural purpose, but the arches don’t actually add to people’s use of it, which is key in architecture.

And at the core of Billington’s category of structural art is intention during the design. To truly be an exhibit of structural art, the designer had to design with the three E’s in mind. No matter whether we look at the renovations themselves, or the original bridge design, the designer in each case definitely did not endeavor to achieve in its entirety elegance, economy, and efficiency. The first designer didn’t care about elegance at all, and the second had no need for achieving economy and efficiency because he/she was limited to the constraint of adding on to the existing bridge. Therefore, I conclude that while this bridge may function as a catalyst for future structural art in Atlanta, it is not structural art itself.

 

Okay, now onto the fun part (hopefully?)… the structural analysis.

Structural Analysis

The design for the Peachtree Street North Bridge was completed as part of a bridge competition for all the bridges Midtown Alliance hoped to construct as part of its $6 million beautification project. The criteria given to the competitors were symbolic, not structural other than the existing bridge had to be able to sustain and integrate the arches into its existing system. The symbolic goals were as previously stated, to make a statement to everyone approaching Metro Atlanta that the city is worth recognizing and noting.

The construction was more complex. As an I-85 connector bridge, traffic is heavy. 42,000 cars travel on the bridge each day, not including pedestrians and bikers, and 300,000 cars pass under the bridge EACH. DAY. Interrupting traffic was not an option. So the 22 arches, consisting of 2,200 linear feet of steel tubing, were assembled off site. The reason for this was two-fold: the contractor needed limit disruption of regular traffic flow on and around the bridge, and the arch assembly required more space than available on the bridge had it been assembled on site. The arches were painted off site. Then, first on the southbound side, the 10 minor arches were erected and bolted in place. Only once southbound major arch was completed did construction proceed on with the northbound side. Notably uneconomical and inefficient, each segment, angle, joint and weld was assembled uniquely and individually. While somewhat small-scale when compared to the Eiffel Tower or even the Bank of America plaza (or more properly known to Georgia Tech students the Pencil Building), Midtown Alliance has stated that this is “one of the most sophisticated structural engineering projects [they] have ever undertaken” [1]. To show the progress made, here are monthly taken photographs of the construction of the Peachtree Street North Bridge as provided on the Midtown Alliance web page.

Figure 5: Offsite Arch Assembly [1]

Figure 6: Beginning of Onsite Minor Arch Assembly on Southbound Side [1]

Figure 7: Southbound Arch Assembly cont’d [1]

Figure 8: Southbound Arch Assembly cont’d [1]

Figure 9: End of Minor Arch Southbound Assembly [1]

Figure 10: Completed Minor Arch Construction [1]

Figure 11: Southbound Major Arch Construction [1]

Figure 12: Beginning of Northbound Major Arch Construction [1]

Figure 13: Completed Bridge Construction [1]

Structural Systems:

  • Footings
  • Columns supports
  • Concrete beams

Figure 13: Footings, Columns, and Concrete Beam [3]

  • Rectangular steel girders
  • Deck
  • Steel tube arches

Figure 14: Steel Girders, Deck, and Steel Tube arches [1]

Load Path:

Steel Tube Arches (uniform weight load) –> Deck (point loads + self weight + LL) –> Steel Girders (Line Load) –> Concrete Beams (Point Loads) –> Columns (compressive point loads) –> Footings (line load) –> Ground (surface load)

 

Mechanics of Load Distribution:

**Many assumptions were made on the dimensions of all elements except for the steel tube arches as those are the only new structures so the information is readily available. Simplifications were also made to the shapes to make calculations more straight forward. Also, based on the simplifications to basic beams and such, I found that the bridge only experienced axial forces, and bending is not an issue when modeling the dynamic load of the cars and pedestrians as a uniform load and ignoring weather conditions. The bridge is also not high enough to warrant wind force analysis.

Figure 15: Steel Tubed Arch Calculations and Models

Figure 16: Deck, Girder, and Beam Calculations

Figure 17: Footing Calculations

CAD drawings were absolutely instrumental in the successful design and implementation of the steel tubed arches. According to the Midtown Alliance, 1 million data points were used to model the BIM and CAD drawings effectively for the arches. Due to the massive public participation in this project, the drawings needed to be as accurate and easy to read as possible to get as many people on board to fund and politically support the project.

Personal Response

Overall, my drive was definitely improved by the addition of these arches to my trip. While the bridge might not hold up to my New York standards of bridges (the George Washington Bridge is the ultimate structure), my dorky-ness couldn’t help but shine through

References

  1. https://www.midtownatl.com/about/programs-and-projects/capital-improvements/peachtree-street-north-bridge
  2. https://atlanta.curbed.com/2016/5/6/11605204/atlanta-bridge-architecture-fancy
  3. https://www.ajc.com/news/local-govt–politics/peachtree-bridge-sign-put-premier-street-lights/1RxpKqHukolDoQTUJvWoeO/
  4. https://atlanta.curbed.com/2017/3/23/15031664/peachtree-street-bridge-midtown-arches
  5. https://www.midtownatl.com/about/programs-and-projects/capital-improvements/peachtree-street-north-bridge
  6. https://en.wikipedia.org/wiki/Sydney_Harbour_Bridgehttps://atlanta.curbed.com/2013/7/10/10221698/peachtree-street-bridges-to-be-beautified
  7. https://atlanta.curbed.com/2015/11/24/9897018/midtown-peachtree-bridge-upgrade-aims-make-statement