Blog 1
Peachtree Street North Bridge

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

Comments

  1. I don’t think this is structural art only because they added those arches for ornament and for any structural support. And according to Bellington structural should have unnecessary oranement on it.