Blog 1
Bankhead Highway Bridge

Bankhead Highway Bridge

I saw this dilapidated bridge from Tech Parkway, and walked by later along Northside Drive to get a closer look. Although somewhat nondescript, it caught my attention because I was surprised to see an abandoned structure of its size in the middle of the city. The simplicity and eccentricity of the bridge led me to delve further into its history and decide that it would make for an excellent blog post.

Figure 1 – Bridge Against Atlanta Skyline

Structure Information

The Bankhead Highway Bridge was built in 1912 to carry Bankhead Highway (roughly modern day US 29) over the Norfolk Southern and CSX railroads. It was most likely funded by GDOT and designed by a contractor they hired [1]. At the time of its construction, both the railroad and the highway saw very heavy usage, but eventually highway reroutes caused the bridge to become extraneous. Along with high maintenance costs this led to its abandonment and ultimately the destruction of one of the approach ramps. [2]

Figure 2 – Bridge Location

Historical Significance

This bridge is not at all innovative in either construction or design, but is an excellent example of a trussed steel bridge from the time period. It can be viewed as the typical quick and easy solution for land based spans that needed to carry only the load of cars during the early 1900s [1].

Cultural Significance

During active usage, the bridge provided a major causeway for access to the center of Atlanta which otherwise would have been obstructed by the railway. This railway was not for passengers, but instead long commodity filled trains ran along it. In the early 1900s these lines were an important artery for goods transportation to and from Atlanta [3]. Today the bridge is banned from public access (both vehicle and foot traffic) due to extreme structural integrity problems, and the deck, superstructure, and substructure have all been rated “Imminent Failure” in inspections since 1991 [2]. There is also a missing approach ramp and the bridge terminates abruptly at that side with no guard railing or warnings to keep people from falling. However this does not stop graffiti artists and homeless people from climbing onto it, and these are the only people who currently utilize the structure for anything other than the background of grungy Instagram pics.

Structural Art

The three ideals of Structural Art are efficiency, economy, and elegance, and I would argue that the Bankhead Highway Bridge accomplishes the first two, and closely approaches the third. The steel truss structure itself is composed of smaller trusses, creating a light but very strong superstructure and using a minimum of materials. The trusses support a span made of concrete that rests on reinforced concrete pillars, both span and pillars using a reasonable amount of materials. The bridge is therefore quite efficient, and uses materials that were cheap and commonly produced during the time period. It was also built using fairly quick and easy construction processes, as the land based nature of the span eliminates many of the challenges seen in bridges over bodies of water. Both of these factors lead to the conclusion that the bridge also fills out the economic ideal. When it comes to elegance however the bridge is weakest, and I’m sure Billington would hate it, but I personally appreciate its appearance. The bridge is skewed, meaning that the side trusses are the same length, but displaced a single truss length so that from above the bridge is parallelogram shaped. This is an interesting aspect and drew my eye initially as it can create a subtle optical illusion. I also believe that the truss structure connects solidly with the concrete base and together look simple, but strong. Along with the clearly visible load paths from truss to concrete span to pillars (and laterally through the top truss) I believe that the Bankhead Highway Bridge is Structural Art, although Billington may have disagreed based upon its lack of innovation and heavy looking form.

Figure 3 – View of Bridge from Below Missing Approach Ramp

Structural Analysis

The bridge approaches were simply supported cast in place concrete slabs on concrete pillars, and the truss structure is made of steel and supports the concrete middle span of 99.7 feet. This concrete span is 47.9 feet wide and approximately 2 feet thick and the trusses have a vertical clearance of 13.1 feet [2]. The truss structure is a Warren Truss (equilateral triangles) with added verticals and the trusses themselves are also smaller Warren Trusses but without verticals. The top chords and non vertical horizontal trusses have a hollow rectangular cross section with two sides consisting of trusses and all the other members are just single trussed beams. The trusses are riveted together and it is a through truss, so motor vehicles would pass between the upper and lower chords [1]. The truss is skewed as it crosses the railroad at diagonal angle, and from an elevation view looks like a long parallelogram. The top cord is also trussed in the same manner to provide lateral stability, although due to the small span, stiff base, and minimal footprint from an elevation view, the lateral stiffening is somewhat redundant. There is also a concrete sidewalk cantilevered off both sides of the bridge deck. The building techniques of the time were pretty similar to current bridge building techniques (other than the new automatic bridge building machines), and involved wooden form and scaffolding to pour the concrete and the steel was Bessemer mass produced [1].

As the bridge is not currently in use, the only important load is dead load from the self weight of the concrete span. This load is carried by the truss structure which supports the weight of the large concrete slab through members in both compression and members in tension. The weight is ultimately carried by two thick concrete pillars on either side of the span. In the single remaining approach span, there is no truss structure, so its entirely supported by large columns. Below is a simplified truss structure that represents the sides of the bridge to show in a basic way which members take compressive or tensile forces. The green members are experiencing compressive force and the red members are taking tensile force, while the white member has forces that balance out to zero. When the self weight (simplified in the picture [4]) is applied across the bottom chord,  the max stress in any member is in the outside diagonals and is approximately 0.75x the total force. The total force on the bridge due to the self weight of the concrete using a density of 145 lbs/ft^3 and previously stated measurements is 1,384,932.7 lbs which means that the max force in a member is 1,038,699.5 lbs (the total multiplied by 0.75). The compressive strength of old steel is about 36,000 psi (from an internet database), and the end diagonals on the bridge are the only non-trussed members, I had to assume a cross sectional area of about 50 in^2 from the photos. Using these values and the formula (F/A) the normal compressive stress on the outside diagonals is 27,698.7 lbs/in^2 which is less than the compressive strength of steel but only barely. If any live loads from vehicles were added to the bridge failure would rapidly occur, making it abundantly clear that closing the bridge was the correct choice.

Figure 4 – Simplified Warren Truss with Verticals

Another area of possible failure is in the concrete support columns, which could buckle or crumble from compressive bearing stress as shown in the photo below as green arrows. The four columns each have a symmetrical tributary area of 1/4 of the bridge and so must each support 346,233.2 lbs (total weight/4), and are 20 ft tall (previously stated). The columns are slightly tapered squares, and from pictures I will assume that the area at the top of the beam is 3×3 ft and is about 3.5×3.5 ft at the middle. Using these values (and F/A) the bearing stress on each column is 267.2 lbs/in^2, and the compressive strength of concrete is higher than that by at least a factor of ten, so there is no danger of failure from crumbling at the supports. The critical buckling load would be at the red line halfway down the column in the picture below. Using a modulus of elasticity of 2.9 x 10^6 psi (from internet database) and an estimated moment of inertia of 139,968 in^4 (I=b(h^3)/12 using 3×3 ft approximation) comes out to be 6,9551,102.2 lbs (Pcr = (pi^2)EI/(L^2)), a simply massive number that the bridge would never reach. The Bankhead Highway Bridge therefore is in no danger of failure due to its columns, but instead its trussed superstructure and concrete span.

Figure 5 – Compressive Bearing Stress and Hypothetical Buckling Location

Personal Response

It’s somewhat difficult to see from the pictures, but when I was actually walking around the bridge I was fascinated by the skewed truss. It hadn’t occurred to me that such a design was an option, maybe I had seen some in the past but never really took notice, but it was an exciting departure from the normal truss bridge I’ve always seen. I’ve always had some difficulties with trusses (ever since statics) and it was very interesting to try and trace the load paths in person, and then check my answers through equations during the analysis, and my understanding of the joint and section methods has definitely improved. I would love to go up on the bridge, but it’s kind of hard to get to, probably dangerous, and had some homeless people camped out on it, so I may not actually go for it.

References

  1. http://historicbridges.org/bridges/browser/?bridgebrowser=georgia/bankhead/
  2. https://bridgereports.com/1096608
  3. https://bridgehunter.com/ga/fulton/12151210/
  4. http://ivanmarkov.com/truss-simulator.html

Comments

  1. rclayton30 says

    If this bridge was complete, do you think it would enhance/ benefit the community?

    • zcollinson3 says

      Hmmmmm, I doubt it would do anything to reduce congestion, but maybe if it were added as a part of the beltline? That could be cool if someone restored it but changed it into a pedestrian area… Maybe a slide at the broken end could be fun for kids?

  2. kkpetsu3 says

    Great analysis! I do agree with you on the fact the 3rd ideal (elegance)is definitely not met to qualify this bridge as a structural art, especially not in the actual conditions of the bridge. As you’ve said, I wonder why the bridge hasn’t been submitted to demolition since it’s no longer in use. The bridge, in my opinion, represent a serious safety hazard for the community since it’s apparently lacking maintenance.

    • zcollinson3 says

      Thanks! Yeah it seems like the bridge met the bear minimum requirements in terms of function, with almost no attention paid to how it looked. This means it was both economic and efficient, but certainly not elegant. I wasn’t able to find anything on the lack of demolition plans, but my guess would be expense, and probably no one in the community has cared enough to make a stink about it to the local government. I think it looks kind of cool looming over the abandoned area though…