Blog 1
Park Drive Bridge

Park Drive Bridge

I found this bridge while walking in Piedmont Park this weekend to break in my hiking boots and immediately thought of how great it would be to write a blog post about. I took some pictures (shown below) and explained to my friend how we could see the load paths (which went right over his head).

Figure 1: Park Drive Bridge


Structure Information

Figure 2: Location of Park Drive Bridge [2]

Upon further research (pulling up Google Maps), I figured out the bridge was on Park Drive. The bridge is called the Park Drive Bridge, previously known as the Piedmont Park Boulevard Bridge. The structure was built in 1916 and designed by O.F. Kauffman, who was a city engineer working at the Department of Bridges and Estimates. The purpose of building this bridge was to connect neighborhoods to the park without having to walk over the railroad tracks that ran along the park, which is now the Atlanta Beltline (see map to the left). The bridge was funded by four sources: City of Atlanta, Fulton County, Southern Railway, and Northern Boulevard Park Corporation. [1]

 


Historical Significance

Since the bridge was built long after the invention of reinforced concrete and the arch bridge, there was nothing significant about the Park Drive Bridge’s design or how it was built. In fact, since the bridge is over land rather than water, it was easier to build than many of the bridges we have learned about in class so far.


Cultural Significance

Figure 3: Mural under the Park Drive Bridge [2]

This bridge previously served as a connection from the developing Druid Hills neighborhoods into the park over the railroad [1]. After the Piedmont Park parking lot was build, the bridge was used for parking lot access until the parking area for the park was moved. Now, the bridge is not open to the public for vehicles, but is still open for pedestrians and bicyclists, although there is no railroad to worry about crossing over anymore. Additionally, the Park Drive Bridge has been incorporated into the Art of the Atlanta Beltline project and features a mural.


Figure 4: Close-Up View of the Middle of Park Drive Bridge

Structural Art

 

From far away, this bridge can very well show structural art. The load paths seem clear, there doesn’t seem to be any extra beams or columns, and you can see through the bridge, as Billington commonly uses as a requirement.

Once closer to the bridge, however, you notice the tiles for decoration on the entire bridge and the heavy-looking deck with thick, ornamental bricks as a railing. You can also see small beams connecting the spandrels, which do not seem to be load-bearing given the large girders underneath the deck. These beams are most likely for decorative purposes or because the engineer was worried that the girders could not hold the entire load. This extra material and the cost to add these decorations go against the values of economy and efficiency needed for a structure to be structural art.

 

 


Structural Analysis

This bridge was built as an arch bridge in the middle and simply supported sections on the sides out of reinforced concrete. The construction was contracted to Case & Cothran and cost $28,904.75. The brick section at the top was laid and plastered after the completion of the bridge and the 7 foot deep slab. [1]

In the middle section, the longer span, an arch is in compression against two abutments. The spandrels above the arch are also in compression. There are beams connecting both arches across the bridge as well as beams running between the spandrels in line with the arches. The girders lie perpendicular to the arches and connect to the spandrels that are above the arch. These components can be seen in Figure 4.

Figure 5: Load Path on Park Drive Bridge

The load goes from the slab to the girders. These loads are then transferred to the spandrels down to the arches. The arches transfer the load to the large abutments, which finally take the load to the ground.

The load comes from both the dead load of the railings and the very small live load from the pedestrians walking across the bridge; however, there used to be a larger live load when vehicles were allowed on the bridge.

The braces connecting the arches are there for stability to make sure they do not start leaning. The beams between the spandrels are either for decoration or to provide extra support for the variable side of the girders at the edges of the deck of the bridge.

Figure 6: Full Arch Diagram

 

If the span of the bridge between the abutments is 500 ft, and the load

Figure 7: Free Body Diagram of the Left Side of the Arch

on the deck is 1000 lb/ft, the reaction forces on the abutments can be calculated, neglecting the self-weight of the structure, seen in figure 6.  The reactions in the y-directions can be found to be 250 kips.

 

 

Figure 8: Free Body Diagram of Left-Most Point of Arch

To find the reaction in the x-direction, you must split the arch in half, and find the sum of the moments about L/2 (see figure 7).

The reactions in the x-direction are towards the arch and equal 625 kips if the maximum height of the arch is assumed to be 50 ft.

To find the maximum stress of the arch, the maximum force must first be found. This is done by finding the internal compressive force at the end of the arch. A free body diagram of the edge of the arch can be seen in Figure 8. The maximum force is equal to 673.15 kips. Assuming an area of 8 square feet, the maximum stress in the arch is 84.14 kips/square foot.


Personal Response

I have walked by this bridge many times and never thought it was anything special. I think that the ornamental tiles and brick wall on top make it look like it belongs in another time period. If the bridge were cleaned up and these parts were removed, I think I would like it a lot more. Overall, I think it was very cool to see a bridge that looks similar to the sort of designs we’ve been learning about in class. Seeing this in-person, and realizing on-the-spot that I could follow the load paths, was very cool.

Comments

  1. Good job with the analysis. Tracing out the load path, finding all of the reaction forces, finding the max force, and calculating the max stress gives a really thorough overview of what’s going on in the structure. You made good simplifying assumptions and found some meaningful information.