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The Granada Bridge

The Granada Bridge

Structure Information

The Rockefeller Memorial Bridge, known by locals as The Granada Bridge, spans the Halifax River or Intracoastal Waterway. The bridge links the peninsula and mainland parts of Ormond Beach, Florida, USA. The current standing bridge was built in 1983, however, the current bridge is the fourth bridge to exist in this approximate location since 1887. The Granada Bridge is shown in Figure 1 below.

Figure 1: The Granada Bridge [1]

The purpose of the bridge is to carry highway, pedestrian, and cycling traffic over the Intercoastal Waterway. The bridge was funded and is currently maintained by the Florida Department of Transportation. The designer of the bridge is unknown due to the lack historical construction plans and records accessible to the public through the Florida Department of Transportation.

Historical Significance

The structural engineering design of the Granada Bridge is not an innovative design in a historical context. The Granada Bridge is a stringer or multi-beam bridge [2]. The beams making up each span are supported at each end by a box girder which sits atop the vertically supportive piers. There are large abutments at each end of the bridge. The structural principle of this bridge design has been used for millennia, and the most common type of highway bridges  in Florida have this same structural design. Additionally, the bridge was built using foundation and general construction technology that had been used before. The foundations for the piers were built using cofferdams installed where the piers exist in the river. The foundations, columns, and girders are cast-in-place concrete, the beams are pre-stressed concrete and the deck is cast-place concrete. The best existing example of this type of structural design is the Lake Pontchartrain Causeway over Lake Pontchartrain in Southern Louisiana, USA. The Lake Pontchartrain Causeway spans a total of 23.83 miles. It consists of four traffic lanes and has the same structural design principles as the Granada Bridge, but is 65 times longer. Figure 2 below shows an abbreviated view of the causeway.

Figure 2: Lake Pontchartrain Causeway [3]

Since the Granada Bridge is such a standard highway bridge, the actual structural design should not be considered a model for any future bridges. However, its function as a connection between the peninsula and mainland parts of Ormond Beach since 1887 has inspired the construction of multiple bridges in the greater Daytona and Ormond Beach area.

Cultural Significance

The history surrounding the Granada Bridge is fascinating. The progress and construction of the series of bridges leading to the existing bridge is reflective of the progress of industrialization and influx of population to the area. The first bridge built in the location where the existing bridge stands was the result of a competition between builders in Daytona Beach and Ormond Beach to see who could cross the Halifax River the fastest. It was a wooden bridge with a drawbridge device finished in 1887. In 1890, Henry Flagler, owner of Florida East Coast Railway bought out all shares of the Ormond Hotel, and in 1905 built a second bridge near the first that could support rail and carry passengers directly to the hotel. Figure 3 below shows the Hotel with the wooden bridge in the bottom right hand corner. Flagler later redesigned the bridge for automobiles to be able to drive to the hotel, and the first wooden bridge was demolished shortly after [4].

Figure 3: Vintage Ormond Hotel postcard with bridge in lower right-hand corner [5].

Henry Flagler was not the only notable man to leave his mark on Ormond Beach. The man known as the richest in modern history, John D. Rockefeller made his summer and retirement home at the southeast corner of the bridge. After Flagler’s redesigned railroad bridge became too old to be maintained, a new two-lane wooden bascule bridge was built in its place and named the John D. Rockefeller Memorial Bridge opening in 1952 [4]. Eventually the bridge had to be rebuilt to accommodate the widening of the connecting roads. This new bridge was constructed in 1983 and stands as the Granada Bridge today.

The first bridge built made it possible for the world’s first automobile race to take place on Ormond Beach in 1903. People loved the bridge because it allowed them to indulge in the luxury of the Ormond Hotel and the excitment of industrialization and the recreation that it made possible. Since then, the bridge has remained a north star for residents and tourists in Ormond Beach. Although there are only minor historical events surrounding the Granada Bridge, it means a lot to the residents of Ormond Beach, myself included. I was born and raised in Ormond Beach, and I know that I am home every time I reach the apex of the bridge looking towards the Atlantic Ocean. The combination of the Intracoastal Waterway and the succeeding vast ocean in front of it reminds me of watching fireworks sitting on the sloped bridge abutments on the 4th of July, or running up the steepest side during high school cross country practice.

The building of a bridge that has done so much for Ormond Beach did not come without hardships. During the construction of the existing bridge in the 1980’s, three workers fell from a scaffolded platform on to a barge below. One of the workers lost his life [6]. In addition, there have been multiple fatalities associated with normal traffic usage of the bridge. Traffic accidents are common in highly trafficked areas, and with an ever-increasing daily vehicle count using the bridge to cross the Halifax River, this is unfortunately expected. In addition to vehicular traffic, the bridge has pedestrian pathways on each side of the deck which connect four different recreational parks on each corner of the bridge.

Structural Art

By David Billington’s definition, The Granada Bridge does demonstrate some degree of structural art. The load path from the deck to the foundations is discernable. This is representative of some degree of efficiency. In addition, the bridge is owned and maintained by the Florida Department of Transportation, which indicates that the structural design was performed under certain economic constraints. Designing for a publicly funded piece of infrastructure means that the design that is sufficient at the lowest cost will be built. According to Billington, structural art only flourishes under the constraint of economy. This pillar of structural art is present in this bridge. I may be biased because the bridge is so central to my adolescence, but I think that the Granada Bridge is an elegant one. I think its elegance comes from the transition you experience when you cross it. The columns and deck are thin and intentional when compared to the vast, chaotic ocean that comes in to view as you cross over the high point of the bridge. Although you cannot do the actual experience justice, Figure 4 below shows the view of the ocean as you descend the bridge.

Figure 4: View of the Atlantic Ocean descending the Granada Bridge [6]

Even though the bridge does demonstrate some degree of structural art, I think that it generally cannot be considered structural art by the criteria provided by David Billington. I think this is mainly because of the nature of materials used and the form of the bridge deck. I think that structures made from prestressed concrete are inherently not structural art. Because the concrete is in compression before it is under service load, the true load path in final form is never actually realized. In addition, the curvature of the deck is asymmetrical relative to each land mass, meaning the apex of the bridge is closer to one end. If the bridge was symmetrical I think it would be more elegant, thus, closer to the ideal for structural art. Figure 5 below shows the asymmetry in form of the granada bridge.

Figure 5: Asymmetry of apex of Granada Bridge [6]

Structural Analysis

The Granada Bridge is a stringer/multi-beam or girder bridge. The superstructure consists of 19 spans of average length of 101 ft [2]. Each span consists of thirteen prestressed concrete I-beams. Atop of these I-beams is a cast-in-place concrete deck reinforced by steel. The ends of each beam sits atop a bearing pad which are flush with the corrugated top of a modified cast-in-place reinforced concrete box girder. The girder is the start of the substructure. The girders are supported by two cast-in-place reinforced concrete columns which are round in cross-section. The columns located closer to the center of the bridge are taller than those at the bridge ends, so they are laterally braced together at their midspan to prevent buckling. The columns are supported by cast-in-place reinforced concrete foundations located in the water only. The foundations are rectangular in cross-section. At each end of the bridge there are massive abutments. The abutments are covered in grass, but it is assumed that they are constructed using mass concrete or reinforced concrete. The construction method used to build this bridge is typical of girder bridges. The foundation for one bridge bent is built using a cofferdam. Cofferdams are watertight structures constructed using metal sheet piling or similar material which are pumped dry to permit construction below the waterline [2]. The reinforcing cage for each column is then set on the foundation and concrete is poured. The reinforcing cage for the modified box girder is then set on top of the two columns and concrete is poured. This process is a completed bent. A bearing pad is set on the girder at the location of each beam. The beams are prestressed and constructed off-site then transported to the site. The beams are set and tied to each other and to each bent that they are set on. One span consists of thirteen beams connected at each end by a bent. The bridge was built one span at a time. It is assumed that the first span would start at one end abutment and connect to the first bent. A reinforced concrete deck is then poured in sections, usually a span at a time.

The structural systems employed in this structure begin at the bridge deck. The reinforced concrete deck is statically loaded by primarily vehicular traffic. Additional load comes from the self weight of the deck. The deck transmits a compressive uniform line load on to each beam based on the tributary area of each beam. Each beam is modeled as a simply supported beam because there is no moment transmitted at supports. The beam transfers the load from the deck as well as its own self weight as point loads to each bearing pad that it sits atop of, and the bearing pad transmits that point load to the girder. The girder transmits that load  as well the load of its own self weight as two point loads, one going to each supporting column. The columns then transfers that load and the load of its own self weight to the foundations which distribute the total load to the ground. It should be noted that the abutments carry the axial force transmitted by the beams at each end of the bridge. The structural system of one bent is shown in Figure 6 below.

Figure 6: Load path on structural system

The structural system can be broken down in to structural elements with load path applied to analyze the structure. The Granada Bridge is 1923 feet in length with 19 main spans, indicating an average span length of 101 ft. The edge-to-out width is 94.5 ft. Therefore the tributary area with beams shown can be modeled as shown in Figure 7 below.

Figure 7: Model of tributary area for one span

The tributary area for Beam 1 and Beam 13 is the same are found using the calculations below

The tributary area from the remainder of the beams is the same and can be found using the calculations below.

The design load used for this bridge was AASHTO Specification HS 20 [2] which assumes an axle load of 32,000 lbs and a tire load of 16,000 lbs with a tire contact area of 200 square inches. Combined with the load of the self weight of the concrete having density assumed to be 150 pcf, the total surface load transmitted by the deck to the beams is as shown below.

Traffic Load: (32,000+16,000) lb/(200 in^2/144 in^2) = 34560 lb/ft^2

Assuming deck thickness of 1 ft,

Self-weight load: (150 pcf)*(1 ft) = 150 lb/ft^2

Total Area Load = (34560+150) lb/ft^2 = 34710 lb/ft^2

To find distributed load on Beams 1 and 13,

w=(34710 lb/ft^2)*(1/2)*(93.4 ft/13) = 124689 lb/ft

To find distributed load on remainder of Beams (2-12),

w=(34710 lb/ft^2)*(93.4 ft/13)=249378 lb/ft

Using the distributed load on Beams 1 and 13, the beam can be modeled as being simply supported as shown in Figure 9 below.

Figure 9: Beams 1 and 13 modeled as a simply supported beam

Using structural analysis and symmetry, the reactions, Ra = Rb = 629694.5 lb

Using the distributed load on Beams 2-12, the beam can be modeled as being simply supported as shown in Figure 10 below.

Figure 10: Beams 1-12 modeled as simply supported beam

Using structural analysis and symmetry, the reactions, Rc = Rd = 12593589 lb

These reastions exist as point loads on the girder as shown in Figure 11 below.

Figure 11: Girder modeled as beam

The self weight of the girder can be modeled as density of concrete time area of cross section assuming a width of 2 ft,

Self Weight = 150 pcf*(93.5 ft * 2 ft)=28050 lb/ft

Using Mastan, the reactions are found to be, C1 = 1.046E8 lb, C2 = -3.026E7 lb, and C3 = 1.046E8 lb. Assuming that the columns are not fixed to the girder, the reaction at C2 can be considered negligible, and the load will not be used in further analysis.

Using the reactions, the axial load in the columns are equal to C1, and C3. Assuming column diameter as 5 ft, the stress in the columns can therefore be defined as

stress1, stress 3 = F/A = 1.046E8 lb/2827.4 in^2=36994.7 psi

To find the load on the foundation and therefore the load transmitted to the ground, the self weight of the column can be calculated assuming an average column height of 65 ft (clearance requirement)

Self-weight = 150 pcf (pi/4)*(5 ft)^2*(65 ft) = 191440.8 lb

Therefore total load on outer foundations = (1.046E8+191440.8) lb = 1.048E8 lb.

Assuming a square cross-section of dimension 8 ft x 8 ft, the stress in the outer foundations is found using the following calculations,

stress = F/A = 1.048E8 lb/9216 in^2 = 11370.6 psi

Therefore, the strength of the soil has to be greater than or equal to 11370.6 psi.

Deformation in the columns can be found using the formula:

deflection = (PL)/(EA) = 5.7 in

5.7 inches is 0.73% of the total column height, making this deflection acceptable.

The tallest columns exist at the center of the bridge and maximum height is equal to 65 ft, therefore these columns are laterally braced for buckling. The critical buckling load can be found using the following equation,

Pcr = (pi^2*E*I)/L^2 = 7.54E9 psi

Therefore it seems that lateral bracing is not needed for stability in this analysis.

Since this bridge was constructed in the 1980’s, it is assumed that construction and design plans which follow the specifications of the Florida Department of Transportation were used to communicate the design principles used.

Personal Reaction

For years I have run, walked and driven over this bridge. To me it has always symbolized my homecoming. Looking at it from a structural engineering perspective gave me insights that I never would have gained about the history of the Granada bridge and its true significance to the growth and development of my hometown.

References

[1] https://rosebone.deviantart.com/art/Granada-Bridge-210251435

[2] http://www.city-data.com/bridges/bridges-Ormond-Beach-Florida.html#790132

[3] https://carynschulenberg.com/2015/08/lake-pontchartrain-causeway/

[4] https://ormondhistory.org/a-brief-history-of-ormond-beach-2/

[5] https://www.florida-backroads-travel.com/ormond-beach-florida.html

[6] https://www.redbubble.com/people/dbenoit/works/6631477-ormond-beach-bridge

 

Comments

  1. ezaruvinsky3 says

    You made the comment “I think that structures made from prestressed concrete are inherently not structural art.” After learning about Maillart’s work with reinforced concrete and how no one thought that you could use concrete for elegant designs, would you change your opinion? Do you think that prestressed could be structural art if designed right?

    • skyrazis3 says

      After todays lecture I do think that prestressed concrete should be considered structural art. I think the idea of using less material while maintaining ability to carry load is a criterion of efficinecy that would fit in to Billington’s ideas about what is and what is not structural art. I also think that prestressed concrete structure can be very elegant, and this has to do in part with the ability of prestressed concrete to be very thin yet sufficient in use.

  2. nzukerman3 says

    Do you think that the Granada bridge as it stands today is the best of the three solutions that have been in its place? I wonder if the race to build first was an inspiring or limiting constraint?

    • skyrazis3 says

      I think that in the context of modern times it is the best of the options built in the past just because of the modern technology that has made it strong and serviceable. I think the race to build the bridge in the 1800s could be both an inspiring and limiting constraint. On one hand a designer might be pushed to develop new forms or construction techniques to be able to span the river quickly. On the other, the time constraint could have led designers to feel trapped in one specific category of design or form that they thought they knew would work to get the job done.