Royal Gorge Bridge: Structural Elements of the Highest Bridge in the US

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The Royal Gorge Bridge is situated in the Rocky Mountains close to Canon City in Colorado, US. It was constructed across the Arkansas River and Royal Gorge in 1929 and was the highest bridge in the world at that time.

The bridge was constructed at the core of an amusement park and was planned as much as a tourist destination as a means of transportation. The stability and stiffness of the bridge came from an irregular wind cable system underneath the bridge deck, which reduced the swaying under vibratory motion.

Key Features:

1. The total height of the bridge from the deck to the river surface is 321 m.
2. The railroad track of the Royal Gorge Bridge is located at the bottom of the gorge.
3. The length of the bridge is 366 m, with central and side spans of 268 m and 40 m, respectively.
4. The width of the deck is 5.5 m and the heights of the south and north towers are 46 m and 34 m, respectively.
5. The structural components of the bridge consist of steel towers and steel cables carrying a thin steel structure with a timber deck.
6. The bridge was designed to carry both vehicular and pedestrian loading.

1. Aesthetics of the Royal Gorge Bridge

The Royal Gorge Bridge doesn’t meet a few of Leonhardt’s standards and the open braced steel frame utilized for the towers is not a suitable choice from an aesthetic point of view. Though not remarkable, the design of the bridge is impressive and still suits the surrounding area of the amusement park. 

2. Structural Elements 

Typically, a cable-supported bridge is not adopted to construct a bridge across a valley as very tall towers would be required to acquire adequate foundation strength. The towers of the Royal Gorge Bridge were constructed at the edge of the gorge due to the availability of strong granite rocks which make up the gorge walls.

If the gorge walls were less solid, there would have been a danger of occurrence of slip because of the vertical overburden pressure which is applied on the edge of the gorge.

The area above the gorge ensures that the bridge will be exposed to high-intensity winds. Therefore, the suspension bridge was not chosen during the preliminary design because of its complexity. For this reason, the bridge was designed for a wind cable framework to provide stability to the deck. Particularly before the renovation and repositioning of the wind cable framework, the stability and swaying of the deck were considerable during windy conditions.

The utilization of wind cables permits the design to be liberated from the more common truss supports for lateral bracing. In this manner, the aesthetics and stability of the bridge deck were improved.

2.1 Foundations

Usually, for a suspension bridge, the foundation for the towers will be very large and would require greater depth to provide an adequately strong base. But the strong granite rocks were available at the surface and thus, the shallow foundation was sufficient for the construction of the Royal Gorge Bridge towers. Concrete abutments set into the encompassing rocks were used as foundations for the towers. Abutments were placed to match the existing rock layout. However, this design resulted in the creation of unsymmetrical towers for each pier.

2.2 Piers

The towers and piers of the Royal Gorge Bridge were constructed using a lightweight steel structure, mainly consisting of bolted angle sections. Cross bracings were provided to increase the stiffness of vertical members and bolted gusset plate connection was used at the joints. Also, gusset plates were provided to maintain the continuity of the crossing members at the joints.

Cross bracing also enhanced the stability in two vertical directions, whereas horizontal stability was provided with the use of steel rods in each tower. The bolted connection was used to connect the steel rods to angle sections in the corners of the tower. Additionally, flat plates were provided for supporting cross bracing between towers and above the roadway.

2.3 Main Cables

The diameter of the main cables used in the Royal Gorge Bridge is 230 mm. Each cable was made up of 2100 galvanized steel wires. The ultimate strength provided by the main cables was in the range of 800-830 N/mm². However, the ultimate strength specified for the structural cables used at present is in the range of 1300-2200 N/mm².

The cables are irregular as they are not secured by circumferential wires wrapped around the cables. This can be attributed to the mild conditions offered by a good environment and the absence of severe pollutants in provincial Colorado.

2.4 Cable Anchors

Cable anchorages are provided to support the main cable so that the forces from it can easily be transferred to the foundation. Trenches were excavated to provide the anchorages. The length of the trench was 23 m and provided up to a depth of 7.5 m. The trench width was 1.2 m and had two lines of openings bored at a distance of 0.5 m.

In 100 openings of each trench, a 50 mm steel pipe was grouted into 1 m² area of the trench, and 21 of the 2100 wires in each cable were then wrapped around the pipe. The wires and the anchoring cables were secured by filling the trench with concrete after connecting all the wires.

2.5 Hangers

Initially, the hangers were comprised of steel rods manufactured using tempering and forging. A pin connection was provided between the steel rods and the bridge deck. After retrofitting, the hangers were comprised of rope suspenders, which were pin connected to the main cable bands and deck with shut-end fittings. The rod is made up of steel and its diameter is 40 mm.

The center to center spacing between the hangers is 3 m, connecting to every main beam. The hangers were provided at a small angle to reach the deck as the main cables were farther than the deck at the piers. However, this angle is less noticeable towards the focal point of the bridge as the main cable is pulled in marginally.

2.6 Deck

The deck of the Royal Gorge Bridge is made up of timber and supported by steel joists, which run parallel to the length of the bridge. Further, the joists are supported by I-section beams in the perpendicular direction, which are held by the pin connection at the hanger. Also, a single steel beam was provided on either side of the deck to reduce the longitudinal deflection due to cantilever action at the edge of the deck.

There are 1300 wood boards along the length of the deck; of these, around 250 require replacement every year. Although the bridge needs to be closed temporarily for replacing these wood boards, the bolted connection of wood boards with deck permits faster substitution of only the woods which are not suitable.

2.7 Wind Cables

After the opening of the bridge, the wind cable system was provided to the bridge. It was already planned to incorporate a wind cable system connected to the deck to improve the stability; otherwise, the deck would be very unstable. Wire rope wind cables are secured to the canyon walls on one or the other side of the bridge and bent towards the deck at the focal point of the bridge.

The wind cables are made up of galvanized steel wire with a diameter of 38 mm. These wind cables were mainly used to connect the primary beams to the deck along the longer span. The wind cables were also pre-stressed to ensure that both the deck and the main cable will not become slack.

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