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West End Sidescan Mapping

Today the center span and other debris lie on the floor of Puget Sound, where they fell. A site plan of these remains was developed from sonar soundings of the Narrows. The remains of the center span and anchor blocks are readily identifiable. The side scan information was then used to develop the documentary "Gertie Gallops Again," prepared by Tacoma Municipal Television for the show "CityScape".  In 2000, this workup of the ruins was used to develop a site plan for the new Tacoma Narrows Bridge.

 

1940 Tower1950 Tower

The two main channel piers were undamaged during the failure of the original bridge and were used to support the second bridge with only minor modifications to the concrete pedestals under the new steel tower legs. The original construction of Pier 5 was one of the most difficult ever attempted, establishing a world record for depth of water in which a caisson had ever been landed. Water depth, along with tidal currents of nine miles per hour at the site, required extreme caution and ingenuity on the part of the engineers and contractors. Thirty concrete anchors, each weighing approximately 600 tons and connected to the caisson with 90 one-inch diameter wire cables, were required to hold the caisson against the current. At times the level of water at one end of the caisson would be seven to eight feet higher than at the other. Holding construction barges alongside the caisson was extremely difficult. Steel cutting edges were attached to the bottom of the caisson. After the caisson was lowered to the bottom of the channel in 135 feet of water, the cutting edges assisted in penetrating through 90 feet of sand, gravel, and boulders where the bottom of the caisson was finally positioned at a depth of 225 feet below mean low tide. Lightweight concrete was used in the roadway deck of the present bridge to lessen the load on the piers. However, the total superstructure weight of the present bridge exceeded the first bridge by approximately 1.6 times per lineal foot. The designers determined that this additional weight would not cause excessive overloading of the foundations. The original anchor blocks were also used in the second bridge, but were modified substantially due to the greater weight of the superstructure. This weight increased the horizontal force in the main cables from the original 28 million pounds to 36 million pounds. In addition, the new cables were spaced 60 feet apart compared to the original 39 feet. The concrete anchorage was modified by removing the sides and blasting to the top of the footing blocks. Then, new anchor bars were installed for the main cables. In addition eight feet of concrete was added on each side, and extending the back of the anchor 20 feet to its full height. The additional width and length, in effect, provided a yolk or "U" configuration around the old concrete core that remained.

 


1940 Roadway

Every reasonable precaution was taken to stabilize the structure and insure against any possibility of noticeable motion. One of those precautions, and a feature unique to this bridge, is the design of the roadway deck. Open steel grid slots were installed between each of the four traffic lanes and at both curbs. These open steel gratings function as vents to relieve oscillations created by passing wind. The gratings are bonded to the concrete to preserve slab continuity across the full roadway. In tests the use of these slots was an effective method of reducing the forces which cause oscillation and, consequently, constitute a large factor in stabilizing the structure.

1950 Roadway

 

Categories Galloping Gertie ( 1940 ) Tacoma Narrows Bridge ( 1950 )
Total Structure Length 5,939 feet 5,979 feet
Suspension Bridge Section 5,000 feet 5,000 feet
Center Span 2,800 feet 2,800 feet
Shore Suspension Spans (2), each 1,100 feet 1,100 feet
East Approach and Anchorage 345 feet 365 feet
West Approach and Anchorage 594 feet 614 feet
Center Span Height Above Water 195 feet 187.5 feet
Width of Roadway 26 feet 49 feet 10 inches
Width of Sidewalks(2), each 5 feet 3 feet 10 inches
Diameter of Main Suspension Cable 17.5 inches 20.25 inches
Weight of Main Suspension Cable 3,817 tons 5,441 tons
Weight Sustained by Cables 11,250 tons 18,160 tons
Numver of No. 6 Wires Each Cable 6,308 8,705
Weight of Shore Anchors 52,500 tons 66,000 tons
Total Length of Wire   20,000 miles
Towers:    
Height Above Piers 425 feet 467 feet
Weight of Each Tower 1,927 tons 2,675 tons
Piers:    
Area 118 feet, 11 inches by 65 feet, 11 inches 118 feet, 11 inches by 65 feet, 11 inches
East Pier, Total Height 247 feet 265 feet
East Pier, Depth of Water 140 feet 140 feet
East Pier, Penetration at Bottom 90 feet 90 feet
West Pier, Total Height 198 feet 215 feet
West Pier, Depth of Water 120 feet 120 feet
West Pier, Penetration at Bottom 55 feet 55 feet

 

Full map from sidescan data

These days most are aware of the location, identification and recoveries from the Titanic. The Titanic was a large ship but in comparison it would take 5.56 Titanic’s end for end to occupy the space that Galloping Gertie now occupies. In raw materials the bridge contained 5.0 times that of Titanic. These figures make Galloping Gertie the largest man made structure ever lost at sea. In addition the current swept bottom of the Narrows has now become the largest single man made reef supporting an abundance of marine life.

 

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