Saturday, May 1, 2021

1926 US-30 Benjamin Franklin Bridge over Delaware River at Philadelphia, PA

(Bridge Hunter; Historic Bridges; 3D Satellite)

The length of the longest span is 1,750' and the total length is 8,170' (1.5 mi.). It was designed by Ralph Modjeski, and, in 1991, it had an average daily traffic of 87,158. [Bridge Hunter] It claimed the "world's longest suspension bridge" title from the 1924 Bear Mountain Bridge, which had a navigation span of 1,632'.

Historic Bridges rates this bridge 10/10 for both national and local historic significance. Not too many bridges get a double-10.

LC-DIG-highsm- 57137
Credit line: Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
Carol M. Highsmith's photographs are in the public domain.


phillyroads (Figure by Paul Phillipe Cret and Rudolphe Modjeski.)
The towers are 385' above mean high water and the foundations are up to 170' below. They provide a clearance of 135' and hold two cables, each of which are 30" in diameter and contain 18,666 wires.
 
Delaware River Port Authority - DRPA posted
#WaybackWednesday pic of the "Delaware River Bridge" looking east towards New Jersey at the middle of the center span on 9/8/1924. The bridge would later be renamed the Ben Franklin Bridge.
Bridges Now and Then shared
Dave Frieder: Engineer of design, Ralph Modjeski. This is a "Cousin" of the Manhattan bridge.

DRPA-rehabilitation via DRPA-ben-franklin
 
Bridges Now and Then posted
Philadelphia-Camden's Delaware River Bridge, renamed the Benjamin Franklin Bridge in 1955, is seen under construction, June 13, 1924. (Free Library of Philadelphia)
 
John Kathman commented on BN&T's post
1927

Bridge Hunter
[I tried two different web sites to get a higher resolution photo. Both were broken. I've noticed that the deck bends upwards when the cables have only a partial load.]

Street View
The anchorages on either side give the bridge an appearance of massiveness. Each of the granite anchorages to which the steel cables are attached covers three-quarters of an acre. The two anchorages, which together required 216,000 tons of masonry, rise 175 feet above the ground. Their "toeholds" sink into the earth 65 feet into the Philadelphia side, and 105 feet into the Camden side. [phillyroads]
So anchorages are like piers in that much of the work is below the surface. Cable-stay bridges don't need anchorages, but they probably do need taller towers. I wonder how the costs trade off.
Street View

Most suspension bridges are deck trusses. The Williamsburg Bridge was a through truss. This bridge appears to be a pony truss where the truss does not have a top and the sides are both above and below the deck.
Street View

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