THE PRINCIPAL SUSPENSION BRIDGES OF THE USA
Past president Barry Hiscox continues the Letter from America series
Although this brief article discusses the most important achievements in suspension bridge building here in North America, to any non-engineers it is perhaps necessary to identify the main differences between cable-stayed and suspension bridge design. A cable-stayed design sees vertical loads on the deck supported by diagonal cable stays that transfer the loads to the towers and hence to the tower foundations. A suspension design, on the other hand, uses cables to carry the vertical loads through curved cables in tension. Although, in general, cable-stayed bridges can be built more rapidly than suspension bridges, they are effectively limited by span length (suggested 500 – 3,000ft) and loads that they can realistically carry. Despite more modern advances in material technology, ultimate cost considerations come into play and, despite longer time to build, suspension bridges have, in theory, no such limitations regarding span distance.
The Bay Bridge, connecting the cities of Oakland and San Francisco, California.
The Humber bridge, with its span of 4,630ft and an overall length of 7,283ft, was the longest suspension bridge in the world when opened to traffic on 24 June 1981, but was later surpassed by the Akashim Kaikyo bridge between Kobe, Honshu Island and Awaji Island in Japan, with a main span of 6,532ft. This was the longest suspension span until overtaken in March of 2022 by the current longest span in the world, which is the 1915 Cannakkale bridge in Türkiye, with a staggering suspension span of 2,033m/1.257km/6,637ft.
The first modern (iron) suspension bridge was the Menai bridge, built in the UK between 1819 and 1826, connecting north Wales (County Gwynedd) and the island of Anglesey across the Menai Straits and designed by the famous Scottish civil engineer and architect, Thomas Telford. The suspended span is 579ft (177m) long, stands 100ft above water and is supported by 16 huge chains. It is still in use but no longer a toll road and free to cross, except for larger buses because of size and weight limitations.
The Benjamin Franklin bridge, which connects Philadelphia and Camden.
According to the Bridgemaster inventory, there are (or have been to present day) a staggering 1,459 true suspension bridges in the USA, with the first built with iron chain and a span of 70ft at Jacob’s Creek, Ironbridge, Mount Pleasant, Pennsylvania in 1801. It was removed in 1833. The Jacob’s Creek bridge was designed by James Finlay, a local judge and inventor. The contract with Finlay was signed in 1801 with the two local counties of Fayette and Westmorland committing to half of the original $600 cost ($10,000 with current inflation).
Design of the suspension bridge has come a long way, such that in the USA the top 15 major suspension bridges in excess of 1,600ft span have all been built since the turn of the century.
John Fulton and Andrew Oliphant constructed the bridge with iron supplied by Isaac Meason (Finlay’s friend who owned the nearby Union and Mount Vernon furnaces). Two chain cables of iron bar in links between five and 10ft long were anchored to the ground at each end and stretched over 14ft tall pyramid-shaped piers built on either side of the creek.
Design of the suspension bridge has indeed come a long way since those first faltering efforts, such that in the USA the 15 major suspension bridges in excess of 1,600ft span have all been built since the turn of the 20th century.
San Francisco’s Golden Gate bridge.
1. Verrazzano Narrows: 4,260ft
Opened in 1964 in New York City, connecting Staten Island with Brooklyn. It was the longest in the world until 1981 when its length was surpassed by the Humber bridge in Great Britain.
2. Golden Gate: 4,200ft
San Francisco’s famous bridge was the longest in the world from 1937 to 1964, when it was surpassed by the Verrazzano in New York City.
3. Mackina: 3,800ft
Connects the upper and lower peninsulas of Michigan. At an overall length of almost five miles, the total is technically the world’s longest suspension bridge between anchorages.
4. George Washington: 3,500ft
Connects Manhattan in New York City with Fort Lee New Jersey. When opened in 1932, it was, until 1937, the world’s longest suspension bridge.
5. Tacoma Narrows: 2,800ft
Parallell twin spans crossing the Tacoma Narrows of the Puget Sound in Washington State. The older of the two bridges opened in 1950, with the newest opening in 2007. The original 1940 Tacoma Narrows bridge collapsed four months after opening due to a design fault exposed during high winds.
6. Carquinez: 2,390ft
Crossing the Carquinez Strait at the north east end of San Francisco Bay, it runs parallel to an older cantilever bridge (both collectively referred to and operated as the Carquinez bridge). The main span is known as the Alfred Zampa Memorial bridge.
7. San Francisco – Oakland Bay: 2,310ft
Each of the twin Western Spans make up the seventh-longest suspension bridge in the USA, connecting the cities of San Francisco and Oakland, California, over San Francisco Bay via Yerba Bueno Island.
8. Bronx – Whitestone: 2,300ft
Connecting Whitestone, Queens with the Bronx – two miles west of the Throgs Neck bridge, crossing the East River. Plans for a third fixed crossing to Long Island have not come to fruition.
9. Delaware Memorial: 2,150ft
Comprises twin parallel main spans between Delaware and New Jersey, the first opened in 1951, with the second opening in 1968.
10. Walt Whitman: 2,000ft
Connecting Philadelphia, Pennsylvania and Gloucester City, New Jersey across the Delaware River.
11. Ambassador: 1,850ft
International crossing of Detroit River, from Detroit, Michigan, USA to Windsor, Ontario, Canada. The longest suspension bridge in the US until 1931.
12. Throgs Neck: 1,800ft
Connecting Bay Terrace, Queens with Throggs Neck in the Bronx, New York City. Opened in 1961 at the northern mouth of the East River.
13. Benjamin Franklin: 1,750ft
Connecting Philadelphia, Pennsylvania and Camden, New Jersey, crossing the Delaware River. It was the world’s longest until surpassed in 1929.
14. Bear Mountain: 1,632ft
Crossing the Hudson River in New York, it connects Bear Mountain State Park and Westchester County, 30 miles north of New York City. Opened in 1924, it was the world’s longest suspension bridge for 19 months until surpassed by the Benjamin Franklin bridge.
15. Williamsburg: 1,600ft
Crossing the East River in New York City, it connects Williamsburg in Brooklyn and Manhattan’s lower east side. It opened in 1903, surpassing the Brooklyn bridge and becoming largest in the world until 1924.