SUSPENSION BRIDGES

SUSPENSION BRIDGES

Information About Suspension Bridges:

used for long spans--usually 1000 feet or more
can span over 2,000 to 7,000 feet, which is longer than any other type of bridge
aesthetically pleasing, while very lightweight and strong
tend to be the most expensive to build
roadway able to suspend at a low elevation
constructed easily and needed material transported easily
most aerodynamically vulnerable
expensive to build: over 50 years ago it cost between 2 million and 103 million (depending on length)
cables, towers and anchors are the 3 main supporting parts

Materials Used to Build Suspension Bridges:

TRUSSES: usually structural steel but silicon steel or alloy steels can be used
CABLES: galvanized steel wires with an ultimate strength of 215,000 to 230,000 psi.
TOWERS: structural steel but stone, concrete, or timber have been used
SUSPENDERS: galvanized steel ropes with a diameter ranging from 1 inch to 3 1/4 inches.

Considerations and the Design of Suspension Bridges:

Traffic flow is considered when determining the width.
TRUSSES: They serve to distribute concentrated loads (like snow, concrete, etc.) and secure aerodynamic stability. Bridge width establishes the spacing of the trusses, but is usually no less than 1/50 of the span. Diagonals are close to 45º.
CABLE SYSTEMS: The bridge wires are placed parallel or in strands and then compacted and then connected to the suspenders with cable bands. For larger section, twin cables are used.
TOWERS: Towers can consist of a variety of frames and the base of the tower is either hinged or fixed.
FLOOR: Floor beams tend to be plate girders or trusses. About every 100 feet, joints should be placed so that the deck doesn't obstruct the main structure. Most importantly, the goals of the deck include lessening the dead load and resisting wind.
ANCHORAGES: Safety depends on the security of the anchorages. Many are placed below ground level and then connected to the nearest tower. It is important that the weight on the anchorages does not exceed what is allowed on the material of the anchorage.

TWO FAMOUS EXAMPLES OF SUSPENSION BRIDGES

Golden Gate Bridge

ABOUT THE GOLDEN GATE BRIDGE:

The chief engineer was Joseph B. Strauss
It is located in San Francisco, CA USA
The bridge took 4 years to build (1933-1937)
One of the longest bridges in the world, it is 8,981 feet long!
At the time, the bridge cost $27 million.
Strauss completed the bridge only five months after the promised date and $1.3 million under budget. For his efforts, Strauss received $1 million and a lifetime bridge pass.
The length of the steel wires used in the cables of the bridge is enough to circle the earth three times!
The bridge sits less than eight miles from the epicenter of the most catastrophic earthquake in history.
This suspension bridge is made of steel and concrete.

Tacoma Narrows Bridge

Not one of the most successful examples of suspension bridges, in November 7, 1940 the bridge collapsed.

ABOUT THE TACOMA NARROWS BRIDGE:

It collapsed due to wind vibrations.
The bridge was located near Tacoma, WA USA, and had only been open a few months when it collapsed.
Only taking two years to build (1938-1940), the bridge was labeled an essential economic and military portal to the Olympic peninsula.
Before collapsing, the bridge was the third longest suspension bridge (853 m) in the world and had the longest single span in the country.
The bridge cost just under $6.6 million.
Between 1948 and 1950, the bridge was rebuilt after spending the last 3 years studying aerodynamics so history would not be repeated. It still stands today.