News-Star.com
By Bob Allison, Contributing Writer
Posted Jul 17, 2011 @ 08:36 PM
SHAWNEE, Okla. —
Disasters
The night of May 26, 2002 a 580-foot section of the bridge on I-40 near Webbers Falls, Oklahoma collapsed into the Arkansas River causing vehicles to plunge into inky space at 70 mph-killing 14 motorists. A tugboat captain blacked out causing the barge he was controlling to collide with the bridge support. Twenty thousand vehicles a day were re-routed daily for two months while the bridge was repaired. During the evening rush hour August 1, 2007 an eight-lane bridge over the Mississippi River in downtown Minneapolis collapsed, killing 13 people. For thirteen months 140,000 vehicles a day were re-routed during its replacement. Ordinarily we take bridges for granted, and it takes such deadly yoo-hoo-hoo experiences to remind us just how important bridges are to our convenience and safety.
Mechanics
Bridges span gaps, and the greater the gap spanned the stronger the design and material required. Stepping stones and fallen trees were probably the earliest bridges. The first bridges constructed by modern engineers were simple steel beams resting on piers. Because of the weight of the beam, they are generally 250 feet or less.
Merely changing the shape of materials increases their load-bearing strength—thereby increasing the length or ‘span’ of the bridge. Take a piece of cardboard and place it across the gap between two books as in a beam-type bridge. It supports little weight as a beam. Bend the cardboard into an arch, and it will support a book on top. Roman multiple-arch bridges [aqueducts] carried water within and road traffic on top.
Fold cardboard and turn it sideways, and it can support two books. Steel beams configured in X and N-shapes create equilateral and isosceles-shaped spaces that strengthen—and thus lengthen—spans. If the piers holding the truss are heightened considerably and a cables attached at their tops bridges can be held up by ‘suspenders’ from the cable to the roadway creates a ‘suspension’ bridge like the Brooklyn bridge. The easiest way to envision a ‘cantilever’ bridge is to see the Firth of Forth Bridge in Scotland. You can locate the point of greatest stress on a bridge by finding where engineers have placed the most structural supports like piers, cables, trusses, etc... FULL ARTICLE
Posted on
Sun, July 17, 2011
by John Cox