How to Take Out a Suspension Bridge
How would one take out a suspension bridge? You could ram an airplane into it, but it's harder than ever to commandeer an airplane these days. Also, unlike tall buildings that fill the skyline, where you have a pretty good chance of hitting your target, bridges are low to the ground and don't have much of a vertical cross-section. Trying to hit one with a 747 wouldn't be impossible, but it would be difficult.
For an explosion to do maximum damage, it has to be done in a confined area. Tim McVeigh drove his Ryder truck under an awning in front of the Federal Building in Oaklahoma City, and that helped guide the blast of the explosion into the building, increasing the damage done. The first attack on the World Trade Center was done several years ago with a truck bomb parked in the enclosed basement garage. Being inside the basement caused the full force of the truck bomb to be directed at the structure of the building.
If you took a truck loaded with explosives, parked it on the bridge, and detonated the truck, you might take out part of the roadway, but you probably wouldn't do much permanent damage to the bridge. If you parked one on the bottom deck of a double-decker bridge, like the SF-Oakland Bay Bridge, you might take out some of the roadway above and below you, but since the sides of the bridge are open, most of the power of the blast would be directed sideways and out, doing little damage.
The recent (unsubstantiated) terrorist threat specified that the target would be a "suspension bridge". A suspension bridge has a roadway which is held up by cables. The cables are moored into something solid, such as the bedrock at either end of the bridge. In the Bay Area, the San Francisco side of the SF-Oakland Bay Bridge is a suspension bridge, as is the Golden Gate Bridge. The San Mateo and Dumbarton bridges are not suspension bridges.
The cables that hold up a suspension bridge are under enormous tension, since they are holding up a roadway (and the cars that cross it) that weighs many tons. Each main cable that holds up a suspension bridge is made up of hundreds of smaller wire steel cables, wrapped tightly together to form an incredibly strong structure with extremely high tensile strength. If anything were to reduce that tensile strength, the cable would snap and the bridge would collapse.
Heat is the most obvious thing that causes a steel cable to lose tensile strength. Heat a steel cable up to 3000 degrees F and the steel will turn to liquid. However, just heating steel to 500 degrees F or so can cause it to lose tensile strength. Heating up a bridge cable hot enough would cause it to lose tensile strength and the bridge would collapse.
Heating and then rapidly cooling steel can also cause it to lose tensile strength and become brittle. Heating up a bridge cable and then rapidly cooling it without actually causing structural failure could be just as damaging to the bridge -- the cable could become brittle and the bridge rendered unsafe for use, even though the bridge itself is still standing.
You could heat up a bridge cable by detonating a truck full of gasoline on the bridge. If detergent is mixed with the gasoline first, it becomes "jellied", like napalm, and sticks to whatever is being burned, concentrating the heat on the gasoline-covered surface. You could also heat up a bridge cable with just about any flammable substance that has a high heat-concentration, such as thermite.
Unfortunately for Californians, our governor has only seen fit to post California National Guard units on bridges with guns, and you can't stop a fire with bullets. If Governor Davis was a little more on the ball, he'd post California National Guard units on bridges with guns, fire-fighting equipment and fire-retardent foam, and make sure that the units are trained as fire-fighters. That way, anyone attempting to melt a cable on the Golden Gate Bridge would find themselves and their fires quickly extinguished.
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