One cable holds the bridge up.
San Francisco has its share of massive earthquakes, but the Bay Bridge, one of the city’s main transit arteries, is not as quake-safe as you’d hope. That’s why, alongside it, the state is building a massive new replacement structure—the largest self-supporting suspension bridge ever built. Jim Giles at New Scientist went to visit the bridge and provides a primer on its engineering:
In a regular suspension bridge, the cables that support the roadway are hung between two or more towers, like a hammock between trees, and anchored at each end by a connection to land. The new bridge is more like a sling. A single cable loops under the roadway, over the tower and beneath the roadway on the other side of the tower. The enormous forces placed on the cable by the road cancel out, leaving a structure that is balanced but not directly supported by a land anchor…
As the [road] segment fell into place it revealed the full length of tower that stands behind it, an elegant structure made up of four concrete pillars. These drop into enormous steel foundations, parts of which were built in Texas and shipped to California via the Panama canal. The pillars are connected by “shear beams”—relatively weak steel components that are designed to break if the towers move. The two roadways, one each for east and westbound traffic, hang from the cables but are not attached directly to the tower. This arrangement means that the four pillars and two roadways will sway when a quake hits, but remain intact even through the strongest shaking that geologists expect the region to experience over the next 1500 years.
Read more at New Scientist.
Image courtesy of Bay Bridge Information Office.
After a 10-to-1 vote in the Board of Supervisors, San Francisco stands poised to force cellphone makers to display the level of radiation their phones emit next the phone’s display in a store. It would become the first American city to do so.
The mayor, Gavin Newsom, supported the measure and will probably sign it into law. If he does, the rules will be phased in February next year, and violators who don’t provide the information will be charged up to $300. Predictably, the move is lauded as progressive and pro-consumer if you ask supporters and cast as a misleading ordinance if you ask the Cellular Telecommunications and Internet Association. But what does it actually mean?
The number in question is called the specific absorption rate, or SAR. It’s a measure of how much of the phone’s radio frequency energy gets absorbed into a person’s tissue. Although no clear picture has emerged of what effects cell phone radiation has on health (though there are plenty of interesting and contradictory studies), there are already legal upper limits for the SAR of phones. Here in the United States, the Federal Communications Commission is responsible for testing those levels and won’t allow anything higher than 1.6 watts per kilogram of body weight. In Europe the cap’s a little higher—2 watts per kg.