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"  The designers of the east span of the San Francisco\u2013Oakland Bay Bridge\u2014opening Labor Day weekend as the result of a 13-year, $6.42 billion project\u2014didn&amp;#039;t have the luxury of anchoring the new bridge into bedrock. Instead, the 2047-foot span will become the world&amp;#039;s longest self-anchored suspension bridge, an engineering marvel that must endure not only Bay Area traffic but also the inevitability of major earthquakes here. In a traditional suspension bridge like San Francisco&amp;#039;s iconic Golden Gate, engineers use inches-wide steel cables to hang thousands of tons of concrete and steel over water, and those big cables anchor to the shore. The new eastern span of Bay Bridge (it&amp;#039;s divided into two spans), designed by the joint venture of T.Y. Lin International and Moffatt &amp;amp;amp; Nichol, can&amp;#039;t rely on the too-small nearby rock or the surrounding muddy ground\u2014which amplifies seismic movement\u2014for support. So the bridge goes it alone, anchoring its 35,200-ton side-by-side steel decks to itself. &amp;quot;There was no room for anchorage on Yerba Beuna Island in that space,&amp;quot; explains California Department of Transportation chief bridge engineer Brian Maroney. &amp;quot;We had to go to the unusual west-end loop, the most fantastic part of the design.&amp;quot; The bridge will use a single 2.6-foot-diameter main cable looped around the roadway and held high by a single 525-foot tower. The 10.6-million-pound cable is composed of 137 wire bundles, each consisting of 127 5.4-millimeter-wide steel wires (that is 17,399 total wires for those scoring at home). The cable acts like a giant rubber band holding the entire structure together. The tower, located slightly closer to the west end of the bridge, does the bulk of the heavy lifting. It supports 90 percent of the bridge&amp;#039;s weight. From the main cable, 200 steel wire suspender ropes grab hold of the 28 steel roadway decks. The anchorage house, or &amp;quot;the box&amp;quot;\u2014engineer shorthand for the concrete room anchoring the cable in the bridge deck, discernible only from beneath the bridge\u2014plays its own role in keeping the steel decks suspended over San Francisco Bay. The anchorage house, located inside the east end of the bridge deck, has &amp;quot;strategically placed bore holes,&amp;quot; in a 15-foot-tall wall, Maroney says. This is where the single cable splays into the bundles to more easily manage the load as the cable wraps the deck and heads back toward the top of the tower. In all, the cable travels from the east end, up over the single tower, wrapping around the west end of the bridge deck, and back along the same path to anchor into the east end. If the rubber band metaphor doesn&amp;#039;t work for you, think of it as a giant sling. Building the anchorage house required incredible precision. &amp;quot;For each strand, only one spot is okay,&amp;quot; Maroney says. &amp;quot;All others are wrong. There is only one orientation that is acceptable. We had to color-code the wires in the strand so they were oriented in the right angle. It was absolutely surgical.&amp;quot; Crews even used soft-metal tools, softer than the wires, to avoid damaging the strands.  To stay strong during foundation-shaking earthquakes, the tower splits into four steel legs, and the ones that are farther east on the bridge were driven at an angle through more than 300 feet through mud to rest on bedrock. The tower legs are joined by plates called shear links, which stiffen when the wind gusts to keep the bridge stable. In an earthquake, the shear links absorb the earth&amp;#039;s movement. They&amp;#039;re designed to break if the quake is strong enough, to prevent damage to the rest of the bridge.  &amp;quot;These links provide additional damping to the bridge by dissipating energy while going through the inelastic cycles, thereby reducing the seismic forces and displacements,&amp;quot; says T.Y. Lin International lead design engineer Marwan Nader. &amp;quot;At the expansion joints the roadway box girders are connected with two hinge pipe beams. The hinge pipe beams are designed to allows the structures to slide relative to each other.&amp;quot; The new system can also withhold the differing frequencies of any seismic event. On the east side, the bridge sits in soils, with a lower frequency than on the west side, where there is rock. &amp;quot;The two ends may experience approximately the same event, but it will arrive at different times with different frequencies,&amp;quot; Maroney says. Not everything has gone as planned, though. The bridge&amp;#039;s long-planned Labor Day opening was in jeopardy as recently as early August when a portion of the high-strength steel bolts that hold the earthquake shock absorbers to the bridge were found broken after tightening. A quick fix has crews installing steel plates where the bolts broke. Crews will move forward with a new steel saddle to replace the bolt action to ensure seismic stability for the expected 150-year life span of the bridge. That&amp;#039;s a long time for the new Bay Bridge to suspend its steel. And it must do it alone.  Tim Newcomb covers sports and infrastructure for Popular Mechanics. Follow him on Twitter at @tdnewcomb "^^<http://www.w3.org/2001/XMLSchema#string>
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