Buildings that Roll with the Rattle and Shake
Designing structures to effectively withstand earthquakes has long been an intriguing engineering challenge. Putting those designs into practice outside of the laboratory is another hurdle altogether.
Civil and environmental engineering professor Gregory Deierlein has taken on both. He is working with the national building code committee to create guidelines for implementing an earthquake-resistant building system designed by his research team.
Three elements distinguish their system. The first is the braced frames that form the skeleton of the building [A]. In a building with conventional braced frames, Deierlein says, "when a large earthquake hits, typically the braces will buckle and eventually fracture." The braces in his design are allowed to rock on their foundation, preventing the structure from becoming overloaded in the event of a quake.
The second is a series of high-strength steel cables [B] that run the height of the building, reducing the effects of "residual drift." If you look at post-earthquake images from Japan or New Zealand, Deierlein explains, you will see buildings that did not collapse, but that have a permanent lean to them. "At this point, these leaning buildings usually have to be demolished. Our key goal is to protect the structural elements and also provide, through these cables, the ability for the building to self-center."
The final component of the system is a set of self-contained steel fuses designed to dissipate the energy of the earthquake [C]. The force will buckle the fuses, which can be replaced, but leave the rest of the building intact.
By collaborating with architects and "the kind of pioneering engineers" who are working with building code officials to certify the system, Deierlein is beginning to see his research implemented. Engineer David Mar is building a new Packard Foundation building in Los Altos that makes use of the cables, fuses and rocking frames, and Gregory Luth, MS '75, Eng '79, PhD '91, is constructing a casino in Cape Girardeau, Mo., that also utilizes the system. —Helen Anderson, '14
An Orca-inspired Mic for Listening in the Deep
If asked to name the species with the best sense of hearing, the jackrabbit, owl or even man's best friend might come to mind. But don't discount the denizens of the deep. Because light can only travel some 100 meters below the water's surface, ocean-dwelling creatures rely on sound not just to communicate but also to navigate. It was the sonar system of the orca whale that inspired Stanford electrical engineers to create a highly sensitive underwater microphone.
What we perceive as sounds are actually fluctuations in pressure. On land, where the air pressure is relatively constant, it's simple enough to design sensitive microphones that can pick out minute pressure changes from the background. But underwater, the pressure increases by the equivalent of an additional atmosphere with every 10 meters of depth, making faint sounds increasingly difficult to detect the further down you go.
This hampered previous attempts to design a hydrophone capable of sensing sound over a range of amplitudes and frequencies that can be deployed at variable depths. "The only way to make a sensor that can detect very small fluctuations in pressure against such immense range in background pressure is to fill the sensor with water," explains postdoctoral researcher Onur Kilic, PhD '08, who with his team designed the pea-sized device.
To measure the pressure changes caused by sounds that range from particles ejected by the sun plunging into the ocean to underwater explosions, Kilic's team employed a laser and fiberoptic cable to detect the microscopic movements of three tiny diaphragms—each tuned to a different range of the spectrum. In addition to research applications such as seafloor mapping and object-tracking, the hydrophone may also have uses on dry land.
"So far, we have a sensor for sound," says collaborator Olav Solgaard, MS '87, PhD '92, an associate professor of electrical engineering. "But we also have a point pressure system—it is exciting to think about possible applications in medicine and biology that could combine microscopes with the sensor for early cancer detection or stem cell research." —Katy Storch, '12