November 1, 2007 Engineers have designed windows and walls that will not break when shaken by the intense motion caused by earthquakes. By installing glass panels that do not touch the corners of the frame, researchers increased the drift capacity of the window frame. Adding a gap between the walls and frame allows for some movement, especially when reinforced by "fuses" that connect the two structures.
- Elastic-rebound theory of earthquakes
- North Anatolian Fault
- Moment magnitude scale
- Earthquake liquefaction
People in California know all too well the aftermath of a powerful earthquake. Despite major improvements to building codes, and existing structures, there is still the threat of serious damage and possibly, loss of life.
Engineers are working to design better buildings, able to withstand whatever Mother Nature has to offer.
On a specially built "shake" table, architectural engineers study historical quakes on a mini-scale. Then in a basement laboratory they test a life-size wall, mounted on a special rack. The goal here is not to improve the frame -- but everything that fills it. Shards of glass and other debris pose a serious threat.
"Most of the damage or breakage in the glass starts at the corners where the glass panels interact with the holding frame," Ali Memari, Ph.D. architectural engineering professor at Penn State University, told Ivanhoe.
These engineers designed glass panels that don˝t touch the corners of the frame. By rounding the glass corners and polishing the edges, researchers found the glass curtain walls would have fifty-percent more "drift capacity," meaning the frame could move fifty percent more before the glass would break.
Researchers are also testing infill walls -- the bricks and masonry inside a frame. A tight fit between the infill and the frame means there's a higher chance the wall will be damaged during a quake. But if builders leave a gap, the wall loses support. That's why professor Memari has designed what he calls "fuses" for the walls -- tiny lumber or concrete discs mounted between the wall and the frame. The fuses provide support, but still allow some wiggle room.
"When the force that is transferred from the frame to the wall exceeds the capacity of the fuse, the fuse breaks and the wall is safe. You can change the fuse after an earthquake," Dr. Memari said.
Penn State engineers have a patent pending on the glass curtain walls. They are still researching the infill wall "fuse" system.
The American Geophysical Union, the American Society of Civil Engineers, the IEEE-USA, the Incorporated Research Institutions for Seismology, Inc., and the Materials Research Society contributed to the information contained in the TV portion of this report.
BACKGROUND: In natural disasters such as earthquakes or hurricanes, the biggest danger doesn't come directly from the shaking or the winds, but from debris. In both cases, broken glass becomes deadly shrapnel. Architectural engineers at Pennsylvania State University are investigating the behavior of glass and walls during earthquakes to come up with better strategies for building windows and walls able to withstand earthquakes and other natural disasters.
WINDOW TREATMENT: During an earthquake, a building's frame or skeleton will sway with the tremors. The frames that hold the glass window planes in place change shape as they shift along their joints. Yet the glass remains rigid. Pressure builds on the corners of the glass, and chips and cracks begin to form. The Penn State researchers built a model wall and window frame and used an actuator to simulate the effects of an earthquake, noting points of failure. They concluded that if the corners of a sheet of glass are rounded, and the edges processed, this shifts the point of stress concentration so the window frame can move without breaking the glass.
OFF THE WALL: The engineers have devised another system to help prevent the collapse of certain types of walls during an earthquake. Usually masonry infill walls are used in conjunction with beams and columns to form the structure's "skeleton," thereby bearing the weight. But an earthquake gives rise to the same kinds of stresses exhibited by glass window panes. A potential solution is to add steel reinforcement into the infill wall and tie the wall to the building's frame. Another solution is to add a gap between the walls and the frame. This prevents interactions during earthquakes, although one would sacrifice some of the stiffening power added by infill walls. The Penn State team opted for middle ground: instead of isolating the walls completely, they used a hockey-puck-like fuse to connect the wall to the frame, providing support, but also serving as a buffer during seismic events.
WHAT CAUSES EARTHQUAKES? An earthquake is the result of a sudden release of stored energy in the Earth's crust triggered by shifting tectonic plates. The Earth's lithosphere is an elaborate network of interconnected plates that move constantly -- far too slow for us to be aware of them, but moving, nonetheless. Occasionally they lock up at the boundaries, and this creates frictional stress. When that gets to be too large a strain, the rocks give way and break and slide along fault lines. This can give rise to a violent displacement of the Earth's crust, which we feel as vibrations or tremors as the pent-up energy is released. However, only 10% or so of the total energy is released in the seismic waves. The rest is converted into heat, or released as friction.