Shaking a 275-ton Building 110
Roland Piquepaille writes "If you want to predict how a tall building can resist to an earthquake, some researchers have better tools than others. Engineers from the San Diego Supercomputer Center (SDSC) have built a full-size 275-ton building and really shaken it to obtain earthshaking images. The building was equipped with some 600 sensors and filmed as the shake table simulated the 1994 Northridge earthquake in Los Angeles, California. It gave so much data to the engineers to analyze that they needed a supercomputer to help them. Now they hope their study will yield to better structure performance for future buildings in case of earthquakes."
Any structural engineers around? (Score:2, Insightful)
Who makes that software?
How much does it cost?
Link that works...for now. (Score:4, Insightful)
"Wednesday, Apr 11 @ 13:13 PDT The powerful earthquake struck suddenly, shaking the seven-story building so hard it bent, cracked and swayed in response. But this was no ordinary earthquake. In a groundbreaking series of tests, engineering researchers from UC San Diego's Jacobs School of Engineering jarred a full-size 275-ton building erected on a shake table, duplicating ground motions recorded during the January 17, 1994 Northridge earthquake in Los Angeles, California. To record the impact on the building, the structure was fitted with some 600 sensors and filmed as the shake table simulated the earthquake, yielding a flood of data including stress, strain, and acceleration -- so much information that engineers were having a hard time making sense of it all. That's where visualization experts from the San Diego Supercomputer Center (SDSC) at UC San Diego came in. "
The only problem is (Score:5, Insightful)
Hence what these guys are doing: a good old fashioned experiment, involving an actual building on a giant table that shakes, reproducing the exact movements recorded in an actual earthquake. That's how you find out if your model and simulation are actually the right ones. If the building behaves like in the assumed models, then all's well, if not, well, someone will have to come up with a better model.
It might seem that wth, we already know the laws of mechanics well enough, we don't need experiments to test them. The problem is that any model is based on some simplifications, since you just don't have the computing power to even account for all waves, reflections and interferences in a big building with hundreds of joints and thousands of metal bars, pipes, whatever other discontinuities through the walls. So physicists get to decide what are the important parts to simulate, and which should at best be lost in the decimals.
E.g., if you want to know if a horse floats, you can just as well imagine it to be a sphere or a cube. (As the wisecrack goes, "you know you're an engineering student if you approximate a horse as a sphere, because it makes the math easier.") Actually, wisecrack aside, for that you won't even imagine it to have any shape at all, since shape is irrelevant. It doesn't really matter what exact shape it is, just the mass and the volume. E.g., if you want to know how fast a rocket reaches the moon, you don't need to know the exact shape or colour of the rocket, you can just think it's a point. Etc.
That's how we solve problems nowadays. We get to decide what is really important, and what can be safely ignored in the model.
Unfortunately, if you to be really sure that you did the right choices, you have to compare it to what happens in real life. Does your simulation really behave like the real thing in that situation? Or did your approximating the horse as a sphere lead you to a wrong solution like rolling it along the race track to win?
That's, in a nutshell, what these guys did.
Re:Too bad (Score:3, Insightful)
Think about why the World Trade Center towers collapsed. (Hint: something to do with the effect of sustained high temperature kerosene fire on the strength of structural steel.)
Re:Too bad (Score:1, Insightful)
Re:The only problem is (Score:4, Insightful)
Since my father was one of the team that sent men to the moon. I know a bit more about simulation and testing than the average bird. On Redstone Arsenal (near Huntsville, Alabama) stands a building where they did full mock up shake testing of the Saturn V rocket. I appreciate the intent of people who wish to do full computer simulations. These are getting very good and they delete with the need for many simple tests. Nothing substitutes for the real thing and doing real tests. This was a 37 story rocket they were launching.
The remark about decimal points is valid. Everyone forgets that the only math that truly exists is integer math. We enjoy using approximations using floating point math but that is all that these are. They are approximations. The list of errors that arises out of these approximations is long. This math only operates well within about 3 decimal places and then it begins to develop progressive errors.
In Apollo mission computer programming there was a decision made not to attempt over 5 decimal places in navigation and simply to do correction measurement over time. It worked like a charm. It was possible to calculate much more finely but in reality the measurements were not more valuable. Nothing was to be gained by the determination of 11 decimal places that the mission required for accuracy.
Shaking a 275 ton building will hold as a good approximation for that size range but will not do too well in estimation of a 275,000,000 ton building. It will require actual measuring of such a building. There are many such approximations that come up that people do not consider. For example the velocity of the top of a building is different than the bottom. In really big buildings level and plumb have to bend for the earth. In really big buildings what is the pull of the tide? All sorts of things like that begin to have significant value