Shake TableA shake table can be used to test the resistance of structures to seismic shaking. It can also be used to demonstrate the sensitivity of structures of different heights to the frequency of the ground motion.The shake table pictured below, which was designed and built by John Lahr with expert advice from Tom Tompkins. It is based on a lightweight platform that is constrained to move in just one direction. The platform is made from a 10 inch by 10 inch piece of foam-core board. Hollow metal tubes are connected to the left and right edges with hot glue. Metal rods (made from welding rods) are slid through the tubes and the ends of the rods are held firmly by wooden blocks. The blocks are hot glued to a plywood base. Click for larger image At this point in the construction, the shake table platform is able to slide about an inch back and forth on the rods. It should move freely, without binding on the rods. I use a variable speed drill to shake the table. An alternative to a variable speed drill is a single speed drill connected to an electronic speed control, such as the one sold by MLCS Ltd. Their speed control unit is shown on their web site: http:/www.mlcswoodworking.com/Pages/speedcon.html , or they may be contacted at 1-800-533-9298.WARNING:I want to thank GregNowak (gregorysroomyahoo.com) for the message that he sent on 2/22/2001 from concerning leaving the shake table running for a long period of time. The table should not be operated unattended and the 110 V power should always be disconnected when not in use.Paraphrasing Greg: There should be a big warning on any plans for an earth quake simulation mechanism that uses a motor of any sort, as it could be dangerous if not designed and operated properly. About 10 years ago at a Cal Poly State University SLO open house (Poly Royal) an Architecture or Engineering student used a power drill set on low speed and left his earthquake machine running. It overheated and a fire erupted that basically burned down a few class rooms.A rod with a U-shaped bend is mounted parallel to one side of the platform so that it is free to rotate. One end of a stiff wire is bent in a loop around the bottom of the U while the other end is shaped like a hook and is attached with strapping tape to the edge of the table. A more durable and flexible alternative to strapping tape is to attach a square piece of automobile inner tube with dimensions of about 2 x 2 inches to the edge of the table. The rubber is folded over and glued to the top and bottom of the table. Hot glue has worked well for me. Let about 3/4 inch of the rubber stick out beyond the edge of the table and poke a hole through this flap. Then the stiff wire is bent through the hole in the rubber. This arrangement will cause the platform to move back and forth as the rod is rotated. The variable speed drill is attached to the rod by its chuck. The handle of the drill is also firmly attached to the plywood base. Construction details are posted for two shake table designs.To demonstrate the need for a diagonal element to give a wall shear resistance, I use 6-inch long pieces of wood or plastic with magnets mounted at each end. I have attached Velcro to the platform and mating Velcro to a base block. The base block has magnets mounted at each end. A basic wall, consisting of two verticals and a horizontal element, can be built on the base. This wall will fall down during an earthquake. If, however, a diagonal element is added (made from a longer magnet stick, which is provided) then the wall will resist strong shaking. I usually provide one long magnet stick to be used as the diagonal and three short magnet sticks and challenge the students to build a strong wall. Eventually they discover that a diagonal element is crucial. Here is a photograph of the shake table. To the left of the table is a piece of plastic (cut from a fluorescent light diffuser) with four fingers of varying length. When this is vibrated on the shake table, one can experiment with the effect of shaking frequency on the amplitude of motion of buildings of varying heights. Below are three photos of the shake table: one with the magnetic sticks and two with a flexible building.Click here for some additional information on use and construction of magnetic sticks.At the May, 2000, open house of the USGS in Menlo Park, CA, Bob Page demonstrated some additional building issues using the shake table.(Click to enlarge) He added some posts to one side of the building so that a string could be wrapped to form x-braces for each floor. Below are pictures of the building with and without the x-bracing. (Click to enlarge) Bob also demonstrated a design feature of some modern buildings, namely base isolation. The foundation of a building with base isolation allows the ground to move rapidly back an forth while the building stands nearly still. To illustrate this,