April 19, 2017

# World’s Largest Shake Table Puts Six Story Building to Test

Question: what are the two worst engineering nightmares? Answer: resonance and heat! And just to add one more pain on an engineer’s already throbbing head, build the structure out of thin-walled members!

In this video, a group of researchers has put a cold-formed, six-story steel frame building under seismic excitations and fire testing using the world’s largest outdoor shake table at the University of California in San Diego. The aim of this scaled testing is to apply the resulting data to larger projects and improve current building designs against earthquakes, fire, and other disasters.

This particular test is very interesting as the structure is made out of cold-formed steel (CFS). As an engineer, this is perhaps the most difficult type of material you can design as it is basically a thin-walled material. Meaning, you are working with a material that is only in the range of a few millimeters where conventional engineering principles don’t apply anymore. Designing cold-formed steel members for simple loading actions is a nightmare on its own, nevermind designing them for seismic and fire resistance.

The team made use of drones to capture the before, during, and the after behavior of the wall-braced building. They are attempting to map the damage of the structure so they can develop better techniques in immediately assessing infrastructures for cracks and faults when a disaster occurs. In order to gain more realistic results, the team has incorporated domestic appliances that could potentially ignite a fire during an earthquake. The appliances were braced and freestanding to find out which restraint methods are effective.

So why use a complicated material to construct earthquake resistant structures? Simply because CFS is much lighter in weight compared to conventional materials like concrete. To explain this better here is a very simple, yet fundamental equation you need to know when dealing with earthquake loadings.

Omega^2 = k/m

Omega is frequency, k is stiffness and m is mass. If you have a larger mass you will end up with low frequency and a smaller mass will give higher frequency. In earthquake engineering, low frequencies are the most destructive type of wave loadings while higher frequencies are not detrimental. So, using CFS in structural design is ideal.