Allison Bogisich
Adriana Velasco
Building for an Earthquake
Introduction
Earthquakes are an ever-present worry for Californians. This problem is always at
the back of our minds, especially of building engineers. Buildings need to be safe before,
during, and after an earthquake to protect residents. Earthquakes are unpredictable; you
never know which direction or how much force one they will apply. Our objective, for
this experiment, was to create an earthquake resistant building that would be limber and
move with the force of the earthquake, without falling over and killing the people inside.
Methods
The first aspect of building that we needed to understand was natural frequency
and the period of a structure. How does the mass and height of a structure affect these?
We first experimented with straws and play dough to simulate the affects of earthquakes
on building. From this were able to conclude that the taller the building the longer the
period is, the lower the frequency is. Another simulation was done using simple
pendulums, which was done to derive period versus height and mass. The problem was
that mass wouldn’t enter the equation since the string we used had very little resistance.
Next, we watched videos and looked over photos of collapsed buildings due to
earthquakes. This was done so we could see what the possible modes of failure were.
There were also videos of buildings that had been designed in such a way that they were
able to withstand earthquake stimulations. We needed to study failures in order to plan
how our buildings would fail. Base isolation, and seismic dampers are two ways of
retrofitting building to resist earthquakes. However, just making sure that you are
building with the right materials is a good start. We each built our own individual tower,
using K’nex as our only building material. Since we also wanted our buildings to have a
chance against an actual earthquake simulator, we needed to design our buildings so that
they would be rigid enough to stand, but not so much as to have a brittle failure during an
earthquake. We made cross braces so that when our buildings were shaken, the
compression/tension in the braces would absorb the shock. Another aspect of design was
taking into consideration how you would want your force flow to be during an
earthquake. This is where a builder will purposely make one beam weaker than the other,
so it will give first. Then the flow of energy will be transferred efficiently through one
pathway, back towards the ground. All of these ideas were taken into our own design
ideas.
Results
After completing our model buildings, we took them to the Richmond field
station, a UC Berkeley lab, and tested our structures on their shake table. Natural
frequency is the frequency of free oscillation in cycles per unit of time. Allison’s building
was able to stay standing with a natural frequency of about 10 hertz, and then the top
level, that was not supported with cross beams, collapsed. However, the rest of the
building survived. Then, with a natural frequency of about 14 hertz, Adriana’s building
experienced brittle failure at its base, causing the whole structure to fall over. Both of our
buildings were able to withstand a small earthquake, but the larger simulation was too
much for our buildings.
Discussion
After reviewing the damage done to our buildings, we were glad to find that our
buildings survived all of the small shakings, and didn’t collapse till the forces were
increased to the magnitude of a large earthquake. Allison’s building experienced failure
on a specific level of her building, and that was because there was a difference in
stiffness on the top level because there were no cross braces to support it. Adriana’s
building failed because there weren’t enough braces and supports on the bottom level.
The flow of the earthquake’s force was disrupted, causing our buildings to collapse.
Another cause of the failures could also be dependent on the materials we used to build.
Our connections were weaker than a real building’s braces, frames, and walls, which
would be mostly steel and concrete, since they are the best combination for a balance
between stiffness and flexibility. In conclusion, our K’nex buildings gave us an idea of
what the effects of an earthquake will be like, so that buildings can be better prepared for
tomorrow.