How Does the Height of a Building Affect Its Stability in an Earthquake?

Many times in history, we have experienced natural disasters across the world. These could include tsunamis, flooding, hurricanes, droughts… and earthquakes. The average rate of deaths caused by earthquakes, since 1990, has been approximately 27,000 across the entire world. However, many lives have been saved due to strict government laws about newly built buildings. In the UK, we call these “Seismic Building Codes”. In brief, these laws ensure a certain level of protection against earthquakes. This was the point where I decided to focus on factors that affect how sturdy a building is during an earthquake. The foundations, the shape and the height of the building are but a few contributing factors. But out of these 3 factors, I have decided to focus more deeply on how the height of a building affects its ability to withstand an earthquake.

EQUIPMENT:

•   Strictly Briks Blocks (For Building)

•   3 Layers of Base Plates (The number of layers can be changed depending on how much of a height difference you desire) (For Building)

•   2 Equivalent Rectangles of Plexiglass (For the Shake Table)

•   4 Small Rubber Bouncy Balls (For the Shake Table)

•   2 Large Heavy-Duty Rubber Bands (For the Shake Table)

• Measuring Stick (OPTIONAL: Just to confirm the building's height, which is our independent variable)

• Stopwatch (To measure our dependent variable: time taken for the building to collapse)

(Although I have mentioned one particular brand: Strictly Briks, any other similar material can be used to model a building)

HYPOTHESIS:

I predict that the tallest building would be the least likely to withstand an earthquake (which I will demonstrate using a shake table). This is because structures that are “tall and skinny”, are generally more exposed to the lateral shaking movements of the ground beneath the building - which are basically the type of movement earthquakes make. This also means that I believe that the shortest building would be the most likely to withstand an earthquake.

METHOD

1. Assemble your shake table using the below steps:

STEPS FOR ASSEMBLING SHAKE TABLE:

a) Place the two plexiglass sheets on top of one another.

b) Stretch a rubber band around each end of the plexiglass sheets, about 1 inch from the edges.

c) Insert the rubber balls between the boards at each of the 4 corners, placing them directly under the rubber bands - to help prevent the sheets from bending.

Your shake table is now assembled!

2. Attach the Base Plate to the top of the shake table. Secure the Base Plate by slipping it underneath the rubber bands.

3. Build a series of towers using the Strictly Briks on top of the base plate. The only difference between each structure should be their height (keep it in mind, that the tower’s footprint must remain the same).

4. Measure the height of each tower using a measuring stick.

5. Next, measure the distance of the table displacement:

 6. Lastly, use your phone to measure the ‘earthquake’ strength using your phone, to replicate the use of a seismometer (which is a scientific tool used to measure the earthquake strength). I used an app called “Arduino Science Journal” to do this. To measure the force you applied to the Shake Table, you could build a space for your phone on top of the plexiglass. This step is not entirely necessary and is only there for keeping in with my control variables: applying the same amount of force each time.

7. Finally, keep a record of your measurements in a table & plot all your variables onto a graph.

ANALYSIS

In my hypothesis, I predicted that my tallest structure (which in this case was 30cm) would take the least amount of time to collapse. This was proved by the mean amount of 19 seconds that the 30cm structure took when compared to the 33.3 seconds of the 20cm structure and 43.6 seconds of the 10cm structure. From these mean results, we can also see that the 10cm structure withstood my shake table for the longest period of time: 43.6 seconds. All of this evidence is proved to be fair, as one of my control variables was to repeat this same experiment 3 times. So I believe that this proves that my hypothesis is correct and also provable through a practical.

(Sorry about the crumpled table - Unfortunately, it was the only picture of it I could find)

EVALUATION

Out of all of the results I produced during this experiment, the largest difference in time had a maximum of 9 seconds, which provides a controversial aspect to how equal and fair I tried to make the tests. Fortunately, the large time difference is only true with the 20cm structure. Meanwhile, the 10cm and 30cm structures have a difference of 4 and 5 seconds (in that respective order) between their shortest and longest times taken to collapse, which shows that the experiment was largely accurate. Thus, I believe that the project had a repeatable aspect to it and that more repeats were unnecessary. Although, if I was offered to repeat the experiment with one of the 3 buildings again, I would test the 20cm structure again.

I also believe that this experiment is reproducible - as in I or another person could repeat this experiment using similar materials and obtain similar results. Unfortunately, this cannot be proved as I do not know of another person doing this same experiment & buying different materials to construct another Shake Table appears to be both time-consuming and resource-consuming. But I can predict this through a similar contrast of results based on height differences which I have found online, on a website called Science Buddies.

Although my results, fortunately, turned out to be mostly similar, I found difficulty in finding suitable materials. First, I discovered that Lego was very difficult to destroy. Then, I discovered that the plexiglass I was to use, was actually rather too small to fit my phone (on which I am recording the force applied to my Shake Table). This meant that I had to make quite a few drastic adaptations to my method. First, I substituted Lego for Strictly Bricks and a structure that was easier to destroy. Next, I tried placing my phone in different sections on the top of my Shake Table. Every time I tried this, it weighed down a certain side of the Shake Table, making the test unfair. In the end, I decided to place the phone underneath the Shake Table. Regrettably, this meant that the vibration measuring app couldn’t measure the force I applied, to its highest potential. Perhaps next time, I could search for a larger piece of plexiglass to help measure the force better.

Lastly, I was finally able to access what minuscule amount of force my phone was able to measure. There, it showed that the shake table moved at an average of -0.1m/s², 0.1m/s², -0.09m/s² (for the 10cm, 20cm, 30cm structures respectively). This signals that the Shake Table virtually distributed the force equally (with 0 being perfectly distributed). Also, as I pulled my hardest on each of the structures (to try making each of the tests as fair as possible), every single of my 9 tests began with “7.5m/s² & -7.4m/s²”, which I found to be a highly notable mention.

CONCLUSION

In this project, I have explored the topic title “How does the height of a building affect its stability in an earthquake?” Personally, I have found this topic quite interesting, what with creating my very own Shake Table and demonstrating a practical. Although I discovered through trial and error that phones are much heavier than plexiglass and that Lego is in fact very hard to smash apart, I felt that the project was an overall success. This is largely because I was able to prove the prediction I made earlier on in the experiment, using clear statistics (see above graph). In short, I found that contrary to my (to be honest, quite pessimistic) expectations, my control variables were well managed (symbolising that I was able to make it a fair test) and I was able to prove my prediction. 

To conclude, the taller a structure is, the least time it takes to collapse.

BIBLIOGRAPHY

https://www.usgs.gov/programs/earthquake-hazards/how-seismic-waves-affect-different-size-buildings

https://www.scientificamerican.com/article/bring-science-home-earthquake-proof-engineering/#

https://www.nytimes.com/interactive/2019/06/03/us/earthquake-preparedness-usa-japan.html

https://blogs.scientificamerican.com/history-of-geology/january-17-1995-the-kobe-earthquake-and-early-antiseismic-architecture/

https://www.bbc.com/future/article/20190114-how-japans-skyscrapers-are-built-to-survive-earthquakes

https://www.bigrentz.com/blog/earthquake-proof-buildings

https://www.sciencebuddies.org/science-fair-projects/project-ideas/CE_p013/civil-engineering/building-the-tallest-tower