Science Buddies: "Ask an Expert"

My project is titled the Effects of Seismic activity on diffent types of skyscrapers. I have my shake table started and need to start working on my skyscrapers. I have searched the web and can't seem to find a way to scale down a skyscraper so I can build out of balsa wood. Do you have any suggestions as to where I can look? I really want this project to be successful. I appreciate any help or guidance.

The art of modeling something in order to evaluate how it behaves is NOT a simple task.

I doubt you can model a steel girder and concrete floor pan type of skyscraper with balsa and predict the failure modes accurately.

Have you done any research into the elasticity of materials, resonance, joint design, soil properties, foundations? If not, you need to do some additional research before you can get started.

Rather than trying to model something large with lots of failure points inaccurately, you might want to model a smaller portion more accurately.

Thank you for your response but I appologize for not being clearer in my question. We are looking to build skyscraper frames out of balsa wood. If we were to make a 10 story building how would we determine how to build it to scale. example: cm or meters to stories or feet in height. We are looking to observe the "sway" of the differnet designs when subjected to the same seismic activity.

We are looking to observe the "sway" of the differnet designs when subjected to the same seismic activity.

Sway in a structure is NOT just a response to seismic activity. Wind can also cause sway.

You need to do some research on oscillations and how they relate to mass and spring forces if you want to attempt to model a skyscraper well enough to predict structural behavior. Getting the dimensions to scale is a lot easier than getting the mass and beam deflection spring forces scaled. There is also the problem of modeling the joints and any "fasteners". If the structure you are modeling is bolted together vs welded it will be harder to model. The behavior at the joints is usually critical to getting the model to accurately predict behavior. Coming up with appropriate sized fasterners in a model that accurately reflect a scaled version is difficult.

Engineers and architects gave up on physically modeling buildings for seismic properties and went to computer models back in the early 1970's. In order to get accurate data for their computer models, they would run experiments on a single joint or single component and then use the computer system to build a composit model. For wind induced sways, they still use low speed wind tunnels and physical models today because the fluid dynamics and interactions can't be decomposed easily and modeled based on a composit of individual pieces.

Much of the rigidity of a skyscraper comes from the skin and the flooring systems and how joints are made and where cross bracing is placed. For example, if you take a 3cu ft cardboard moving box, how you apply the tape to the joints has a significant effect on how well the box retains its shape. Further, what you pack into it and how you wrap and add crumpled paper affects how the weight is distributed and how easily the box can be deformed. The same thing applies to your building model.

The behavior of an empty skyscraper compared to an occupied skyscraper can be significantly different.

erikws,

I think that this is a really interesting project idea! I think that a good experiment to look at without having to worry about trying to represent the building materials it to look at how the building dimensions (maybe height?) can be affected by seismic activity. In terms of building an accurate model of a skyscraper, I didn't find much about building with balsa wood, but here is an interesting website: http://skyscrapermodels.us/models/model_main.html. It has 21 models of skyscrapers from around the world built out of paper. The best thing is that you can download and print them all for free and then fold them into a 3D model. If you use a heavy paper and maybe figure out a way to weigh it down (maybe constructing a bottom and filling each building with sand?), it could be a pretty nice model system.

I also found this pretty simple experiment with skyscraper models and seismic activity, but maybe it would be helpful to look at: http://www.fire.lacounty.gov/ProgramsEv ... Shaker.pdf

Good luck with your experiments!
tzforbes