Period of Vibration
Ground Accelerates to Left
Ground Accelerates to Right
Ground & Building at Rest
- Talked about how height and weight affect building. Now discuss type of motion that the house sees.
- Consider cycling back and forth – vibrates back, forth
- Especially important when talking about shear walls (which will be discussed later in this course), and why you need hold-downs on both sides of the walls.
- Earthquakes send out shock waves that travel in all directions up to the surface of the earth.
- These waves impart horizontal and vertical forces on buildings.
- The vertical forces ordinarily cause the building to move up and down safely with the ground.
- However, horizontal force waves cause buildings to move laterally and vibrate back and forth during an earthquake.
- Unless the building is properly constructed, these horizontal forces will cause damage.
- Period of Vibration
- The time an object takes to vibrate back and forth one complete cycle is known as its period of vibration.
- Two important points:
- Buildings do vibrate
- Go through motion multiple times
- The figure shows a complete period of vibration.
- The period of vibration is one of the most important factors determining how a structure will respond to ground shaking.
- Important to note: The ground is moving, not the building.
- For one- or two-story wood frame buildings, the period of vibration is about ½ a second.
- For an earthquake 15 seconds long, the building will go through this shaking approximately 30 times.