Out-of-phase movement occurs when adjacent parts of a building, or adjacent buildings, move or vibrate in different directions or at different times during an earthquake, windstorm or other dynamic event. It happens because the parts differ in stiffness, mass or natural frequency, so each responds to the same shaking in its own way.
Parking structures are especially exposed because they are long, heavy and often built in separate sections divided by joints, or placed against an adjoining building. Where two sections can swing toward each other at the same moment, the gap between them must be wide enough to prevent pounding, and everything that crosses the gap must be able to move.
Where it matters in parking
- Seismic and expansion joints. Joint width is set by the combined movement of both sides, not one. See expansion joint.
- Adjacent buildings. A garage attached to a hotel or office tower behaves differently from it; separation or careful connection is required.
- Ramps and bridges. Elements spanning between sections need slip or sliding supports so they are not torn apart or crushed.
- Utilities and finishes. Pipes, conduit, sprinkler lines and barriers that cross a joint need flexible connections.
- Split-level and irregular structures. Changes in stiffness, such as a stiff stair core at one end, can concentrate movement. See seismic loads and shear walls.
The Parkonomics view
A joint is a promise that two structures will not meet when the ground moves. Out-of-phase movement is why that promise has to be sized for both sides at once.
Related terms: Seismic Loads, Expansion Joint, Lateral Loads, Shear Wall, Floor Diaphragm