Structure Response

Residential structure response to blast vibration has been researched extensively (Example:  USBM RI 8507 and RI 8896). And frequency is a very important component of ground vibration because it affects how structures respond. The general types of response within a structure caused by external vibrations are:

  • Foundation Structure Response or the structure vibration below the ground level is equal to the incoming ground vibrations.
  • Whole-Structure Response or the racking motions, of the above ground part of the building, that respond to frequencies of 4 Hertz to 12 Hertz.
  • Mid-Wall Response or motions within individual panels or components of the above ground part of the  building, normally out of plane with walls, responsive to frequencies of 12 Hertz to 20 Hertz.

In the whole structure response, the above ground portion of the structure is free standing, and moves more than the below ground portion because the foundation is fixed. Differential motions between the upper and lower corners cause racking responses. Mid-wall responses are typically responsible for window rattling, picture tilting, etc.

Structure-responseWhen the vibration frequency closely matches a natural or fundamental frequency of a structure or structural component, the structure or component will tend to respond more vigorously and the incoming ground vibrations are amplified in the upper portion of the structure. Alternatively, if the ground vibration frequency does not match the natural frequency, very little seismic energy transfers into the structure, and there will be little, if any, response.