The USBM recommended limits are published in Report of Investigations No. 8507. The “Z-curve or Siskind curve” is the information most often cited, which was published in 1980 as a result of an extensive study conducted in the late 1970s. The study was completed using numerous blasts, throughout several states, while monitoring at different types of structures. The graph shows recommended limits to reduce the damage potential for structures when comparing ground velocity with its associated frequency. The USBM curve is like a speed limit. If vibration levels do not exceed the curve, damage is not likely. For those cases over this limit it does not automatically mean vibration has caused damage; however, it does indicate a thorough evaluation should be made. The graph shows the limits recommended by the USBM to preclude cosmetic damage to plaster and drywall, the most fragile building materials.
It was determined by the evaluation that each of the structure types (single, 1 ½ and 2-story homes) have a varying natural frequency. Blast vibration matching the frequency of the structure of concern may amplify the shaking within the house. If the particle velocity is high enough, cracks can occur or may be extended. This cosmetic, threshold damage potential is alleviated when the limits are maintained. As the frequency of the ground motion changes from low to high (1-40 Hertz) the structure responds less and the limits increase. A safe level of 0.50 ips for plaster and 0.75 ips for drywall that exists for 4 – 15 Hertz increases as the frequency increases to a maximum velocity of 2.00 ips.