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STRUCTURAL RESPONSE TO VIBRATIONS The human response studies discussed above involved people in a test situation rather than in the'v own homes. Absent from the above equation was people's feazs of damage to then homes, house rattle, and the startle of the sudden, unexpected vibration. Undoubtedly, the addition of these effects lowers the thresholds at which people react. The objective of this section will be to begin a discussion on how structures respond to vibrations so as to lay a foundation for the establishment of a rational vibration criteria. When ground vibrations emanating from a blast encounter a structure, that structure will begin to vibrate as well. The chazacteristics of the structure and its interaction with the ground vibrations r determine how the structure will respond to these vibrations. The amplitude and duration of the structure's vibration, and the frequency of the vibration, are all dependent upon both ground vibration and the structure's properties. The most important properties of a structure in determining how it will vibrate are its size, stiffness, and damping. Damping is a measure of a building's tendency to return to rest once set into motion. The size and stiffness of the structure determine the fundamental frequency at which it will vibrate freely. Residential structures are complex structures. The many components of a residential structure mean that at almost any vibration frequency, some element of a structure will respond. This is why the vibrations produced by walking in a room may cause the dishes in a cupboard to rattle but not the pictures on the opposite wall. These vibrations aze generally not considered damaging to the structure by the occupants though, because only a small. component of the whole structure is vibrating and the structure was designed to safely withstand the vibrations produced by everyday human activity. Similarly, structures are designed to safely withstand events that cause the whole structure to vibrate such as wind force and elastic ground vibration. When an entire structure vibrates as a unit, it shakes at some fundamental oscillation frequency. When the structure is excited by a blast induced ground vibration, it must move at the same _ frequency as the ground. If the frequency of the ground vibrations match the fundamental frequency of the structure, the structure may, depending upon its damping, magnify these vibrations. Engineers may determine how much the whole structure will vibrate from the ground _.. vibrations by calculating the response of the house as a damped single-degree-of-freedom system. A single-degree-of-freedom system means that the house will vibrate in the direction of the ground vibration, such as vertically from vertical ground vibrations or laterally from lateral ground vibrations. A damped system will return to rest on its own at some time after it is excited; it will not continue to vibrate forever once the ground vibration stops. Figure 2, on the following page, `'s shows the solution to an under damped single-degree-of-freedom response for a typical two story ~° structure vibrated from its base. This represents a common residential structure type in Parkdale, Freemont County, Colorado near the proposed quarry. The relative vibration amplitude is determined by the structure's damping ratio. The fundamental frequency of this structure was assumed to be 6.9 Hz, typical for a two story residential structure as per the research of Dr. ;• to G