Not So Good Vibrations (Part II)
In our previous blog post, Not So Good Vibrations (Part I), we discussed waves related to blasting type applications as well as why we perceive vibrations from blasting inside our home and not outside. In this blog post we will discuss the question, can vibration responses from nearby blasting activity cause damage to my residence?
Before we discuss potential damage, let us first discuss the concept of natural frequency and how it is related to the structural response of a residential structure. Natural frequency is the frequency at which the residence, in this case, will tend to vibrate when subjected to certain external forces. The natural frequency of your residence is dependent on the way it is built, the building materials used and the height as well as other various factors. Research completed by the U.S. Bureau of Mines (USBM) determined that the natural frequency of common residential structures can vary between 4 to 12 hertz. The natural frequency of your residence can be different depending on the orientation or direction, front to back or right to left. The USBM also determined that the natural frequency at the middle of the walls (midwalls) within residential structures were in an elevated range between 11 to 24 hertz.
As previously discussed, waves from blasting type activities can travel through the air in the form of airblast waves and travel through the ground in the form of ground vibrations. When these waves reach your residence, they have a variable amplitude and frequency that will take place over a period of time. The amplitude of the waves will cause the structure to respond in racking and/or midwall bending. The amount of response depends on the specific amplitude. The time duration determines how long the structure will be subject to amplitudes of the incoming waves which influence the amount and duration of the structural response. However, if the waves contain a frequency close to the natural frequency of the residence or the natural frequency of the midwalls, the structure could experience an amplified response where the vibration (amplitude) is two to five times higher than the amplitude of the incoming waves. This phenomenon is known as resonance, a dynamic force that drives a structure to vibrate at its natural frequency that can produce a higher amplitude vibration response. Frequencies of the incoming waves that are lower than the natural frequency can produce an equivalent amount of vibration of the structure, however, frequencies higher than the natural frequency can produce a smaller amount of vibration than the incoming waves. Therefore, the frequency property of the incoming waves is an important property as related to the damage potential of a blast.
Can the structural response of my residence from waves related to blasting activities cause damage to my residence?
Well, it depends. Every case is different with many variables to consider; an evaluation is needed on a case-by-case basis. Research completed by the USBM determined that damage would most likely be observed first within the interior finishes (drywall) of the residence in the form of cracks. Due to racking responses, cracks have the potential to develop where excessive strain is produced within the planes of the walls or between the walls at corners. However, the type of cracking within the interior finishes that has the potential to develop from blasting is consistent with cracks produced from changes in temperature or common human activity within a residence, such as slamming doors. Any cracks within interior finishes would need to be carefully analyzed before concluding that they were caused by blasting. The USBM determined that midwall bending responses have less cracking potential or less potential to damage interior finishes than racking responses, even though midwall bending responses are more often perceived. The USBM research also references data that shows higher levels of vibration are needed to result in damage to foundation components such as concrete or masonry than interior finishes, but again, an evaluation is needed on a case-by-case basis.
The party conducting the blasting should be aware of the variables that influence the amount of wave energy released from the blast and configure the blast to decrease the impact on any surrounding structures. An example is using a sequence of millisecond delays between detonations to limit all the explosive energy from being released at one time. With so many variables, the same blast may generate waves that do not result in a human perceived response at one residence but may cause fear of damage or annoyance in another residence at a different location. Therefore, as previously stated it is important for any concerns to be evaluated on a case-by-case basis as every situation is different.
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About the Author
Matthew Shuler, P.E. is a Consulting Engineer in our Charlotte, NC Office. Mr. Shuler provides consultation in the areas of assessment of residential, commercial, and industrial structures including root cause investigation, scope of damage, building code compliance, construction evaluation, structural system assessment, and evaluation of structural components. You may contact him for your forensic engineering needs at firstname.lastname@example.org or (704) 523-2520.