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Vibration and Damage
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Vibration doesn't always cause damage to homes or buildings, even if the vibration is felt by the residents. There are many, often interacting, factors which can affect the likelihood (probability) of vibration damage from construction or other sources. Estimating the probability of damage (or the probability that damage was caused) to a given house or other structure is considerably more complicated than simply comparing the peak particle velocity (PPV) of a ground vibration with an, often, poorly-chosen vibration standard and, thereby, declaring the vibration "safe" or "allowable". Unfortunately, such scientifically-unfounded and questionable uses of vibration data and standards are much too common. Here, I will discuss, in a reasonably non-technical manner, what some of the factors contributing to damage are and how they can affect damage probabilities. Human Perception and Vibration Damage Because people are more sensitive in perceiving vibration than structures are to damage from vibration[1],[2], the mere presence of construction-caused vibration is neither proof of caused damage nor suggestive, by itself, of forthcoming damage. Humans begin to perceive vibration at around 0.005 in/sec peak particle velocity,[12] while one of the lowest proposed allowable ground vibration velocities for structures is at 0.05 in/sec - a factor of ten higher. The longer a vibration of a given peak velocity lasts, the more disturbing people will find it.[8] Other Damage Causes Most houses more than ten years old or so will have a few pre-existing (i.e. before construction start) hairline cosmetic cracks in drywall, stucco, etc. A few examples of cosmetic cracking can arise from slight settling, ground movement, temperature and humidity cycling, and even, in extreme cases (hurricanes, tornadoes), wind loading.[18] Hairline drywall cracks are often very hard to see, especially if you don't make a specific effort to find them. Thus, merely discovering a small, hairline drywall crack is not, by itself, a sign of a construction vibration cause, even if construction is occurring and producing felt vibration. However, if you suddenly find cracks in almost every room in the home, especially diagonal cracks at corners of windows and doors, with construction ongoing or recently completed, you should look into construction vibration as a possible contributor or cause. Factors Affecting Damage Probability In the broadest sense, ground vibration velocity (PPV), duration, number of repetitions and repetition rate of the vibration all relate directly to the likelihood (probability) of damage caused by vibration. Frequency distribution of the vibration also affects the probability, but inversely (i.e. the lower the frequency, the higher the probability of damage, all other factors being equal). Each type of vibration and vibration source has its own mix of these factors present at the same time. The interactions among these variables are not fully understood, although there is at least some knowledge of the effects of each of the variables separately. The CVDG Pro's Vibration and Homes chapter explores some of what is known about these interactions. To give a sense of one aspect of damage contributor interaction, let's look at an example. A vibration whose frequency overlaps the resonance frequency of a home has an enhanced capability for damage over vibrations whose frequencies do not have such direct overlaps. However, a resonant vibration whose duration is longer than the persistence time of the vibrations created in the home by the vibration, like those created by construction, has a greatly enhanced chance of creating damage over one which is short-lived (e.g. a blasting-related vibration). In this circumstance, new vibrations reach the house before the old ones have died away, increasing the degree of resonant amplification of the ground vibration in the house structure. This example is relevant in a comparison of construction with blasting vibrations, because construction vibrations last far longer than the structure vibrations, while blasting vibrations persist for considerably less time than the house vibrations created by them. Vibration damage probability, as with many other quantities in science, fundamentally follows an S-shaped "sigmoid curve", of the idealized sort shown at right, as vibration velocity (a measure of the vibration "intensity") increases.[7] Over a range of very low vibration velocities, no houses are damaged. At these low velocities, people may be able to feel the vibration, even though there is no visible effect on their homes. At the highest vibration velocities, virtually all homes experiencing the vibration are visibly damaged. Essentially all the people feeling such a high intensity vibration will be made distinctly uncomfortable by it. That point at the vertical center of the sigmoid curve, which represents 50% probability of damage in this setting, can occur at different vibration velocity values, depending on the type and severity of damage examined and the vibration source. For example, information from USBM RI 8507 indicates that, for a single, low or high frequency, blasting-caused, vibration lasting less than "a few seconds", 50% of homes will experience "threshold" damage (see below for a discussion of the definitions of damage descriptions) at a peak particle velocity (PPV) of about 2 in/sec. For "minor" damage, that 50% point is at about 5 in/sec, while for "major" damage, it is at about 6 in/sec.[15] At the 5% probability level ("two standard deviation confidence level", see Vibration Standards and Statistics and Vibration Damage in the CVDG Pro for more explanation of the role of statistical analysis in setting and applying vibration limits), the PPV for threshold damage from blasting vibrations is in the 0.5 - 0.7 in/sec range, based on the same data. The OSM and USBM RI 8507 limit of 0.75 in/sec (0.5 in/sec in USBM RI 8507 for homes with plastered walls) for mid-frequencies is, thus, set at a level which has approximately a 5% probability of bringing about damage from a single, blasting-caused, direct ground vibration lasting less than a "few seconds".
Construction Damage Probabilities Blasting vibrations are both better studied and better suited to meaningful statistical analysis than construction vibrations.[16] Vibration sources, velocities, frequency distributions, operations and ways in which they are carried out are far more variable in construction vibration than in blasting vibration. For example, the vibration frequency distributions, PPV's and durations of vibratory compactor operations are utterly different from those from blasting or pile driving (see the CVDG Pro chapter, Vibration Signatures, for some real life examples). There are no studies of construction vibration comparable to those of blasting which provide either the 5% or 50% damage probability velocity for any specific type or source of construction vibration damage. It is widely understood in the scientific literature that the velocities for a specific probability of damage (or some other similar measure related to damage likelihood) are likely considerably lower than those for blasting (i.e. the damage probability is higher for a given ground vibration PPV associated with construction work than for the same PPV velocity associated with blasting).[20], [22] Thus, construction vibration standards set much lower "acceptable" vibration peak particle velocities (PPV's) for homes - typically, about one quarter of those set for blasting.[21], [24] Construction Operation Vibrations Some construction operations like blasting, demolition, pile driving and compaction have special vibration damage concerns
attached to them, as I have indicated in the CVDG chapter,
Vibration Potential. However, even these need not cause damage, if properly carried out far enough away from homes and guided by carefully-done vibration monitoring. The complicated relationship between distance and damage potential is discussed in much more detail in the CVDG chapter, Vibration and Distance. It is also highly desirable to have seismograph or accelerometer (e.g. from a smart phone or tablet computer) data showing the characteristics of the vibration itself in supporting an argument for damage causation (see DIY vibration monitoring chapters in the CVDG Pro for more). Since vibration monitoring data are either not recorded or not reported in most construction projects, it is often difficult to know if the vibrations warrant further investigation. To that end, Judging Vibrations (CVDG PDF and Pro) has some rough, non-numeric guidelines for separating those vibrations which are usually non-damaging from those whose damage potential warrants more inquiry. Damage potential of a given vibration is often assumed, even by those who do vibration monitoring and consulting, to be governed only by the peak particle velocity ("PPV") of the vibration. However, the detailed frequency component makeup of the vibration, its duration and the number of times it is repeated all contribute to damage probability. In the graphic at right are shown a video frame grab of a vibratory compactor passage during a paving operation, its corresponding transverse axis vibration waveform and the corresponding FFT-derived frequency spectrum at bottom. The measured vibration has a peak particle velocity in violation of the FTA vibration standard for frame homes, with the peak velocity frequency below 40 Hz (see Vibration Frequencies for more on the significance of vibration frequencies below 40 Hz), even though this compactor has a nominal vibration frequency of 66.7 Hz. The duration of the vibration is considerably longer than the five seconds recorded in the waveform trace in the middle of the graphic. The frequency makeup and the duration can interact in several senses to increase still more the potential for causing damage, as described in the CVDG chapter, Resonance/Fatigue. Structures respond differently to vibrations, both in the specifics of how they vibrate and their susceptibility to damage from vibration. The response of different structural types and materials to vibration has been fairly well-studied, especially for blasting vibration.[3] The building materials used, the building design, the building age, the nature of the soil on which the building rests and the level of maintenance of the structure are some of the important structural determinants of vibration damage resistance. Engineered, steel-reinforced buildings are more resilient to vibration damage than engineered, non-reinforced structures.[4] These are both more vibration resistant than the timber frame or masonry homes which most people have in the U.S. and in other places worldwide. Once significantly damaged by vibration, buildings become more susceptible to further damage from subsequent vibration.[26] Thus, a modern home damaged by vibration should be considered more susceptible to future vibration damage until repaired; a correspondingly lower allowable vibration limit must be employed for it.[22]
Vibration Effects on Historic Structures and Cultural Artifacts Historic structures, whether on the U.S. National Register of Historic Places or not, are usually even more prone to vibration damage than typical wood-frame homes. The greater concerns over historic structures arise from the design, structure age, building materials and building methods used. Many historic or old homes in the Eastern part of the U.S. are built on rubble foundations, which are substantially more subject to damage from vibration than modern, concrete slab-on-grade or foundation wall construction. In the U.S. Southwest, for example, there are many structures of historical interest which are over 1000 years old (e.g. Casa Rinconada, shown at right, in Chaco Culture National Historical Park in New Mexico),[10] built by pre-European native peoples of stone masonry or adobe.[25] Such irreplaceable buildings require special preservation techniques and vibration standards set at the lower end of allowable vibration intensities.[2],[10] It has been proposed that the "Swiss standards" provide appropriate limits for protecting artifacts from vibration damage in museums housing fine art, although those authors indicate a necessity for a case-by-case evaluation, with attention to resonance effects.[12],[13] Protection of historic structures in the U.S. is governed by the National Historic Preservation Act (NHPA) of 1966, as amended in 1992, as well as state and municipal laws and ordinances. Section 106 of NHPA provides a mechanism for review of proposed construction and other impacts on historic structures, whether registered on the U.S. National Register of Historic Places or not. Such "Section 106 reviews" can be powerful tools in assuring that the lower vibration limits for historic structures are known and observed during construction. For more on Section 106 reviews, see the CVDG chapter, Pre-construction. Non-technical Vibration Damage Classifications In litigation and other non-technical settings, vibration damage is usually classified as "structural" or "cosmetic" (sometimes seen as "architectural"). "Structural damage" is often construed to mean any damage adjudged (usually by an engineer or scientist) to have compromised the stability of the building. Damage to major home systems (e.g. plumbing, heating) should also be considered as structural damage, since they compromise the use of the home and often necessitate vacating the home for months at a time to effect repairs, just as other structural damage often does. "Cosmetic damage" is used to describe any other damage which affects "only" the appearance of the home, without endangering its ability to withstand the forces (e.g. gravity, wind-loading, etc.) normally applied to it or to function in the manner expected by the owner. Cosmetic damage can appear over, and mask, more serious underlying structural damage. Scientific Vibration Damage Classifications The scientific literature of vibration damage rarely uses the "structural" and "cosmetic" classifications. Instead, the terms "threshold", "minor" and "major" are commonly used there to classify damage. Unfortunately, even these terms are not used in the same way in all studies, nor is there any accepted set of rules for applying them. This lack of consistent definition, precision, and coverage of all damage types means that it can be difficult to compare results from different studies due to different applications of these descriptions. The phrase "threshold damage" is usually applied in cases where the "only" apparent damage is limited hairline (i.e. barely visible) cracking in drywall or plaster. However, some studies apply the term "threshold damage" to any occurrence of cracking, no matter how numerous, extensive, widespread or serious (e.g. cracking in the bodies of drywall sheets, cracks with different vertical positions of the two sides of the crack, cracks with large separations between the sides of the cracks). In many of the studies which use this terminology, a crack with visible separation, vertical offset between crack sides and running from floor to ceiling is placed in the same category as a barely visible hairline crack a few inches long. "Minor damage" is often construed to include cases where there is extensive cracking of drywall, but no structural compromise of the home. Some uses of this terminology include examples of cracking of concrete blocks and monolithic (poured in a single piece) concrete, despite the fact that such damage to concrete requires vibration velocities far in excess of any U.S. vibration standard to produce.[5] Since most homes in the U.S. are built on poured concrete slabs, damage to exterior concrete is often accompanied by hidden slab damage. "Major damage" in some studies means major structural damage only. In other studies, it includes damage to monolithic concrete and concrete blocks or even large amounts of cosmetic damage. For example, in one construction damage example, there were over 600 new cracks, extensive through-cracking of concrete blocks and mortar in property walls and monolithic concrete slabs (example photo above), and damage to multiple mechanical systems - all in one home of many damaged similarly. Some studies would classify such damage as minor, while others would classify it as major damage. The blasting vibration damage study, USBM RI 8507, provides what it identifies as a "uniform classification" of damage,[6] using these terms, but its criteria neither agree with those used in some other studies cited by it nor are they sufficiently detailed to allow a clear and indisputable classification in all examples. For example, the USBM RI 8507 "uniform classification" criteria are silent on non-preexisting damage to concrete, concrete blocks and mortar.[11] The OSMRE Blasting Guidance Manual[14] indicates that such damage to concrete is one result of extreme vibration velocities, usually accompanied by other, obvious damage. It is important to note that the use of these terms is almost entirely based on visible condition observations, rather than on any measurable quantity which can be related clearly and directly to damage severity or, by implication, vibration velocity. In practice, the severity of vibration-caused cracking is usually also takes into consideration the length, breadth, location (e.g. through a drywall sheet, as opposed to at a sheet join), overall number of cracks seen and distribution of them (e.g. confined to a single location within a home vs. widespread through out all the rooms in the home). Along with the presence and effect of any other types of damage present and reasonably attributable to construction operations, these factors should taken into account in characterizing the degree of damage severity. Damage Amounts The inherent imprecision of all the terms commonly used for damage classification leads to considerable disagreement in their use and interpretation, even in the more precise scientific literature. I suggest that, for non-scientific settings (e.g. litigation), all these terms and their implications should be avoided. The only practical, quantified criterion of damage level in non-technical settings is cost of repair to pre-damage condition. The cost of repair is unlikely to be grossly understated or overstated, since repair contractors must examine the damage carefully and without bias to reach a competitive estimate for repair (see CVDG Pro chapter, Damage Repair, for more on this topic). If the repair requires vacating the house during the repair, the cost of moving and alternate housing should be considered a part of the "cost of repair". Repair cost is the criterion which should govern one's approach to damage, not ill-defined, subjective, non-standardized terminology, the implications of which may not be the ones intended by those performing the underlying scientific studies. I propose that damage with repair costs under $10000 be characterized as "minimal damage", damage with repair costs between $10000 and $40,000 be termed as "substantial damage", and any damage whose repair cost is above $40,000 be categorized as "extensive damage". The intent of this terminology is not to characterize damage appearance and amount from a scientific standpoint or for a scientific use, but to provide a quantitative means of comparing and discussing damage degrees in non-scientific settings. The numbers associated with these characterizations will change over time,[17] with inflation in building costs, but the necessary adjustments can be done easily from economic data. A Better Outcome Ultimately, the ideal outcome for both homeowners and contractors is to achieve an effective degree of vibration "safety", which either eliminates vibration damage or confines it to threshold damage in small areas (i.e. not house-wide). Our CVDG Pro chapter, Vibration Safety, discusses methods of assessing and accomplishing a reasonable degree of vibration safety. Absolute and complete safety from vibration damage in construction may not be attainable in all examples, due to the constraints of the job and/or the locale. However, the large number and worldwide distribution of construction vibration damage examples suggests that we can do a considerably better job of avoiding unnecessary damage than we are currently.
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It is also recognized in the vibration scientific literature, and in the vibration standards derived from that literature, that the longer a vibration lasts, the greater the probability of it causing damage, all other 
The sigmoidal shape of the damage probability vs. vibration velocity curve has another important implication. The 5% damage probability level, where blasting standard limits are set, lies near the point on the sigmoidal curve where the damage probability begins to increase dramatically with slight increases in PPV. When one exceeds the standard PPV limit, the probability of damage rises very rapidly. This can be seen well in the diagram at
right, re-plotted from blasting damage probability data in 
Thus, attributing damage with certainty to a specific vibration cause is often difficult, unless the formation of the damage is observed, documented and can be linked to known and documented actions causing vibration. 
Materials of building construction also show widely different resistance to vibration damage. Monolithic concrete, mortar and concrete blocks in good
condition all require extreme vibration velocities to crack (see example at left of such a crack in a monolithic concrete home slab, caused by repeated pounding on asphalt pavement with a large excavator bucket 180 feet away). Plaster interior wall finishes are less resistant to vibration-caused cracking than drywall.
Maintenance can be an issue in some cases, as well. 
