Vibration and Damage
Vibration doesn't always cause damage to homes, even if the vibration is felt by the residents. There are many, often interacting, factors which can affect the likelihood of vibration damage. Here I will discuss, in a non-technical manner, what some of these are and how they can affect damage probabilities. As I will show, estimating the probability of damage (or the probability that damage was caused) to a given structure is considerably more complicated than simply comparing the peak particle velocity (PPV) of a ground vibration with an arbitrarily-chosen vibration standard and, thereby, declaring the vibration "safe" or "allowable". Unfortunately, such scientifically-unfounded and questionable uses of vibration data and standards are all too common.
Human Perception and Vibration Damage
Because people are more sensitive in perceiving vibration than houses are to damage from vibration1,2, 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 vibration levels 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 It is 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 things being equal.9
Vibration damage probability, as with many other quantities in science, roughly follows an S-shaped "sigmoid curve", of the sort shown at right, as a function of vibration intensity.7 Over a range of low vibration intensities, no houses are damaged. At these low intensities, people may be able to feel the vibration, even though no visible damage is done. At the highest intensities, 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 center of the sigmoid curve, which represents 50% probability of damage, can occur at different vibration velocity (intensity) values, depending on the type of damage examined and its source. For example, information from USBM RI 8507 suggests that, for blasting-caused vibrations, 50% of homes will experience "threshold damage" (see below for a discussion of the definitions of damage descriptions) at a velocity of about 2 in/sec (or "intensity", as above). For "minor" damage, that 50% point is at about 3 in/sec, while for "major" damage, it is at about 4 in/sec.15 Blasting vibrations are both better studied and better suited to meaningful statistical analysis than construction vibrations.16 There are no comparable studies of construction vibration which provide the 50% damage probability velocity for any type of construction vibration damage, although it is widely acknowledged to be considerably lower than that for blasting. Thus, construction vibration standards set much lower "acceptable" vibration velocities, typically, about 20% of those set for blasting.
Some construction operations like demolition, pile driving and vibratory compaction have special vibration damage concerns attached to them, as I have indicated in the CVDG chapter, Vibration Potential. However, even these may not cause damage, if properly done far enough away from homes and guided by careful vibration monitoring. The complicated relationship between distance and damage potential is discussed in more detail in the CVDG chapter, Vibration and Distance. 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.
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 (CVDG Pro). Since there are no vibration monitoring data recorded in most construction projects, it is often difficult to know if the vibrations warrant further investigation. To that end, Judging Vibrations has some rough 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, to be governed only by the peak particle velocity ("intensity", "size") 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 its potential for causing damage.
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 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), for a compactor with a nominal vibration frequency of 66.7 Hz, and a duration considerably longer than the five seconds of the waveform trace. The frequency makeup and the duration can interact in several senses to increase still more the potential for causing damage, as described on the CVDG page, Resonance/Fatigue.
Building Construction Considerations
Structures respond differently to vibrations. The response of different structural types and materials to vibration has been fairly well-studied, especially for blasting vibration.3 Building materials used, the building design, its age, the nature of the soil on which the building rests and level of maintenance of the structure are some of the important structural determinants of vibration damage resistance.
Engineered, steel-reinforced buildings are more resistant 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. Once significantly damaged by vibration, buildings often become more susceptible to further damage from subsequent vibration. 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.
Materials of building construction show widely different resistance to vibration damage. Monolithic concrete, mortar and concrete blocks 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 pavement with a large excavator). Plaster interior wall finishes are less resistant to vibration-caused cracking than drywall.1 For more discussion of material susceptibility to vibration damage and what damage might tell us about the vibrations responsible for it, see Recognizing Damage. Each instance of damage must be carefully and scientifically evaluated, taking into account all the data, to draw conclusions about the cause of the damage.
Vibration Effects on Historic Structures
Historic structures, whether on the U.S. National Register of Historic Places or not, are usually even more prone to vibration damage than typical homes. The greater concerns over historic structures arise from the design, structure age, building materials and building methods used. Maintenance can be an issue in some cases, as well.
Many historic 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 slab-on-grade 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. 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" are an appropriate limit 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
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) 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. "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. It is often true that cosmetic damage appears over underlying structural damage.
Scientific Vibration Damage Classifications
The scientific literature of vibration damage rarely uses these two 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. 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.
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 hairline 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). 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 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
"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 terms 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 criteria are silent on non-preexisting damage to concrete, concrete blocks and mortar, all of which require vibration velocities well in excess of any U.S. ground vibration standard to cause.11 The OSMRE Blasting Guidance Manual14 indicates that such damage to concrete is usually accompanied by other, obvious damage.
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 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 $5000 be characterized as "minimal damage", damage with repair costs between $5000 and $20,000 be termed as "substantial damage", and any damage whose repair cost is above $20,000 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, with inflation in building costs, but the necessary adjustments can be done easily from economic data.
Ultimately, the ideal outcome for both homeowners and contractors is to achieve an effective degree of vibration "safety". Our CVDG Pro section, Vibration Safety, discusses methods of assessing and accomplishing a reasonable degree of vibration safety. Even if complete safety from vibration damage in construction can't be attained, the large number and worldwide distribution of construction vibration damage examples suggests that we can do considerably better than we are currently.
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