Blast Vibration Basics Post #5
Effect of Nonblasting Sources on Residential Structures
This is the fifth post in a series on blast vibration basics. Blast vibrations are an important concept to understand because all blasts produce vibrations and, in some cases, those vibrations will annoy neighbors or potentially damage nearby structures. However, in today’s environment, blast vibration damage is extremely rare due to the regulatory limits placed on blast vibrations from blasting and other construction activities.
This post focuses on natural sources and their effect on structures. The post discusses construction, natural events, and human activity. This information is an excerpt from a study I coauthored for the Florida State Fire Marshal on whether or not the US Bureau of Mines (USBM) RI 8507 vibration limit recommendations apply to Florida construction materials mining activities.
Aerial close-proximity blast site (image from: https://www.tri-cityherald.com/news/local/article240658796.html).
Cracks in structures can occur because of various influencing nonblasting factors over time. The following list from USBM RI 8896 [Stagg et al., 1984] quotes potential causes of cracking in structures:
1. Differential thermal expansion
2. Structural overloading
3. Chemical changes to mortar, bricks, plaster, and stucco
4. Shrinkage and swelling of wood and wood-paper products
5. Fatigue and aging of wall coverings
6. Differential foundation settlement.
Prestraining the materials can also influence the impact of the transient vibrations and affect material failure levels. Prestraining from static stresses includes any weight on the material, including unplanned sources such as the impact of root systems, consolidation of soil, and differential foundation settlement [Stagg et al., 1984]. Additionally, any differential decay of materials caused by the environment could cause prestraining.
The following describe nonblasting sources of structural damage. Nonblasting sources are grouped into the following categories: construction, natural events, and human activity.
Construction
Many crack-inducing factors to houses are related to design and construction. Oriard [1999] provides a suitable summary of construction factors that can cause cracking. Cracking can occur because of normal curing and aging of building materials, such as lumber warping and shrinkage during curing [Oriard, 1999]. Shrinkage caused by curing and aging can occur to lumber, concrete, plaster, stucco, blocks, bricks, mortar, and other materials [Oriard, 1999; Stagg et al., 1984]. High temperatures and humidity can accelerate the deterioration, shrinkage, and swelling of materials, which increase the chance of cracking. Site preparation, inadequate building foundation design, or building on fill can also cause cracking.
Apart from construction practices, certain locations within a residential structure can have concentrated stresses because of the physics of a static structure. For example, stress can concentrate in corners, such as window and door openings. Corners are likely areas to find cracks, regardless of the cause [Crum, 1997].
Natural Events
All homes are cracked from natural events [Siskind et al., 1980]. Cracks expand on an hourly basis from natural causes, including temperature and humidity changes, wind, and groundwater and soil settlement. For example, weather (e.g., temperature, humidity, and wind) can cause internal strains equivalent to 0.5 in/sec [Siskind et al., 1993]. In general, weather or environmentally induced displacements can be over 10 times the displacements caused by large blast-induced displacements [Dowding and Lucole, 1988]. Natural events can also affect homes by the normal curing and aging of building materials, effects of site preparation, building foundations, external water, and quality of materials and workmanship as discussed previously in Section 2.2.1 [Oriard, 1999].
Oriard [1999] dedicated half of his textbook to the effects of environmental forces on structures. Typical environmental forces that affect structures include changes in moisture content, temperature changes and thermal shock, daily weather cycles, and extended periods of dry or wet weather. All of these environmental factors can also affect moisture content in soils. Siskind et al. [1980] state that “Soil moisture changes are notorious for causing foundation cracks.” Additionally, certain clay soils can shrink or swell as a function of changes to water content [Holtz and Kovacs, 2010].
Comparison of Strain Levels by Daily Environmental Changes, Household Activities, and Blasting [Stagg et al., 1984]
This table shows the maximum strains caused by natural events [Stagg et al., 1984] and compares those strains to the corresponding blast vibration level. Natural changes include cyclic changes in temperature, humidity, and wind. These data were taken during limited blasting activity on the USBM test structure during the RI 8896 study. The corresponding blast vibration levels were taken from a strain-versus-ground particle velocity envelope that was created by using empirical data. The corresponding ground vibration levels are the worst-case predictions based on the strain-producing ground vibrations. The table shows that daily environmental changes can have the same effect as blast vibrations from 1.2 to 3.0 in/sec.
Comparison of Crack Responses Caused by Weather Effects, Human Activities, and Blast Vibrations [Kosnik, 2009].
Another study, which took place in Florida, reviewed the response of cracks in a southern Florida residence to natural events, weather, and blasting [Kosnik, 2009]. Kosnik [2009] collected data during 2007 and showed that the maximum frontal effect, which is “the absolute value of the difference between peak 24 hour central moving average values and the 30 day central moving averages,” and maximum daily effect of weather have significantly higher crack response than human activities and blast-induced ground motion. Koznik’s data are illustrated in this graph.
In Kosnik’s study, temperature changes caused crack responses of over 10,000 µin. The August 21 event (0.087 in/sec PPV, 456 µin crack response) was the largest blast vibration-induced crack response (nondamage) recorded during the study, while the September 7 event (0.037 in/sec PPV, 456 µin max crack response) represents a more typical blast vibration event (nondamage). This study supports Dowding and Lucole [1998] in that displacement caused by seasonal changes of temperature and humidity are at least 10 times the displacement caused by blast vibrations in the range of blast vibration amplitudes recorded by Kosnik [2009]. The maximum daily effect of the crack response shown in Kosnik’s figure is over 20 times the crack response caused by the highest recorded blast-induced crack response.
All houses, regardless of location, are subject to daily environmental fluctuations, such as temperature and humidity. These variables can vary even more significantly during heavy wind, rain, and heat events. These events only accelerate the aging of the residential structures.
Human Activity
Everyday human activity causes a significant amount of vibration to residential structures [Siskind et al., 1980]. The USBM published tables in RI 8507 that compared mining blast vibration strains to strains caused by human activity in various stress concentration locations of residential structures. The USBM also compared blast vibration PPVs that were caused by mining to those caused by human activity in various stress concentration locations of residential structures. The next two tables are examples of these comparisons from RI 8507.
Strains in Test Residential Structure From Blasting and Human Activity [Siskind et al., 1980]
As shown the first of these two tables, induced strains from blast vibrations on the structures ranged from 11 to 43 µin/in. The blast-induced ground vibration PPVs ranged from 0.21 to 0.47 in/sec. Comparatively, human-induced responses caused strains that ranged from 3.2 µin/in and lower to 140 µin/in. Door slamming, jumping, and nail pounding produced the highest strains, which, in these cases, were sometimes significantly higher than the strains induced by the mine blasting.
Structure Vibrations in Test Residential Structure From Blasting and Human Activity [Siskind et al., 1980]
The second of these two tables shows that those same blasts caused structural vibrations that ranged from 0.85 to 1.37 in/sec. Human-induced responses caused PPVs that ranged from 0.03 to 10.1 in/sec. Not including midfloors (which are not representative of whole structure response), the maximum structural vibrations were 3.81 in/sec (caused by nail hammering). Jumping, door slamming, and nail hammering produced the highest maximum structure vibrations; however, low-impact events, such as walking, still caused significant structural vibrations.
Comparison of Strain Levels by Daily Environmental Changes, Household Activities, and Blasting [Stagg et al., 1984]
The USBM confirmed that household activities can induce strains in residential structures of up to nearly 1 in/sec-equivalent during the RI 8896 study, which showed that household activities can range from 0.03 to 0.88 in/sec, as seen in this last table [Stagg et al., 1984]. Other reports also suggest that everyday human activities such as door closing and walking can cause internal strains equivalent to 0.5 in/sec (RI 9455 and RI 8896). For example, a front door slam could produce significant strains over an entire residence (RI 8507).
Summary
The USBM and other authors have shown that many factors affect residential structures, including construction, natural events, and human activity. In most cases, nonblasting events cause significantly more strain on residential structures than typical blast events.
Construction practices can affect residential structures by causing prestraining, which affects the integrity of the structure. Aging can cause shrinkage in many construction materials that are used today, including lumber, concrete, plaster, stucco, blocks, bricks, mortar, and other materials. Additionally, building a residential structure to code does not prevent cracking because all structures crack over time from the curing of materials and settlement.
Natural events can cause significant strains in residential structures through daily and monthly fluctuations in factors such as temperature, humidity, and soil moisture. Multiple studies have shown that natural events can have over 10 times the impact on residential structures as typical blast vibrations. Daily environmental changes can affect cracks equal to or more than blast vibrations of 1.2–3.0 in/sec. Human activity is also a significant cause of transient strains to residential structures. Household activities can cause equivalent PPVs in the structure of up to 3.81 in/sec.
The studies discussed in this section show that any residential structure will undergo significant strains on a daily basis. Daily strains, which can be caused by human activity and natural events, can cause equivalent strains to blast vibrations over 3.0 in/sec.
