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`~ RESEARCH ON REPEATED VIBRATIONS <br />Homeowner reaction to the reseazch used to develop the Appendix B curve in RI-8507 is <br />typically met with skepticism. Residents often counter with the fact that their homes are <br />repeatedly being subjected to vibration loads and that there must be a cumulative effect on the <br />structure. In 1984, the USBM published RI-8896 entitled, "Effects of Repeated Blasting on a <br />Wood Frame House". This study was the fast to document long term strain response of a house. <br />Long term strain measurements allowed blast-induced strains to be compared with those <br />produced by changes in environmental factors such as temperature, humidity, and human activity. <br />During this study the Bureau arranged to have awood-frame test house built in the path of an <br />advancing surface coal mine so that the effects of repeated blasting on a residential house could be <br />~ studied. In a two year test period 587 production blasts were fired with peak particle velocities <br />ranging from 0.10 in./sec to 6.94 in./sec. Later the ent've house was shaken mechanically to <br />produce fatigue cracking. The first crack appeared after 56,000 cycles. This is the equivalent of <br />28 years of shaking by blast-generated ground motions of 0.50 in./sec twice a day. The following <br />significant facts were concluded from this study. <br />"' • Numerous hairline cracks (0.01 to O.I mm wide) appeared in the test house during <br />construction and also during periods when no blasts were detonated. <br />~~ Gypsum wallboard fails at a strain of approximately 1000 µmlin, and visible cracks appeaz at <br />strains slightly above this point. <br />• Concrete block shows visible localized cracks at mortaryoints with strains of approximately <br />3000 µi~n. <br />• Temperature and humidity induced strains across wallboard taped joints were as high as 149 <br />_ and 385 µin/in. respectively. <br />• Door slamming produced strains of up to 140 µin/m. in wallboard. <br />• The smallest ground vibrations that would produce the equivalent of environmental and door <br />k , slamming strains in walls are 1.2 in./sec and 0.50 in./sec respectively. <br />k. <br />Threshold-type cracks appeared with and without blasting. Therefore, changes in the rate of <br />threshold crack occurrences are better indicators of the effects of blasting on cracking than <br />observations of individual cracks. The rate of threshold cracking when ground motions were <br />below 0.50 in./sec was not significantly different than when motions were between 0.50 and 1.0 <br />`` in./sec. However, when ground motions exceeded I.0 in./sec, the rate of crack formation was <br />more than three times the rate observed when motions were less than I.0 in./sec. <br />15 <br /> <br />