Cracking Open Construction Vibrations
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Cracking Open Construction Vibrations Glenn Stewart, M.E., P.E.
Published online: 29 June 2007 Ó Engineering Design & Testing Corp. 2005
We live in a world where there is construction activity all around us. Schools are being constructed, bridges built, residential areas developed, streets widened and roads repaired. All this activity involves heavy equipment such as earthmoving machinery, large trucks and compactors. Amidst all this activity is loud noise, dust and ground vibrations. Ground vibrations, in particular, pose conditions that rightly are of concern to contractors, construction companies and neighboring property owners. The common concern is that ground vibrations produced by construction activity can harm surrounding property. This concern has led to a body of technical knowledge correlating measured levels of ground vibration to structural damage. The same concern has led to procedures that can be followed at a construction site to reduce the likelihood of damage to surrounding property and to address issues of damage should such arise.
What is Ground Vibration?
The vibration source is the truck going by. It could have been a jackhammer or a pile driver or a blasting operation. Whatever the source, what is created is seismic energy in the form of waves that propagate outward from the source and force the ground surface to move. Specific points in the ground (referred to in the language of ground vibrations as ‘‘particles’’) experience displacement, velocity and acceleration. These parameters are capable of being measured with a much greater level of sensitivity than you can with your feet. Research has shown that, with respect to correlating ground vibrations to structural damage, there are two parameters to be considered: peak particle velocity and frequency of the vibration waves. By way of further description, think in terms of how fast the surface of the ground is moving as it travels up and down. (Peak particle velocity is expressed in inches per second, or ips.) Think also in terms of how many of these up and down cycles occur in a short period of time. (Frequency is expressed in cycles per second, which is the same as saying ‘‘Hertz,’’ or ‘‘HZ’’.)
Stand near a roadway when a heavy truck passes by and hits a bump or two. You may not bounce up and down as a result, but you are likely to feel the ground vibrate at your feet. Your feet, however, are not going to be sensitive enough to quantify the vibrations that are felt.
Reprinted from The Stress Point, with permission from Engineering Design & Testing Corp. G. Stewart, M.E., P.E. (&) Columbia District Office, Engineering Design & Testing Corp., Columbia, SC, USA e-mail: [email protected]
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What is the Correlation? Over the years, the results of a lot of research have allowed correlations to be made between both ground vibration peak
‘‘Galloping Gertie,’’ the 1940 Tacoma Narrows Bridge, just before failure. Photograph courtesy of Ed Elliott, The Camera Shop, Tacoma, WA; www.camerashopt
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