This is the inaugural post in Stephen Repetski's new Metro news column, Metro Reasons. Follow along for investigative reporting, breaking news, and analysis.
A report Metro commissioned confirms what residents along the middle of the Green Line have been reporting: trains are causing noticeable vibrations. The report says while the vibrations are noticeable and that the 7000-series cars can cause more of them, there’s no risk of structural or cosmetic damage and thus nothing more needs to be done.
The report, issued January 22, 2018 by noise and vibration consultant Wilson Ihrig and recently posted online by Metro, wraps up a study which began after residents along the Green Line complained of feeling increased vibrations in their homes. Some residents noted that their homes were shaking more than they had in the past, which seemed to correlate with Metro’s increased use of the new heavier 7000-series railcars.
Vibration and noise measurements were taken at 10 residences along the Green Line for the study in June and August 2017, which were selected from 48 residences that had complained to Metro. Ihrig also placed some sensors outside homes.
Of the 10 residences, vibration criteria (from when the Green Line was designed) was exceeded between 8 percent and 29 percent of the time at four of them. The report says that none of the other six residences saw any design criteria exceedances.
Ground-borne noise generated by the system would have to exceed 35-40 A-weighted decibels (dBA) or vibration would have to exceed 70 vibration decibels (VdB) in order to be significant, according to the Green Line design criteria. Federal Transit Administration guidelines say vibration criteria for residences near train lines with more than 70 trains per day is 72 VdB and ground-borne noise criteria for those same homes is 35 dBA.
Noticeable vibration is not a concern, study says
While the study corroborated the vibration, it says there’s no potential harm to buildings in the area because of it.
“All measured vibration levels are well below that at which structural or [cosmetic] damage…could occur,” notes the report. “There is a large difference between vibration levels that can be felt and vibration levels that can cause even minor damage.”
The vibration “annoyance criteria” like the ones noticeable by residents and found by the study are “an order of magnitude lower than vibration damage thresholds, so if the annoyance criteria are not exceeded, there is no possibility of physical damage.”
The greatest “peak particle velocities” that produce noise/vibration were 0.0801 ips PPV at one home in the longitudinal direction and 0.034 ips PPV at another in the transverse, whereas the typical train produced 0.001 to 0.01 ips PPV. The report says that only once the PPV reaches 0.5 ips might cosmetic cracking damage occur.
A 1971 study referenced gives a 0.02 ips PPV threshold for possible cracking of plastered walls or ceilings, but no structural damage. Anything under 0.1 ips PPV according to the study poses “‘virtually’ no risk of architectural damage to normal buildings, and a limit of 0.08 ips PPV…is no risk to ruins or ancient monuments.”
WAMU last year noted that Metro “has not agreed with the assertion that the new trains…are responsible” for the greater vibration.
However, according to the report the 7000-series cars give off “noticeably louder” noise than the other legacy train cars in the Metro fleet. It stipulates that while the noise and vibration are higher than average, the vibration levels in most residences in the area are “still below the level that is widely considered to be the threshold of perceptibility.”
Metro’s contractor looked into whether the rails themselves might be part of the issue as well. Rails which are rough, chipped, or have other defects might cause trains to produce greater noise or more vibration. After investigating and performing visual inspections, they didn’t find any significant defects which might contribute to the issue.
Residents are likely to notice vibration for the forseeable future
An email from DC Councilmember Brandon Todd’s Director of Communications, Joshua Fleitman, notes that while residents complaints have been at least partially validated by the study, “at this point, our office has exhausted every tool at our disposal.” The email quote posted by Petworth News notes that the councilmember “can’t force WMATA to acknowledge the problem and fix it.”
Without having violated known regulations or laws by causing these more noticeable vibrations, it doesn’t appear that Metro will be required to do anything to mitigate resident complaints.
Metro Reasons is a regular breaking news, investigative reporting, and analysis column by Stephen Repetski about everything Metro. Please send tips to Metro Reasons.