Mystery Vibrations Question 3

[271828]

I'm looking for the source of vibrations in a building and would appreciate any advice.

Four story multi-unit residential. Residents in multiple units feel disturbing floor vibrations that persist for about a minute a few times per day. Daytime or evening. All year round.

Fairly light structural system with steel-framing with concrete slab. Parking on the slab on ground below.

Each residential unit has its own HVAC with an air handler in a closet and ductwork going to the roof. Each residential unit has a mechanical unit on the roof; some are heat pumps. Mid-Atlantic state in the US, so it gets fairly cold in the winter and fairly warm in the summer. Nothing very extreme.

I measured vibrations for 12 hours recently. The measured natural frequency of the bay in which we spent most of the time was just under 12 Hz. The units were almost all empty. Most of the HVAC units were not running for most of the day, but it is possible that the AHUs were running some of the time. We activated and deactivated the HVAC (heating and cooling separately) for almost all of the units, one at a time, and there was no obvious culprit.

For most of the tests, the waveform amplitudes were about 1.0 mg and spectral accelerations were about 100 ug RMS.

There were three very perceptible vibration events in about an 8 hour span. The waveform amplitudes were about 4 mg and the spectral accelerations were about 2000 ug. In each of these three events, there was an extremely clean spectral peak at between 11.5 Hz and 12.5 Hz, suspiciously close to the natural frequency. The events lasted at least 30 seconds. The waveforms look almost like a single tone sinusoid.

During the last two events, I was able to run outside and verify there was no motor vehicle idling nearby.

There is a hydraulic elevator. It was not operating during the events. We ran it up and down a few times and it didn't seem to be the culprit.

I'm fairly close to being at a loss on this one. Am I missing something easy? Any ideas, even guesses about what might cause a roughly 12 Hz steady-state vibration in a multi-unit residential structure, would be appreciated.

 

[JStephen]

Long ago, I was standing maybe 50-75 yards from a railroad track when a unit coal train went through and I noticed pretty distinct vibrations in the ground- my impression at the time was that it was similar to ocean waves. Never noticed that before or since. Could it be something like that a half-mile away? Three events in 8 hours, more than 30 seconds would both work for that.

 

[271828]

JStephen, thanks for the response. No rail anywhere close.

 

[GregLocock]

Can you measure the structural intensity of the vibration? This uses pairs of accelerometers to determine the energy flow of the vibration. Frankly it is not much help inside the structure, but at least it might point the finger at an external source.

Another option would be to have an all day barbecue for the occupants and switch the power to the entire building off.

OK, I'll admit I haven't got the faintest idea!



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[electricpete]

I have not much to contribute, but just thinking out loud.

You've got the acceleration waveforms... did the increased vibration appear and disappear gradually or abruptly?
(Gradual beginning would generally signal resonance, which doesn't help much, I know).

Are you sure that what you captured was the same type of event that the residents were talking about? From my admittedly limited experience, I'm under the impression that the noticeable resonant vibrations felt in floors and buildings usually tend to be lower in frequency, maybe 1-5hz. I wonder if there might be some lower frequency events hiding in your data that were not brought to the forefront with the particular measurement / processing / accelerometer sensitivity choices used. I'd be inclined to review the measurement in displacement if the software allows (to emphasize the lower frequency components).

=====================================
(2B)+(2B)' ?

 

[3DDave]

Hmm - I had plates rattle in the middle of the night because a vibratory compactor (steam roller type) was used about 1 mile from my house. Gotta do the roadwork while people are sleeping I guess. Of course that was continuous for a hours at a time.

I would also be interested if anyone was using a power hammer on a timer to reset their etch-a-sketch.

Sure, some industrial process is far more likely, but people do have hobbies.

I missed it - did you put the accelerometers on the basement supports to see if the input is lateral or vertical?

 

[GregLocock]

ep - 12 hz is a common problem frequency in cars as both wheelhop (60 mph out of balance tires) and engine rigid body modes are around that, so you can certainly feel it.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[271828]

You've got the acceleration waveforms... did the increased vibration appear and disappear gradually or abruptly?

Can't see that. I felt the vibration and then hit Record.

Are you sure that what you captured was the same type of event that the residents were talking about? From my admittedly limited experience, I'm under the impression that the noticeable resonant vibrations felt in floors and buildings usually tend to be lower in frequency, maybe 1-5hz. I wonder if there might be some lower frequency events hiding in your data that were not brought to the forefront with the particular measurement / processing / accelerometer sensitivity choices used. I'd be inclined to review the measurement in displacement if the software allows (to emphasize the lower frequency components).

I'm pretty sure the three events are the same type, but it's hard to be sure. The environment went form zero perceptible vibrations to very perceptible vibrations that lasted about a minute. I can't be sure how long they lasted because I hit record and was up running around looking for potential sources.

Humans are maximally sensitive between 4 Hz and 8 Hz. This is a bit above that, but not much. 4 mg was definitely perceptible.

The spectrum is amazingly clear, with almost nothing at any frequency except at 11.4 Hz on the first event and 12.3 Hz on the other two. Low responses between 30 Hz and 60 Hz, but nothing below 10 Hz.

 

[271828]

Hmm - I had plates rattle in the middle of the night because a vibratory compactor (steam roller type) was used about 1 mile from my house. Gotta do the roadwork while people are sleeping I guess. Of course that was continuous for a hours at a time.

I would also be interested if anyone was using a power hammer on a timer to reset their etch-a-sketch.

I don't think the source is something like that. This has been going on seemingly randomly for a while, probably about five years. It's hard to say how long, though. One resident apparently noticed it first. Then, when they started talking about it, others thought they might've been noticing the intermittent vibe for a while.

The building has another condo on one side, beach on one side, hotel across the street on one side, and parking garage across the street on the other. The resident who seems to feel it most is on the hotel side. In the hotel, the thing closest to our bldg is the restaurant. I'm pretty sure the commercial laundry isn't on the side near our building.

I missed it - did you put the accelerometers on the basement supports to see if the input is lateral or vertical?

No, I only measured vibrations at a few floor locations and near the rooftop units. All vertical accelerations.

 

[271828]

Can you measure the structural intensity of the vibration? This uses pairs of accelerometers to determine the energy flow of the vibration. Frankly it is not much help inside the structure, but at least it might point the finger at an external source.

Another option would be to have an all day barbecue for the occupants and switch the power to the entire building off.

OK, I'll admit I haven't got the faintest idea!

If I get another chance to visit the site, I'll do that. I could set up two or three accelerometers. If there is an event, then the relative magnitudes would surely tell me something.

I am pretty sure the vibe is coming from inside this building. It seems possible that it's coming from the hotel across the street, but that seems unlikely to me. These structures are about 15 m apart. If the vibe source causes 4 mg in our bldg, then I would expect it to be causing much more than that in the hotel. Then again, the bay I measured had a vertical vibration mode at about this frequency. Maybe there is amplification in our bldg but not in the hotel. Ideas on that?

My current ideas are along the lines of your BBW idea. The head of the condo association is a EE and is interested in the problem, so he might be helpful. One resident claims to feel it multiple times per day. I'm considering asking for the HVAC breaker to be turned off for one condo unit for one day, and then switch to a second condo unit, working through most or all of them. If the vibe is there every day except when condo unit 4 (say) had no HVAC then that would help.

 

[271828]

Thanks guys for the responses. They're helping.

If this was rattling around only in my head, that wouldn't be good.

 

[electricpete]

Let's think about what could be the frequencies if it's a simple ac motor (without vfd, gearbox or belt)

What is the electrical frequency in your area?

If you live in 60hz area like the US, then 4 pole motor sync speed is 15hz, 6 pole motor is 10hz. 12hz doesn't compute

If you live in a 50hz area like Europe, then 4 pole motor sync speed is 12.5hz and 12 hz might be a reasonable loaded running speed.

It's just a thought exercize. But it doesn't get you very far since of course there are a lot of other possibilities like belt driven fan in hvac unit or vfd or something that isn't driven by an ac motor at all.





=====================================
(2B)+(2B)' ?

 

[BrianE22]

Can you put a data monitor on the electrical current coming into the building and one of the accelerometers? That might show if there are big motors turning on.

 

[RMiller77]

Where are the clothes washing machines located, or is there an adjacent laundry cleaners? An industrial front load washing machine in the spin cycle was moving coffee cups on tables on the second floor of an adjacent building in the strip mall.