Vibrations heard when a cradle ascends for cleaning a high rise building

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Discussion Overview

The discussion revolves around the vibrations and humming sounds heard by residents near the top floor of a high-rise building when an electric hoist cradle is used for cleaning the exterior windows. The inquiry focuses on identifying the cause of these vibrations and exploring potential solutions, considering factors such as wind effects and structural transmission of sound.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the vibrations may be due to the cables vibrating in the wind, a phenomenon referred to as "strumming."
  • One participant proposes that adding mass to the cables could change their resonant frequency, although this may not be practical.
  • Another participant notes that the vibrations might be transmitted through the building's steel frame, suggesting that the sound is not solely airborne but rather structural.
  • A later reply introduces the idea that harmonics could be involved, particularly with overlapping cables forming loops, and questions whether increasing the number of brackets could mitigate the vibrations transmitted to the building's structure.

Areas of Agreement / Disagreement

Participants generally agree that the vibrations are likely related to the cables and their interaction with wind, but there is no consensus on effective solutions or the exact mechanisms of sound transmission into the building.

Contextual Notes

Participants express uncertainty regarding the effectiveness of potential solutions, such as adding weight or increasing the number of brackets, and the discussion remains open to further exploration of these ideas.

polka129
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hello all.there is a electric hoist cradle(Temporary personnel lifting;make fixator) which one of our contractor uses to clean the outside glasses of a high rise building.the cables are suspended up at the top railing with fasteners and then the electric motor escalates the cradle for use.the problem is that this building is right next to the sea and powerful wind blows all around.people near the top floor complain that when this cradle is in use(some where near or between the top floor) they HEAR periodic VIBRATION or a humming sound.i have set out to investigate the cause of this and how to rectify it.please help!

i have attached the snap. https://www.physicsforums.com/attachment.php?attachmentid=52485&stc=1&d=1351615658

thank you
 

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My first guess is the folks near the top floor are hearing the cables vibrating in the wind, sometimes called "strumming". it would be difficult to inhibit this because of the constant winds. One could attach masses, for example, to the cables which would change their resonant frequency, but not very practical or simple. Just as with a guitar string, adding tension increases the frequency of vibration. So, if the cables and the mechanism can safely tolerate more weight, you might try adding weight to the platforms. My quick analysis is: tell them to get used to the sound.

Here is an example of a similar problem, but this cable being towed through the water:
“Cable fairing is a structure attached to a towed cable designed to streamline the flow around the cable, primarily in marine environments.
Cables are faired primarily for two reasons: (1) to reduce normal drag and thus achieve more depth for a given cable scope and speed; and (2) to eliminate cable vibration caused by vortex shedding, commonly known as cable strum.”
http://en.wikipedia.org/wiki/Cable_fairing
 
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As post #2 said you probably can't stop the cables vibrating, but you might be able to isolate the vibrations from getting into the structure of the building. These noises are often transmitted through the steel frame of the building and then into the rooms through the walls, not as sounds traveling through the air.

That could explain why the complaints are coming from the top floor, closest to the cable attachment point.
 
thank you bobby and alpha zero for your kind replies..it did however strike me that this problems relates more to harmonics ..cause overlapping cables can form LOOPS and the blizzards can make them vibrate...you may have also seen the RAILINGS on the top on which the cables are SUSPENDED.what can be done to STOP these vibrations being tranmsitted to the steel structure of the building as alphazero rightly pointed out..?..will increasing the number of brackets do a good deal?railings and brackets are indicated on the diagram...

thank you a lot