Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Oct 30, 2012 #1
    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
     

    Attached Files:

  2. jcsd
  3. Oct 30, 2012 #2

    Bobbywhy

    User Avatar
    Gold Member

    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
     
    Last edited: Oct 30, 2012
  4. Oct 30, 2012 #3

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    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 travelling through the air.

    That could explain why the complaints are coming from the top floor, closest to the cable attachment point.
     
  5. Nov 1, 2012 #4
    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook