huangyhg

church bell tower
i've been asked to take a look at a church bell tower. i was told that every time the bell rings, plaster falls off the ceiling. they want me to come down and take a look to make sure its not a structural problem.
doesnt sound like a structural problem to me, but i certainly will take a look. it sounds inevitable that vibrations from the bell after years of use would eventually shake some finishes loose.
does anyone have any thoughts about this? think i'm on the right track? maybe the bell supports should have some sort of vibration damper installed? would that alter the tone of the bell?
i would suggest considering the following questions:
1. is this a new or existing church. if existing, when did the plaster start falling? is is due to water intrusion weakening the plaster, then the bell vibration being the final toll? <g>
2. i did work on a bell tower years ago that had a similar problem. it turned out that there was so much pigeon manure in the bell tower that the added load from the wet manure pile caused deflection in the wood joist and cracked the plaster. really! there were areas 4' thick of manure! (yes, it was a sh--ty job to remove it). church custodians don't get up in the bell towers nearly enough.
3. if the church is new, then i would look more toward a structural vibration problem. perhaps, the bell support structure needs to be stiffened?
4. before adding vibration isolators to a bell, i would talk to the bell manufacturer.
just some thoughts to consider.
good luck
ps guano makes good flower garden fertilizer!
pylko,
i'm sure that you didn't intend this to be the case, but your post sounds just as if you want to find the solution before you actually see/hear what is happening for yourself.
my knowledge of acoustics is very primitive [my chosen musical instrument as a youth was the trumpet - that should tell you something ], but i would be astonished if modifying the bell support would change the ring tone. rather like sitting the 1st violinists on rubber cushions - no audible effect.
providing the plaster is damaged from water etc and there isn't some change, you may want to consider acoustics. from my experience in acoustics, the bell or the generator or whatever will have discrete frequencies, based on the musical note of the bell or the electrical frequency, etc. if nearby structure has the same natural freqency, it will be excited and vibrate. the answer is not to change the bell, but the structure so it's not excited. if its a wooden structure, over the years the beam may have dried and shifted stiffness/frequency. the addition of significant mass, like the bird manure can also shift the frequency response, not just the stress and strain. was new structure; walls, etc recently added or some "excess" material cut away to install new ventilation ducts, etc. in the loft area? bte, you find many strange little lumps of metal hanging off the exhaust systems or powertrains of new cars to tune out resonances that contribute to noise, vibration and harshness (nvh). perhaps bracing of the ceiling supports will change the repsonse and solve the problem.
blacksmith
i meant to say "providing the plaster isn't damaged from water etc and there isn't....
blacksmith
pylko
your bell must be vibrating the tower either because it emits a frequency that finds something in your tower that gets excited, or the rotational ringing of the bell provides sufficient moment as it swings back as to sway the framing.
it would seem likely that the framing may be light.
i must admit i have designed many churches with bells and without, and never had this problem. it is interesting.
evank

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