Halo
Well-known member
Talking of quaint customs has anyone seen the Penny Hedge in Whitby ?
P S it can sometimes be found outside the Middle Earth tavernTalking of quaint customs has anyone seen the Penny Hedge in Whitby ?
I understood movement it is part of the design brief, but what do I know?Yes, but there was a good reason behind it. The phenomenon that caused it to wobble was unknown prior to the opening of that bridge. Resonance was known to happen with certain circumstances on bridges (eg troops being told to break march and walk), but the resonance on the millennium bridge happened with people just walking independently over it.
What happened was a result of the geometry of the bridge causing the resonant frequency to fall within the range of human walking pace. As people walk they transfer their weight from one foot to the other. This causes an alternating sideways force on the bridge from the feet. This is small, and even though they are not stepping in sync it is not fully cancelled out, and when many people are walking at the same time, there is a resultant force which deflects the bridge sideways. When the bridge returns to its normal position, this imparts a small and imperceptible force on the feet of the people walking.
Although they are not aware of it, people subtly adjust their gait to balance in response to this force, and because it is applying to everyone simultaneously it nudges them into walking more in sync, which caused a positive feedback loop, amplifying the movement of the bridge until it could be seen and felt to wobble.
The fix was to add damping for the sideways movement to reduce the movement and modify the resonant frequency so it was less affected.
It was also a safety issue. People less steady on their feet could end up being knocked over by the motion, which it why they had to modify it.I understood movement it is part of the design brief, but what do I know?
What happened was it was a wee bit too wobbly for landlubbers and they got upset.
I do miss the world of feedback loops now I am retired.