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Addendum to wind load calculations



Hi Everyone,
                        if you have been reading my email about windloading
on structures, I thought I better add that wind is a dynamic load, note just
a static load and so wind induced vibration should also be avoided.

    The most hazardous mechanical resonant frequency to avoid is 0.3 Hz
(dynamic wind loads have most of there energy at this frequency - ie a gust
every 3 seconds) but if in doubt, is is better to have all mechanical
resonant frequencies above 10 Hz as there is usually almost no significant
component of wind loading above that frequency.

    The problem arrises particularly with metal structures because metal
structures behave very elastically (low energy loss - mechanically) and so
at resonance, they often have damping factors of 1% or less. The electrical
equivilent is a resonant circuit with a Q of 50 or more. (ie television IF
band pass filter would have that sort of Q)

That means a cylclic force which is applied to a structure of this sort at
the structure's resonant frequency will have 50 times the affect that the
same force would staticly! There is an old black and white film of a bridge
in the USA (Tocoma Narrows - or a name like that) which shown a bridge which
was otherwise well designed and constructed, oscillating and eventually
collapsing in a moderate breeze. This film graphically illustrates this
effect.

Regards,
Murray Peterson
VK2KGM
(Yes - I am a Structural Engineer)

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