Thread: Bridge Pressure????
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Unread 23-06-2002, 23:19
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#0047 (ChiefDelphi)
  
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Re: Bridge Pressure????
Posted by Raul at 1/21/2001 6:25 PM EST


Engineer on team #111, Wildstang, from Rolling Meadows & Wheeling HS and Motorola.


In Reply to: Re: Bridge Pressure????
Posted by Dodd Stacy on 1/20/2001 5:56 PM EST:



OK, so if the 10lbf is correct then the bridge weighs roughly 174 lbs.

Using a simple static force model: F = Wx/l

F = force to lift or lower from horizontal
W = weight of bridge structure
x = distance from bridge CG to edge of fulcrum
l - distance from edge of fulcrum to end of bridge

So, at start, x is half of fulcrum width which I assume is 5.875" (5.635 lumber + 2x .125 angle iron) which makes x = 2.9375".

The distance 'l' depends on whether you are lifting or lowering the bridge. Notice that 'l' is greater for lifting, so that force should be just slightly lower - we'll ignore that for this rough calculation.
Use the average length of 51" since Dodd gave us a rough force of 10lbs in both directions.

So W = Fl/x = 10x51/2.9375 = roughly 174 lbs

Raul

: Roughly 10 lbf to push up or down from the horizontal balance point, roughly 1 or 2 lbf to move it from the tipped positions.


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