Bridge Pressure????

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Posted by Brian at 1/20/2001 5:30 PM EST


Student on team #670, HHSRC, from Homestead High School.



Does anybody know the force required to push the bridge down? Maybe the weight or the measurement given by the strain gauge.

Thanks in advance....

[archiver]

Posted by nick237 at 1/20/2001 5:52 PM EST


Engineer on team #237, sie h2o bots, from Watertown high school ct and sieman co.


In Reply to: Bridge Pressure????
Posted by Brian on 1/20/2001 5:30 PM EST:



The problem with the question is who`s bridge?. because each bridge is different then each answer will change.
Some bridges will be heavier due to the lumber used by each mock bridge. The moisture content will change depending on the enviroment that each is stored, also unless you use expensive strain gauges to test the weight then again the # will change.
Most people consider a strain guage to be like the scale used to weigh meat or fish but the acuracy is about the same as a good guess.
There are so many variables that the info would not really help.
nick237



: Does anybody know the force required to push the bridge down? Maybe the weight or the measurement given by the strain gauge.

: Thanks in advance....

[archiver]

Posted by Dodd Stacy at 1/20/2001 5:56 PM EST


Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.


In Reply to: Bridge Pressure????
Posted by Brian on 1/20/2001 5:30 PM EST:



: Does anybody know the force required to push the bridge down? Maybe the weight or the measurement given by the strain gauge.

: Thanks in advance....

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.

Also, we determined today, finally, that the goals will accept 13 black balls without them touching a big ball set on the top, 14 and still hold the big ball, but a bit precariously if you tip the goal. For you scoring strategists out there.

Dodd

[archiver]

Posted by Dave Butler at 1/20/2001 6:15 PM EST


Student on team #188, Blizzard, from Woburn C. I. and Ontario Power Gen..


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



: : Does anybody know the force required to push the bridge down? Maybe the weight or the measurement given by the strain gauge.

: : Thanks in advance....

: 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.

: Also, we determined today, finally, that the goals will accept 13 black balls without them touching a big ball set on the top, 14 and still hold the big ball, but a bit precariously if you tip the goal. For you scoring strategists out there.

: Dodd

when you got the 13 balls in were the pvc pipes, around the side, "streched" out...bent (i guess) out of shape????

or when you had the 13 balls in were the pipes the same as before the balls where put in...

[archiver]

Posted by Dodd Stacy at 1/20/2001 8:25 PM EST


Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.


In Reply to: Re: Bridge Pressure????
Posted by Dave Butler on 1/20/2001 6:15 PM EST:




: : Also, we determined today, finally, that the goals will accept 13 black balls without them touching a big ball set on the top, 14 and still hold the big ball, but a bit precariously if you tip the goal. For you scoring strategists out there.

: : Dodd

: when you got the 13 balls in were the pvc pipes, around the side, "streched" out...bent (i guess) out of shape????

: or when you had the 13 balls in were the pipes the same as before the balls where put in...

No. The balls were simply dropped in over the top. The PVC verticals were not bent, nor did we "stuff" the black balls forcefully in. We lofted them in repeatedly to get a minimum repeatable number.

Dodd

[archiver]

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.