(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

I'm designing a Pratt Bridge (http://www.garrettsbridges.com/photos/pratt-truss-bridge/attachment/pratt-truss-bridge-2/). It must be able to hold 10,000 lbs at pins every 5 feet and be 40 feet long.

2. Relevant equations/ 3. The attempt at a solution

I know how to approach this problem, I'm just looking for an explanation of the concepts behind this problem...

So, I have the upward y-force (Ay) from the support on one end up the bridge (along with an x force which is just 0), and just an upward force from the roller at the other end (Py). I use 5000 as the weight in the y direction, because I'm only considering one side of the bridge.

ƩFy=0=Ay+Py-5000

ƩM=0=-5B-10C-15D-20E-25F-30G-35H-40I+40Py

Depending on which two pins the 5000 lb weight is distributed on (2500 per pin), I solve for Py, and in turn solve for Ay.

Basically, as the weight moves farther from the support (closer to the roller), Py increases, and it in turn increases (or decreases if you consider the negative) the value of Ay. Also, the tension in the trusses is often much greater than 5000 lb. Some trusses (verticals) are always 0, and only have tension when the weight is on their respective pin.

My questions:

How is it possible that the tension in the tresses is significantly higher than the only force in the negative y-direction?

Is it possible for a truss to have 0 tension on a bridge (not literally 0, but in a hypothetical case study, where we are neglecting the weight of the bridge)?

Also, if Ay is a negative value, should I use this negative in future calculations that involve Ay, or is it just indicating that the tension is compressive? So I should use the absolute value?

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# Homework Help: Bridge design conceptual questions

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