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View Full Version : Force vectors on a toothpick bridge

Jeff_Rice
10-21-2004, 06:30 PM
I am working on a toothpick bridge and would like to determine the forces on individual members and joints. What are the steps to doing this? (if you don't want to explain, links would be appreciated as well) I just want to know how to know how, not for the assignment, which is simply to build a bridge.

colt527
10-21-2004, 06:55 PM
I am working on a toothpick bridge and would like to determine the forces on individual members and joints. What are the steps to doing this? (if you don't want to explain, links would be appreciated as well) I just want to know how to know how, not for the assignment, which is simply to build a bridge.

Hey! We learned this in my intro to engineering class! Its a long and tedius process that can be done by a nice computer program called Working Model.

Heres the link to the demo:
http://www.krev.com/demo.html

you just need to create circles for each place a toothpick joint would be then connect them with tension rods located on the bottom of the tool bar on the left. Then after its alll done and modeled click the rods you want to measuse the force on and go to Measure -> Tension. And there you go. I know its a very breaf explination but using help and searching google for tutorials will get u there.

Doing it by hand for a complex bridge is much to tedious and time consuming to be practical.

CrazyCarl461
10-21-2004, 07:19 PM
Pish posh. Real engineers don't use computer software (ok maybe they do). But I would do it by hand the old fashioned way like this:

Assuming you have a bridge with two planar sides, disregard the members connecting the sides and do analysis on one side. Put in the reaction forces on either end. If the bridge sits on four points, each one will be equal to the hanging weight divided by four. Then, just do some good ol' fashioned joint analysis. Start at a joint with a reaction force and work your way around. Although not super-accurate, it is probably best to assume that all of the members are in pure tension or compression (no bending moments and certainly no torsion). Basically, draw each joint and sum the forces going in and out equal to zero using trigonometry to find x and y component forces. If the structure is complicated (compound joints or several angled members) you may have to divide the whole structure into a couple sections by 'cutting' the members (not joints) and start summing some moments equal to zero. When doing that, you can ignore the all forces except for reaction forces and members that have been 'cut'. Try and sum moments about joints that have the most unknown forces, since all forces at that point provide no moment arm and thus drop out.

This way isn't even close to being as accurate as bridge-building software which can take into account beam bending and torsion, but this way is more fun and looks way cooler on paper. I'd hunt down a good mechanics textbook and some green engineering paper and teach yourself some really cool and useful stuff. Who knows, it may come in handy someday on an ME270 exam.