Today was the culmination of first-year Eng Sci's CIV102 Bridge Design Project, where we are given 0.8 m^2 of matboard and told to make a bridge that will sustain as much weight as possible. The first part of testing (not shown here) is running a 400 N (41 kg) train over the bridge; the second part is adding weight until the bridge fails. Most designs are either straight beams (the wrong design) or variable-depth sections, where the beam is thinnest at the support and gets deeper as it gets closer to where the load is applied (think "half a hexagon"). My team decided to take the variable-depth idea a step further and optimize for compression; this generated a parabolic shape that allowed us to hit a depth of 260 mm at mid-span, about 80 mm deeper than the next-largest bridge.
Our calculations yielded a fail load of 1521 N (155 kg), though a series of small construction errors meant that we failed at 1165 N (119 kg) due to compressive buckling in the deck; this was still good enough for second place overall in the competition. This bridge is a rare example of an unorthodox design actually paying off, and I'd like to impart a few tips for any future Eng Sci's bold enough to try it out (believe me, it's not easy):
1) It's a pi beam, completely open from underneath.
2) Aim for a more modest fail load; try 1200, and if you build it correctly, it'll surpass your predictions and probably win you the competition (As Professor Collins said, "most years, a strength-to-weight ratio of 161 would've won"). We were somewhat short on material, and needed to reinforce the weak points with scraps.
3) To build the curve, find the value for I that you need at a few points between the support and the load, and extrapolate from there.
4) Reinforce the hell out of those supports; Collins told us similar designs in the past failed due to shear, right outside the supports, because the I value there is so low (we were down to 42 mm over the supports).
===
And some general tips for every future Eng Sci:
1) Construction is ABSOLUTELY KEY. Many a good design will fail due to poor construction, and Collins personally inspects every bridge to give it a quality mark out of 10 (usually between 7 and 8.5, though we got a 9, probably for originality and really solid construction)
2) Expect to work for 12 hours straight building your bridge if you want it to get anywhere near the kilonewton club (1T5's collection of bridges include a grand total of three new inductees).
3) Doubling up the deck is a waste of material.
4) Reinforce, reinforce, reinforce. If you ever have a break in the material, have a contact plate on each side of the split, both a total width of 30-40 mm. Double up the material at the supports to prevent buckling, and make sure you have a large contact surface between your web members and your deck.
5) More reinforcing. Diaphragms under the supports, under where the loads are applied, and under the holes in your deck that the wires go through.
6) Have I mentioned that the number one factor in how well your bridge does is how meticulously it's constructed?
7) Good luck, have fun, see you in the kilonewton club!
Loading...
3:51
MythBusters - Duct Tape Hour 2: 102 Uses For Duct Tape - Duct Tape Bridgeby BelldandyMai15,480 views
Link to this comment:
All Comments (0)