blue cobra wrote:
Structural analysis followed by experimentation. As for the individual members, you weigh them.
Absolutely right - a couple things to add:
Experimentation- if you understand what you are varying- changing one thing, rather than a lot of things at once. Testing is a key; overall structure testing with a safety tower so you know exactly what broke, how, and individual piece testing,so you know exactly what it takes for each piece to hold the load it needs to. It takes some work, but testing apparatus to do both compression and tension testing can be made (see Gallery).
Weighing - yes; you should know weights precisely, and match them carefully, but weight alone will not tell you what you need to know.
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Well, it was a good day at State yesterday.....
As we all know, no matter how much work, and testing, and checking, .....things can happen.
Yesterday was a case of things coming down.....exactly as planned; great feeling for the kids, (and the coach).
At our Regionals, we'd taken 1st and 2nd, with the interesting twist that our 2nd team beat our first. Our second team was working with the same design my son had developed for out first. At Regionals, Team 1 was at 11.1 gr – held 11.7; Team 2 was at 13.4; held 14.7. 3/32 bass. Third place was about 750. Both bridges failed in the lower leg segment. The leg on the Team 1 bridge that failed was about 10% lower density than the other 3. We knew that in the Southern Region, 1st was about 750 (that team didn't make it to State), followed by about 650.
After some discussion; a major factor of which was many time demands – other events, they decided to take a conservative approach; instead of development time to bring weight down. We knew from how the T-2 bridge did that the upper part was solid- the issue was the legs. What they did was add angle-iron lamination on the lower part of the legs (technique discussed in previous posts). Carefully matched weight on the leg wood at the density of the 3 legs that didn't fail on the T-1 bridge, with 1/64th by a little less than 3/32nds laminated on the underside and inside. Weights came out at 11.6 and 13.3, and “the plan” was that both would hold full load, and that nobody else would get above 1,000. My son's building experience over the years had given him a very good sense of what it took to get into the 750 range, and to ramp that up to 1,000....1,300, 1,400. His confidence in that, and that adding the leg stiffening would produce a bridge that would carry full and have a.....comfortable margin-gained over 6 years of learning- was pretty neat. He carried that into a time management decision to pass up the temptation to chase a significantly higher efficiency.
Both bridges held full load (1293 and 1128); third place was 694;4th was about 630. Tight-in video on both showed no distortion other than what was supposed to happen. As discussed in previous posts, a key design approach was building so that the bridge starts out “distorted”, and settles in at full load to “design geometry.” With our design, this is primarily in the main tension members. At 11kg design load, they stretch about 1.3mm. That means the lower leg ends move out a little over 2mm per side. The bridge stars out with about 1mm clearance on the 45cm free span, and ends up approaching a 49cm span. The whole truss section is bowed a bit at no-load, and flattens/straightens near full load. Wish we'd had the time to really push it to max efficiency; my best guess is it could be refined into the....1,600 to 1,800 range. National winner, no way; but respectable.
The design and jig-building work earlier in the season meant a....predictable bridge took about 5 hrs to build. Both teams built 2 bridges, a Regional, and a State
Along with coaching building events for our HS,I've had the pleasure of working with 2 middle schools in town on their bridges. They were 1st and 2nd at State (both a bit over 1,300), and one of them will be coming to Nationals. Looking forward to working with them to see what they can do to crank their efficiency up.....
