Truly "blind" trial and error would definitely be time-consuming, but I don't think uninstructive. On the contrary, like Dragonfly said, I think to put a design in wood and get it through its paces on the blocks is the only way to really see what's going on. I agree with Aia - we built close to 40 bridges this season as a team, and we learned something from every single one, all the way down to the bridges built in the week between states and nationals.
Since we have a number of people building bridges, what we did to start the season was to let everyone try whatever they wanted based on what they knew from past experience and their understanding of the physics involved. We'd all get together to watch tests, give critiques, and talk about what we could learn from the bridge and how it loaded. It took us a couple months, but eventually we hit upon a good design. Then, we spent the rest of the year perfecting it. We tried changing one variable at a time - one person would try using a different cross-section for the main compressions while another person would try changing the angle of the legs while another person would try tapering the sides together, etc. We'd regroup, watch the tests, talk about what we found, and then move on to a new round. One person would try .15g for the compressions while another tried .21. Rinse and repeat.
The point of this description: we tried consulting the truss-bulider program, but it couldn't actually teach us how to build a successful bridge. It might show some simple versions of sturdy triangle arrangements, and it can show relative loads... but how can you make that work in balsa wood? In the end, building 30+ bridges taught us a TON during the course of this past season - what works, what doesn't, and I know more for it. Not only do I know more about that each particular design, but I learned things about engineering in general and how structures and forces work. I was able to see in a very tactile, real way laid out in front of me in splintered balsa fragments what I should do next time and how we could improve. This was infinitely more real and useful to me than a numbers readout from a program that really doesn't teach me anything about how to make the next bridge better than the one we just broke. Without trial and error, how can you improve? Build one bridge, and then another, but learn nothing in between from the trial and error process? That seems insane to me. It's an absolutely essential part of the design process, to my way of thinking, and one that cannot be replaced by working with fancy spreadsheets or stress-analysis programs. For us high schoolers, this is critical to learning about how things work. It's building something, trying it out, and improving our design and methods that's really instructional - not just simply heeding the advice of a structural engineer we know or a statics program we couldn't emulate ourselves or a spreadsheet that we didn't write ourselves. To me, that would be far more "blind" than being methodical with trial and error, like we were.
Seems to work too: like I said before, none of the top bridges (the C bridges, at least) would have held up to that JHU app, but clearly they were successful nonetheless. Our bridge sits proudly atop my desk, and I'm happy when I look at it and see each individual piece of it, and I can remember the bridge that we built last season that taught me the lesson that resulted in it being exactly how it is now. I see "oh yeah, that gusset was added after it broke such a way" and "oh we moved that there after we almost hit minimum specs" and "we added that extra piece after it kept breaking at the top" and "we decided on .19g for that part since the ones heavier held everything and the ones lighter broke early" and so and and so forth. The final product was absolutely a result of a full year's hard work, with things learned at every step of the way through rigorous trial and error. While I may not fully understand static analysis, I don't think that means I know nothing about structural engineering, nor do I think that my method was weak. Above all though, I know it was as far as you get from uninstructive.
Aside from that rant, rjm, the clarification about how that statics program doesn't show deflection makes a lot of sense to me. I knew that was the case, but I didn't put it all together to come to that conclusion properly - thanks. That makes my misunderstanding of the inverted V and the rest of the issues much more clear - I get that all now. Thanks!
Harriton '10, UVA '14
Event Supervisor in MA (prev. VA and NorCal)