Nejanimb, thank you for the further insights into your approach. Discussions like this are really great for all of us. Many different things under discussion, but let me pick up today on one area.
We have a number of posts that are essentially saying, “wow, I don’t really have any clear ideas, or understanding how to get from…..30, or 20, or 15 gr to…..say 10…..how do you do it???”
My last post was so long, I didn’t carefully read it all before I posted, and I only realized after I sent it, I didn’t finish my Step #4-Refining/Improving. Let me provide some real detail on exactly how the refining and improving process went for us this year. Folk can draw their own conclusions, and maybe it will provide some insights for those that are wondering exactly how you can…….optimize a design (a basic configuration).
Let’s start with a sketch of the design for this year’s bridge. Its posted in the picture gallery:
Members in compression are in blue; those in tension in red; forces shown are on members at 15 kg load. Members in black have essentially no load, they just act as column bracing (actually, there’s about 0.25 kg of compression on the black members extending out to the legs from the bottom, main tension members).
As I …..left off in Step 4, the first build was using bass for the compression members, (and the essentially unloaded column bracing pieces – the black ones, above), and 1/64th strips for tension members (high density balsa-20 to 25 #/cf range, paired, i.e., 2 strips/member, one on each side of the members they join). It initially came out at 15.3gr. It was intended to be just a …..confirmation of buildability – the joint where the main tension members, leg tension members, compression piece down from the load block, and tension member out to the upper corners of the truss come together is obviously a challenge. Suffice it to say, it worked. A quick test – using the “safety tower” I discussed in an earlier post – confirmed it easily held 15kg.
Let me digress briefly to point out once again what I think is probably the most useful tool/approach we have discovered over the years- testing with a “safety tower.” The safety tower provides a way to limit how far the load block can fall at structure failure (like to about 1/8th inch). What that gives you is a) you know exactly what failed, and how it failed, b) the damage is limited to that primary/initial failure mode, c) you can patch/fix/reinforce the failed piece/joint and re-load, and see what fails next, and repeat that process till you get to full load. Without the knowledge (as opposed to guessing) of what failed, and how, you are really shooting in the dark with any “tweaks”/”fixes.”
Now, on with the development story. As is often the case with Science-O, there were …..time issues; when we got the Regional events schedule, we had a number of conflicts requiring moving people to different events; Peter had to pick up an additional event, and the time to build another bridge was……something we wanted to avoid. In about half an hour, Peter was able to cut out the upper black column bracing pieces and replace them with light balsa. Weight went down to 14.8; no need for testing- the replaced pieces were under no significant load. When I say “light balsa”, btw, I’m talking 0.7 gr/36”; light enough you have to be very careful handling it so as not to crush it. At Regionals, the bridge held full load (1013 efficiency), and won by…..a wide margin.
The second bridge was also built……taking a conservative approach. Using the spreadsheet for estimating bridge weight I discussed earlier, it showed that replacing the main top compression members – the pieces that form the top of the bridge, along with the compression members that run down from the load block to the “5-way joint” with laminated balsa would get the weight down to just about 10 gr. This estimate reflected a safety factor (based on compression testing) of a little over 1.4 on the center section of the top member, and about 1.5 for the outer portions of the top member, and the compression members running down from the block to the 5-way joint. The build came out at 10.0 gr. It was not load tested. Putting a load block on, and pushing down by hand confirmed – really by feel - the structure was…..about as rigid as the Regional bridge. This “hand testing” was probably to about half load. As reported earlier, the bridge held full load at State; it won with a 1500 efficiency, second place was a little over 1000, 3rd was a little under 750. Had Peter not gotten sick, and really time-stressed, we probably would have gone for a third bridge for State. Pulling safety factors (on the top members and compression members running down from the block) down to 1.2, the estimated weight goes down to just under 9gr. Replacing the legs (which are bass in both bridges) with laminated balsa, with a safety factor of 1.2, produces an estimated weight about 7.1gr – 2112 efficiency. Pulling the safety factor down to 1.1 on compression pieces, and down to 1.2 on tension members (which are pretty light, so even a big percentage change doesn’t mean a lot of weight difference) shows an estimated weight of 6.8 gr – an efficiency just under 2206. I sure wish we’d had the time to try that…. may just do it this summer for fun.
One additional side comment re: lamination. Based on breaking a lot of pieces of wood in compression testing, there is value – an advantage to be realized – in using laminated construction in compression pieces. Specifically, in the load range between 4 and 8 kg, using 1/8 square sticks, at exposed column lengths between 4 and 7cm; comparing the density/weight of un-laminated balsa, and light 1/8th core with 3/32 (1/64th thick – 24 #/cf density) “angle iron” laminations, we’re seeing the laminated construction will carry a given load at …..at least 20% less weight than un-laminated. “Light” balsa core – 0.8, 0.9 gr/36.” “Angle iron” lamination – putting the lamination strips on adjoining sides, with the edge of one overlapping the edge of the other; using slow CA glue; a thin but full layer, being sure to have glue at where the edges overlap.
I’ve laid out this detail for two reasons-
First, I hope this may provide some concrete food for thought, and some specific hard design information/ideas for folk that are still in competition (State, or Nationals); I’m pretty confident 2200+ for a C bridge is do-able, and that confidence is based on …..real data.
Second, and I believe more importantly, this provides specifics, in a scientific process, of how you can develop/refine a design to a point that……approaches maximum efficiency. This is Science-O; there is a scientific method; there is an engineering design process; it involves math, and control of variables. A structural engineer, who has been at it for years, can look at a design problem/a structure and see, comprehensively understand, “what is going on”- what forces are at work where; what materials properties, and construction precision are needed to carry a given load. For those of us who aren’t structural engineers, many of these things are neither obvious, nor, often, intuitive. Following scientific methods, and established engineering processes provides a path to understanding. Understanding provides a path to……refining and optimizing. It is one thing to “change the angle in one place, or change the member construction in another”, and see what happens (in a full structure test), or to just happen on a design that is really good; it is something very different to do that with knowledge and purpose; to know that that change in angle increases load from 5.5 to 6.5 kg, and that to carry that additional load, the density of the piece at that cross-section has to go up …..40%.
As nejanimb and I have both said, there are a lot of ways to get……a level of success, and success, however it is reached must be respected. I certainly, and fully, respect what he and his team have accomplished.
I simply ……offer for everyone’s consideration, what I perceive as the value of the engineering process. It is the existence (and substance) of this process that I try to teach to our kids. I’ve learned pieces of it from various folk over the years who have been willing to share, and I continue to learn every day. Its that learning, and the sharing of learning, that have kept me involved in Science-O, and a supporter of what it provides for our kids
Croman and alirog, really looking forward to hearing your latest test results! Wishing you well.
Len Joeris
Fort Collins, CO
