Elevated Bridge B/C

[croman74]

Thanks for the compliment, Croman!

I think with B division it's harder. Unfortunately, you're by necessity dealing with students that have less experience and, usually, less maturity.

(no offense meant whatsoever - it's obvious that you've got a pretty good idea of how things ought to work.)

Having teammates who are less experienced prototype new ideas is extremely useful. This allows the more experienced builder to focus on making competition bridges, but everyone still gets to contribute to the team's final product.

The maturity thing is kind of true about some kids. What I'm going to do is give the other kids some designs that I think would be interesting to play around with. I wish I had more time, but there's only 2 weeks and 3 days before states.

[sewforlife]

Croman, how did your bridge do?
and also, what is everyone's opinion on angled bridges?
lots of people have told me that having a bridge with no incline is great if ur testing is perfect (no bucket swing, etc.) and that's probably true. also Balsa Man said it, but I want to know what others think.

[Balsa Man]

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

[croman74]

<Snip>
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.
<Snip>
Len Joeris
Fort Collins, CO

I'm not sure if I understand this correctly. Are you saying that the laminated wood held 20% less than the un-laminated. And I know I'm going to sound stupid asking this, but how do you laminate? Do you use regular laminating paper?

[andrewwski]

Laminating refers to gluing two pieces together lengthwise.

[croman74]

Oh, ok.
I'll be testing my bridge in a few hours. I went to go test now, but one small part fell off. I reglued it, I'll just wait for it to dry. When I reweighed it, the bridge was 15 grams. Hopefully it holds it all!

[Balsa Man]

I'm not sure if I understand this correctly. Are you saying that the laminated wood held 20% less than the un-laminated. And I know I'm going to sound stupid asking this, but how do you laminate? Do you use regular laminating paper?

Not a stupid question at all.

First, I'm saying test data show that a laminated piece will be at at least 20% lighter than the un-laminated one- carrying the same load (done as, and within the limits, stated). Glue is a major variable you have to control. The strips you laminate on have to have a layer of glue their entire length and width - a thin layer, but a complete layer

How? Wish I could just draw a picture; let's try it in words (gee, this is like like WIDI):
Visualize - and make yourself a drawing looking end-on; in cross section - you're looking at 1/8th inch square. Start at the upper left corner. From that corner, along the top side, you will have a strip 3/23 wide; glued on the top side of the 1/8 stick; left edge of the strip aligned w/ the left edge of the stick. You now have a 1/8th" x 9/64th" rectangle cross section. Now, again starting from the upper left corner, glue a 3/32 wide strip on the left side. The upper edge of that strip is aligned with the top. You have put an angle iron of 1/64th along the upper left corner of the stick. The edge of one lamination strip has to be up against the side of the other, and its critical that edge have glue on it.

In terms of actually how to do this glue-up. Do the lamination on a stick piece that is, oh, a couple centimeters longer than final length. Make your lamination strips a couple centimeters longer than that. You can cut the ends after laminating. For the first strip, have a piece of waxed paper down on a flat surface. Put your glue on the strip, out to within about a centimeter from the ends. The glue-free ends give you a way to handle it. Put the stick down on the wax paper. Glue the strip on one side. Be sure to get the bottom edge of the strip down to the waxed paper; flush with the bottom side of the stick. Once its down, edge flush with the bottom of the stick, and the glue has.....grabbed it, use something, a basswood stick, or a steel ruler (wrapped with like GladWrap, so it won't stick) to push the lamination strip up against the stick along the full length. You want the strip tightly up against the stick the full length. Its helpful to use an emory board or sanding block to carefully smooth up the bottom side; the side that the edge of your lamination strip is flush with- this gets any glue that may have squeezed out out of the way so the second strip can be tightly laminated on.

Now for the second strip, same basic process. Orient the stick so that the first laminated strip is on the bottom. Put the second strip on so that the bottom edge of it adjoins the edge of the first strip.

Go through this process without glue first to.....figure out the handling. Then I'd try it on some scrap before using/potentially wasting "real bridge pieces."

That's how it works....
If this doesn't make sense when you draw it out, let me know, and see if I can post a drawing.

Len Joeris
Fort Collins, CO

[Allirog24]

My test results were not great...only held like 4 kg. Pathetic. I don't know what went wrong. It had like a little more than a 500 efficiency.

[Balsa Man]

I don't know what went wrong.

Sorry to hear that, Alirog- both that it didn't do as well as you'd hoped, and that you don't know why.
Did you, or others watching see anything to give you a clue what happened?

Len Joeris
Fort Collins

[croman74]

Thanks for the help Balsa Man. In my opinion laminating would take too long. I guess I'm just lazy.
Allirog, did you see where it broke? Was it wood or at a joint? Maybe we could help.