Elevated Bridge B/C

[Balsa Man]

Careful, for a square stick you can't really use A, B, or C to define the stick. It applies to the faces, not the bulk properties......

YES - important clarification, absolutely correct.

[sr243]

please respond
My bridge for competition has been broken by someone, think i know who, a person who has a grudge on me. Anyway i need to make one before the competition tomorrow. There is about 20 hours before competition and I always have used wood glue. CA has failed me before so i stopped. Anyone who uses CA, can u tell me whether it is good or not. I have Zap-a-gap, medium viscosity? Please respond. thanks. Otherwise do u think wood glue which dries in 24 hours should be strong and light enough in 20 hours?

[Balsa Man]

please respond

Bad scene- hang in there.

With 20 hrs, I think CA is the only realistic way you have. Wood glue is not going to be dried to near full strength. CA can and does work- used in many of the top bridges.

Good luck!

[croman74]

Careful, for a square stick you can't really use A, B, or C to define the stick. It applies to the faces, not the bulk properties......

YES - important clarification, absolutely correct.

True, I forgot to mention that. Thanks for the clarification.

If i am correct there different grades of wood for the same size peices...right?

Yeah. I put a bit of info in the wiki on this. There are 3 main grains: A, B, and C. A grain is more flexible than the others and is probably better for tension. C grain is good for compression as it is stiffer. And B grain is a mix of the two.

Croman is right-on about the different grain types. When you speak of "grades", I'm guessing you may be talking about "weight-grades." Couple things on that.

Balsa, as discussed on this forum and elsewhere, comes in a wide range of density- from around 5 pounds per cubic foot to....mid 20s pounds per cubic foot. It all depends on where out of a log the wood is cut from. So, for a given size - let's use 1/8th square sticks - depending on the density, you can get sticks of widely varying weight. Your local hobby store will have a...bin of sticks; the only way you know what they weigh is to weigh them. Places like Specialized Balsa (check them out on the Web) sell "weight-graded" wood- they weigh sticks, and you can by by weight. Using our 1/8th square exmple, you can buy 36" sticks in 1/10th gr increments - from 0.8gr to 4.4 gr. That doesn't mean each stick weighs exactly 1.0 gr, by the way. If you were to order say six 1.0 gr sticks, you would find they all are close to 1 gr - probably between 0.95 and 1.05.
Hope that answers your question.

Oh, whoops, he said grade. I thought he said grain.

[sr243]

please respond

Bad scene- hang in there.

With 20 hrs, I think CA is the only realistic way you have. Wood glue is not going to be dried to near full strength. CA can and does work- used in many of the top bridges.

Good luck!

Thanks, I just seem to always have bad joints with CA. I will be extra careful with CA today.
anyone know how long CA takes to dry to full strength

[jander14indoor]

Couple of things about CA.

First there are different grades and qualities. The stuff hobby stores carry is typically better than what you can get in hardware stores. In addition, age degrades CA properties unless properly stored. Use fresh for important projects/joints.

Second, its not really the glue that's weak or strong in these events, but glue joint design as a system. Discussed several times previously in this string. See also: http://soinc.org/sites/default/files/up ... weight.pdf a long discussion I wrote for Wright Stuff some years back. Still applies, though some changes in joint design are appropriate for bridges from Wright Stuff. Key points, maximize glue area. Try for long grain to long grain, especially on tension or torsion joints. Make sure the wood fights tight, no glues fill gaps well, in spite of the names. think about using gussets.

Third, a big mistake using any glue, but especially CA as it doesn't dry and lighten, is to use too much. Any more glue than needed to very thinly cover the joint area is just added weight. Don't apply whole drops direct from the bottle to each joint.

As to drying time for CA, depends. If there's much humidity, or you use an accelerant, it doesn't take long at all for CA to reach max strength, minutes maybe. If on the other hand you live in one of the desert regions of the US (or its winter in the north and your house is dry from heat), it can take an hour or more. In that case you just about have to use some kind of accelerant. But don't spray it! This just wastes it, fills the air, and eventually sets the glue in your bottle hard! Instead, use an eyedropper or pipette.

Hope that helps some

Jeff Anderson
Livonia, MI

[JimY]

OK, time to put some real numbers in here. I coached elevated bridge for three schools this season: the two middle school teams in my town, plus the high school team. One of the three is going to nationals. I'll start with the two teams that aren't going to nats. Both won first place at state in elevated bridge (Indiana). The B division entry was between 7.1 and 7.2 grams and was loaded to 15 kg (it actually failled at 15.3 kg). The C division entry weighted 6.86 grams and was loaded to 15 kg without failing for a 2186 efficiency.

Second place in B division was the other middle school team that I coach. Their's weighted between 7.3 and 7.4 grams and broke somewhere in the 13 kg range. Second place in C division was about 1200 efficiency.

The winning state B and C division designs had two things in common. First, they both used 9 structural nodes per truss and second, the truss designs for both were completed back in October and not changed since. The second place B division bridge had 11 structural nodes and it was also designed back in October and not changed since. I didn't see the second place C division bridge, however, in general, the bridges that I've seen in competitions have been extremely complicated in comparison with ours. They have dozens of structural nodes per truss. Why are they so complicated?? I hope the readers out there at getting the picture that simpler may be significantly better.

So, the team that I'm working with for nats is taking the 9 node design and the build is about 1.5 hours from being completed. The total number of individual pieces of wood on the completed bridge will be 98. 88 of these are bass. Yes, bass. Note the almost complete lack of balsa. Since it isn't completed yet, I don't know the final mass; however, I expect it to be a shade under 6 grams. I also expect it to be able to do somewhere between 13 and 15 kg of load.

I have experimented with balsa this season, but am staying away from it for nats. One reason is Georgia humidity. Another is limited success. I've built 5 prototypes in balsa this season (4 in B and 1 in C). All 4 in B division broke somewhere between 10 and 12.2 kg of load (and were 4.78 to 5.65 g in mass). Three of these were the at the lower end of this mass range. Three (not the same three as the previous sentence) also used the stiff balsa that is discussed throughout this thread. Also, the designs had either 86 or 90 individual pieces of wood, and of this total, either 20 or 24 were balsa. The lone C prototype used 24 pieces of balsa and I stopped the loading at about 14 kg because it was about ready to fail. It weighted 6.1 to 6.2 grams.

So, I'm hoping that everyone that is going to nats and/or is reading this thread is getting the idea how good the best bridges may be. For B, expect top ones to be somewhere slightly over 2500 efficiency. In an earlier post on this thread, I predicted the best would push 3000. I'm backing off from this, mainly because I can't seem to get there. However, I'm holding out hope that one or two teams can still approach it. On C, I still expect the top ones to push close to 2500 efficiency.

[fli754]

Is the team that's going to nats B or C division?

[Allirog24]

States is in four days, and we are testing a bridge on Monday. Depending on how it does, we are building our final bridge on Monday/Tuesday. I am so excited!

[croman74]

States is in four days, and we are testing a bridge on Monday. Depending on how it does, we are building our final bridge on Monday/Tuesday. I am so excited!

Good luck! Do you have an estimate of what your score will be?