Engineers, help me out?

Crown · Post by **Crown** » 2007-09-25 10:14am

CJvR wrote:How about an arc? It would distribute the load better that a simple beam design. IIRC you can bend heated wood fairly well.

An arc suffers from its own problems, namely it will need a buttress on the ends to stop them from spreading out, and I'm not sure if the rig will do that.

From the diagram (sorry I'm at work and am just skimming here), it looks like an idealised pin support structure, so I don't imagine clamps will be holding the beam. It also places into question whether it will be cheating if he makes an arc out of the wood, but then the clamps are providing the role of the support, where strictly speaking the butress is part of the design of an arc, so he should be using the wood (that is if I'm remembering my arcs correctly, I could be talking out of my ass, so someone feel free to pull me up on that).

Post by **Darth Wong** » 2007-09-25 10:38pm

I don't think an arc is at all practical in this case. You still have to cover the same span, so the moment arm is the same, and you don't have enough wood available to really play with the shape of the arc. It all has to be plausibly cut out of a single 30cm 2x4. The same goes for a pyramidal tower design, similar to an electrical transmission tower; sure, it's a good design and a cool idea, but we're working with a limited amount of raw material and unknown construction skills.

Post by **Beowulf** » 2007-09-25 11:10pm

I suppose the question is: is the increase in strength from forming the wood into an arc, more than enough to offset the decrease in strength from having a thinner piece of wood. A bit of research show that the arc needs to be in the shape of a catenary to have a good amount of strength. Of course, making it into an arc will make it massively more complicated. It's kinda his choice whether he wants to spend the time to make it into an arc.

Lisa · Post by **Lisa** » 2007-09-26 12:06am

would a series of triangles do the required job? from what i understand the bottom of the bridge wouldn't even have to be flat.

General Trelane (Retired) · Post by **General Trelane (Retired)** » 2007-09-26 04:56pm

The key design element will be the glue joints. Because glue is the only fastener allowed, trusses are out.

Wood glue will not hold on end grain, so all joints will need to be side grain with sufficient overlap to get a strength comparable to the base wood. Make sure to clamp the joints snugly but not too tight (that would just squeeze the glue out resulting in a weak, glue-starved joint).

Question: are you limited to only the one 30cm length of 2X4? Or do you have more that are simply all precut to 30cm?

General Trelane (Retired) · Post by **General Trelane (Retired)** » 2007-09-26 05:01pm

On second thought, trusses are right out, but they would be tricky. You would need to glue reinforcing pieces on either side of each joint; these reinforcers would need to be large enough to ensure suitable contact surface with each truss member meeting at the joint. Make sure you orient the grain on these reinforcers so avoid easy splitting.

Seggybop · Post by **Seggybop** » 2007-09-26 08:08pm

Well, my bridge has been crushed.

I ended up making a truss bridge essentially as General Trelane described in the last post, with the members scaled according to the way I calculated the forces would spread and massive gussets surrounding all joints.

The test seemed ok initially, but then support members I glued between the two sides began popping off and shooting into the air. Shortly thereafter the whole thing began to twist and the bottom chords cracked. T_T'

It looks like my problem was that I overestimated the strength of the glue joints (or rather, the ones I was capable of producing given my relative lack of experience). A truss design, being very dependent on the strength of the joints, was very unhappy given this.

I think I would have been a lot better off making I-beam variation. I had wanted to do a truss because that's apparently what did the best the previous year, but it seems apparent now that the deciding factor for that is really craftmanship.

It's alright though, I still did much better than most of the people, and I have by far the most detailed plans and design history to turn in (in no small part due to the help from this thread; I'm pretty sure I'm the only one who bothered to do significant calculations) so I'll likely get a pretty good grade. I'm still going to try a rebuild with an I-beam and I'll post how that goes.

So anyway, massive thanks to everyone who provided assistance. I really appreciate it and I'm sure I'll end up with a better grade than I would have otherwise.

Hawkwings · Post by **Hawkwings** » 2007-09-26 08:38pm

how much force before your bridge gave out?

Post by **Beowulf** » 2007-09-26 08:50pm

Technically fasteners could have been used if they were made out of wood (wood peg). Drill a hole into the beam, then hammer a glue coated peg into the hole. One of the keys to a strong glue joint is a large surface area. Which is why you frequently see the a joint that looks like the ends are serrated, kinda.

Post by **Darth Wong** » 2007-09-26 10:55pm

Seggybop wrote:Well, my bridge has been crushed.

I ended up making a truss bridge essentially as General Trelane described in the last post, with the members scaled according to the way I calculated the forces would spread and massive gussets surrounding all joints.

The test seemed ok initially, but then support members I glued between the two sides began popping off and shooting into the air. Shortly thereafter the whole thing began to twist and the bottom chords cracked. T_T'

It looks like my problem was that I overestimated the strength of the glue joints (or rather, the ones I was capable of producing given my relative lack of experience). A truss design, being very dependent on the strength of the joints, was very unhappy given this.

I think I would have been a lot better off making I-beam variation. I had wanted to do a truss because that's apparently what did the best the previous year, but it seems apparent now that the deciding factor for that is really craftmanship.

Always remember the KISS principle.

It's alright though, I still did much better than most of the people, and I have by far the most detailed plans and design history to turn in (in no small part due to the help from this thread; I'm pretty sure I'm the only one who bothered to do significant calculations) so I'll likely get a pretty good grade. I'm still going to try a rebuild with an I-beam and I'll post how that goes.

So anyway, massive thanks to everyone who provided assistance. I really appreciate it and I'm sure I'll end up with a better grade than I would have otherwise.

Good to hear.

General Trelane (Retired) · Post by **General Trelane (Retired)** » 2007-09-28 03:16pm

Hawkwings wrote:how much force before your bridge gave out?

Yes, please do tell. And how much did the 'winning' bridge take? What was its design?

Beowulf wrote:Technically fasteners could have been used if they were made out of wood (wood peg). Drill a hole into the beam, then hammer a glue coated peg into the hole. One of the keys to a strong glue joint is a large surface area. Which is why you frequently see the a joint that looks like the ends are serrated, kinda.

With the limited amount of material, making pinned joints would have been a problem anyway. But yeah, with glued joints, the key is surface contact (and not starving the joints--I learned the hard way that I have a penchant for using too much clamp force). Using serrated ends is called finger-joining, and it both increases the surface area and avoids end grain on end-to-end joints.