Designs B/C

[retired1]

If your parents say OK, PM me your regular email address and I will share some bridge thoughts/plans with you rather than everyone.

[someusername]

Haha.. I don't have a coach.. the teacher supervisor is a biology teacher so he only knows how to help the more-sciency events.

Do you recommend building the sides and then connecting them, or building the top and working around? Also, to solve the problem of the bracings interfering with the loading block on the compression members, would there be any noticeable impact on the performance if the bracings were on the bottom side of the compression members rather than on the top, and having an even number of bracings so the chain goes through the bracings?

I'm not as certain on techniques as these guys are, but I would recommend building the sides of the bridge first then connecting them with some form of a jig. unfortunately, I cannot accurately tell you if putting bracing on the top of the beam or bottom of the beam would make any difference. In my head it feels like the beam would be more "unstable" because you're resting the force away from where the connection is (again do not take my word on that, I'm terrible about knowing about things like this). Lastly, having an even number of bracings I believe transfers the load more evenly across the structure and doesn't give one point more stress than another.

[UQOnyx]

Is anyone else using 1/16" balsa for trusses?

[fanjiatian]

Seems to me the rules are very clear... at any cross sectional point along the 350 mm bridge span, there must be at least one part of the bridge that is at least 5 cm wide... it could be the top chord at one point, and the bottom chord at others... What concerns me is that a bridge could be constructed that is 3 CM wide, with very small 2 CM outriggers glued to one side or the other with a super light 1/64" member running from one to the other, serving no purpose other than to meet the width requirement.

This would be a correct interpretation based on what was discussed at the coaches conference this summer. You will see bridges that are less than 5cm wide with "very small outriggers glued to one side or the other with a super light 1/64" member running from one to the other" to fulfill the 5cm requirement.

Actually I don't think people would be able to get away with just a 1/64" member unless if their main members were also 1/64". The rules state that the bridge has to be at least 5 cm along its entire span at any height. If you positioned your trusses 4 and 62/64 " apart and attached one 1/64" member on each side, it'd only satisfy the rules for that one height (unless if I'm reading the rules incorrectly)

I kind of want to revisit this discussion briefly haha; what if you did the inverse? If you positioned the outside edges of the trusses barely over 5 cm apart and glued a thin member on the inside flat with the top? I wonder if it'd make a difference. It could be worse if the bridge widens under stress at the loading block area; then the weight would rest on the thin glued member if not glued perfectly flat. But it could also be lighter (if I interpreted the part I mentioned above correctly)

[chinesesushi]

Seems to me the rules are very clear... at any cross sectional point along the 350 mm bridge span, there must be at least one part of the bridge that is at least 5 cm wide... it could be the top chord at one point, and the bottom chord at others... What concerns me is that a bridge could be constructed that is 3 CM wide, with very small 2 CM outriggers glued to one side or the other with a super light 1/64" member running from one to the other, serving no purpose other than to meet the width requirement.

This would be a correct interpretation based on what was discussed at the coaches conference this summer. You will see bridges that are less than 5cm wide with "very small outriggers glued to one side or the other with a super light 1/64" member running from one to the other" to fulfill the 5cm requirement.

Actually I don't think people would be able to get away with just a 1/64" member unless if their main members were also 1/64". The rules state that the bridge has to be at least 5 cm along its entire span at any height. If you positioned your trusses 4 and 62/64 " apart and attached one 1/64" member on each side, it'd only satisfy the rules for that one height (unless if I'm reading the rules incorrectly)

I kind of want to revisit this discussion briefly haha; what if you did the inverse? If you positioned the outside edges of the trusses barely over 5 cm apart and glued a thin member on the inside flat with the top? I wonder if it'd make a difference. It could be worse if the bridge widens under stress at the loading block area; then the weight would rest on the thin glued member if not glued perfectly flat. But it could also be lighter (if I interpreted the part I mentioned above correctly)

I suggest you look at the rules clarifications posted about this. They address the any height along the span and whether or not you're allowed to use outriggers to meet the requirement of 5 cm.

[fanjiatian]

My b I didn't realize clarifications were up so soon! Good to see
Just to verify I read it correctly: "At any height" just refers to a single flat horizontal plane at an arbitrary height for which the width is at least 5 cm? So the rest of the bridge could be less? Good to know
My other question about outriggers wasn't a question about the rules so much as a design question. I guess it wouldn't really matter if the outrigger were thin enough.

[bernard]

My b I didn't realize clarifications were up so soon! Good to see
Just to verify I read it correctly: "At any height" just refers to a single flat horizontal plane at an arbitrary height for which the width is at least 5 cm? So the rest of the bridge could be less? Good to know

Yes - the event supervisor should be able to pick any section along the span of your bridge and at least one point along it would measure 5 cm. So if you were to look at a bridge from above, you would see a bridge that is at least 5 cm wide along the entire span. For this bridge, you would see a 5 cm by 46 cm large rectangle when looking from above, so it would be allowed.

[embokim]

I finished my 25 design and after much sweat got my score to 1000! My bridge weighed 12 grams and held 11.6 kilograms. I used the baltimore truss and it worked as a compression bridge. Also I lightened my bridge by just taking out some crosshairs and by making it shorter. Any tips are welcome.

[UQOnyx]

I'm having an extremely confusing problem. I know this place isn't the right place to get clarifications, bla bla, but I think I'm interpreting a rule completely wrong. When i looked at the rule regarding the width of the bridge (3.e), it says that the bridge must be a width of 5 cm OR MORE. However, the loading block's dimensions are 5cm x 5cm (4.c.i). Doesn't that mean that if the width of the bridge is greater than 5 cm, the loading block would fall through the bridge.. Am I wrong?

[bernard]

I'm having an extremely confusing problem. I know this place isn't the right place to get clarifications, bla bla, but I think I'm interpreting a rule completely wrong. When i looked at the rule regarding the width of the bridge (3.e), it says that the bridge must be a width of 5 cm OR MORE. However, the loading block's dimensions are 5cm x 5cm (4.c.i). Doesn't that mean that if the width of the bridge is greater than 5 cm, the loading block would fall through the bridge.. Am I wrong?

There are several ways the loading block would not fall through a bridge that is wider than 5.0 cm. I've illustrated some of them below. Feel free to ask questions if any of them are unclear.