Elevated Bridge B/C

[nejanimb]

Wow - I must say I'm surprised by the change. I guess this just makes things "more elevated" but it feels like it makes things closer to a tower... oh well. I think taking away the rule about maximum cross section of pieces is GREAT - that was always sort of a silly restriction, I thought.

Time to get to work!

[rjm]

The bridges are taller, but the clearance space is also taller, and wider. The change isn't so dramatic for Div. C, but for Div. B the supports will be more indirect. The available truss depth is the same as last year. I don't think these will seem much like towers. At any rate, second-year changes in a new event are incremental, not fundamental. The testing apparatus did not change.

The cross section rule was overdue to be dumped. It may not make much difference to most builders, since large cross sections really aren't needed, but it will give you some freedom on gussets and joints. It will make judging easier, too.

Bob Monetza
Grand Haven, MI

[JimY]

Hey AlphaTauri,

Why do you call a bridge that broke at 14.87 kg an epic failure? Unless your state did something strange in judging the event, like adding a top tier for only the bridges that took a full 15 kg, it is more like an epic success! There was no top tier in the national rules last year for bridges that met all the engineering requirements and were loaded to 15 kg. The top tier was only for bridges that met all engineering requirements regardless of how far into 15 kg they were loaded. It's probably the same this time around.

rjm,

I too felt that the 0.25" rule was overdue for elimination. So, this gives many teams the option to build with things like unaltered popsicle sticks and still make tier 1. We'll have to see what this change brings. My bet is that we'll see some heavy structures, especially in B division, but fewer builds in tier 2.

nejanimb,

Some preliminary calcs show that the overall stresses in the center section for this year's rules (both B and C) can be about the same as last year's if the nodes are placed well. The overall stresses are somewhat proportional to 1/H, where H is max 5.0 cm for B and 7.5 cm for C, same as last season. The stresses on the legs themselves will be lower only if their slope increases. No change if they are vertical.

[andrewwski]

Hey AlphaTauri,

Why do you call a bridge that broke at 14.87 kg an epic failure? Unless your state did something strange in judging the event, like adding a top tier for only the bridges that took a full 15 kg, it is more like an epic success! There was no top tier in the national rules last year for bridges that met all the engineering requirements and were loaded to 15 kg. The top tier was only for bridges that met all engineering requirements regardless of how far into 15 kg they were loaded. It's probably the same this time around.

Yes, I'd say it's definitely about as much as an epic success as you could get. You broke about as close to 15 kg as you could get - which is what you want to maximize your efficiency. Unless you can get your bridge to break at exactly 15 kg, having it hold 15 kg means you're overbuilding - you could in fact hold more - which is lost efficiency.

I still don't understand the fascination with building a bridge that holds the entire weight. The balsa events have not been tiered for years. Thus building a bridge that holds the entire weight is essentially throwing away points for efficiency.

Anyway, trying to figure out the rules from what's been posted, do I have them correct for Division C?
-Clear span 45 cm
-5 cm bearing zones on each end
-Maximum height 20 cm (as opposed to 15 last year)
-Standard minimum clearance - 30 cm long x 12.5 cm high (as opposed to 25.0 cm x 7.5 cm last year)

Time to fool around with some designs even though I'm not in S.O. anymore. Oh, the addiction.

[nejanimb]

When I was first hearing about changes through the grapevine about new changes, I didn't hear that the max height would go up, and that it would just be 12.5x30, and that sounded extremely painful. But, JimY, you're right, this shouldn't be too much harder than last year. There's obviously a bigger emphasis on the legs this time around, so we'll have to figure that out!

Have some ideas I want to sketch today. Looking forward to it!

[sewforlife]

Hey AlphaTauri,

Why do you call a bridge that broke at 14.87 kg an epic failure? Unless your state did something strange in judging the event, like adding a top tier for only the bridges that took a full 15 kg, it is more like an epic success! There was no top tier in the national rules last year for bridges that met all the engineering requirements and were loaded to 15 kg. The top tier was only for bridges that met all engineering requirements regardless of how far into 15 kg they were loaded. It's probably the same this time around.

I would say that that's an awesome success too, unless the weight of the bridge is heavy, and then that cancels out a good efficiency.

[AlphaTauri]

Hey AlphaTauri,

Why do you call a bridge that broke at 14.87 kg an epic failure? Unless your state did something strange in judging the event, like adding a top tier for only the bridges that took a full 15 kg, it is more like an epic success! There was no top tier in the national rules last year for bridges that met all the engineering requirements and were loaded to 15 kg. The top tier was only for bridges that met all engineering requirements regardless of how far into 15 kg they were loaded. It's probably the same this time around.

Yes, I'd say it's definitely about as much as an epic success as you could get. You broke about as close to 15 kg as you could get - which is what you want to maximize your efficiency. Unless you can get your bridge to break at exactly 15 kg, having it hold 15 kg means you're overbuilding - you could in fact hold more - which is lost efficiency.

I still don't understand the fascination with building a bridge that holds the entire weight. The balsa events have not been tiered for years. Thus building a bridge that holds the entire weight is essentially throwing away points for efficiency.

JimY and andrewwski,

My region (PA Central) did add a top tier for bridges that met all requirements and held 15kg. Matter of fact, my partner and I thought we did really well, because it broke right before 15kg- until our coach told us about the extra tier. And that was why my school didn't medal in EB.

[dragonfly]

My region (PA Central) did add a top tier for bridges that met all requirements and held 15kg. Matter of fact, my partner and I thought we did really well, because it broke right before 15kg- until our coach told us about the extra tier. And that was why my school didn't medal in EB.

Wait....They made a top tier ONLY for people who held all the weight?

[jazzy009]

My region (PA Central) did add a top tier for bridges that met all requirements and held 15kg. Matter of fact, my partner and I thought we did really well, because it broke right before 15kg- until our coach told us about the extra tier. And that was why my school didn't medal in EB.

Wait....They made a top tier ONLY for people who held all the weight?

That's ridiculous! then lame 40g bridges would beat awesome 7g ones!

[andrewwski]

Was your team notified at least 30 days prior to the competition in writing about the rule change? If not, you had a legitimate reason to make things ugly for them.

The balsa events used to be tiered, where holding all the weight was the top tier, but I believe that ended the second-last year of tower.