Designs B/C

Locked
User avatar
bernard
Administrator
Administrator
Posts: 2222
Joined: January 5th, 2014, 3:12 pm
Division: Grad
State: WA
Pronouns: He/Him/His
Location: Seattle, WA
Has thanked: 127 times
Been thanked: 395 times
Contact:

Re: Designs B/C

Post by bernard » February 24th, 2015, 8:28 pm

bearasmith wrote:
bernard wrote:
noobforce wrote:Okay. I'll start building some "normal" bridges and see how they go.

Also, what kind of tension members are you using? if you are running single tension members? So far I've been experimenting with 1/8x1/16 balsa and they seem to fail also a lot, way before the compression members' max load. Maybe it's because I am cutting them by hand. My shipment for machine-cut balsa will not come for a few days so I will be pressed on building/testing time.
How muchyour bridge hold when your tension fails? For one of my best bridges so far, it failed at 21 kg with 1/16" x 1/16" balsa tension. I don't think its because you're cutting your tension by hand. It probably has towith the density of the tension you are using.
How light were the 1/16" x 1/16" tension members?
I don't have my wood with me right now, but they definitely weren't squishy. I would say they were medium density (10-14 pounds per cubic foot). Pretty stiff.
"One of the ways that I believe people express their appreciation to the rest of humanity is to make something wonderful and put it out there."

bearasmith
Member
Member
Posts: 30
Joined: October 22nd, 2013, 7:31 pm
Division: C
State: MD
Has thanked: 0
Been thanked: 0

Re: Designs B/C

Post by bearasmith » February 24th, 2015, 8:36 pm

I was just wondering how dense the wood was, because every time I used balsa wood for tension it broke. But now that I use bass wood, the tension members are much heavier.
Medal Count: 52

User avatar
bernard
Administrator
Administrator
Posts: 2222
Joined: January 5th, 2014, 3:12 pm
Division: Grad
State: WA
Pronouns: He/Him/His
Location: Seattle, WA
Has thanked: 127 times
Been thanked: 395 times
Contact:

Re: Designs B/C

Post by bernard » February 24th, 2015, 8:43 pm

bearasmith wrote:I was just ing how dense the wood was, because every time I used balsa wood for tension it broke. But now that I use bass wood, the tension members are much heavier.
If your efficiency is greater even with heavier tension members, it is a good trade-off. Try using more dense balsa tension members if you want to keep tension light but also strong.
"One of the ways that I believe people express their appreciation to the rest of humanity is to make something wonderful and put it out there."

IvySpear
Member
Member
Posts: 18
Joined: January 26th, 2015, 5:38 pm
Division: C
State: NC
Has thanked: 0
Been thanked: 0

Re: Designs B/C

Post by IvySpear » February 25th, 2015, 6:46 pm

Hey this may sound like a very churlish question, but when you buy wood, where do you see the density number? Or do you just somehow feel by touch? And what counts as a "high density"? I just classify wood into Balsa, Bass, foamy, decent, and stiff. (Not including thickness).
Self-actualization is an irony, for when you achieve it, you realize you have not achieved it.

User avatar
bernard
Administrator
Administrator
Posts: 2222
Joined: January 5th, 2014, 3:12 pm
Division: Grad
State: WA
Pronouns: He/Him/His
Location: Seattle, WA
Has thanked: 127 times
Been thanked: 395 times
Contact:

Re: Designs B/C

Post by bernard » February 25th, 2015, 6:58 pm

IvySpear wrote:Hey this may sound like a very churlish question, but when you buy wood, where do you see the density number? Or do you just somehow feel by touch? And what counts as a "high density"? I just classify wood into Balsa, Bass, foamy, decent, and stiff. (Not including thickness).
Almost always, the density of the wood won't be written on it. You might be able to feel how dense it is, but it's better to bring a scale to the hobby shop to weight wood to find the densities you want. I use the density classifications on the Specialized Balsa website: Light Balsa at (6-10 pounds per cubic foot), Medium Balsa at (10-14 pounds per cubic foot), & Heavy Balsa at (14-19 pounds per cubic foot); Extra Light Balsa at (6 or less pounds per cubic foot) & Extra Heavy Balsa at (19+ pounds per cubic foot).

Your way of classifying wood also works well since you would want to be sure your wood seems stiff if that's how you want it.
"One of the ways that I believe people express their appreciation to the rest of humanity is to make something wonderful and put it out there."

sjwon3789
Member
Member
Posts: 107
Joined: December 31st, 2012, 3:45 pm
Division: C
State: VA
Location: Virginia
Has thanked: 0
Been thanked: 0

Re: Designs B/C

Post by sjwon3789 » February 26th, 2015, 10:43 am

http://i.ytimg.com/vi/kkXi8OjXnWk/hqdefault.jpg

What's the advantage to putting the loading block in the center in the bridge like that, rather than on the top?
Seems like it'll be more stabilized if it was on the top...
2013 Events: Boomilever, Keep the Heat, WIDI
2014 Events: Boomilever, Geologic Mapping, Mission Possible, Scrambler
2015 Events: Air Trajectory, Bridge Building, Mission Possible

DoctaDave
Member
Member
Posts: 167
Joined: December 28th, 2013, 10:59 pm
Division: Grad
State: CA
Has thanked: 0
Been thanked: 0

Re: Designs B/C

Post by DoctaDave » February 26th, 2015, 11:06 am

It's actually more stable on the bottom since the center of gravity is lower. You may have seen many bridges that are loaded from the top that topple over because they have to be built near perfectly in order to be stable. Loading from the bottom like in that picture can be more consistent and reliable but the trade off is that you have to design the bridge in such a way to transfer the load from the bottom to the top pieces in compression. I've built and seen bridges do very well with both designs however so you'll just have to try different designs and see what you do better with

sjwon3789
Member
Member
Posts: 107
Joined: December 31st, 2012, 3:45 pm
Division: C
State: VA
Location: Virginia
Has thanked: 0
Been thanked: 0

Re: Designs B/C

Post by sjwon3789 » March 3rd, 2015, 2:51 pm

Is this glue issue or just the thickness of the wood?

http://i.imgur.com/z2lBI5G.jpg

That was the only place that broke at 10kg mark. Seems like a lot of pressure was centered there. I'm retesting it w/ thicker wood but there may be variable since it may have become weakened (nothing broke except all joints - although one piece of wood broke at the VERY tip but I can just replace that).
2013 Events: Boomilever, Keep the Heat, WIDI
2014 Events: Boomilever, Geologic Mapping, Mission Possible, Scrambler
2015 Events: Air Trajectory, Bridge Building, Mission Possible

Friedoyster3
Member
Member
Posts: 99
Joined: February 1st, 2009, 6:38 am
Division: C
State: IN
Has thanked: 0
Been thanked: 0

Re: Designs B/C

Post by Friedoyster3 » March 3rd, 2015, 3:28 pm

sjwon3789 wrote:Is this glue issue or just the thickness of the wood?

http://i.imgur.com/z2lBI5G.jpg

That was the only place that broke at 10kg mark. Seems like a lot of pressure was centered there. I'm retesting it w/ thicker wood but there may be variable since it may have become weakened (nothing broke except all joints - although one piece of wood broke at the VERY tip but I can just replace that).
Glue problem. The way you have that set up, the compression forces are nearly parallel to the joint, if that makes sense. If I had to guess, it essentially sheared the joint apart. My suggestion would be to redesign the join so that the diagonal compression member makes a butt joint to the middle comprehensive piece. That is, the contact plane between the two pieces is perpendicular to the ground. That's what I've been doing and I've had great success with it.
University of Michigan Aerospace Engineering
Munster High School, Indiana
2015 Nationals: Bridge 12th; Air Traj 7th; Scrambler 6th; Geo Maps 5th; Team 6th!
"Science Olympiad is, was, and always will be one of the greatest things that ever happened to me."

Azn
Member
Member
Posts: 25
Joined: May 31st, 2012, 7:57 pm
Division: Grad
State: PA
Has thanked: 0
Been thanked: 0

Re: Designs B/C

Post by Azn » March 4th, 2015, 8:51 am

sjwon3789 wrote:Is this glue issue or just the thickness of the wood?

http://i.imgur.com/z2lBI5G.jpg

That was the only place that broke at 10kg mark. Seems like a lot of pressure was centered there. I'm retesting it w/ thicker wood but there may be variable since it may have become weakened (nothing broke except all joints - although one piece of wood broke at the VERY tip but I can just replace that).
I would agree with Friedoyster that it is a glue problem, but I would suggest using gussets to increase the joint strength. This would mean taking a thin piece of wood and laying it over the side of the joint (covering the joint), you could even do this on both sides .

Based on the way your joint looks, this should greatly increase the joint strength without making the design that much heavier.

Edit: This method is sometimes also known as "sandwiching". I think a good starting place would be to put a piece of 1/16 x 1/4 along both sides of the top compression piece that are long enough so that they completely cover both joints

Locked

Return to “Bridge Building B/C”

Who is online

Users browsing this forum: No registered users and 1 guest