Designs B/C

Friedoyster3
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Re: Designs B/C

Post by Friedoyster3 » March 4th, 2015, 9:25 am

Azn wrote:
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
Yes! I forgot to mention that I use gussets too. Personally, I haven't even need to sandwich the joint, one side of each joint has been enough for my technique.
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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."

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Re: Designs B/C

Post by someusername » March 4th, 2015, 1:02 pm

Friedoyster3 wrote:
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.
I'm confused as to why you wold want the joint cut perpendicular to the ground. Wouldn't the joint be stronger if the contact plane was parallel to the ground? then the structure would naturally sit on on the other beam if the glue was taken away (sorry I'm terrible at explaining things without showing it). I feel like having the plane perpendicular to the ground would make the joint weaker. I've been making bridges with the plane parallel to the ground and I have yet to have a glue issue.

If I'm interpreting this wrong (which is easily a possibility) please correct me.
simplicity is key...sometimes

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Re: Designs B/C

Post by Friedoyster3 » March 4th, 2015, 4:52 pm

someusername wrote:
Friedoyster3 wrote:
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.
I'm confused as to why you wold want the joint cut perpendicular to the ground. Wouldn't the joint be stronger if the contact plane was parallel to the ground? then the structure would naturally sit on on the other beam if the glue was taken away (sorry I'm terrible at explaining things without showing it). I feel like having the plane perpendicular to the ground would make the joint weaker. I've been making bridges with the plane parallel to the ground and I have yet to have a glue issue.

If I'm interpreting this wrong (which is easily a possibility) please correct me.
Think about the components of the force acting on the middle piece. Sure there is a large force directed downward from the loading block, but there also has to be a large horizontal component from the diagonal compression members acting parallel to the joint, which was the force component that caused sjwon3789's failure. I'm not saying that making the plane parallel to the ground won't work--you have proof that it can--but it is possible that it might not be the best way to do it in all situations.
In fact, the more that I think about this, I'm starting to realize that the best possible design could be to make the joint perpendicular to the compression member. With that, the middle piece becomes a wedge shape, if that makes sense.
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2015 Nationals: Bridge 12th; Air Traj 7th; Scrambler 6th; Geo Maps 5th; Team 6th!
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Re: Designs B/C

Post by Ripplestar » March 7th, 2015, 4:07 pm

sjwon3789 wrote: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...
It would seem stronger if you place it on the top so it's sitting directly on the truss. I did that for my regionals bridge and I got a score of 3610

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Re: Designs B/C

Post by thsom » March 8th, 2015, 10:56 am

How do you deal with/solve a torque issue? My bridge design's angled legs (the 'ladder parts') are twisting. They aren't breaking perpendicularly or parallel to the actual compression pieces, but they pieces twist and the truss snaps. I'm not sure what to do for that :/

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Re: Designs B/C

Post by chinesesushi » March 8th, 2015, 12:58 pm

thsom wrote:How do you deal with/solve a torque issue? My bridge design's angled legs (the 'ladder parts') are twisting. They aren't breaking perpendicularly or parallel to the actual compression pieces, but they pieces twist and the truss snaps. I'm not sure what to do for that :/
diagonal x-members between compression and tension would solve this problem. like, a piece of wood from top on one side to the bottom and the other side, and vice-versa to form a 'x'
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Re: Designs B/C

Post by thsom » March 8th, 2015, 5:03 pm

chinesesushi wrote:
thsom wrote:How do you deal with/solve a torque issue? My bridge design's angled legs (the 'ladder parts') are twisting. They aren't breaking perpendicularly or parallel to the actual compression pieces, but they pieces twist and the truss snaps. I'm not sure what to do for that :/
diagonal x-members between compression and tension would solve this problem. like, a piece of wood from top on one side to the bottom and the other side, and vice-versa to form a 'x'
Thank you!

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Re: Designs B/C

Post by SO_2015 » March 15th, 2015, 8:43 pm

Hi,

I have a question related to testing the bridge before the event.

Lets say ,the 'final'(the one which I will bring to the event) bridge can hold 14kg of weight . So before bringing it to the event, with what weight should I test the bridge ?

Is it 7kg(half of weight which it can handle) or full 14 kg(which I don't feel comfortable) or some other weight enough to test the strength?

thanks

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Re: Designs B/C

Post by chinesesushi » March 15th, 2015, 9:30 pm

SO_2015 wrote:Hi,

I have a question related to testing the bridge before the event.

Lets say ,the 'final'(the one which I will bring to the event) bridge can hold 14kg of weight . So before bringing it to the event, with what weight should I test the bridge ?

Is it 7kg(half of weight which it can handle) or full 14 kg(which I don't feel comfortable) or some other weight enough to test the strength?

thanks
Some have stated 50%-60% of the maximum load you think the bridge will holds, so in this case that would be 7-8.4 kg. I would say test it to a score like 1000, or 1500 or something that you feel comfortable with so you know that your bridge can at the very least hold that amount.
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Re: Designs B/C

Post by SO_2015 » March 15th, 2015, 10:55 pm

Thank you .

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