Designs B/C

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Re: Designs B/C

Post by iwonder » March 16th, 2015, 2:26 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
If you want to test your bridge (no one says you have to, but it's saved me a few embarrassing moments). Then easing it up to about halfway like chinesesushi said is a good benchmark. Listen for any major creaks or pops as it loads (make sure it's really quiet in the room), these could be signs that it won't hold as much load at the contest. Some small noises aren't too bad, since that's mostly just the bridge 'settling' (which is actually a good thing). And of course if it fails when you put the bucket on because you forgot to glue a joint... well now you can fix it. (unlike me, at the contest... whoops).
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Re: Designs B/C

Post by SO_2015 » March 16th, 2015, 2:36 pm

Thank you .

Like you mentioned, I want to avoid the 'surprise' at event but was debating between that and making bridge weak.

Not to take liberty but may I ask you( and other Guru's) one more question- it seems one way to lighten the bridge is to sand it. In my experience sanding a bridge has it made it weak. Is there a trick to sanding or it is the common experience.

Thanks for taking time out to help.

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Re: Designs B/C

Post by iwonder » March 16th, 2015, 2:39 pm

SO_2015 wrote:Thank you .

Like you mentioned, I want to avoid the 'surprise' at event but was debating between that and making bridge weak.

Not to take liberty but may I ask you( and other Guru's) one more question- it seems one way to lighten the bridge is to sand it. In my experience sanding a bridge has it made it weak. Is there a trick to sanding or it is the common experience.

Thanks for taking time out to help.
It's just a matter of where you're sanding. If you sand on the members then yes, it will get weaker, if you choose the right portions of joints then you'll be just fine. It might save you 0.1-0.3g depending on densities and how your joints are formed.
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Re: Designs B/C

Post by SO_2015 » March 16th, 2015, 7:46 pm

Thank you.

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Re: Designs B/C

Post by SO_2015 » March 19th, 2015, 10:31 pm

I have another question related to the 'outside width' equals to 5 cm.
In my case I am using balsa wood which has a thickness of 1.2 cm (05.inch). This piece of wood is on each side of the bridge.
So, when we calculate 5 cm width, should we include this to calculate the 5 cm or just the 'empty' space between the two sides should be 5 cm.
In latter case, my end to end width would become , 7.4 cm. Which one is correct?

Thanks

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Re: Designs B/C

Post by bernard » March 19th, 2015, 11:05 pm

SO_2015 wrote:I have another question related to the 'outside width' equals to 5 cm.
In my case I am using balsa wood which has a thickness of 1.2 cm (05.inch). This piece of wood is on each side of the bridge.
So, when we calculate 5 cm width, should we include this to calculate the 5 cm or just the 'empty' space between the two sides should be 5 cm.
In latter case, my end to end width would become , 7.4 cm. Which one is correct?

Thanks
The 7.4cm is correct. Just to make sure I'm understanding you correctly: the orange in the image below is included in the width measurement.

Image
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Re: Designs B/C

Post by SO_2015 » March 19th, 2015, 11:30 pm

Thank you Bernard.

My use case is second type or middle one, per your picture.
The block sits on the wood on either side, but is not overshooting the boundary. So by your illustration , I should be ok. right?

So, in the above scenario, the 'empty space between the two wood edges( depicted by orange color) would be 5 cm - 2*(thickness/width of wood).

I am just trying to understand - is the 5 cm from Orange to orange OR between Orange ?

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Re: Designs B/C

Post by SO_2015 » March 19th, 2015, 11:35 pm

Sorry... I may have jumped to conclusion...

I read your response again. ... I think you have said that 'WOOD aka Orange' is included in the width.

Maybe .. I could have phrased my question better-- Does wood width count towards 5 cm?

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Re: Designs B/C

Post by bernard » March 20th, 2015, 12:18 am

SO_2015 wrote:Sorry... I may have jumped to conclusion...

I read your response again. ... I think you have said that 'WOOD aka Orange' is included in the width.

Maybe .. I could have phrased my question better-- Does wood width count towards 5 cm?
Yes, wood width counts towards 5cm. The orange is wood, and the orange is included in the measurement, which is why the second illustration in the image is allowed (with a 5.0cm loading block, the empty space between the wood would be less than 5.0cm).
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Re: Designs B/C

Post by Azn » March 22nd, 2015, 2:08 pm

Obviously everyone is entitled to their own opinion, and a great thing about an event like bridges is that there isn't one single sure-fire way to succeed.
When it comes to pre-testing, i've always been against it, believing that even testing to just 50% would compromise the efficiency of the device. One important challenge in the event was being able to build consistently to avoid "surprises", and testing each device only once (to failure). Then again, I understand that some people like to make sure their devices can "settle in" before the real test, so I suppose there is a level of personal preference.

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