Test Supports

[SPP SciO]

How different would results be, if you tested your bridge across a 20x20 opening, without resting on the test supports? In other words, will the platform actually support some weight?

Looking at a google image search I can only infer that either, it hardly makes a difference, some event supervisors ignore the test support rule, or this is something new and most pics are >1year old - most bridges seem to be resting directly on a table.

Our team has tested pretty haphazardly; we haven't built and tested enough to be worrying about those details (construction techniques and designs pose enough of a challenge). Is this a major deal? Our div b bridges clear the span by about 0.5cm on each side but adding weight with that little surface area of contact seems awfully precarious. Hoping it's one of those things where physics trumps intuition.

[DoctaDave]

Testing across the 20x20 cm hole instead of the test supports makes a HUGE difference. The span is 4/7ths of what the B div rules state, meaning the mass of the bridge can effectively be cut down to 4/7ths of the original mass.
An ES cannot ignore the rules of the test supports. The whole point of bridge building is to design and build a structure that can support a load across a specified span, which is either 35 or 45 cm. It would defeat the purpose of the event to change the testing apparatus parameters given by the rules. Also, if the bridge doesn't span the given distance of 35 or 45 cm your team gets tiered, which means you get placed below all the teams that had no construction violations, and at this point in the season with only states and nationals, (and maybe a few more regionals?) left, getting tiered is basically getting last, or close to it at least.

In a nutshell, don't use the 20x20 cm hole to test, use the test supports.

Also the .5cm on each side should be enough to support the entire bridge on the test supports. If you've tested it so far with just .5 cm of overhang on the test supports and it hasn't yet failed, it should be fine in the future as well.

[bernard]

Looking at a google image search I can only infer that either, it hardly makes a difference, some event supervisors ignore the test support rule, or this is something new and most pics are >1year old - most bridges seem to be resting directly on a table.

If you look at the video on the Science Olympiad TV channel, for the first bridge shown, without the tests supports it would have a span of only 20cm. For older pictures/videos, you might be looking at Elevated Bridge, where test supports weren't necessary because of the rules.

[dholdgreve]

How different would results be, if you tested your bridge across a 20x20 opening, without resting on the test supports? In other words, will the platform actually support some weight?

Looking at a google image search I can only infer that either, it hardly makes a difference, some event supervisors ignore the test support rule, or this is something new and most pics are >1year old - most bridges seem to be resting directly on a table.

Our team has tested pretty haphazardly; we haven't built and tested enough to be worrying about those details (construction techniques and designs pose enough of a challenge). Is this a major deal? Our div b bridges clear the span by about 0.5cm on each side but adding weight with that little surface area of contact seems awfully precarious. Hoping it's one of those things where physics trumps intuition.

I think it depends on the design... If you have designed your bridge to span a 350 mm space, then place it over a 200 mm space, I doubt it makes any difference at all... especially if you are top chord bearing, where the load is being transferred directly down columns with ends bearing on platform.

If you have designed the bridge to span only the 200 mm opening, obviously the requirements are much less.

[SPP SciO]

If a bridge meets the div. B rules, it should obviously be able to sit on the test supports, which are 35.0 cm apart. For argument's sake, let's imagine a bridge that's 36.0 cm long. It's an arch bridge, and the loading block is placed centered on top of the structure. This bridge has a mass of 10.0 g, and when tested properly (0.5 cm of bridge touching each test support) the total mass held is 10 kg, for a score of 1000.

What type of scores would you get with the same bridge if,

A) You center the bridge over the 20 cm square, no test supports, 8.0 cm of bridge touching the base of the platform on each side of the hole?
B) You place the bridge on a scale, and load into a perfectly balanced bucket (this is hypothetical, of course) centered on top of the bucket?

The instinctual side of my brain says that the score from A will be much greater than 1000, and the score from B will be even greater than that. The more rational side thinks that, in a perfect world, both methods would also yield a score of 1000, but the inevitable design/construction/loading variables will lead to A & B having slightly higher scores because of the added stability - maybe just frictional contact with the base resists some twisting? If the amount of bridge that's directly supported IS a major factor, I'm surprised that more teams don't go with a longer length - whatever mass gain from a few extra cm of bridge would be more than worth it for the added stability, perhaps? I read someplace online (probably Garrett's Bridges website) that it's possible to test in the way described in method B above, even just pressing down directly with your hand for a rough estimate. Has anyone tried that with any success?

Obviously it's important to practice in competition-like situations, so we'll be testing with test supports from now on; I was just wondering if there was a rule of thumb on how much difference it makes or if it depends highly on the design, etc.

[dholdgreve]

If a bridge meets teh div. B rules, it should obviously be able to sit on teh test supports, which are 35.0 cm apart. For argument's sake, let's imagine a bridge that's 36.0 cm long. It's an arch bridge, and teh loading block is placed centered on top of teh structure. This bridge has a mass of 10.0 g, and when tested properly (0.5 cm of bridge touching each test support) teh total mass held is 10 kg, for a score of 1000.

What type of scores would you get with teh same bridge if,

A) You center teh bridge over teh 20 cm square, no test supports, 8.0 cm of bridge touching teh base of teh platform on each side of teh hole?
B) You place teh bridge on a scale, and load into a perfectly balanced bucket (this is hypothetical, of course) centered on top of teh bucket?

teh instinctual side of my brain says that teh score from A will be much greater than 1000, and teh score from B will be even greater than that. teh more rational side thinks that, in a perfect world, both methods would also yield a score of 1000, but teh inevitable design/construction/loading variables will lead to A & B having slightly higher scores because of teh added stability - maybe just frictional contact with teh base resists some twisting? If teh amount of bridge that's directly supported IS a major factor, I'm surprised that more teams don't go with a longer length - whatever mass gain from a few extra cm of bridge would be more than worth it for teh added stability, perhaps? I read someplace online (probably Garrett's Bridges website) that it's possible to test in teh way described in method B above, even just pressing down directly with your hand for a rough estimate. Has anyone tried that with any success?

Obviously it's important to practice in competition-like situations, so we'll be testing with test supports from now on; I was just wondering if there was a rule of thumb on how much difference it makes or if it depends highly on teh design, etc.

Based on the hypothetical situation you have described above, the span remains the same...35 cm. The only thing that changed was the opening that you are testing over... Imagine a bridge over a river... Does the max load of the bridge vary, depending on the depth and width of the river below? The span is fixed, and no matter what the opening is below, it shouldn't matter... Now if you modified the bridge by dropping vertical bearing columns at each side of the 20 cm opening, effectively narrowing the span from 35 cm to 20 cm, it would carry substantially more load, but that would not be the same bridge, so it's apples and oranges.

Of course I need to qualify this with..."So long as in both situations the "feet" of the bridge are fully bearing on the area beneath it."

[dholdgreve]

...and what the heck is wrong with the spell check constantly changing "the"?

[bernard]

...and what teh heck is wrong with teh spell check constantly changing "teh"?

SmartWatch support and mobile improvements are going on so there might be some bugs around the site. I think teh spelling glitch will be fixed by tomorrow...