Bridge tester design (for competition)

[mike_302]

I have to design a bridge tester for a tech design class.
In researching the design, I need to decide on which material to use, but there are so many factors in this one choice that it's near impossible to do it all alone. First off, I understand the idea behind the common bridge building competition, but I do not quite understand the numbers enough to make any definite statements or decisions. The major question I guess would be: How much pressure could be applied to a high school or college level bridge in a competition? Assume that a pressure plate is applying pressure to a large (if not the whole) top surface of the bridge.

By Newton's third law, I guess I can assume that whatever amount of pressure is applied to the bridge, I will need to design a frame for our tester to withstand that amount of outward force acting upon it (NOTE: Our tester will closely resemble the one here: http://www.sciteacher.com/bridges/tester.html , with the exception that some sort of motor will be used in place of the jack, and the motor will turn a bolt that will apply presure to the plate)

Any help would be appreciated. Thanks

 

[madmike159]

Well a lot of bridges are made of steel because its strong and cheep. I don't know what kind of tool and meterials you have in your class. If you going to do it out of wood then man made boards like ply wood are very strong and you could use a solid hardwood for supports eg. oak. You need to put in more detail of the kind of bridge you are going to make (single span, suspention etc) and what toold and meterails you are able to use.

 

[mike_302]

Oh, oops, sorry. Perhaps I didn't make it clear enough.

You must've heard of those popsicle stick bridge contests before, correct? YouTube it: there's plenty. But most colleges and high schools use a simple (and inaccurate method) of applying weights on a string to the bridge when they test it. Not accurate at all, nor is it consistent. So our tech design class ISU project is to DESIGN, but not build (because: Congrats to our school board, funding for engineering students is a nono) this bridge tester.

So our main idea is to use a drill, fastened to this frame (similar to the one shown in the link on the original post) that turns a bolt. The bolt will turn inside a (not sure what it's called exactly, but its one of those elongated nuts. they're threaded on the inside like a nut, but they can come in... We'll, I'e seen 6 inch before) nut. The nut is attached to a plate which will, with the turn of the drill, raise and lower upon the victimized bridge underneath it.

There is a bit more automation to the design, but for now that is not as important as understanding how exactly the frame of the bridge crusher should be built... Which will rely upon how much pressure it will have to withstand. Do you guys understand what I'm designing or am I being vague :S ? Its my fault if I am! Sorry :)

 

[DyslexicHobo]

I definitely agree that the standard bridge-testing methods are usually not consistent between trials.

Perhaps my gripes from last year's bridge-building competition in AP Physics can help you even though I have no definitive ideas to offer. One of the biggest problems was where to apply the strain. Should it be along the entire bed of the bridge? Half of it? If you are using the usual method of placing a piece of wood on the bed and hanging stuff from it, how can you ensure that the apparatus will not come into contact with any roadbed supports or substructure while maintaining equal force across the bridge? How can you add more weight without transferring momentum (the way we did it, we had to place 1kg iron weights into a bucket. It was really hard not to drop them; and if someone did drop it their bridge would sustain significant damage)?

Good luck. It sounds like an interesting project! Perhaps you will find a new, revolutionary method to make physics students less angry at their teachers for having unfair bridge competition judgements. :D