Truss bridge help

[oG x TEDDY]

I have been tasked with creating a truss bridge in my applied physics class.
the bridge must span 50 centimeters must be 10 centimeters wide and 6 inches tall
i am being given 90 drinking straws and 2.5 meters of tape.
my instructor said he will be placing 20 pounds onto the bridge and more so until it breaks,
i posted this for some help on a design that can withstand 20 or more pounds.
if you have some suggestions please post
thank you

 

[Jonluw]

Does the bridge have to be a construction you can move from place to place, or may you construct it on site?

I'm the kind to take a look at your building materials and think "How am I supposed to get the straws to stick together in any useful way? Aw, fuck it, I'll just span the tape over whichever gap they want me to bridge and see how it holds up."

In any case, here's a tip: Use triangles.

I seem to recall from a similar experiment that a bridge with a triangular cross-section was an effective solution. It's hard to apply weight to such a bridge though.

 

[GundamSentinel]

Drinking straws can carry far more load in tension than in compression. Play to that strength, an inverted bridge (road above the truss) will be quite a bit stronger than a normal one.

 

[oG x TEDDY]

Does the bridge have to be a construction you can move from place to place, or may you construct it on site?

I'm the kind to take a look at your building materials and think "How am I supposed to get the straws to stick together in any useful way? Aw, fuck it, I'll just span the tape over whichever gap they want me to bridge and see how it holds up."

In any case, here's a tip: Use triangles.

I seem to recall from a similar experiment that a bridge with a triangular cross-section was an effective solution. It's hard to apply weight to such a bridge though.

the bridge will be placed across a span of 50 centimeters once it's finished

Drinking straws can carry far more load in tension than in compression. Play to that strength, an inverted bridge (road above the truss) will be quite a bit stronger than a normal one.

there wont be a road, only the truss, but thanks for the tip

 

[oG x TEDDY]

Ignore this

 

[Batou667]

Like other posters have said: triangles all the way, each with a side length of one straw. Use the tape sparingly and use any excess to strengthen the length of the bridge.

 

[LiberalSquirrel]

Drinking straws can carry far more load in tension than in compression. Play to that strength, an inverted bridge (road above the truss) will be quite a bit stronger than a normal one.

there wont be a road, only the truss, but thanks for the tip

He/she is giving you the right advice, though. Even without a road, persay, you're still going to have to apply the load somewhere. Here, let me attempt to show what he/she's saying.

_____ top of bridge here. load point also here _____
edge | rest of bridge here, below loading point | edge
edge | built from trianglesi need a spacer here|
edge |giant spacer of doom, ignore completely|

As you can no doubt tell, I am not exactly a master of ASCII art, but I tried. Call my art skills into question as much as you want: but I majored in civil engineering for 3 years at university. So using these tips... make your own idea. It's the only way to learn. Test, test, and test again.

 

[oG x TEDDY]

Drinking straws can carry far more load in tension than in compression. Play to that strength, an inverted bridge (road above the truss) will be quite a bit stronger than a normal one.

there wont be a road, only the truss, but thanks for the tip

He/she is giving you the right advice, though. Even without a road, persay, you're still going to have to apply the load somewhere. Here, let me attempt to show what he/she's saying.

_____ top of bridge here. load point also here _____
edge | rest of bridge here, below loading point | edge
edge | built from trianglesi need a spacer here|
edge |giant spacer of doom, ignore completely|

As you can no doubt tell, I am not exactly a master of ASCII art, but I tried. Call my art skills into question as much as you want: but I majored in civil engineering for 3 years at university. So using these tips... make your own idea. It's the only way to learn. Test, test, and test again.

Thank you
this is the first project in this class and iv'e only had about 5 classes and non of which taught me much about physics, we watched stuff about the big dig in Boston and a documentary about bridges. i had guessed that using triangles was the best idea from the start but never really thought about compression or tension nor where the load would be place. i am very interested in this class and hope i pass it, thank you for the suggestion