Beam Design Question?

hils0005

for one of my drafting classes I have to design a beam. The beam needs to carry a concentrated load at the midpoint.
some parameters:
4ft span
total materials must weigh under 12oz
Pin reactions at the ends

the person last year that won(highest load) made a simple I joist out of Balsa. I'm wondering if anyone has any experience in this project.
I am thinking of making a Vtype webtruss out of pine, I know mod. of Elasticity for pine is greater than balsa, but more dense which I may run into problems with the weight.

Any suggestions?

Studiot

Pin reactions at both ends?

hils0005

sorry, simply supported rigid beam, reaction only in the y direction, no x or moment

Studiot

Castellated beam or box?

Torsion

hils0005: a couple things to consider.

How is it loaded, will it be 4point or 2 point bending, if 4point and region of constant moment midspan on a 4ft span a 'V' might see a quicker load path but have higher stress do to the off center applied loading.

Why a simple I section is so effective is because it's maximizing 2nd moment of area while minimizing connections. Your truss system may theoretically have a higher capacity but the constructability of all those pieces joining increases your chances of never seeing the full loading potential simply because of how it was made.

Recommendation: if there are no material restrictions get a hold of some Fiber Reinforced Polymer wrap and make another simple I section, but epoxy the wrap to your section. Making sure the direction of the fiber matches the principal plane of loading. FRP is very light and could be cut, if you find your I section is getting close to the 12oz marker and can't wrap the entire member, wrap the critical sections specifically the two web members.


Hope this helps.

joelupchurch

Try this site:

http://www.dragonplate.com/ecart/categories.asp?cID=84

They actually sell carbon fiber I-beams and a lot of other stuff.