Will a 12' 2x6 be strong enough for the arm of a trebuchet with 400-500 lbs of counterweight?

Question

The long end of the trebuchet arm will be 9' long, and the short end will be 2.5' long. Our calculations say it should hold (with the 6" side oriented vertically), but when you look at the 2x6 it just doesn't seem like enough to support that load on one end. Any civil engineers out there who can help?

Answer 1

A few years back a guy who was using just about this same setup reported that it was marginal for strength. It didn't break but he was afraid it would. A 2X8 would be better, with triple thickness from the end of the short side extended 2 1/2 to 3 forward of the axle. This is pretty much what the Mixed Nuts trebuchet uses in competition. Mixed Nuts is a King Arthur style treb, which stresses the arm more than a standard design.    

Answer 2

No.  Plus, you will have a serious twist problem.  You need a twist resistant structure like a box beam.

Answer 3

no. 500lbs moving at around 200 fps? no. those specs should release about the same amount of power as a small car hitting a wall at 20mph. that is large enough to break through a brick house. you will hurt yourselves because unless you bolt it to a 1000lb trailer you wont be able to keep it from flipping over. evergreens grow too fast to have wood dense enough to handle the brutal acceleration on the 9' end. remember, trebuchets were generally made from the heartwood of oak trees. it is very dense and has very small knots(defects) it. ideally, you guys should use something like a ten foot long 2*6 hollow aluminum extruded square stock. it is not cheap but it could probably handle the stress from 150-200lbs. that will fling a 6lbs cantaloupe 50 ft. if you get it right without hurting yourself,scale it up. you really need to make scale models or you wont have a clue what to expect.

ps. big wheels mounted on the side the unit is slinging to really improves the aim. the french found trying to hold it in place is a bad idea. dont fight the force it produces; get a reallllyy long trigger cord!  i saw someone above me said to bolt multiple boards together. do you want to place a hole in the arm that is taking all the stress? i think you can figure that out.

Answer 4

It would be strong enough in theory, provided it doesn't buckle, which it likely will.  When it buckles, watch out - toothpick city.

You should make your beam more square so it won't twist and splinter.  Lag s***w/bolt three 2X6's together to make a beam 5.5" X 4.5" in cross section.  Maybe 2 would even be enough - a lot better than one, certainly.  

Remember the old adage: "When it doubt, build it stout!"