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Buckling of the Popsicle Crane Boom

A beam buckling

We have put quite a bit of effort in understanding the forces on the crane boom. Now we need to start figuring out what we are going to do about them.  (Refer to the post crane-boom-analysis-part-2) The fact that the whole boom is under compression loading explains a lot to me as to why real cranes are designed the way they are, but it also creates some special problems for designing one made out of popsicle sticks.  Initial testing will be building a tower and testing the tower with a force pressing straight down.

The tower we will be building is four vertical columns tied together in such a way that this assembly also becomes a column.  A long column under compression has the problem of buckling.   The assumptions are that the column is loaded with the force pushing directly downward on the column and that the force is not enough to actually crush the column.  It is also assumed in the simple case that the column is made of a continuous material of constant width and thickness.  The key word there is assumption: that word should always set a red flag in your mind.

No material, no matter how perfectly manufactured, will meet those assumptions.  Our popsicle sticks with the lap-joints certainly does not meet those assumptions.  Once a weak part is found the material will tend to bend as shown in the first picture.  Once the material bends, more stress will be introduced in the weak part because the forces are no-longer symmetric throughout the material.  This can lead to a quick, catastrophic failure.   This happens when the material buckles and in the case of metal kinks.   In the case of wood the outer fibers are stressed to the breaking point and then the fault quickly continues because there are fewer remaining fibers to hold the load and the wood splits.

Basically, there is no way we can calculate the stresses on our popsicle stick columns because there is too much unknown.  However, we can get a feel for what is helpful and what is not. A beam subjected to a sideways force, a bending force, will oppose that force depending upon the strength of the material, the cross section area of the material and the distance of the material from the centerline of the beam (the width of the beam).  The distance is very important because it increases the lever arm for this opposing force.

The top of a crane boom from some government archive pictures.

The way we will deal with the problem is first to make the four corner columns of our boom as strong as possible and then just like the  real crane we will “lattice” the corner columns together with cross bracing.

I created a video showing the problem and several drinking straws experiencing the buckling failure.   I also explain how I intend to build the first test columns and some more of the assumptions I am making as well about the reasons for making the beams out of popsicle sticks instead of square dowel rod.   (It sure would be a lot easier to use square dowel rod, but that doesn’t require any finesse… and learning.)

Gary

Announcement:   I am still working on changing the Categories on this blog.  I will check how that affects people that have registered as users and have more control on the e-mails they receive and send out a special e-mail once I have completed the task.

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