Balsa or Bass

[havenguy]

I have another question about stiffness. We used the stiffest peices of wood for our tower for states, and our tower ended up weighing about 2 grams more than our previous ones. So does more stiffness mean greater mass?

In structural engineering, stiffness is defined as the quantity that relates displacement to force in a member.

For example, if a member is subjected to an axial force (either compression or tension), then the relationship between the axial displacement (shortening or elongation of the member) and the force can be written as:

F = k d

where F is the axial force, d is the axial displacement and k is the stiffness coefficient of the member.

From the above equation, you can see that the stiffer the member (the larger the coefficient) the less the member is going to displace when it is subjected to the force F. This concept also applied to buckling; the stiffer the member, the larger the force has to be to cause the member to buckle (to displace).

But how is stiffness related to the weight of a member?

In general, an stiffness coefficient is determined based on several properties of the member such as its size and its modulus of elasticity. For wood, since modulus of elasticity is directly related to wood density, then we know that stiffer members have a higher density. And since, a member's weight is directly related to its density, then the denser the member, the more it weighs.

Alright, a bit confusing, but denser members means greater stiffness, resulting in greater mass. That makes sense. Thanks

[jander14indoor]

<SNIP>In general, an stiffness coefficient is determined based on several properties of the member such as its size and its modulus of elasticity. For wood, since modulus of elasticity is directly related to wood density, then we know that stiffer members have a higher density. And since, a member's weight is directly related to its density, then the denser the member, the more it weighs.

Weeeellll now, there IS a big "Now that depends..." and a "Yeah but..." to the generality that stiffer wood is denser wood.

A VERY important thing to understand about that statement is the "In general,..." part. For wood, especially balsa, while the trend is correct, there is a LOT of variability around it. For a given density stick, you can find a VERY wide range of stiffness. Much more than two to one. Of course extremely stiff light wood is rare, it can be found and SHOULD be evaluated. Realize that if you don't check, you also have a good chance of selecting floppy wood of a given density, disaster

For the very best towers, you must select wood that is on the stiff end for the given density you select, it will increase your load carrying lots and efficiency LOTS because you can use less bracing, thinner sections, etc.

Note, this is one of the reasons why two seemingly identical towers (including weight) can have very different efficiencies. The tower with floppy (or inconsistent) wood will fail far before the tower with stiff wood, SAME DENSITY!!

Jeff Anderson
Livonia, MI

[lucwilder42]

No one likes floppy wood

[havenguy]

Yeah, we are forced to use bad wood because of the availability of the balsa wood in our area.

[Frogger4907]

Order it online?

[havenguy]

Order it online?

We also do that, but we have a lot of people working on towers, so we are kind of forced to spare because we have to split the wood between 3 or 4 groups.

[Littleboy]

give you the best wood?

[havenguy]

Uh, kind of an unclear question. Do you mean I should take the best wood from the stores, the stores give us the best wood