[TowerTalk] Is A Tower Weaker in Some Directions?

[Jim Lux]

>
>       Now consider the case when the wind is from the single leg side of the
> tower and towards the opposite face.  The single windward leg will be in
> tension and the two opposite legs will be in compression.  Unlike the first
> example with the wind applied on the face, the two leeward legs of the tower
> resist the downward force from the wind with twice the counteracting force
> of a single leg.  To get this tower's two legs to buckle will require twice
> the wind force on the opposite leg vs the single leg example above.
>

Actually, this isn't necessarily the case, because straight tension 
failure is very different than straight compression failure is very 
different from buckling failure.

The tower leg that in tension will fail because the material strength is 
not enough: it's all about cross sectional area * failure load per unit 
area.

The tower leg that is in compression can fail either by straight 
material failure or by buckling. Buckling is more about the 
length/diameter ratio than the material properties, and a lot about 
symmetry and lack of dents.

We can see a practical demonstration of this:
I would think that it is very difficult for a person to pull an aluminum 
can apart (without twisting and tearing it first), but it is easy to 
crush an aluminum can (column failure by buckling), long before it fails 
by simple material failure.

It is true that an Engineer may design a lattice tower so that the 
individual members receive loads that will buckle at about the same 
overall structural loading that they would fail by straight 
compression/tension.  Efficient designs tend to have all failure modes 
occur at the same time/load, and excessive strength in one mode often 
means excessive stiffness in some way, which can lead to premature 
failure.  (the classic example is a 1000 lb breaking strength rope and 
1000 lb breaking strength piano wire in parallel, not holding 2000 lbs)