DIYbanter - View Single Post

(Robert Bonomi) wrote:

Engineering for systems under stress, particularly dynamic stresses, is a
_complex_ and _complicated_ subject.

This is a key distinction. I would guess that issues of dynamic stress
drop to insignificance in a structure like a house, though. (But I am
not an engineer, nor do I play one on tv, so take that with a grain of
salt.)

*ALL* the components have to be considered, =both= singly, and in combination.
'Over-building' _one_ component can result in excessive transfer of stress
to _other_ components, Leading to failure of _that_ component under conditions
that are _less_ severe -- as measured for the overall system -- than the
original design was spec'ed to handle.

There are numerous real-world instances of this *exact* thing happening.
One of the easiest places to find them is in the world of home-built, plans-
built, aircraft.

I was thinking of exactly the same example, but it seems different in
that dynamic stress is a very big factor here.

So far, I've done this only as a thought experiment, but I'm tempted:
Go to a hobby shop and buy some balsa and build 4 "structures": 2 24"
beams consisting of a 1/4" square piece of balsa, and two more such
beams where 20" on one end is stiffened by gluing another 1/4" square
above and below the primary one. Clamp all 4 structures to your bench
so that 2" is held rigidly, and the remaining 22" is without support.

Now, from the ends of one of each type of beam, hang increasing
weights until they break. My guess is that the breaking point will be
pretty close to the same. (you might have to adjust for the decreased
arm over which the force is applied in the case of the bending beam if
that gets significant.)

Part two is to take 1/2 that weight, attached by string to the end of
the beams, and drop it from various heights. My guess is that here the
stiffened beam will break much sooner, as the more flexible beam
absorbs the dynamic stress with the springiness over its length.

If reality matches my thought experiment, that would say that
increasing the strength of a header in a house is probably a harmless
waste of materials, while increasing the strength of a wing may well
be disastrous.

Another dynamic example is automobile suspensions: if you are going to
significantly strengthen (stiffen) the springs, you should make sure
that the structure to which the suspension is attached has the
strength to accept the increased dynamic load.
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.