Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Lateral Brace Deflection

[Subject Prev][Subject Next][Thread Prev][Thread Next]
This thread is about lateral stability of the otherwise unbraced curved top
chord of those recycled steel bowstring roof trusses for golf course bridge
use, isn't it?  

The exchange by Paul and Mark about distributed, somewhat flexible lateral
bracing, that's provided in abundance, reminds me of the analogous example
of the classical spoked bicycle wheel. The rim is strongly in compression
due to spoke pretensioning, and is in need of lateral bracing just as is the
bowstring truss's top chord. The source of lateral bracing is the spokes,
that is, it is the lateral component of the outward angle the spokes make
toward the wheel hub's flanges. The wider apart the flanges, the stiffer the
array of spokes is for lateral stability. The rim is laterally stiff enough
so that many individual spokes contribute to a stretch of rim not going
sideways. 

Vertically, the rim is less stiff than laterally, but the spoking is very
much more stiff than in the lateral direction. Webs in the bowstring truss
are the equivalent for vertical stability. 

In both directions in the wheel, the Timoshenko -described "beam on elastic
foundation" theory is appropriate. At the time I was an engineering school
senior, the subject intrigued me for both practical and academic reasons, as
I was avidly racing bicycles on wheels I'd assembled myself from chosen
components. I measured up and liberally strain-gaged a sample rear wheel and
loaded it vertically, laterally, and with torque on the hub's core.
Timoshenko's method correlated very well indeed. And I concluded that
enhanced stiffness in the rim in both directions, plus torsionally, and a
wide spread of spoke angle laterally, were good things for me to favor.
(Stiffer, heavier spokes were not, due to attracting greater vertical load
fluctuations to fewer of them at any instant, and thus incurring earlier
fatigue failures.)

In 1981 the excellent Jobst Brandt book The Bicycle Wheel came out and by
computer analysis confirmed my findings. How wheels came to their standard
configuration ages earlier is the result of empirical understandings and
experience with them in use. (Kind of like with residential wood framing:
practice before theory.)

Charles O. Greenlaw, SE
California Road Champion, 1965