Jim Lux wrote:
> I think that the problems, if any, would come from changing the distribution
> of stresses, not from the increased stress at the base, for instance.
> There's a classic exercise in structures textbooks where adding a gusset to
> a "T" junction makes it much weaker, because the increased stiffness from
> the gusset increases the loads just beyond the gusset. There's another
> interesting example with parallel piano wire and steel cables, where the
> fact that one is much stiffer than the other makes the load transfer not as
> intutively expected. In theatrical rigging, there's always concerns about
> "overguying" (particularly when used to "stiffen" a horizontal member
> holding up a lot of lighting instruments) making that piece of the structure
> too stiff, relative to the rest of the structure.
> I think the take home message is that when it comes to structural analysis,
> simple analyses of a modification might miss some key factor, especially if
> you don't understand what went the design of the original structure. A
> casual inspection (unless you happen to be in the tower designing business)
> might not reveal where the "load limiting" joints or members are.
OK. In the case of the thread subject, I still don't see the worry. The
change in stress distribution is mainly one of relief. What component of
the tower can see an increase in stress? The very top (just below the
guy wires) will have some vertical compression that it never had before,
but everything below will see a big reduction in peak stress, I believe.
Perhaps if someone could illustrate the issue with an exaggerated design
of some sort.....
Steve K8LX
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