[TowerTalk] concrete bases for freestanding towers

[Jim Lux]

On 5/13/11 5:18 AM, WA8JXM wrote:
>
> Is my memory faulty, or have the recommended bases grown over the years?  Were the old recommendations inadequate, or has everyone grown super conservative over the years?   "If one yard is adequate, three will be better, so let's use five yards"???
>
The other thing that's changed is the design wind load.  I suspect 
(although haven't gone back to look at old versions) that the wind speed 
for which the tower installation is designed has increased over the years.

This is one of those fairly subtle things, and also fits in with the 
whole "definition of failure" issue.  The mfr may consider failure any 
permanent deformation, while the ham may consider anything that doesn't 
result in a pile of scrap metal as perfectly fine.

The "failure probability" might have been driven down for some reasons. 
  Maybe before, the design covered 2/3s of the typical installations (in 
terms of material properties, soil bearing strength, etc.) and now the 
design covers 99%, so it has more "margin" to cover the edge cases.

One thing driving that is city regulations. A LOT more cities are 
requiring "engineered" installations: that is, backed up by calculations 
and analysis rather than empirical recommendations from the mfr.


And, of course, there's the economics.  If you're buying everything from 
scratch, adding $100 for another yard of concrete to a $10,000 tower 
project isn't a big percentage change in the project cost. That's the 
sort of rationale that would go into a generic engineering design: 
what's the penalty for making it a bit bigger and making the design more 
robust.

On the other hand, if you're putting up a tower you already have, and 
you're doing your own labor, etc., then the $100 is a bigger fraction of 
your cash outlay.