[Towertalk] Tower base "solution"

[K7LXC@aol.com]

In a message dated 4/4/02 3:48:45 PM Pacific Standard Time, 
rhodes@evertek.net writes:

> Possibly because Steve gave no real answer other than his usual & prudent 
>  "do what the manufacturer says". I think the man was trying to ask "Why 
>  doesn't the manufacturer say that this is OK?" Steve gave a rather flip 
>  answer to a serious question. Maybe someone else out there can fill us in 
>  with a little more background. I am no engineer, just a pill pusher, but 
it 
>  seems like if you can use forms in the ground & backfill around the base, 
>  you could just as well take a base that was formed elsewhere and use it as 
>  well. But like Steve says, you have to get it engineered. Can anyone out 
>  there give us a reasoned answer? I am sure that it would require more 
>  concrete. How much more?
>  
    Part of the answer is that tower base specs typically call for them to be 
installed in "undisturbed" earth. Using some sort of precast concrete in the 
hole or using forms is not generally acceptable UNLESS you get an engineer to 
bless it. 

    Much of the integrity of the tower base comes from the interface of the 
concrete and earth so the engineer makes certain assumptions about the 
holding capacity of the dirt. Yes, there are some times when you might use a 
form in the hole but then you'd need to be careful about compacting the soil 
so that the soil will actually contribute something. 

    Yes, the tower base consists of a whole bunch of weight to get the center 
of gravity down low so you could come up with other schemes to do so. Will 
they work? Maybe. Would you make such 'back-of-envelope-engineering' 
decisiions for YOUR tower? Would those decisions enable you to sleep during 
that *big* storm that'll be rolling through with no thoughts as to the 
reliability of the installation? 

    And, yes, the forces acting on a traffic light are MUCH different than 
those on the typical ham tower; i.e. height above ground, total weight, total 
windload, torque, etc. 

Cheers,
Steve    K7LXC
TOWER TECH - 
Professional tower services for industry and amateurs