[TowerTalk] field-day mast anchoring question

[jimlux]

On 1/22/16 7:31 AM, dw wrote:
>
> For those familiar with the AB-621 military launcher mast units.
> Lets say there was an interest in using this on a field day event, where
> there is a concern that the ground is too soft to safely establish
> anchors.
> Is there a possibility of using concrete wall-barrier blocks bolted
> together, as dead-weights to safely hold the anchors?

Sure..

The catering/party equipment rental places do this all the time, as do 
riggers for theatrical/music performances.  They have big concrete 
blocks with a big loop sticking out of the top (rebar or a piece of pipe 
bent in a U) that they connect their guys to. We've got a 90 foot-ish 
structure in a parking lot at JPL with a bunch of 5000 lb blocks like 
this holding it in place.

I've also seen portable anchors that you fill with water.  You're 
basically relying on bulk mass to hold things in place.   IN some 
places, dragging a hose to fill a kiddie pool or trash cans or trash 
bags is easier than dragging concrete blocks around.

You could also look into what are called FIBC (Flexible Intermediate 
Bulk Carriers) which are those big fabric bags full of just about 
everything you see on the back of a semi flatbed truck.  They hold about 
a cubic yard, they cost about a dollar (empty), and you might be able to 
rent/borrow/buy things like a yard of gravel or dirt in a FIBC.  If you 
have something that can move a couple of tons around.
They're pretty nice because they have a strong fabric strap handle (how 
you pick it up with a crane, forklift, etc.) that you can hook your guy 
to.  And, depending on what it's filled with, it will spread out and 
conform to the ground surface.

I've not tried it, but I'll bet you could get one of these things, line 
it with polyethylene sheet, and fill it with a ton of water pretty 
easily. They're really strong, really cheap, and store very compact when 
empty.  (actually, just looking up prices, I see that they actually make 
liners for these things so they can hold liquids).  Looks like they're 
about $15-20 each on amazon when you buy just one, brand new.





You need to calculate what sort of loads you're going to see, and make 
sure you have enough mass to hold it.  You can probably figure something 
like a friction coefficient of 0.3 or something for the sliding 
motion/horizontal component.

https://www.nzta.govt.nz/assets/resources/surface-friction-conference-2005/8/docs/frictional-characteristics-roadside-grass-types.pdf

has a handy table of sliding friction of a concrete filled tyre (in NZ) 
across grass and dirt.  (It's all about locked wheel skids and 
distances).  As you might imagine, grass is slippery when wet.



> For example, I think I can come up with a number of 60lbs cement
> wall-barrier slabs that can be bolted together for each anchor point.
> If this would be a viable option, how much weight on the anchor blocks
> would guarantee a safe installation?
> The intent is to put up a 40 meter 2 element yagi, without a rotor, at a
> height of 50 feet.
>
> Thanks in advance.
> N1BBR
>