Tuesday, April 14, 2009

Tower Footing Laid

I laid the concrete footing for the tower on Easter Monday, so I have started checking some of the To Do's off the list!
The ground was pretty rock hard where the tower needs to be placed - which I assume is a mixture of the mountain (bedrock) being just a few feet down and perhaps the odd bit of hardcore left over from when the house was built/extended. I decided I didn't need to go overboard with the float, so it's only about 3' long x 2' wide x 1'6" deep.

Even though the scale of the footing was hardly anything to write home about, not being the kind of guy whole engages in too many construction projects it still gave me plenty to ponder, check and recheck. Where to position the base laterally? How to determine the distance from the wall (given the stand-off bracket to be assembled 2 storeys up at the eaves)? Luckily, I believe the project doesn't require millimetre precision, as there's a long rise of tower and some tolerance for small misalignment. Nevertheless, a mistake will be costly in time and materials (to say nothing of frustration). So, time will tell, but hopefully nothings too pear-shaped just yet!

One of the things I've never done before is mount anchor bolts into concrete. I was advised to try to find a kind of bolt used to create formers for concrete called a ready-bolt. The local hardware store had none however, so instead I decided to risk a 6" long heavy duty coach bolt. I'm assuming that nicely cured, well mixed concrete ought to hold a regular bolt of this kind with enough buried length for good adhesion, with the head providing enough of an additional anchor to prevent turning. Again, time will tell.
A practical problem was how to embed the bolts in fresh concrete at the correct positions for mating with the tower base. Eventually I suspended the base over the pit, with the bolts loosely fastened with nuts through the base plate, dangling into pit at the correct location. Then I partially filled the pit with concrete to about halfway up the bolts and left the concrete for about 30 minutes to set just enough to offer some support to the bolts. Then I carefully unbolted and removed the base plate and with better access then proceeded to complete the footing and level it with a slightly wetter mix. This approach seems to have worked - at least the concrete is now nicely curing with the bolts looking just right.

I should probable leave the concrete to cure as much as possible before attempting to properly bolt down the base (maybe a week). In the meantime, the important business of assembling the entire tower, complete with the connecting mast, antenna and its feed line needs a bit of planning.

The next independent step will be to attach the stand-off bracket to the eaves right under the roof apex, and also to install a couple of rings to the wall under the eaves though which rope can be passed to help steady the tower (and maybe take a little load) when it comes time to raise it.

I had wondered earlier when considering this project whether the tower could be constructed from the bottom up, section by section, and then finally the mast and antenna added at the top. The challenge of taking a 9 meter antenna up a three storey high ladder, then transferring it to the top of a tower that is meters above seems however a little too great. The only practical alternative is to assemble the whole shooting match on the ground and then rotate the whole thing up to the vertical and manhandle it into position on the base before making firm the attachment to the stand-off bracket under the eaves.

To be safe, that operation will require a many pairs of strong hands, and a way to temporarily anchor the bottom of the tower to a spot on the ground so it doesn't lift when the fulcrum (the burley guys pushing the tower up) moves past the centre of gravity. Hopefully, with the addition of ropes passed through the high rings for added stability and some additional leverage, the tower can be lofted to the vertical without incident. Anyway, that's the current plan, but having never erected a tower before this is all by nature of invention than of experience!

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