Greetings,
I am halfway into erecting my first tower. It consists of two commercial
6,5 meter (length) triangular, 450 mm (diameter) galvanized steel sections
giving me 13 meters of height plus another 3 meters of a 6 meter pole above
the top bearing. Low by USA standards (I think) but 1 meter above the legal
(without planning permission) height in South Africa :) On this will go a 6
element Log Periodic by ATN driven by a YAESU SDX 1000 rotator? The base
consists of two T - hinge sections where one T - section bolts onto a
heavy-duty cage set into 1 square meter of concrete. The other T - section
bolts via two hinges to the base T - section on which the three legs of the
tower slides and gets bolted into three (50 mm diameter, 5 mm thickness and
200 mm length) tubes welded to the T - section.
I have not been sleeping contemplating all the possibilities of tower
failure and consequences....
Can the engineers and experts please advise on:
The concrete is made up on the 3:2:1 principal. 18 x builder wheelbarrow
size pebbles, 12-x builder wheelbarrow size sand & 6 x (50kg) cement, all in
a 1 square meter hole. I put this into the hole three weeks ago. Since then
I have kept the concrete wet and intend waiting until the end of the month
before raising the tower.
- Is this mixture I poured into the base sufficient for the tower (tower
weight +/- 180 kg) and do I need to wait until end of month of curing before
raising (would be 21 days then)?
I have raised the possibility of the three steel tubes (45 mm diameter,)
that is welded to the one T - section onto which the three tower legs slide,
failing at the weld. The welding was done by a professional and is about
1.5 cm around the tubing. I am being told not to confuse bulk with strength
and that thicker steel will rather add weight than strength. Also that the
triangular configuration will tend to balance the forces and weight of the
tower. Still these three tubes will be taking all of the weight of the
tower and still worries me as a weak point in the process.
- Can someone explain to me the forces of gravity and wind involved in the
case of a triangular tower and where the stress points are? Will the base
tubes take these stresses (laterally and downwards)?
The base T - section hinges right at the bottom about 100 mm above the
concrete, where the intention is that I will use a 500 kg capable winch
mounted in concrete level with the base plate, about 3 meters behind the
pointed section of the triangular tower (flat side pointing away) to raise
the tower. This will entail manually lifting the tower some height until
there are enough angle to enable the winch to function. The idea is also to
lift to the point where the antenna can be assembled onto the mast and then
take the tower to full erection.
- What will be the angle where I can expect the winch to take the dead
weight and raise the tower, (the winch have a heavy duty brake section and
is rated for 3,000 kg overload), and what will the forces be on the three
T - section tubes that I discussed in the previous question. As the tower
will near its vertical position succumbs to inertia how do I best prevent it
crashing into its base... I thought to have heavy-duty ropes pulling into
the opposite direction of the hinge movement?
The bolts supplied for the hinges were 20 mm diameter for a 30 mm hole.
This would result in the T -section being able to laterally move in the
hinges if the tower is moving due to wind etc. I have acquired high-tension
steel bolts that fit tightly into the holes and now prevents any lateral
movement.
- Would there be a reason for allowing the movement or am I correct in my
assumption that such movement will put unnecessary strain on the bottom T -
sections and hinges. Some advice given says that a tower should be allowed
to have some movement?
Due to property constraints I cannot utilise the standard 120-degree spacing
for the guys. One guy will be taken from the sharp point of the triangle
backwards and then the other two guys will then be taken sideways from the
flat side.
- Will this work or am I creating unequal forces on the tower?
Thank you very much
Wynand
ZS6ARF
zs6arf@telkomsa.net
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