I have been following this thread since it started.
In 2009 I was forced to replace a 'wind altered' tower with a new one. I
decided to try to use the existing base for the new tower which was going to
be the same height with the same antenna wind load.
When I went to get the building permit I was informed that the new tower
installation would need to conform to the newly approved tower code for the
State of Idaho. I contacted US Towers and got their specs which I then
submitted. The application was rejected because the US Tower specs were not
shown to meet the latest IBC code. That was remedied by US Tower updating
their specifications to conform to their current tower specifications. Then,
I was informed I would need to have a structural engineer [with an Idaho PE
Stamp] review and approve my plan for using the existing base. As an
alternative I could install a new base that met the IBC code.
After significant dollars invested in structural engineering consulting it
was determined that using new bolts epoxied into the old base would not
'pass'. The bolts were going to be too close to each other when one used the
US Tower Tee Base plate. Apparently one looks at a cone of force pulling up
on the bolt from the bottom. The cone expands in circumference as you move
from below the surface to the surface of the concrete. If the force cones
for two bolts overlap their load capacity must be reduced. That does not
apply for the situation where the base is a new pour and the bolts are cured
in place. With different spacing epoxied bolts could meet the spec but they
would not fit the US Tower TEE Base. One would need to get an approved
design fabricated for the different bolt placement. The correct economic
decision [for me] was to install a new base.
I was informed that in either situation the length of the bolt beyond about
18" in the base contributed nothing to the load capability of the bolt. One
of those places where longer is not better -- larger diameter (bolt) does
continue to be better.
I rather suspect that Skip, KJ6Y, is correct in his assessment that epoxied
bolts will work just fine in practice. But they may not have the safety
factor that locale and/or IBC code requires. If I had not been in a location
where the local building code was 'important' to me, I might very well have
used the Hiltie epoxy method. As it was I dug a new hole, built a new rebar
cage and poured new concrete. I continue to sleep undisturbed, but
significantly poorer.
Tod, K0TO
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