[TowerTalk] FW: More crank-up questions

[Matt]

Regarding the 3.5x3.5x7.5 base dimension, a copy of Tri-ex hand calculations
dated from 1965 shows the assumption was 4,660 psi soil bearing strength @
7' below grade and 1,550  @ 28" below grade.  Their assumed foundation load
from the tower was 29,900 ft-lbs and 800 lbs shear.  The 3.5x3.5x7.5 base
was designed for 1.5x overturning moment at these conditions.  The design
notes allow for soil conditions of  333 psi wich they defined as "sand well
graded".  Regarding foundation dimensions I guess it depends on which school
one subscribes to.   Conservative is to size for no tip-over without taking
any credit for any contribution of soil bearing strength.   If one has some
compressive soil numbers, then the foundation size can be reduced to take
credit for the coil's contribution.

I calculated base reactions of ~26,000 ft-lbs moment @ design load and
46,500 ft-lbs @ tower failure.  I used the later value and a conservative
safety factor for the design of the new adjustable base plate.

I believe that Taj still sells the original design Triex rebar cages with
the welded ears and which are tied with additional #3 bars down as you
described.  This effectively combines the functions of steel concrete
reinforcement with the tower leg supports.  These functions are separated in
my foundation.  

Steel concrete reinforcement is double #5's at the corners and midway on
each side (16 total).   The vertical corner bars are tied together with #3
horizontal square loops at 6" on center.  The middle side vertical bars are
tied together with #3 diamond loops at 6" on center.   The horizontal loops
and diamonds loops are doubled up (3" on center) at the top of the cage
(where the pad extends above the ground).   The cage is sized for 3" to 6"
of concrete cover over the bar at all points.  The cage was placed on bricks
and blocks to keep the steel out of the soil at the bottom of the
excavation.   There are pics of this on my site.

The tower legs bolt to a fabricated base.  I'll eventually get some
dimensions up on the WEB for this but there are some pics there now.   The
base is anchored to the foundation with (6) 1" x 36" F1554 grade 55 anchor
bolts.

Matt
KM5VI


>> I'll try again...

>> Comparing the new base (yours) to the old base (mine), I'm wondering how
different they are.

>> The old rebar cage is a simple affair consisting of a bunch of straight
pieces tied together.  There are no bends.

>> The hole is 3'6" X 3'6" X 6'6" deep which appears minimal to me.

>> There are no bolts holding the base ears.  They are welded to #10 (inch
and a quarter) 60" long rerod that extends to the top of the ears.  #3 rerod
extends them to within 6" of the bottom of the hole.  It does not give the
latitude you have with two bolts holding each ear.  It's a strong set up,
but appears much more primitive than your LM470 base

>> 73

>> Roger (K8RI)