[450] i guess i have to fix this

[John Foege]

I have provided a mast strength spreadsheet. I dropped in a table of
standard EMT diameters and thicknesses per trade size.

What you will see in this example is a piece of 1-1/4" trade size EMT
(standard electrical conduit) protruding 10 ft from the top of the
tower with an M2 9 el beam 9.5 ft up on the mast.

The yield strength for galvanized steel (aka EMT conduit) is, sadly,
only around 36,000 PSI. Yield strength is defined as the point where
elastic bending of material stops. Any bending forces higher than the
yield strength will result in a non-elastic excursion, at which point
the tube will not bend back to original shape. It will stay crooked
and/or break if the wind gets strong enough.

So you can see, all I entered was 10ft mast length. 1.51 OD
(corresponds to trade size 1-1/4"). Thickness 0.065 in. Wind speed I
wanted to test at was 90 mph. Mast yield strength 36000. There is a
table to the right where you can list up to 10 antennas on the mast.
You'll only see one. The 1.2 sq ft windload was taken from M2's
datasheet. The drag coefficient of 1.2 is standard for antennas and
masting (rounded material) and doesnt have to be changed.

You will see the results are that the bending stress at 90 mph wind is
~52,000 psi, much greater than the yield strength of 36,000 psi, and
even approaching the ultimate tensile strength of 58,000 psi at which
point the conduit would snap.

The calculator tells you for your current choice of EMT and antennas
on the mast, that the safe windspeed is 75mph. Remember this is the
wind load it can take constantly without ever negatively affecting the
mast. The wind speed would have to get up to about 95-100 mph before
the mast would snap in this example. But bending non-elastically is
just as bad as snapping in my book (and in everyone elses). So thats
why you use yield strength and not ultimate tensile strength for the
calculation.

Hope this will help you decide what size EMT to use and if whatever
your configuration will be will be safe. I guess the next step is to
figure out what the historical maximum gusts are for CT and
specifically your location.

On Thu, Dec 17, 2009 at 8:26 PM, Tyler Conlon <admin at tyconpowered.com> wrote:
> Last night there was a gust here.  It broke the mast at the 2 meter beam.  I
> will probably have to lower it to fix it.  Anyone have a large crane?
>
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