More on the three way vs four way guying question.
WORST CASE load on the anchor is when the wind blows from the same direction
as the anchor (one anchor takes ALL of the wind load). This is the same in a
three way or four way guy ststem. By definition, all other wind directions are
NOT worst case as the wind load is reduced on a given anchor and increased on
the nearest adjacent anchor.
The amount of force on the two adjacent anchors can be determined by
multiplying the wind force by the cosine of the wind angle relative to the
anchor angle. This is the case in any guy system, three way or four way. For
example, assuming a 1000 pound wind force, in a three way guy system, if the
wind is 30 degrees off anchor #1, the cosine of 30 degrees is .866 and the
force on anchor #1 is 866 pounds. At the same time, the wind force on anchor
#2 is cosine of 90 degrees X 1000 lbs which is 707 pounds.
Example #2, same wind force on a four way guy system, anchor #1, cosine 30
degrees =.866 X 1000= 866 pounds. Anchor #2, cosine 60 degrees=.5 X 1000= 500
pounds load. In this case the load on anchor #2 is less than in a three way
system but the load is trivial compared to the design strength of the anchor.
If you were to plot out the loads on the anchors on a piece of graph paper you
would see the shape of the plot to be the familiar sine wave that we encounter
so many times in our study of electronics theory. In the anchor case, the
curve is shifted 90 degrees (that is why it is a cosine function). You will
recall that the curve is somewhat broad on top and falls rapidly down the
sides as you go through the various angles from 0 to 90 degrees. The forces on
the anchors also follow the same shaped curve.
Anchor forces are essentially the same in a three way or four way system. You
still have to design the anchor for the same worst case loads. Any given
anchor will be "loafing" when the wind is blowing in the opposite direction
plus and minus 90 degrees to the anchor. This is the same for a three way or
four way system. In either system the wind load is never applyed to more than
two anchors at any given time as all other anchors are in their "loaf" zone.
Hope this is of interest to some and helps clarify the three way versus four
way guy question.
Tnx/73 de Gerald, K5GW
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