Page 21 - Pipeline Rules of Thumb Handbook

8 Pipeline Rules of Thumb Handbook

F = The total resultant external force acting on the Example—A derrick mast 90ft high is supported by nine
structure equally spaced guys anchored at a horizontal distance of
G = The angle between the horizonatal plane and the 170ft from the mast and the elevations of the guy anchorages
force F are 10ft below the base of the mast. The load on the struc-
h = The height of the structure ture is equivalent to a force of 10,000lbs, acting on an angle
d = The horizontal distance from structure to guy of 10 degrees below the horizontal. What is the maximum pull
anchorage on any single cable?
m = The vertical height of anchorage above or below the
base of the structure From Figure 1—
The horizontal component of the force, F, = Fcosg.
a = The angle whose tangent is (h ± m) ∏ d h = 90ft
m is plus if the anchorage is below the base of the d = 170ft
structure and subtracted if it is above. m = 10ft
g = 10°00¢
P = Fcosg ∏ cosa F = 10,000lbs

As cosa is always less than one, P is always greater than F 90 +10
tana = = 100 = . 0 588
cosg, the horizontal component of force F. 170
It must be remembered that P represents the total pull
acting along the guys at an angle, a, with the horizontal, and a = 30°28¢
in the same vertical plane as the force, F.
If only one guy were used, P would represent the extra F cosg 10 ,000 ¥ .985
0
tension, T. In practice, however, a number of guys are always P = = = 11 ,427 lbs
cosa . 0 862
used and, therefore, the pull on any one guy will not be equal
to P. The following table gives factors for any number of guys From Table 1, T = 11,427 ¥ 0.50 = 5,714lbs
from 3 to 15, equally spaced about a central structure. To find
the maximum extra tension, T, that will be applied to any If erection tension is 10 percent of total working tension,
single guy by the force, F, capable of rotating 360 degrees 5,714 is 90 percent of total working tension. Therefore,
around a vertical axis, it is only necessary to multiply the value 5 714 100
,
¥
,
of P, as determined above, by the factor for the number of working tension = = 6 349 lbs .
90
guys used. It must be clearly understood in using this table
that the guys are uniformly spaced and under equal tension
when no load is acting on the structure.
Table 1
No. of Guys Factors* No. of Guys Factors*

3 1.15 10 0.45
4 1.00 11 0.40
5 0.90 12 0.37
6 0.75 13 0.35
7 0.65 14 0.32
8 0.55 15 0.30
9 0.50
* These factors are for average conditions. If the guys are erected
under accurately measured tensions of not less than 20% of the working
load, the factors for five or more guys may be reduced by 10%. If the
erecting tensions are low or not accurately equalized, the factors for 5
or more guys should be increased 10%.