MecaStack provides a couple of different parameters with regard to guy wires lengths. Here is a typical output:
In this output there is a column "Length Straight Line" ('S' in figure below), this is the straight light distance from the Pin on the stack to the Pin on the turnbuckle (furthest end from Deadman anchor). There is also a "Length Pin to Pin" dimension ('L' in figure below), and this is similar to the "Length Straight Line" except that it includes Sag. (Ignore the dim's shown in the below figure, not relevant to this particular set of data)
The sag calculation ('D' in figure) is made per a publication by National Association of Broadcasters "Engineering Handbook". This equation is as follows:
Using D, we can then calculate L with the following formula (Also taken from the same reference):
Using this info, lets show a manual Calculation for the first guy wire (@ 267 ft) in the above MecaStack Output:
S = 328.767 ft (Taken from MecaStack output)
W = 7.39 lb/ft (Linear Guy Weight is not shown, but provided in MecaStack file)
T = 22.90 Kips Pretension (Not shown, but provided in MecaStack File)
Theta = 53.06 Deg (Guy angle relative to grade, not shown but provided in MecaStack File)
Calculate Sag:
D = (7.39 lb/ft) * (328.767 ft)^2 * Cos(53.06 Deg) / (8*22,900 lbs) = 2.62 ft
Now we can Calculate Pin To Pin Length, L:
L = 328.767 + 8*(2.62^2) / (3*328.767) = 328.767 + 0.05568 = 328.82 ft
Looking at our MecaStack output, you can see that the value 328.82 ft matches what is shown for the first guy wire.
One question we often get is whether the length includes Sag, and as you can see from the above it does include sag; however, the impact of sag to the overall length is generally much lower than what people expect. This is dependent upon pretension, as the pretension decreases the difference between S and L will become greater. For example, in the above calculation is we reduced our pretension to 50% of that used, we would get the following:
Calculate Sag:
D = (7.39 lb/ft) * (328.767 ft)^2 * Cos(53.06 Deg) / (8*11,450 lbs) = 5.24 ft
Now we can Calculate Pin To Pin Length, L:
L = 328.767 + 8*(5.24^2) / (3*328.767) = 328.767 + 0.2227 = 328.99 ft
Even though the sag doubled from 2.62 ft to 5.24 ft, the pin to pin length 'L' only increased by 0.16 ft. This is probably a much smaller increase than most would expect.