Permitted Loads on Standard RUS Distribution Guy Assemblies

Recently RUS re-evaluated the ratings of the mechanical loading on standard RUS distribution guy assemblies. Following is a summary from that re-evaluation. Table 1 summarizes the new calculated "permitted loads" on standard RUS guy assemblies.

The RUS "designated capacities" (the strength to sustain longitudinal loads) of the component parts of standard guy assemblies are as follows:

Guy Assembly Component	RUS Designated Capacity (lbs)	Notes
2-1/4 inch square (flat) washer	4,089	(1)
3-inch square, curved, washer	7,766	(1)
4-inch square, curved, washer	13,779	(1)
5/8 inch machine bolt	8,300	(2)
3/4 inch machine bolt	12,400	(2)
Guy Attachments
Guy Hook type	10,000	(3)
Plate type	10,000	(3)
Wrapped type guy	90% of RBS	(4)
Guy Strain Insulator	10,000	(3)
Guy Wire	90% of RBS	(4)

The designated capacity equals area (in²) of washer (less area of bolt hole) times allowed longitudinal loading on wood. RUS designates the allowable loading on wood to be 900 lbs/in² to avoid the crushing of wood fibers.
Based on ANSI and manufacturers’ rated shear strength.
Based on RUS "Items Requirements" specifications and manufacturers’ test results.
RBS is the "Rated Breaking Strength" (of guy wire) as published by manufacturers. The 2002 Edition of the NESC requires that the RBS be multiplied by a strength factor of 0.90 found in Table 261-1A.

All of the guy assembly component parts tabulated above work in series in standard guy assemblies. The strength rating of a guy assembly is based on its component part with the lowest designated capacity.

RUS specifies that the designated capacity of each component part (excluding the guy wire) be multiplied by the following factors based on Table 2561-1A of the 2002 Edition of the NESC. The result of this multiplication yields the assembly’s "permitted load" in pounds (lbs).

0.85 - NESC Grade C construction
0.65 - NESC Grade B construction

The following table compares the present strength ratings (in Bulletin 1728F-803) of standard RUS guy assemblies with their new calculated permitted loads. The "permitted loads" were calculated by multiplying the designated capacity of the assembly’s component part with the lowest strength rating by a strength factor of 0.85.

TABLE 1 RUS Bulletin 1728F-803 - Specifications and Drawings for 24.9/14.4 kV Line Construction

Assembly Number	Description	Maximum Working Load - Horizontal (lbs)	Permitted Load (lbs) (New)
E1.1	Single Down Guy (Through Bolt Type)	5,200	3,475
E1.01	Single Overhead Guy (Through Bolt Type)	5,200	3,475
E1.02	Single Overhead Guy (Through Bolt Type)	8,500	6,600
E2.1	Single Down Guy - Heavy Duty (Through Bolt Type)	8,500	8,500
E3.1	Single Down Guy (Wrapped Type)	8,500	90 % of RBS or 14,000
E4.1L	Single Down Guy - Large Conductors (Pole Band Type)	10,000	10,000

The tabulated values above should be reduced by 25 percent for NESC Grade B construction.
Applied loads should never exceed 90 percent of the rated breaking strength of the guy wire installed.
The tabulated "maximum working loads" are ratings for the horizontal direction; the "permitted loads" are not to be exceeded in any direction.
Loads applied to guy assemblies need to be multiplied by the appropriate overload factor of Table 253-1 of the 2002 Edition of the NESC

The calculated "permitted loads" of the first three assemblies in the preceding table are based on the area of the washers used; the last three assemblies are based on the ratings of the guy attachment hardware.

There is an error in the material list for assembly "E1.1" in Bulletin 1728F-803. The guy attachment should be installed using a 3-inch, square, curved washer instead of a 2-1/4 inch square washer as listed. The permitted load for this assembly with a 3-inch, square, curved washer is 6,600 pounds.

If you would like more information or have any questions, please contact Jim Bohlk, Electrical Engineer, Distribution Branch, at 202-720-1967 or at jbohlk@rus.usda.gov.