Longitudinal Loading on Crossarm Assemblies

The Rural Utilities Service (RUS) recently re-evaluated the mechanical loading on RUS standard distribution crossarm assemblies. The results of this re-evaluation are documented in a forthcoming RUS Bulletin 1724E-151, "Mechanical Loading on Distribution Crossarms," that contains equations, data and explanatory information. As a result of this work, the "allowable longitudinal loading" values shown in the design parameters on the drawings of RUS Bulletin 1728F-803, "Specifications and Drawings for 24.9/14.4 kV Line Construction," are being revised using the following equations and methodology.

Applied vertical loads need to be considered when determining the permitted longitudinal loading of crossarm deadend assemblies. The following mathematical relationship, which relate vertical and longitudinal loading, has to be satisfied to avoid overstressing the wood fibers of crossarms:

The following applies to RUS standard distribution, deadend, crossarm assemblies:

• Permitted Vertical Moment (Capacity) of Assembly = N x M_v x F_s
• Permitted Longitudinal Moment (Capacity) of Assembly = N x M_h x F_s
• Σ Applied Vertical Moments =

(See Diagram 1 below.)

• Σ Applied Longitudinal Moments =

The units of measure of the above four groups of terms are "ft-lbs." Note that all of the calculations apply to one-half of the crossarm assembly (on either the right or left side of the pole looking parallel to the line). Each conductor attachment location, at a distance D₁ or D₂ from the center of the assembly, has either one conductor attached ("into" the assembly) or has two back-to-back conductors attached (one "into" and one "out from" the assembly).Following are the definitions and values of the variables in the above equations:

• All of the conductor spans "into" a crossarm assembly have the same length; all of the conductor spans "out from" a crossarm assembly have the same length. The length "S," where S = S_in + S_out, is called a "weight span."
• The tensions of all of the conductors into the crossarm assembly (L_in) are the same; the tensions of all of the conductors out from the crossarm assembly (L_out) are the same. "L" is the difference of the conductor tensions (L = L_in - L_out) at each (phase) conductor attachment location on the assembly.
• All of the conductors attached to the crossarm assembly are the same type and size as the largest conductor. Thus in the above equation: W₁ = W₂ = W₃ =W₄ = W.
• A load moment (M_LW) of 250 pounds (which might be attributed to a lineworker, materials or equipment) times 2 feet and times a constant overload factor of 2.0 (the product equals 1,000 ft-lbs) is added to the applied vertical load moments to satisfy NESC Rule 261D4b requirements. (Note: Standard construction practices and RUS discourage lineworkers from standing on crossarms.)

After applying the above assumptions and substitutions, the equation can be simplified and re-written as:

(ft-lbs)

This equation can be solved for "L" as a function of all of the other variables in the equation. Tables 1 and 2 show the calculated permitted unbalanced conductor tensions ("L") for several commonly used distribution conductors versus three different weight spans ("S"), for standard RUS crossarm deadend assemblies and NESC Grade C construction.

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.

TABLE 1

PERMITTED UNBALANCED CONDUCTOR TENSION (Lbs/Phase)*

SINGLE and DOUBLE DEADEND ASSEMBLIES; 1 PHASE EACH SIDE OF POLE - NESC Grade  C

 

NOTES:

Reduce tabulated tensions by 40% for NESC Grade B construction. *(Lbs/Phase) means tension difference at each point on crossarms where conductors are attached. **Weight span equals 1/2 span length into assembly plus 1/2 span length out from assembly. Weight Span for single deadend assembies only equals 1/2 span length into assembly. Calculations assume all conductors same size and type as largest conductor and level spans. Assemblies have been multiplied by strength factor of 0.85 (2002 NESC Table 261-1A).

TABLE 2

PERMITTED UNBALANCED CONDUCTOR TENSION (Lbs/Phase)*

DOUBLE DEADEND ASSEMBLIES; 2 PHASES EACH SIDE OF POLE - NESC Grade  C

 

NOTES:

Reduce tabulated tensions by 40% for NESC Grade B construction. *(Lbs/Phase) means tension difference at each point on crossarms where conductors are attached. **Weight span equals 1/2 span length into assembly plus 1/2 span length out from assembly. Calculations assume all conductors same size and type as largest conductor and level spans. Assemblies have been multiplied by strength factor of 0.85 (2002 NESC Table 261-1A). Applied loads have been multiplied by overload factors (2002 NESC Table 253-1).