Page 105 - McWane Poles Sales Manual 2024

Sales Manual • 2024
105
Reliability-Based Pole Design
Summary
Ductile iron is a material not recognized by the National Electric Safety Code (NESC). McWane’s ductile
iron poles have been designed in PLS POLE software using ASCE 48, which is a standard applicable only
to cold rolled steel structures. McWane needs to establish the nominal strength of the ductile iron pole
using ASCE 111, a manual adopted by NESC that provides methods for deriving and documenting the
strength of utility structures made of new materials. The intent of ASCE 111 is to correct the problem of
inconsistent reliabilities among different pole types. The completion and documentation of this exercise
will provide McWane and potential customers with confidence in the strength of the ductile iron pole.
Test Methods
ASCE 111 defines nominal resistance (Rn) of a utility structure as the 5% lower exclusion limit (LEL). The
5% LEL nominal strength (a 5th percentile) is a capacity that 5% of structures do not meet. For a normal
density function, the 5th percentile is 1.645 standard deviations below the mean:
R5 = m – 1.645 (m x COV) where COV = Std. Dev / Mean
ASCE 111 presents three methods for deriving nominal strength: 1) an empirical analysis based on fullscale
testing, 2) a theoretical analysis with mechanics-based models used in conjunction with Monte Carlo
simulation (if necessary), and 3) a default assignment of material distribution parameters. The Empirical
Method is especially applicable to wood because of the variable nature of wood poles. The Mechanics
Based Models are appropriate for steel and concrete poles because of the relatively uniform material.
Default Assignment is used when there is insufficient data of a pole material. Default Assignment is the
most conservative approach. Because ductile iron is relatively consistent and because assembling a
database of full-sized pole tests (Empirical Method) would be cost prohibitive, the Mechanics-Based Model
is the appropriate test method for deriving the nominal strength of ductile iron poles.
Mechanics-Based Model
Basic material properties (thickness, yield strength, and diameter) can be used in conjunction with a model
to estimate the nominal pole strength of ductile iron poles. If no covariation exists [and none is expected]
between the basic material properties sampled, variance of a strictly linear model can be estimated as the
sum of variances of the individual input parameters, eliminating the need for simulation. Strength will be
defined as the moment capacity of the pole at a given pole cross section. The assumed model for moment
capacity for a ductile iron tube follows:
Bending Moment Capacity (MOM) = Yield strength [FY] x Section Modulus [S]
S = Moment of Inertia [I] / Radius [C]
I = (Pi/64) x (OD^4 – ID^4)
The moment capacity model needs to be confirmed through a full-scale pole breaking test. To confirm the
moment capacity model, the actual moment capacity at the point of ultimate pole failure needs to equal
the calculated moment capacity based on the material properties observed at the point of ultimate failure.
Material Sampling
The greater the sample size, the more accurate the estimation of the pole strength. The proposed initial