Evaluating Strength Loss of Tubular Steel Poles Due to ...

SEPTEMBER 5 - 7, 2018

Evaluating Strength Loss of Tubular Steel Poles Due to CorrosionWesley J. Oliphant, PE, AWS-CWI, F.SEI, F.ASCE

Principal, Chief Technical Officer, Exo Group, LLC. e-mail: [email protected]

mailto:[email protected]


Why evaluate strength loss due to corrosion?


Steel Poles - types and usesSelf supported single pole structures

Lightly loaded poles (tangent and small angle)Heavily loaded poles (medium angle, strain & dead end)

Self supported multi pole structures (incl. framed)Heavily loaded poles (tangent & small angle)

Guyed pole structures (single, framed and multi pole)Heavily loaded poles (tangent, medium angle, strain & dead end)


Steel Poles - in service considerations

Pole load withstandSteel grade and thicknessPole section shape and diameter

Pole weathering protection (Above and below Grade) Hot dip galvanizing (zinc)Corten / weathering steel (alloyed with copper, nickel etc)Painted coatings (urethanes, epoxies etc)


Chemical attackRoad salts Chemicals from agricultural or industrial environments

Corrosion failurePoor paint adhesionCoating degradationSteel section loss

Mechanical failureWeld and joint failureStress cracks and fatigueOverload due to steel section loss

Steel Poles – life expectancy issues


Design – what service loads and conditions are expected?NESC, ASCE, CSA, California GO 95

Materials & Fabrication – specifications?ASTM, AWS, AISC, NACE, SSPC

Maintenance & Inspection – Not much written on how to evaluate?

Steel Poles – standards & guides


Section LossWhat is an acceptable rate of section loss?What areas of the pole are most critical for strength loss?Is buckling becoming critical?What is the remaining pole strength capacity?What is the estimated remaining service life under the existing load?

What is the current state of practice for answering these questions?

Steel Poles


Inspection Methodology & Strength Loss Decision Tree

Perform Inspection & Gather data (per NACE SP0415 / IEEE Std. 1895)

Is corrosion present?

Determine ImportanceFactor of Line

YesNo

No FurtherAction Reqd.



Is Capacity Utilization of Existing Design based

on actual wires and spans < 1.0

Calculate Capacity ReductionBased on Corrosion Losses



where (at the reduced cross section due to corrosion, under analysis):M = applied Moment, in-kipsP = applied Axial load, kipsS = reduced Section Modulus of the cross section due to corrosion* , in3

A = reduced Area of the cross section due to corrosion*, in2

Fa = Allowable Compressive Stress, ksi

* This reduced section modulus and reduced area are not trivial to calculate. A somewhat complex method can be used to calculate corroded area and corroded section modulus with shifted neutral axes. This is something that may not be apparent to engineers trying to quickly perform an analysis.

The formula for % capacity utilization at a given cross section with deterioration becomes Equation (1) below:


Make sure to consider local buckling with any reduced wall thickness

where:

w = pole section flat width, in.t = pole section wall thickness, in.Fa = Allowable design compressive strength of the steel, ksiFy = Yield strength of the steel, ksi


YesNo

Is Sufficient Capacity StillAvailable compared to

Required Design w/no Localized Buckling Issues?

Remediate Corrosion

Design StrengtheningScheme + Remediate

Corrosion

Strengthen & Remediate Corrosion

Evaluating Strength Loss of Tubular Steel Poles Due to ...

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