evaluating strength loss of tubular steel poles due to
TRANSCRIPT
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]
SEPTEMBER 5 - 7, 2018
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)
SEPTEMBER 5 - 7, 2018
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)
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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
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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
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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
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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.
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Inspection Methodology & Strength Loss Decision Tree
Is Capacity Utilization of Existing Design based
on actual wires and spans < 1.0
Calculate Capacity ReductionBased on Corrosion Losses
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Inspection Methodology & Strength Loss Decision Tree
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:
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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
SEPTEMBER 5 - 7, 2018
YesNo
Is Sufficient Capacity StillAvailable compared to
Required Design w/no Localized Buckling Issues?
Remediate Corrosion
Design StrengtheningScheme + Remediate
Corrosion
Strengthen & Remediate Corrosion