Evaluating Strength Loss of Tubular Steel Poles Due to Corrosion Wesley J. Oliphant, PE, AWS-CWI, F.SEI, F.ASCE Principal, Chief Technical Officer, Exo Group, LLC. e-mail: woliphant@exoinc.com SEPTEMBER 5 - 7, 2018
SEPTEMBER 5 - 7, 2018
SEPTEMBER 5 - 7, 2018
SEPTEMBER 5 - 7, 2018
SEPTEMBER 5 - 7, 2018
SEPTEMBER 5 - 7, 2018
Why evaluate strength loss due to corrosion? SEPTEMBER 5 - 7, 2018
Steel Poles - types and uses Self 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 withstand Steel grade and thickness Pole 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) SEPTEMBER 5 - 7, 2018
Steel Poles – life expectancy issues Chemical attack Road salts Chemicals from agricultural or industrial environments Corrosion failure Poor paint adhesion Coating degradation Steel section loss Mechanical failure Weld and joint failure Stress cracks and fatigue Overload due to steel section loss SEPTEMBER 5 - 7, 2018
Steel Poles – standards & guides 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? SEPTEMBER 5 - 7, 2018
Steel Poles Section Loss What 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? SEPTEMBER 5 - 7, 2018
Inspection Methodology & Strength Loss Decision Tree Perform Inspection & Gather data (per NACE SP0415 / IEEE Std. 1895) Is corrosion present? No Yes Determine Importance No Further Factor of Line Action Reqd. SEPTEMBER 5 - 7, 2018
Inspection Methodology & Strength Loss Decision Tree Is Capacity Utilization of Existing Design based on actual wires and spans < 1.0 Calculate Capacity Reduction Based on Corrosion Losses SEPTEMBER 5 - 7, 2018
Inspection Methodology & Strength Loss Decision Tree The formula for % capacity utilization at a given cross section with deterioration becomes Equation (1) below: where (at the reduced cross section due to corrosion, under analysis): M = applied Moment, in-kips P = applied Axial load, kips S = reduced Section Modulus of the cross section due to corrosion* , in 3 A = reduced Area of the cross section due to corrosion*, in 2 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. SEPTEMBER 5 - 7, 2018
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, ksi Fy = Yield strength of the steel, ksi SEPTEMBER 5 - 7, 2018
Is Sufficient Capacity Still Available compared to Required Design w/no Localized Buckling Issues? Yes No Remediate Design Strengthening Corrosion Scheme + Remediate Corrosion Strengthen & Remediate Corrosion SEPTEMBER 5 - 7, 2018
SEPTEMBER 5 - 7, 2018
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