Evaluating Strength Loss of Tubular Steel Poles Due to Corrosion - - PowerPoint PPT Presentation

SEPTEMBER 5 - 7, 2018

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

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

Steel Poles – life expectancy issues

SEPTEMBER 5 - 7, 2018

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

SEPTEMBER 5 - 7, 2018

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?

Steel Poles

SEPTEMBER 5 - 7, 2018

Inspection Methodology & Strength Loss Decision Tree

Perform Inspection & Gather data (per NACE SP0415 / IEEE Std. 1895) Is corrosion present? Determine Importance Factor of Line Yes No No Further Action Reqd.

SEPTEMBER 5 - 7, 2018

Inspection Methodology & Strength Loss Decision Tree

Is Capacity Utilization

• f Existing Design based
• n actual wires and spans < 1.0

Calculate Capacity Reduction Based on Corrosion Losses

SEPTEMBER 5 - 7, 2018

Inspection Methodology & Strength Loss Decision Tree

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* , 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:

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

Yes No Is Sufficient Capacity Still Available compared to Required Design w/no Localized Buckling Issues? Remediate Corrosion Design Strengthening Scheme + Remediate Corrosion Strengthen & Remediate Corrosion

SEPTEMBER 5 - 7, 2018