Remaining life of an ageing 220 kV transmission tower: the damage mechanisms that decide it and how they are found

The remaining life of an ageing 220 kV transmission line tower is set by the damage mechanisms acting on its galvanised steel and its concrete foundation. A Remaining Life Assessment finds each one, measures it, and checks the steel still standing against the loads the current edition of IS 802 requires.

The mechanisms that decide whether a four-decade-old tower stays in service:

• Atmospheric corrosion of the zinc coating: galvanising protects the steel only until the zinc is used up, and on industrial or coastal sites that can happen well inside forty years
• Corrosion of the stub at ground line: the leg section entering the concrete foundation, where moisture and soil sit against the steel and the loss is hidden
• Bolt and joint degradation: loosening, missing bolts, corrosion and wear at the connections that hold the lattice together
• Member deformation: bent, buckled or missing members, from impact, theft or overload, including buckling of the stub itself
• Foundation degradation: cracking, spalling, settlement and erosion around the concrete chimney
• Loss of verticality: a tower that leans or deflects carries its loads off the line it was designed for
• The design-basis gap: a tower erected in the mid-1980s was designed to an edition of IS 802 that has since been revised twice, in 1995 and 2015

Who this is written for, and how each mechanism is treated

The engineers who own and run transmission lines come from electrical, instrumentation, civil and other disciplines. Many have never needed to think about how a steel structure loses strength over time. This article is written for them, on the approach Paresh Haribhakti, Managing Director of TCR Advanced Engineering, sets for this work: keep to the tower's own degradation. Each mechanism is explained in plain terms, and for each one the article sets out four things: how it develops in service, how it is identified in the field, how it is assessed by engineering evaluation, and how it is monitored afterwards.

It draws on a recent proposal. An owner approached us for an "RLA study" of a four-decade-old 220 kV double-circuit line in a high wind zone, carrying suspension and tension towers of three designs. What the owner needed, and what we proposed, was a structural integrity assessment: measured evidence of the condition of every tower, then a structural check of what that evidence leaves standing against today's loads.

A few terms are used throughout. A lattice tower is built from steel angle sections bolted together into a braced frame. Hot-dip galvanising is a coating of zinc bonded to the steel by dipping it in molten zinc; the zinc corrodes in place of the steel. Section loss is the steel thickness, and therefore strength, lost to corrosion. A member buckles when a slender piece in compression bows sideways and stops carrying load, well before the steel itself is overstressed.

1. Atmospheric corrosion and loss of the zinc coating

How it develops. Zinc on a galvanised member corrodes slowly in air, forming a patina that slows it further. Rain, humidity, sulphur compounds from industry and chlorides from the sea all speed it up. The coating thins evenly at first, then wears through at edges, corners and wherever water collects, and the steel below starts to rust. A tower usually passes through recognisable stages: bright or dull grey zinc, then light rust staining, then patches of red rust where the steel is exposed, then flaking rust and measurable steel loss. Once the zinc is gone, the steel corrodes several times faster than the zinc did.

How much zinc there was to lose. IS 4759 sets the minimum zinc coating for hot-dip galvanised structural steel 5 mm thick and over at 610 g/m², and for threaded work of 10 mm diameter and over at 300 g/m². At the density of zinc, 610 g/m² is a layer roughly 85 µm thick and 300 g/m² roughly 42 µm. The standard notes that these minimums are for normal or rural atmospheres; heavier coatings are agreed for marine and industrial sites.

How it is assessed. ISO 9223 classifies atmospheric corrosivity by the first-year corrosion rate of reference metals. Dividing the coating thickness by the zinc rate for each category gives an order-of-magnitude life for the coating:

These are first-year rates, and zinc usually corrodes more slowly once its patina forms, so the arithmetic is a bracket, not a prediction. It still tells an owner where to look first. A line built in the mid-1980s that runs beside a smelter, a captive power plant, a fertiliser complex or the coast sits in C4 or C5, and on those sites the coating on the members has plausibly been consumed, and on the bolts almost certainly. The order of ageing follows from the same numbers: bolts and connections first, then thin bracing members, then the heavier leg angles.

How it is identified and monitored. A drone-based visual survey covers every member of every tower without a climb and grades the coating stage tower by tower and face by face. Where rust has started, ultrasonic thickness measurement on the critical members converts it into a remaining steel thickness in millimetres. Repeating both at a set interval gives a measured rate of loss for that line and that exposure, which replaces the bracket in the table with a number.

2. Corrosion of the stub at ground line and inside the chimney

How it develops. Each tower leg ends in a stub, a length of angle section cast into a concrete foundation. Where the stub leaves the concrete, at the top of the foundation column known as the chimney, soil, standing water, vegetation and debris hold moisture against the steel for long periods. The zinc goes first, then the steel corrodes, and the corrosion can continue down the gap between steel and concrete where nobody can see it. Section loss at this point can run well below the visible collar.

How it is identified. The drone survey shows the visible condition at ground line: rust staining, cracked or broken chimney tops, soil heaped against the steel. Ultrasonic thickness readings on the stub just above the concrete measure what is left. Where the visible condition suggests loss below the collar, excavating around one or more chimneys is the only way to see it, and that is a decision taken with the owner.

How it is assessed and monitored. The stub carries the whole leg load into the foundation, so its measured section goes straight into the structural check described in section 7. The stub is also the location to re-inspect first at each later survey.

3. Bolt and joint degradation

How it develops. A lattice tower is held together by bolts, and the joints carry every load from conductor to foundation. Bolts carry about half the zinc of the members they join, so they lose their coating first and then corrode, thinning the shank and the threads. Wind makes a tower and its conductors move continuously, and over years this repeated movement can loosen nuts, let joints slip, wear the bolt holes and, at heavily worked connections, start fatigue cracks. Missing bolts, from loosening or from theft, put the load on the bolts that remain.

How it is identified. Close drone imagery of each joint picks up missing nuts and bolts, rust at bolt heads, gaps and visible slip between connected members. A tightness check on a sample of bolts needs a climb, so the images decide where one is worth making.

How it is assessed and monitored. A corroded or missing bolt reduces the capacity of the joint, and a slipped joint changes how load is shared between members. Both are recorded against the joint location so the structural check can account for them. Replacing corroded and missing bolts is usually the first repair, and the survey records which joints to revisit.

4. Bent, buckled or missing members

How it develops. Members are bent by vehicle impact, falling trees, careless maintenance and storms. Bracing members are stolen for their steel. Leg members carry compression, and a leg angle that has lost thickness or lost the bracing that held it straight can buckle at a load well below what its drawing assumed. A missing brace lengthens the unsupported span of the leg it was restraining, so a tower can look complete from a distance and be materially weaker than designed.

The stub is a known weak point here too. Research at CSIR-SERC on full-scale tower tests found that premature failure occurred through local buckling of stub members, and the Central Electricity Authority's committee on EHV tower failures has, as reported, identified buckling at stub level as a common route to complete collapse.

How it is identified. The drone survey records every bent, bowed or missing member against the tower drawing. Ultrasonic thickness measurement, one of our conventional NDT methods, measures the remaining section on the leg members, crossarm tips and insulator attachments that carry the conductor tension and the broken-wire load cases.

How it is assessed and monitored. A few millimetres lost from a leg angle costs far more capacity than the same loss from a lightly stressed member, so the check weights members by the load they carry. Missing and bent members go into the structural model as they are found, not as drawn.

5. Foundation degradation

How it develops. The concrete foundation and its chimney weather like any exposed concrete. Water enters cracks, the reinforcement inside corrodes and expands, and the cover concrete cracks and spalls off. Rain and floodwater erode soil from around the chimney. Soft or disturbed ground lets one footing settle more than the others.

How it is identified and assessed. Visual survey records cracking, spalling, exposed reinforcement and erosion. The strength and soundness of the concrete are tested on site by rebound hammer and ultrasonic pulse velocity, both on our NABL site-testing scope under IS 516 (Part 5), with cores taken where the owner allows them. A rebound hammer measures surface hardness as an indicator of strength; ultrasonic pulse velocity times a sound pulse through the concrete, so cracks and voids show up as slower readings.

How it is monitored. Foundations that test sound are re-inspected at the routine interval. Foundations with cracking, low readings or settlement are flagged for repair or further investigation.

6. Loss of verticality and deflection

How it develops. A tower leans when one footing settles, when a leg or stub has lost section or buckled, or after a storm or a conductor failure has overloaded it. A lean moves the tower's own weight and the conductor loads off the axis they were designed around, which adds load to the legs on one side.

How it is identified, assessed and monitored. Verticality and deflection are surveyed tower by tower. A measured lean is read together with the foundation and stub findings to find its cause, and the deflected geometry is carried into the structural check. Repeating the survey shows whether the lean is stable or still moving.

7. The design-basis gap: loads the tower was never designed for

How it develops. This mechanism is not damage to the steel. It is the gap between the loads a tower was designed for and the loads it is now expected to carry. IS 802 (Part 1/Sec 1), the code for loads on transmission line towers, was first published in 1967 and revised in 1973, 1977, 1995 and, as the fourth revision, in 2015. The 1995 edition set out reliability levels with return periods, Level 1 at 50 years covering lines up to 400 kV, and six basic wind speed zones from 33 m/s to 55 m/s. A tower erected in the mid-1980s was designed before either of the last two revisions.

The national record says the gap matters. The Central Electricity Authority's committee on failures of EHV transmission line towers for 2024, as reported in the trade press, recorded 28 incidents involving 76 towers, 30 of them at 220 kV, and recommended that towers built to older codes be assessed and strengthened to IS 802:2015. The Central Electricity Regulatory Commission applies a useful life of 35 years to transmission lines in tariff determination. A line commissioned in 1985 passed that mark in 2020.

How it is assessed. This is where the evidence from sections 1 to 6 comes together. Each tower type is modelled with its measured, not nominal, section on the members that have lost steel, with missing and bent members and any lean as found, and checked against the loads the current code sets for that wind zone. That re-analysis is reviewed and signed by the structural engineer of record.

Each mechanism, and how it is found, assessed and monitored

Of these methods, rebound hammer and ultrasonic pulse velocity on foundation concrete are on our NABL site-testing scope under IS 516 (Part 5). The drone survey, ultrasonic thickness measurement on tower members and the verticality survey are engineering inspection activities carried out under our procedures, and are not presented as accredited tests.

What the owner receives

The assessment ends with a decision per tower, or per representative tower where a type repeats under the same exposure, in one of four forms:

• Continue in service, with an inspection interval matched to the measured corrosion rate
• Repair: replace corroded bolts, missing members and damaged bracing
• Strengthen: add or upgrade members where the measured section fails the current load case
• Investigate further: foundations or members where the evidence is not yet enough to decide

What it deliberately does not do is put a single "years remaining" figure on a tower. A lattice structure does not age along one curve. Its remaining life is the time until the next measured member, at its current corrosion rate, falls below the section the current code requires. That is why a credible answer is a ranked repair and inspection plan, and not a number. The same approach sits behind our work on plant life extension for ageing power generation and infrastructure assets.

This article was written and edited by the TCR Newsroom, the editorial desk of the TCR group, covering materials testing, inspection and asset integrity work across TCR’s laboratories in India and Saudi Arabia and its alliances in Qatar, Japan and beyond.