The Structural Integrity of Steel Lighting Columns

Risk strategies and testing regimes adopted by lighting engineers have ranged between absolutely no action whatsoever to over-kill depending upon individual preferences and available budgets. Rarely are the options assessed for their technical efficacy or true cost effectiveness.

The Condition Assessment proposed in the ILE Technical Report 22 (Managing a vital asset) requires columns at greatest risk and with the most serious potential consequences to be identified. Having made that assessment, it makes sense that the testing regime adopted will identify those that are satisfactory and those that are at real risk of early or imminent failure.

The ILE Technical Report No. 22 identifies column positions where defects due to corrosion are significant and where catastrophic failure can occur. Those positions are:



The causes of corrosion of the base & root are widely known.

At the root, the cause is aggressive ground conditions, de-icing salts and canine urine.

At and below the door, the cause is usually water ingress via ill-fitting doors, bracket defects or ‘tidal’ conditions inside the base.

At the swaged joint, the cause is moisture condensation inside the shaft or water ingress due to a defective bracket connection allowing it to collect in the reservoir formed by the sleeved joint. (See below)

Corrosion progresses from the inner surface and not evident until corrosion penetrates the shaft

N.B. Internal corrosion at this position on a column will only occur in columns manufactured by British Steel /Stewart & Lloyd/ Corus that employ the hot shrunk sleeved joint method.

At the root, the most critical zone is to approximately 100mm below ground level. Any corrosion at lower levels is likely to be less severe and the probability of resultant collapse minimal. Also, if corrosion is visible on the base immediately above ground, it will usually be more severe below ground and, if there is no corrosion above ground it does not follow that all is well below.

In most cases, the presence and extent of corrosion cannot be identified by visual assessment alone because it is either below ground or it proceeds from the inside to the outside of the column. Without an effective testing programme, Engineers are likely to be unaware of a latent problem until the column has collapsed.

In selecting a testing regime, it is important that at least the high-risk zones are addressed and that they are addressed effectively.

A number of techniques are presently employed and whilst data is collected and stored, the reliability and value of that data should often be questioned. The following observations are made about some of those techniques.

VISUAL INSPECTION – Assessment is subjective

• cannot identify corrosion from the inside
• Cannot assess the severity of defects
• Impractical for below ground assessment (root)

HAMMER TAPPING - May identify some serious corrosion

• Difficult to identify minor defects
• Assessment very subjective
• May damage surface coating

ULTRASONIC THICKNESS -Impossible to obtain a reading on externally corroded surfaces without grinding.

• After grinding to enable transducer contact, value may not represent the smallest thickness.
• Able to obtain spot readings only on sound external surfaces.
• Must excavate and prepare surface for testing below ground.
• Difficult to locate weakest/thinnest section where internal corrosion has occurred.

ULTRASONIC GUIDED WAVE - Many columns not testable due to proximity to an obstruction (e.g. Wall) or door too close to ground level.

• Can identify corrosion above and below ground (where accessible).
• Minimal surface preparation required.
• Difficult to determine wall thickness from results.

STATIC LOADING - Assesses the EFFECT ONLY of corrosion and cracking.

• Can assess anchorage of flanged columns.
• Care needed to avoid damage to column and foundation.
• Test only assesses integrity at the time of test.
• No indication of presence or extent of corrosion or how near to critical loss.
• Cannot monitor change in column condition with time.

The only technique designed specifically for addressing the four significant defect positions is LOSS OF SECTION testing about which the TRL comment: -

LOSS OF SECTION - RELATIVE LOSS OF SECTION METER™ - BASE & ROOT

• Can identify corrosion up to 100mm below ground level without excavation.
• Can be used to assess the relative loss of section anywhere on the base or shaft.
• Minimal surface preparation.
• Unaffected by surface coating
• Can detect narrow band full penetration corrosion.
• Can pinpoint position of greatest loss.

SWAGED JOINT ANALYSER™ – SWAGED (sleeved) JOINT

• No surface preparation.
• Unaffected by surface coatings.
• Detects relative loss from the inner surface of sleeved joints.
• Can detect and monitor changes in column and joint condition with time.
• Can be used to compare wall thickness of 10 x 10 mm areas anywhere on the column

THE RELATIVE LOSS OF SECTION TECHNIQUE IS CONSIDERED THE MOST RELIABLE AND COST EFFECTIVE SYSTEM FOR IDENTIFYING THOSE COLUMNS AT HIGH RISK OF FAILURE AND THOSE THAT ARE SATISFACTORY.

RELATIVE LOSS OF SECTION AND SWAGED JOINT TEST METHODS

CMT Instruments developed two unique instruments specifically for assessing the condition at the root, base and swaged joint of straight or tapered, circular and multi-sided steel lighting columns. Both systems are 'dry-coupled' therefore there is no gel to apply or remove and no other surface preparation. The Relative Loss of Section meter (RLS) is specifically designed to estimate the loss of metal from both the inner and the outer surface at the critical zones on a column base (from g/l to the shoulder) and to penetrate most ground materials to measure the loss in the buried 100mm WITHOUT excavation. In most circumstances, the standard search head that scans an area 100 x 200mm in front and 100 x 200mm below the base of the head is appropriate. A smaller head that views an area 70mm wide x 110mm (40mm in front and 70mm below) is used in special circumstances.





Operating procedure – Relative loss of section meter

The operating principle is the same for both instruments. Measurements of wall section in the test areas are relative to column thickness (or section) at a sound location.

In each case, a position on the column identified as being free from significant corrosion is scanned and this provides a reference signal related to the thickness and geometry at that position on the column. The procedure is repeated at the selected test positions and the two signals compared by the embedded computer and a value related to the difference in the two signals are displayed. That value approximates the relative difference in the percentage contiguous metal in that test zone.

Four equally spaced measurements taken round the base scan the full circumference. At ground level the instrument penetrates concrete, asphalt, macadam and soil to assess the loss in the 100mm depth BELOW ground zone.

Take reference

Take measurement at four equally spaced positions Take reference around the base and at the other vulnerable zones

Take measurement at four equally spaced positions around the base and at the other vulnerable zones

By taking a further measurement 100mm above ground level, the loss of metal in the zone Ground level to –100mm is known.

The Swaged Joint Analyser - scans an area 10mm x 10mm in front of the search head.

Using the Swaged Joint Analyser, the highest signal around the circumference of the shaft and approximately 150mm above the joint overlap is logged in as the reference. As the head is drawn vertically down the shaft, the changing display reflects the changing mass of metal in the sensed zone. The most negative value is logged at each of four scans equally spaced around the shaft. As the head passes over the joint, the values become increasingly positive due to the double thickness of the inner and outer tubes.

The demonstration piece below illustrates the procedure,

The appearance of the outer surface conceals the severely corroded inner surface that otherwise would not be detected.





Test results

3.1 The average test result for each column is first calculated and to classify the overall condition, a numeric weightings of 4 is applied where one or more measurements fall within the yellow band (class 3) and a weighting of 8 for measurements that fall in the orange and red bands (classification 4 & 5).

This forms the basis for the recommended action as shown below.

TR22 Category	CMT Class	Average LSU’s	Recommendation	TR22 - Priority Score Impact
2U	5	>-50	Immediate removal or making safe the unit	No Change
1U	4	-25 to -50	Schedule for Removal/Replacement as soon as practicable or in accordance with Operating Authorities Action Matrix	No Change
2G	3	-17 to -24	Re-test within 2 years	Minus 2 from score
3G	2	-11 to -16	Re-test within 3 years	Minus 3 from score
6G	1	0 to -10	Re-test within 6 years	Minus 6 from score

Corrosion is rarely uniform. However, a uniform loss of 25% of column thickness below ground may be just as severe as an irregular loss in where corrosion has fully penetrated the column. The following photographs indicate the condition at the root corresponding to a range of Relative Loss of Section units (percentage losses).