1 Objective

To purpose of this web page is to explain the unique features of high mast light poles and the reasons for increased inspection requirements. [See Fig 1 below].

2 Introduction

High Mast Light Poles (HMLP) are a design of lighting column used for taller lighting columns. i.e. 15 – 60 m. High Mast Light Poles typically support luminaires consisting of multiple light sources.

High mast light poles are attached to a concrete foundation using cast-in holding-down bolts. Most High Mast Light Poles use a Stand-off baseplate design. [See Figs 1, 2 and 3 below].

High Mast Light Poles are normally supported using Stand-off Baseplates. Stand-off baseplates have three inherent design weaknesses that reduce the safe operational lifespan of high mast light poles.

The inherent design weaknesses of stand-off baseplates cannot be easily rectified. In practice, the only option to minimise the risks to public safety is frequent inspection and monitoring.

(The ideal solution would be to convert each stand-off base plate into a structurally grouted base plate with pre-tensioned holding-down bolts. However, changing all stand-off baseplates to grouted baseplates is not practicable. The conversion process would be too time consuming as as solution for all high mast light poles. The only safe alternative for most high mast light poles is to conduct frequent inspections).

Inherent design weaknesses of stand-off baseplates include:

i) It is impossible to calculate, measure or predict the individual static or dynamic loads in each holding-down bolt. i.e. Some holding-down bolts will carry more or less loads than others.

ii) The loads carried by individual holding-down cannot be assumed to remain constant over their life span. Loads carried by some holding-down bolts could increase. Loads carried by others could decrease.

iii) The average load in high mast light pole stand-off baseplate holding-down bolt is often significantly higher than the bolt fatigue endurance limit. i.e. The average safe operating life of each holding-down bolt is dependent on the number and magnitude of the load changes imposed on that particular bolt. The loads carried in some bolts will be greater than average. Some loads will be less.

Without knowledge of individual bolt loads, it is impossible to calculate:

a) The factor of safety in each bolt with regard to static bolt capacity. i.e. How close is the load carried in any particular bolt to its failure under static load.

b) The fatigue life of each bolt and hence safe operational life span.

3 High Mast Light Pole design

High Mast light Poles are a type of lighting column supported using a base plate (flange) welded to the column base. The baseplate is then bolted to a concrete foundation using several holding-down (“anchor”) bolts. This design contrasts with columns that are cast or “planted” into a concrete foundation or directly into the soil.

The base plate flange design is used for larger lighting columns or columns used to support heavy luminaires or signage.

One of two methods are used to connect the base plate to the concrete foundation.

Method 1 Stand-off base plate. Non-preloaded holding-down bolts using levelling nuts. [See Figs 1, 2 and 3 below].

“Stand-off base plates” suspend the structure above the its concreate foundation entirely using the holding-down bolts. i.e. The holding-down bolts carry both tensile and compressive loads loads imposed by the supported structure.

Stand-off base plates are the most common connection method for high mast light poles up to 35 m. Stand-off base plates are simpler and quicker to install.

However, stand-off base plates have very serious disadvantages.

Method 2 Structurally grouted base plates. Preloaded holding-down bolts with vertical compressive loads carried by the grout.

Structurally grouted base plates are rarely used for high mast light poles and are not part of this discussion.

Most discussion below is equally valid for any structure which uses a “stand-off base plate”.

e.g. CCTV towers, flood lights, mobile phone masts etc).

An understanding of the design features is necessary to enable an inspection programme to be developed to assess and monitor the unique risks to public safety.

Fig 1 High Mast Light Pole with Stand-off base plate

The design and dimensions used for this examples are typical of commonly used High Mast Light Pole. i.e. The design represented here is NOT based on that of a particular manufacturer.

The holding-down bolts are directly cast into a concrete foundation. See Fig 2 below. Holding-down bolts are not normally sleeved to prevent adhesion with the concrete.

Fig 2 High Mast Light Pole holding-down bolt cage within concrete foundation

Holding-down bolts are typically retained within the foundation using a cast-in retaining ring.

Note. All compressive and tensile dead weight and imposed wind loads are transmitted through the holding-down (anchor) bolts.

The section of each holding-down (anchor) bolt below the lower levelling is NOT preloaded.

Fig 3 High Mast Light Pole base and holding-down (anchor) bolt notation

4 What are the design challenges for High Mast Light Poles?

4.1 Large imposed repetitive wind loads.

HMLPs are both tall and normally carry large multi-light element luminaires.

The relatively large luminaires and HMLP column height apply very large wind loads onto the base-plate.

The HMLP holding-down (anchor) bolts are subject to high cyclic wind loads. i.e. Wind loads are variable.

4.2 Corrosion

The HMLP holding-down (anchor) bolts are subject to potentially high levels of corrosion. e.g. Pooling water, road salt, practical difficulties achieving perfect weather/water seal. (Zinc galvanising protect steel by sacrificial corrosion. Thicker galvanising will protect steel for longer but will always be “used up” in a corrosive environment). [See Fig 4 below].

4.3 High consequences of failure

HMLPs are normally located in areas with high traffic and / or high occupancy over extended periods. e.g. Motorway junctions, motorway service areas, retail parks, air ports etc. Consequence of collapse in any direction and at any time of day could be particularly severe.

Fig 4 Corroded High Mast Light Pole Holding-Down (Anchor) Bolts.

Corrosion has led to a complete loss of threads. The corroded diameter is less that the “stress area” for the size of bolt.

Note also.

1. Aged and ineffective “Denso Tape”.

2. Excessive free height between underside of levelling nuts and concrete. (As a result of sloping concrete surface).

3. Inadequate thread protrusion though the top nut.

Video 1 Failed 35 m High Mast Light Pole Holding Down (Anchor) Bolt.

(Video copyright Caspian NDT)

The middle bolt has failed below below the levelling nut. [See Fig 7 below].

The looseness of all the nuts would imply one of 3 options.

i) The top nuts have loosened. Top nut loosening is unlikely because of the lock-nuts. ***

ii) The bolt has stretched between top nuts and levelling nuts. Stretch is unlikely given the magnitude of the apparent “bolt stretch” over a distance equal to the base plate flange thickness.

iii) The levelling nuts below the base plate flange have self-loosened due to the dynamic wind loads and subsequent oscillation. Self-loosening could be a plausible reason because the levelling nuts are not fitted with lock nuts or locking mechanism.

*** The addition of lock-nuts to the top nuts is futile because it is not practicable to fit locks nuts to the levelling nut!

4.4 Fatigue cracking and failure structures and machines made from steel.

A steel structure subject to cyclic or repetitive loading could eventually fail by fatigue if the stress range between maximum and minimum loads exceeds the endurance limit of the material from which the component is fabricated.

The rate of crack growth and hence time to failure depends on:

a) The magnitude of the stress range. A higher stress range will cause a crack to grow by a larger distance for each load cycle.

b) The presence and magnitude of stress concentrations in the high stress areas. e.g. “Sharper” re-entrant corners and flaws would cause higher peak stresses than “rounder” re-entrant corners and flaws for the same load applied to the structure.

c) The number of load cycles at each particularly stress range. Each load cycle greater than the endurance limit could cause some microscopic crack growth.

4.5 Fatigue design of threaded bolt. (Applicable to any threaded bolt used in any structure or machine).

The notched shape of bolts threads create a high stress concentration. The size of the stress concentration can be reduced by its method of manufacture. e.g. Thread rolling creates a lower effective stress concentration than thread machining. However, stress concentrations created in rolled threads are are still high when compared with a similar rod without threads.

The endurance limit for threaded rod is typically very low when compared with the bolt static tensile strength. e.g. About 20 MPa. [BS EN 3 Part 1-9]. The allowable design value for the endurance limit of a thread rod is often independent of bolt static tensile strength. i.e. A high strength bolt could have the same design endurance limit as a low strength bolt or even lower!

How is it possible that bolts can be used for high cyclic load applications if thread rod has a low endurance limit?

The answer is a paradox. Bolts are often used without risk of failure in high load cycle applications. For example, wheel studs on a motor vehicle. However, the bolts threads is such cases are rarely subjected to more than a very small fraction of the cycling load applied to the bolted joint. i.e. The stress range in a bolt thread is designed to be much less than its endurance limit.

A low stress range in a bolt thread is achieved by preloading the bolt during installation. The bolt is preloaded in excess of any variable loads likely to be imposed during operation of the joint. The change of bolt stress and strain (stretch) will be minimal if the bolted joint does not “open-up” under any operating conditions. Hence, the bolt load remains nearly constant with minimal stress range.

5 What are the design risks specific to High Mast Light Poles?

5.1 Structural fatigue

Most parts of High Mast Light Poles are subject to high levels of cyclic – repetitive stress and hence at risk from fatigue. e.g. The relatively high stress concentration at the top of each gusset.

Risk of structural fatigue is greater in stand-off base plates when compared with structurally grouted base plates using pre-loaded bolts. i.e. All downwards loads are transmitted through the grout. Anchor bolt tension is always much greater than any expected upwards load.

The increased fatigue risk of a stand-off base plate is caused because the flange ring is supported only at its bolts rather than uniformly across is lower face. The more flexible “point” supports at the bolts allow the support flange to flex. The base plate flexure is much more than for an equivalent base plate supported of its complete surface using structural grout and with contact maintained using preloaded bolts. The increased flexure causes higher peak stresses and hence a greater risk of fatigue failure. Higher peak stresses often occur at the tops of flange gussets. [See Figs 5 and 6 below].

5.2 Holding-down (Anchor) bolt fatigue [See above].

i) Inability to preload holding-down (anchor) bolts below the levelling nuts.

High Mast Light Poles using stand-off base plates are one of the very rare instances where bolt threads are subjected to high cyclic – repetitive loads. i.e. The section of bolt threads immediately below the levelling nut cannot be preloaded.

ii) Unknown static installation load carried by each holding-down (anchor) bolt.

The dead weight installation load carried by each bolt is entirely dependent on the skill and judgement of the installer. i.e. If the levelling nuts are slightly higher or lower than its neighbour, then the installation load could be completely different. Installations loads of some bolts could even be tensile if a one levelling nut is slightly higher than its neighbour. A higher tensile load (or lower compressive load) in one bolt will mean that adjacent bolts are carrying a greater compressive load.

The static installation loads will probably have little effect on the magnitude of the stress range in any bolt when subject to wind loads. However, fatigue damage is increased if more of the stress range is tensile. i.e. A fully tensile stress range of 10 units between stress values of 0 to +10, is more onerous than a fully or partially compressive stress range. For example, from -10 to 0 or -5 to +5 respectively.

iii) Potentially variable holding-down (anchor) bolt axial stiffness

High mast light pole holding bolts are normally cast directly into the concrete foundation. Holding-down (Anchor) bolts are not normally sleeved or wrapped to prevent bonding for the bolt shaft to the concrete.

It is not possible to know or estimate the bond between the concrete and each holding-down (anchor) bolt. The concrete bond will depend on bolt finish, length of threading, corrosion, bond shear caused by bolt stretch, concrete variability etc.

A bolt that is bonded to the concrete for part of its length will be axially stiffer than a bolt that is less well bonded or is sleeved over its full length. i.e. The effective “stretchable” length of a bolt is effectively reduces as the length of bonded to the surrounding concrete increases.

A bolt that has a shorter effective length will therefore be axially stiffer than a bolt with a longer effective length.

An axially stiffer bolt will therefore carry a disproportionately higher load than less stiff neighbouring bolts. A higher load in one bolt compared to an adjacent bolt will cause a higher stress range and shorter fatigue life.

The effective bond between individual bolts and surrounding concrete could change over time. For example, as the bolts cyclically stretched. It is therefore impossible to know the precise load or change of load carried by each bolt. The only option is to inspect bolts frequently to detect fatigue cracks.

Fig 5 Finite Element Analysis results for High Mast Light Pole with stand-off base plate.

Note. The dimensions and imposed wind loads are typical of commonly used High Mast Light Poles.

The purpose of this result is for comparison** with an identical High Mast Light Pole but installed with structural grout and with pre-loaded anchor bolts. [See Fig 5 below].

** Precise calculated stress values are unimportant. A key point to note is that the stresses are significantly higher than fatigue stress infinite life endurance limit.

Stresses denoted in red are 200 MPa and above.

Fig 6 Finite Element Analysis results for High Mast Light pole with Structurally grouted based plate and pre-loaded anchor bolts

Note. The dimensions and imposed wind loads are typical of commonly used High Mast Light Poles.

The purpose of this result is for comparison with and identical High Mast Pole using a Stand-off base plate. [See Fig 5 above]. Precise calculated stress values are unimportant.

Stresses have exactly the same colour scale as Fig 5.

Fig 7 Close up view of Finite Element Analysis results of most highly loaded anchor bolt below the lower levelling nut.

The stress area of the bolt has been used. The purpose of this result is to show that the bolt stresses are relatively high when subject to typical wind loads.

(Relatively high in comparison with allowable endurance limit for bolts threads).

Precise stress values in this example are unimportant. A red colour in this diagram represents calculated stresses of 300 MPa. 300 MPa is reasonable for static loads applied to Grade 8.8 bolts.

Note: 300 MPa is very much higher than that for infinite life (endurance limit) of threaded bolts.

Notes.

1. Calculated stress values in elements touching the intersection of the underside of the levelling are not valid. i.e. The re-entrant “corner” as modelled is a “singularity”. A “singularity” would always display stresses that are incorrectly high.

2. This model does not include the stress concentration of threads. Hence, the calculated stresses displayed are unrealistically low.

6 Failure of structures using stand-off base-plates other than high mast light poles.

6.1 Smaller wind turbines (less that 75 kW approx)

A number of smaller wind turbines using stand-off baseplates have collapsed due to failure of their holding-down bolts below the lower levelling nuts. [Ref 2, Ref 3].

(Larger wind turbines normally use sleeved holding-down bolts that are post-tensioned over their full length).

Fig 6.1 Collapsed Wind Turbine at Bradworthy [Ref 2]

Note: Most holding-down bolts snapped below the lower levelling nuts as shown by most of the nuts being still connected to the base flange.

Fig 6.2 Foundation of Bradworthy wind turbine after collopse [Ref 2]

Note: Two holding-down bolts shown near the left side of the photograph has snapped above the levelling nut. The failure of these bolts above the levelling nuts appears to be consistent with the two missing flange nuts just above ground level in Fig 1 above. i.e. All other flange nuts shown in Fig 1 above could only be held in in place if retained by their lower levelling nuts. The earthing strip is assumed to directed towards the electrical control cabinet in Fig 1.

7 References1

1 CROSS Safety Reports

Collaborative Reporting for Safer Structures UK (CROSS-UK).

CROSS maintain a database of reports concerning the safety of structures in the UK

CROSS Safety Reports associated with High Mast Light Poles and related structures

ID Description
98

Unsafe street lighting columns

A report of failure of a cosmetically grouted stand-off baseplate.  i.e.  Non-structural grout placed under the base plate  for appearance rather than the support of structural loads.

315

Telecommunications towers and resin anchors

Resin fixed holding-down bolts failed after 6 years.  (Comment – A stand-off was used for this installation is inferred by the statement “Some of these anchors do not allow for testing and replacement of critical bolts at the superstructure/substructure interface because of levelling nuts below the base plate”.

610

High mast light poles removed from UK site

Removed from site following an order made by the local authority.  (The author of the report had previously informed the site owner but without any success).  The holding-down bolts were judged to to be excessively  corroded and inadequate nut engagement.

773

Accelerated corrosion of telecoms mast holding-down bolts

“During an examination of a 20m high telecoms mast, a reporter found that there had been accelerated corrosion of the galvanised holding-down bolts in the gap between the base plates and the concrete foundation slab. The loss of section was found to be around 5mm in less than 10 years. The holding-down bolts were difficult to inspect as the gap between the baseplate and the foundation was less than 50mm”.

 

2 HSE Report “Assessment of Endurance Wind Turbine Failure – East Ash Farm, Bradworthy, Devon” 2 December 2013

3 HSE Report “Assessment of Gaia Wind Turbine Failure – Winsdon Farm, North Petherwyn, North Cornwall” 12 September 2013