Fatigue Analysis and Repair of a High Mast Light Pole · Fatigue Analysis and Repair of a High Mast Light Pole Counsell Taplin Thomas Noyes-Brown - 2 - 1.0 Details of the affected

Fatigue Analysis and Repair of a High Mast Light PoleCounsell Taplin Thomas Noyes-Brown - 1 -

Fatigue Analysis and Repair of a High Mast Light Pole

Sarah Counsell and Geoff Taplin, Maunsell AECOM, Melbourne AustraliaMal Thomas and Gary Noyes-Brown, VicRoads Technical Consulting

SynopsisFatigue damage of high mast light poles has been an issue worldwide. There havebeen numerous examples of light poles and other road furniture failing due to fatigue.This paper is describes one case of the fatigue analysis and repair, as an example ofa more general problem.

The interchange of the Western Ring Road, Princes Freeway West and West GateFreeway is a major intersection in Melbourne’s western suburbs. Constructed in1994, the interchange is illuminated by 28 high mast light poles, which range inheight up to 40 metres. The high mast light poles are a proprietary system,comprised of galvanised steel tapered twenty-sided slip-jointed mast sections.

Fatigue cracks were identified in some of the poles in 2007. A desk-top investigationwas subsequently undertaken to review the fatigue and ultimate strengths of the lightpoles, anchor bolts, and foundations in order to decide whether repair or replacementof the poles would provide the lowest whole-of-life cost. As a result of thisinvestigation it was decided to repair the light poles, with modified details to extendtheir fatigue life.

This paper describes the investigations that were undertaken, and the recommendedmodifications that would reduce the stress concentration in the pole mast, and henceextend the fatigue life.

Figure 1 Western Ring Road/Princes Freeway/WestGate Freeway Interchange, Melbourne, Australia

High mast lightpoles


1.0 Details of the affected light polesThe 28 light poles at the freeway interchange were all of similar construction.Figure 2 shows the typical luminaire arrangement at the top of the light poles, andFigure 3 shows a typical light pole base, with significant features being the accessopening, the gusset plates, base plate anchor bolts and grout.

Inspections carried out by VicRoads identified fatigue cracking in 27 of the 28 highmast light poles at the freeway interchange. All fatigue cracking was in the light polemast, at the tip of the gusset stiffeners. Figure 4 shows one of the fatigue cracks thatwas identified.

Inspection also identified that the base plates were not fully grouted, with the grouthaving been applied around the external perimeter only (Figure 5 and Figure 6). Poorgrouting has been known to contribute to fatigue damage of anchor bolts in otherlight poles and sign gantries. Bolts need to be checked for fatigue stress due totension and shear, however additional stress due to bending needs to also beconsidered if the bolts have a free length due to insufficient grout.

Figure 4 Fatigue crack at the top of the gusset

accessopening

slip joint

mast

gusset

anchorbolt

baseplate

Figure 2 Typical light poletop structure

Figure 3 Typical light pole base


Figure 5 Grout around base plate(external view)

Figure 6 Grout around base plate(internal view)

One light pole was selected for detailed structural assessment. The light poleassessed was a 40m high galvanised steel pole. The pole is a twenty sidedpolygonal section with diameter tapering from 811 mm ODAF (outside diameteracross flats) at the base to 506 mm ODAF at the top, and was constructed from foursections joined using slip joints. The wall thickness reduced from 6mm to 4mm overthe height of the pole.

The pole is joined to the foundations using 20 M36 galvanised bolts grade 4.6/S.The base plate of the pole is strengthened by 10 gusset plates (see Figure 3).

2.0 Fatigue Analysis – Loading

As with all structures which are loaded by natural phenomena, neither the magnitudenor the number of load cycles for light poles can be predicted precisely.Consequently it is necessary to use approximate methods in design. A number ofdifferent approaches have been adopted in international practice.

2.1 Highways Agency (UK)The Highways Agency Design Manual for Roads and Bridges (Highways Agency2007), Part BD94/07, requires light poles to be checked for fatigue. The method canbe inferred to assume a fatigue stress range equal to 25% of the characteristicdesign stress, and a number of cycles based upon the pole vibrating at its naturalfrequency for approximately 12 days each year. However, the scope of BD94/07 islimited to light poles less than 20 metres in height.


2.2 AASHTOThe American Association of State Highway and Transportation Officials (AASHTO)have commissioned significant research in the USA in the last five years. Culminatingin the AASHTO “Standard Specifications for Structural Supports for Highway Signs,Luminaires and Traffic Signals” (AASHTO 2006) This standard provides a tool fordesign of light poles and sign gantries (including fatigue design and assessment).This research by AASHTO has been undertaken in response to the failure ofnumerous light poles in the USA. For example, in Iowa in 2003, a 43 metre high lightpoles collapsed (see Figure 7) prompting an extensive investigation into this type ofstructure (Connor 2007, Warpinski 2006)

Figure 7 Collapsed light pole in Iowa, USA

Research by Fisher et al (Fisher 1993) reported that, if less than 0.01% of variableamplitude load ranges exceeded the constant amplitude fatigue limit (CAFL) for aparticular Detail Category, then infinite fatigue life would result. On the basis of thisfinding, Kaczinski et al (Kaczinski 1998) proposed that if they could determine thestress range (and corresponding wind speed) that had a probability of occurrence ofonce in every 10,000 stress cycles (ie 0.01%), then they could assess fatigueperformance by comparing that stress range to the CAFL for the relevant DetailCategory.

They based the fatigue life on the CAFL, rather than on an actual estimated numberof load cycles, because the research indicated that a light pole may be subject toover 100 million cycles in a 25 year life, whereas a weld detail reaches the constantamplitude fatigue limit at 10-20 million cycles. Note that in the Australian standardsAS4100 and AS5100.6 the CAFL is referred to as the constant stress range fatiguelimit (f3).

To determine the applicable stress range, Kaczinski used the following approach:


1. Apply Davenport’s wind force spectrum (Davenport 1961) to determine theresponse of a four typical cantilever structures as a function of mean hourlywind speed.

2. Idealise the wind loading as being comprised of a succession of constant windevents of one hour duration, with wind speed equal to the mean hourly windspeed.

3. Estimate the mean hourly wind speed with 0.01% probability of exceedancefrom the annual mean wind speed, by assuming a Rayleigh distribution of meanhourly wind speeds,

2m

2

V4E e)(P

where,)(PE is the probability of a mean hourly wind speed >

Vm is the annual mean wind speed

Setting )(PE to 0.0001 gives the mean hourly wind speed with 0.01% ofexceedance as 3.42 times the annual mean wind speed.

After studying USA wind speed data, Kaczinski adopted an annual mean wind speedof 5 m/s as representative of conditions across the country. This corresponds to adesign mean hourly wind speed of 17.1 m/s. The stress range due to this windspeed was determined from the response spectrum based upon Davenport’s windforce spectrum. Finally the static wind pressure, that would cause the same stressrange, was back-calculated.

As a result of these calculations, Kaczinski determined equivalent static windpressures ranging from 0.17 kPa to 0.30 kPa for the four typical structures that wereanalysed.

The design methods in AASHTO (2006) are based upon the work of Kaczinski. Thenominated pressure to be used for the fatigue design of light poles subjected tonatural wind gusts according to AASHTO is 0.25 x Cd x IF kPa, where Cd is the dragcoefficient for the cross section, and IF is an importance factor for the structure.

To put this design pressure in the context of Australian design codes, a wind speedof 17.1 m/s at 10 metres height in Terrain Category 2, with a dynamic factor of 1.0,and a drag coefficient of 1.2 equates to a wind pressure of 0.21 kPa.

3.0 Fatigue Analysis – MethodsOnce a fatigue loading spectrum has been decided upon, the question arises as towhich of the available methods of fatigue analysis should be applied to the problem.The common methods are discussed below, and applied to the fatigue assessmentof the light pole.

3.1 Detail Category methodThe most common method for assessing fatigue is the categorised detail method(also referred to as the nominal stress method). The nominal stress range at thedetail location is calculated using simple static methods based upon the nominalsection properties, and compared to a fatigue limit for the particular detail


classification based on the number of cycles of the loading. Stress concentrationsarising as a result of the joint configuration (for example due to weld geometry) areallowed for in the detail classifications.

In many codes of practice, details are categorised based on their experimentallydetermined fatigue strength at 2 million cycles. The fatigue strength at this number ofcycles is known as the Detail Category or fatigue class (FAT). As mentioned, theDetail Category accounts for the local stress concentrations due, for example, to theweld geometry and stress direction. A fatigue S-N curve (stress range versusnumber of cycles) is then established for other numbers of stress cycles based uponan assumed logarithmic relationship between stress range and fatigue strength.

This method has some shortcomings, particularly as Detail Categories do not coverall details, and the method does not allow for an assessment of the improved fatiguestrength where a detail is modified. However, this method is used by most standardsand is used as a comparison here to a more detailed approach.

The Australian standards AS4100 ‘Steel Design’ and AS5100.6 ‘Bridge Design: Steeland Composite Construction’ provide for fatigue assessment via the Detail Categorymethod outlined above. However, they do not include a category for detailsassociated with light poles and other road furniture.

As discussed in Section 2.1 the United Kingdom Highways Agency has publishedstandard BD94/07 ‘’Minor structures’ for design of light poles and various roadfurniture including fatigue, however, it does not provide design guidelines for lightpoles over 20m in height.

As discussed earlier, AASHTO (2006) bases fatigue assessment on the DetailCategory method. This document includes categories for details particular to lightpoles such as base plate stiffener plates, anchor bolt connections and accessopenings in the side of light pole structures. For these reasons, AASHTO (2006) wasused in this project for the assessment of fatigue using the Detail Category method.

3.2 Assessment by Detail Category methodAASHTO (2006) describes four types of wind loading:

galloping vortex shedding natural wind gusts truck-induced gusts

For the three categories of structure considered by AASHTO (2006), (signs, trafficsignals and lighting), advice is provided as to which of the four types of wind loadingmust be considered in the fatigue assessment. For light poles, vortex shedding andnatural wind gusts must be considered.

Vortex shedding was considered, however the critical wind speed for vortex sheddingwas found to be below 5m/s. A wind speed below 5m/s is not considered to containsufficient energy to excite the structure for vortex shedding (AASHTO 2006).Therefore, the critical load case for the fatigue assessment of the light pole wasnatural wind gusts.


As described in Section 2.2 above, AASHTO (2006) assumes an annual mean windspeed of 5 m/s. For the analysis of the light pole, actual wind speed data wasobtained from Laverton Weather Station through the Australian Bureau ofMeteorology (BOM). The Laverton Weather Station is approximately 13km from thelocation of the light pole. The BOM records the average wind speed at 9am and 3pmeach day measured over 10minutes at a height of approximately 10m. Based uponanalysis of this data, an annual mean wind speed of 5.5 m/s was used for theassessment of the light pole.

The nominal stresses (based on 5.5m/sec wind speed) at critical locations on thelight pole, together with the relevant Detail Categories, are summarised in Table 1.The Detail Category is based upon the advice provided in AASHTO (2006), as is theCAFL for each Detail Category. Figure 8 is an extract from AASHTO (2006) showingthe CAFL for each Detail Category. The critical locations are shown in Figure 9 andFigure 10.

Table 1 Nominal Stress Results

Detail Nominalstress

Detail Category &CAFL (AASHTO

2006)Termination of gussetplate

40 MPa E’ – 18MPa

Reinforced accessopening

40 MPa E – 31MPa

Mast at base plate 20 MPa E’ – 18MPaAnchor bolts 32 MPa D – 48 MPa

Figure 8 CAFL for various AASHTO (2006) Detail Categories


Figure 9 Critical Fatigue Location (1) Figure 10 Critical fatigue location (2)

The results indicate that the nominal stress exceeded the CAFL at the termination ofthe gusset plate and, to a lesser extent, the reinforced access opening, and the mastat the base plate.

The fatigue analysis results agree with the location of the fatigue cracks which werefound at the termination of the gusset plate. Based on this analysis, it is also likelythat cracks could form at the reinforced access opening and the mast at the baseplate, and therefore regular inspections were recommended to monitor this.

In order to determine a repair method for the fatigue cracking at the gusset platetermination, a more refined method of fatigue analysis was used, as described below.

3.3 Geometric Stress MethodA more refined method for fatigue assessment uses the geometric stress methodusing finite element analysis to assess the effects of local stress concentrations.This is also known as the hot-spot stress method. The finite element modeldetermines the hot spot or geometric stress (as shown diagrammatically in Figure 11below) which is then compared to a reference fatigue S-N Curve based on DetailCategory 100 (Figure 12).

terminationof gussetplate

connectionbetween

reinforcedopening and

main pole


Figure 11 Definition of geometric stress (Hobbacher 1996)

Figure 12 S-N curve for the geometric stress method

The load used for the finite element analysis was the AASHTO (2006) loading asused for the Detail Category method. Figure 13 and Figure 14 illustrate the finiteelement model of the light pole that was created using Strand7.

FAT 100curve CAFL limit for hot-spot

technique = 74 MPa

design stress

0.4t t


Figure 13 Finite Element model oflight pole

Figure 14 Finite Element model around thegusset plates

3.4 Results of the Geometric Stress MethodThe light pole was assessed using the geometric stress approach, using the finiteelement model. The critical detail was then modified, and the geometric stressapproach was used to determine whether there was an improvement in the fatiguestrength.

The geometric stress method calculates the design stress range (geometric stress)by extrapolation of the finite element results. Plate results at points 0.4t (t=thicknessof the main plate) and 1.0t from the edge of the weld are used to linearly extrapolatea result at the edge of the weld, known as the geometric stress, as shown previouslyin Figure 11.

Figure 15 shows the finite element model of the gusset plate termination in theoriginal condition. The geometric stress was estimated as 175MPa (see Figure 16).The fatigue strength was 74MPa (the CAFL limit for the FAT 100 curve).


Figure 15 Strand7 Finite Element showing stress concentrations

Figure 16 Determination of geometric stress at weld toe

FEcomputedstress

linearextrapolation

termination ofgusset plate

t

extrapolatedstress range

mast wall

weld

gussetweld

t

0.4t

175MPa

geometric stresscalculated in thisregion


4.0 Repair OptionsIn order to reduce the stress range a number of repair options were considered.

Ring shaped stiffener around the top of the gusset plates U shaped stiffener to join adjacent gusset plates Reduce the height of the light pole Jacket the base of the mast in concrete

The first two repair options were modelled in Strand7 in order to assess the effect ofthe modification on the stress range, using the geometric stress method (Figure 17).

Figure 17 - Strand7 model of U-stiffener modification

The results of the fatigue stress range for each of the first three options are shown inTable 2 below.

Table 2 Repair optionsCalculated

fatigue stressrange

CAFL(allowable)

fatigue stressrange

Ratio ofcalculated to

allowablefatigue stress

rangeno change (originalcondition)

175 MPa 74 MPa 2.4

ring shapedstiffener

135 MPa 74 MPa 1.8

U-shaped stiffener 95 MPa 74 MPa 1.3Reduce the overallheight to 21 metres

74 MPa 74 MPa 1

Stress isreduced at topof gusset


Based upon this analysis it was decided to repair all the existing fatigue cracks, andfit U-shaped stiffeners as shown in Figure 18. Although the calculated stress rangeexceeded the CAFL (by a ratio of 1.3), the improvement in fatigue life that could beexpected from the repair should result in a service life post-repair considerably inexcess of the 14 year life of the light poles to date. Coupled with routine inspectionsby the asset owner, this provided the lowest whole-of-life cost outcome.

Figure 18 - Light pole base with U-stiffener modification

5.0 Summary

Fatigue cracking was identified in a light pole and a fatigue analysis was undertaken.The results found that some details did not have an adequate fatigue strength,including the mast at the termination of the gusset plates. U-stiffeners were analysedusing a finite element model and found to reduce the stress concentrations aroundthe top of the gusset plates.

Fatigue strength is an important consideration in the design of light poles and otherroad furniture, particularly for the design of the anchor bolts, gusset plate termination,base plate weld and access openings. The AASHTO standard is recommended toassess those details which may be prone to fatigue damage.

6.0 ReferencesAASHTO 2006 “Standard Specifications for Structural Supports for Highway Signs,Luminaires and Traffic Signals”

Connor 2005 “Field and Laboratory Studies on High-Mast Lighting Towers in Iowa”Robert J. Connor et al. Proc 2007 Mid-Continent Transportation ResearchSymposium, Ames Iowa

Davenport 1961 “The spectrum of horizontal gustiness near the ground in highwinds” Quarterly Journal, Royal Meteorological Society, Vol. 87, London 1961Fisher 1993 “Resistance of welded details under variable amplitude long-life fatigueloading” NCHRP Report 354


Fricke 2002 “Evaluation of hot spot stresses in complex welded structures” Fricke, W.Proc IIW Fatigue Seminar

Highways Agency 2007 “Design Manual for Roads and Bridges” Part BD94/07 –Design of Minor Structures

Hobbacher 1996 “Fatigue design of welded joints and components” Hobbacher A. etal IIW Document XIII-1539-96

Kaczinski 1998 “Fatigue-resistant design of cantilevered signal, sign and lightsupports” NCHRP Report 412

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Fatigue Analysis and Repair of a High Mast Light Pole · Fatigue Analysis and Repair of a High Mast Light Pole Counsell Taplin Thomas Noyes-Brown - 2 - 1.0 Details of the affected