ed

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steel tower baseplate. The research ndings conclude that CFRP materials provide a viable alternative

for strengthening steel monopoles that can be easily designed and installed to increase their exural

telecoth inemai

cellulatoweruipmento bu

cates that high-modulus carbon ber reinforced polymer (CFRP)

stiffness qualities of CFRP offer signicant load carrying improve- round or polygonal shape with shaft thicknesses varying from

ARTICLE IN PRESS

Contents lists availabl

w.e

d

Thin-Walled Structures 47 (2009) 10371047dead, wind, seismic, and ice loads, with most of the applied designment while eliminating welding or bolting steel members to theexisting structure. High-modulus CFRP also has excellent fatigue

15 to 120mm. The structures are either directly embedded intothe concrete foundation or installed atop various concretefoundations through connections with anchor rods. Monopolesare designed and fabricated to resist various combinations of

load resulting from wind and seismic lateral forces. Typical windloads comprise of 8090% of the structure design criteria for these

Corresponding author. Tel.: +19195131733; fax: +19195131765.

E-mail addresses: sami_rizkalla@ncsu.edu, srizkal@ncsu.edu (S. Rizkalla).0263-82

doi:10.1materials can provide an excellent solution to enhance the exuralstrength and stiffness of monopoles. The inherent strength and

Monopoles are typically made of high-strength steel and varybetween 7 and 75m in height. Their cross-section consists of aincreases the strength and stiffness of monopoles.Research conducted at North Carolina State University indi- 1.1. Current design and strengthening practicecommon, so reuse and strengthening of existing towers is vitaland often the only option to support the additional services.As the need to install additional cellular equipment grows,existing monopoles, shown in Fig. 1, are frequently structurallyinadequate to support the cellular equipment expansion due tothe increased lateral wind loads. Thus, there is a need to develop acost effective, durable strengthening system that signicantly

information on the behavior and effectiveness of the three CFRPcongurations at increasing the scaled monopole exural strengthand stiffness, as well as the failure modes. The study also providesinsight into the installation process, presenting the methodologyof material handling and the proper connection to transfer theforces from the shaft to the baseplate of the tower.1. Introduction

During the past two decades, thehas experienced signicant growIntroduction of wireless phones,increased the demand on existingnew consumer services. Monopolesupport the necessary cellular eqantennas. Community resistance31/$ - see front matter Published by Elsevier

016/j.tws.2008.10.010Published by Elsevier Ltd.

mmunications industrythe wireless sector.

l, and internet haver networks to supports are typically used tot, coaxial cables andilding new towers is

and corrosion resistance properties, which could signicantlyimprove serviceability over the lifespan of the monopole.

The main objective of the research summarized in this paper isto determine the effectiveness of high-modulus CFRP in increasingthe exural strength and stiffness of steel monopole towers.This paper discusses design aspects for existing full-scale mono-pole towers based on tests to failure of scaled steel monopoletowers strengthened with three different CFRP strengtheningcongurations. Measured experimental data provides detailedstrength and stiffness.Behavior of steel monopoles strengthen

B. Lanier a, D. Schnerch b, S. Rizkalla c,

a American Tower, 400 Regency Forest Drive, Suite 300, Cary, NC 27518, USAb Wiss, Janney, Elstner Associates, Inc., 245 First Street, Suite 1200, Cambridge, MA 021c Constructed Facilities Laboratory, North Carolina State University, 2414 Campus Shore

a r t i c l e i n f o

Available online 23 December 2008

Keywords:

Elastic modulus

Carbon ber reinforced polymers

Steel structures

Design recommendations

Monopole towers

a b s t r a c t

This paper introduces a str

ber reinforced polymer

investigation including te

materials and connection

procedures are introduced

application of the adhesiv

exural elastic analysis a

recommends specic conn

journal homepage: ww

Thin-WalleLtd.SA

ve, Campus Box 7533, Raleigh, NC 27695-7533, USA

thening technique for steel monopole towers using high-modulus carbon

RP) materials. The technique is based on a theoretical and analytical

g large scale steel monopole towers strengthened with different CFRP

tails. Based on the research ndings, design aspects and installation

e recommended installation procedure describes the surface preparation,

and the sequence of CFRP application. The design aspects are based on

material properties of the CFRP and steel monopole shaft. This paper

ion details to ensure the development of the forces from the CFRP to thewith high-modulus CFRP materials

e at ScienceDirect

lsevier.com/locate/tws

Structures

Tower Upgrade System which utilizes both standard modulus

ARTICLE IN PRESS

StruNomenclature

Mn exural nominal strength of tower shaftSt,min minimum transformed section modulus of tower

shaft

B. Lanier et al. / Thin-Walled1038types of towers, whereas dead loads typically make up less than5% of the structure load. Flexural resistance to wind loading istypically the critical structural loading for monopoles due to theirheight and the loading condition. Monopoles are designedaccording to standard ANSI/TIA-222-F or TIA-222-G. These codesutilize wind, ice and seismic criteria per ASCE7-88 and ASCE7-02,respectively with Revision F utilizing an allowable stress steeldesign while Revision G enforces a load and resistance factor steeldesign.

Several solutions are currently available for strengthening ofexisting eld installed steel monopoles. The primary designobjective is to enhance the exural strength and stiffness bybolting, welding or bonding longitudinal steel plates, bars, tubes,shells or ber composites along the monopole shaft. [1]MorrisonHersheld Group (2004) designed the DualPole system, whichincreases monopole strength by increasing the lateral stiffnessof the structure. Two sections of high-grade steel are fabricatedto t outside shaft surface, completely encasing the monopole.The two new outside shaft sections are then welded at the seamsand baseplate to create a non-composite shell surrounding theexisting monopole, which increases the exural stiffness ofthe overall structure. Through increasing the exural stiffness,the monopole exural strength is proportionally enhanced.

[2]Westower Communications (2004) developed a strengthen-ing system using high-strength, threaded bars installed parallelwith the monopole shaft to increase their exural strengthand stiffness. The threaded bars are manufactured by Dywidag-Systems International and range in diameter from 30 to 45mm.The bars are u-bolted to clip angles which are blind bolted to the

Fig. 1. Typical monopole supporting wireless services.CFRP and steel plates bonded parallel to the existing monopoleshaft to increase both exural strength and stiffness. CFRP is usedwhen higher strength increases are required while steel platesare typically installed when strength demands are lower. Thissolution utilizes an epoxy adhesive for both the CFRP and steelbonds. Ultimately, the AeroSolutions solution demonstrates anadhesive bond can be reliably developed for either steel or CFRPmaterials to support the strengthening system.

Research by Ferreira and Branco [4] has demonstratedsignicant exural and axial strength enhancement can bedeveloped through use of glass ber reinforced polymers (GFRP)for fabrication of new monopoles. Their method utilizes carbonber pretension tendons to reduce the tensile stresses, similar to apre-stressed concrete design, in the monopole shaft. This reduc-tion in tensile stresses, in addition to the enhanced strengthcapacity of glass ber, allows for overall strength increases forthese types of towers.

Fatigue performance of this structural system can be critical fora given variable such as multi-directional wind loading. Whilebolted connections are preferable to welding when consideringthe fatigue performance of a connection, bolted connectionsare often difcult to implement due to the complex geometry ofthe monopole tower. Adhesive connections can provide a reliablealternative to cyclical loading. Scaled steel bridges strengthenedwith high-modulus CFRP have been shown to have excellentfatigue performance without notable deterioration of strength [5].

2. Material properties

The proposed strengthening technique in this paper includehigh-modulus carbon ber materials with a modulus of elasticityapproximately three times that of steel. The bers are typicallyfabricated into solid, rigid, pultruded laminate strips that arebonded to steel structures as an external reinforcement using anepoxy adhesive. Another application method is by wet-lay-up,monopole shaft, as shown in Fig. 2. Intermittent clip angles arespaced to limit the buckling length of the bars per thecompressive strength demands and to adequately transfer theshear forces. The bar strengths are developed at each end thoughuse of extended clip angles with multi u-bolt and blind boltconnections. The bars are developed by grouting into the existingconcrete foundation of the monopole.

AeroSolutions [3] has designed the AeroForce Monopole and

Fy yield strength of tower shaftR ratio of CFRP rupture or crushing strain to tower steel

shaft yield strain

ctures 47 (2009) 10371047whereby the dry bers are fabricated in exible sheet form andare impregnated with resin as they are bonded to the monopole toform a solid, rigid material.

Two types of high-modulus pitch-based carbon bers wereused in the experimental program. Pitch-based carbon berutilizes petroleum pitch ber as its precursor, unlike the morecommon polyacrolonitrile (PAN) based carbon ber. Materialproperties for the two types of carbon ber are listed in Table 1.The rst monopole was strengthened using high-modulus carbonber installed in sheet form and impregnated with resin toincrease the exural capacity of the monopole. The remaining twomonopoles were strengthened using CFRP strips, pultruded from

The rst part of the designation indicates the modulus of thebers used, either high modulus (HM) or intermediate modulus(IM), and the second part of the designation indicates theapplication method, either by wet lay-up of dry-ber sheets(WL) or adhesive bonding of CFRP strips (ST). Each test includedthree loading and unloading cycles. During the initial loadingcycle, the unstrengthened monopole was loaded elastically to 60%of its nominal exural yield capacity and unloaded to serve as acomparison with the later tests. Prior to the second loading cycle,the monopole was strengthened with CFRP materials to achieve a

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Structures 47 (2009) 10371047 1039B. Lanier et al. / Thin-Walledeither the high- or intermediate-modulus carbon ber. Theproperties for the two types of strips are listed in Table 2. Steelcoupons were taken from the unloaded portion of the monopolenear the tip and tested in accordance with ASTM A 370-02, usingplate-type standard specimens. The average yield strength for thesteel and elastic modulus were 455MPa based on the 0.2% offsetmethod and 194GPa, respectively.

3. Experimental program

The experimental program consisted of three tests, referred toas Test HM-WL, HM-ST, IM-ST on three separate steel monopoles.

calculated 2040% increase in the exural strength and allowed tocure. The strengthened monopole was then loaded until thedeection at the mid-span was equal to the deection of the rstcycle. After unloading, the nal loading cycle was up to failureof the strengthened monopole. A single loading exceeding theyield exural capacity of the steel monopole was chosen, sincemonopole shafts are typically designed to remain elastic whileresisting lateral forces per pressures from a single 50 (TIA-222-F)to 500 (TIA-222-G) year wind event. Given the rarity of such anextreme loading event and design criteria, cyclical loading of thestructure beyond its exural yield capacity is not typical of loads

Fig. 2. Monopole strengthened using traditional technique.

Table 1Dry ber material properties for high-modulus carbon bers.

Material property High-modulus carbon

ber (HM-WL)

Tensile strength (MPa) 2600

Tensile modulus (GPa) 640

Ultimate elongation (millistrain) 4.0

Effective sheet thickness (mm) 0.19

Table 2Material properties for pultruded high-modulus CFRP strips.

Material property Intermediate-modulus

CFRP strip

High-modulus

CFRP strip

Fiber volume fraction (%) 55.4 55.2

Tensile strength (MPa) 1224 1186

Tensile modulus (GPa) 229 338

Ultimate elongation (millistrain) 5.08 3.32

Compressive strength (MPa) 471 353

Compressive modulus (GPa) 177 317

Strip thickness (mm) 3.2 1.45encountered for monopole towers in service.Design considerations of the tested specimens included

fabrication of a scaled monopole to represent the behavior offull-scale monopole towers, utilizing a strengthening system thatcould be used for a full-scale monopole. Surface preparation andCFRP installation techniques were selected to simulate methodsthat would be used in a eld application. The applied load wasdesigned to simulate moment and shear forces equivalent to eldwind loading conditions. The diameter to thickness ratio, facewidth to thickness ratio and shaft taper were similar tomonopoles in service.

3.1. Fabrication of scaled monopoles

Tested monopoles were fabricated using A572 Grade 65, 5mmthick, steel plate and cold-formed into two, equally sized, six-sided, cross-sections measuring 6096mm in length. The bendradius between the at sections measured 40mm. The two halvesof the nal closed cross-section were welded together with a5mm, full length, partial penetration E80 weld, creating a closedFig. 3. Monopole cross-section dimensions at base.

twelve-sided polygonal shape, as shown in Fig. 3. The cross-section diameter across ats of the combined sections measured457.2mm across ats at the larger, base end and 330.0mm acrossats at the smaller, free end, resulting in a tapered shape factor of21mm/m. The monopole baseplate was cut from 38mm thick,A572 Grade 50 steel and was 700mm along each side. Thebaseplate was welded to the pole shaft using a 5mm partialpenetration E80 weld and topped with an additional 13mm E80llet weld. Six A325, 32mm diameter, anchor bolts were centered

The reinforcement ratio of the applied strengthening was 7.0%,accounting for the ber volume fraction. Anchorage was providedfor the sheets by continuing the bers past the shaft of themonopole and bending the bers up onto the base plate. Moreresin was applied to the surface of the bers and several steelclip angles were used to mechanically anchor the bers to thebase plate. The clip angles were connected by bolting throughthe anchor bolts at the baseplate, allowing the CFRP sheetsto be clamped between the clip angles and the base plate.

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B. Lanier et al. / Thin-Walled Structures 47 (2009) 103710471040on a 305mm by 610mm bolt square, to allow attachment of thebase plate to the reaction wall. Eight stiffeners were welded to thebase of the HM-ST and IM-ST monopoles. These monopoles wereloaded before and after installation of the stiffeners to isolate thestrengthening effect of the CFRP from the effect of the stiffenersalone. The stiffener length along the shaft was 200mm. The lengthalong the baseplate was 100mm and thickness of the stiffenerswas 13mm. A572 Grade 50 steel was used to fabricate thestiffeners and 7mm E80 llet welds were applied along at allconnecting surfaces.

3.2. Surface preparation

Surface preparation of the steel was conducted to ensurecomplete chemical bonding between the steel and the adhesive.This typically involves the removal of mill scale, rust and anyprotective coatings. It was found that the most effective method toachieve a chemically active surface is the use of grit blasting [6].Surface preparation prior to strengthening was completed bysandblasting the entire monopole and base plate until a rough,white steel surface was achieved. Strengthening was completedwithin 24 hours of sandblasting to minimize the oxidation to thesurface. Dust was removed by blowing with compressed air. Nosurface preparation was required for the unidirectional sheetsused for the strengthening. For the two monopoles strengthenedwith CFRP strips, the strips were lightly abraded with 120 gritsandpaper and cleaning by wiping with methanol until noadditional sanding residue was present.

3.3. Strengthening conguration

Monopole HM-WL was strengthened by wet lay-up of 330mmwide unidirectional, CFRP sheets in both the longitudinal andtransverse directions using a saturating epoxy resin. This processallowed the composite material to conform to the surfaceconguration of the monopole. Strengthening was performed tomatch the demand placed on the monopole due to the cantileverloading condition. From the preliminary analysis, it was foundthat most of the strengthening is required at the base of themonopole and no strengthening was required from mid-span tothe tip. As such, the thickness of the applied CFRP sheets wastapered from four plys of the sheets at the base to one plyterminating at mid-length of the monopole as shown in Fig. 4.Fig. 4. Longitudinal strengthening conThe heels of the clip angles were machined to have a larger radius,to minimize the stress concentration at the change in geometry.Six L6 43/8 clip angles were used in total.

Half-width sheets were used to wrap the longitudinal sheetstransversely to prevent possible premature debonding of thestrengthening applied to the compression side of the monopole.These sheets were wrapped around the cross-section in twohalves such that they overlapped by 100mm at mid-depth of themonopole. The transversely oriented sheets were applied con-tinuously from the base to 1200mm along the length also to delaythe onset of local buckling of the steel on the compression side.From this point to the mid-span, the transversely oriented sheetswere spaced apart from each other.

The two remaining monopoles were strengthened by exter-nally bonding of CFRP strips. Monopole HM-ST used high-modulus CFRP strips and Monopole IM-ST used intermediate-modulus CFRP strips. For both monopoles, the same epoxy resinwas used to bond the strips to the monopole. Six strips wereapplied to three at sides on both the tension and compressionsides of the monopole, using twelve strips in total. Strips wereomitted from the ats at the neutral axis depth to makeeconomical use of the material for testing, but it is expected thatthe strengthening would be applied concentrically about themajor and minor axes since the direction of the wind loadingvaries for monopole towers in the eld. Due to the requirementfor greatest strength increase at the base, the amount ofstrengthening provided decreased with increasing distance fromthe base as shown in the exploded view of the strengtheningsystem in Fig. 5.

For Monopole HM-ST, all twelve CFRP strips had a widthof 75mm. The applied strengthening for Monopole IM-ST usedtwo different widths of strips. The four strips located on the centerats had a width of 75mm, while the eight adjacent strips had awidth of 50mm. This resulted in the two specimens havingapproximately equal values of axial stiffness for the appliedstrengthening material. The reinforcement ratio based on theCFRP axial stiffness for Monopoles HM-ST and IM-ST was 10.0%and 17.3%, respectively.

The two monopoles also had stiffener plates welded from theshaft of the monopole to the base plate. This technique was usedto increase the exural stiffness of the monopole shaft at the base,allowing the forces at the ends of the strips to be transferredthrough the stiffeners into the base plate. Research by Lanier andRizkalla [7] has indicated full development of the strips can beguration of Monopole HM-WL.

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StruB. Lanier et al. / Thin-Walledachieved through use of stiffener plates. The monopole specimenswere loaded to 60% of the yield stress initially before installationof the stiffeners or CFRP strengthening. The monopole specimenswere then tested to the same load following installation of thestiffeners, but before application of the CFRP strengthening. Thisallowed the increased lateral stiffness due to the stiffeners aloneto be determined. The stiffener plates increased the lateralstiffness of the monopole by approximately 6%.

3.4. Testing conguration and instrumentation

The loading applied for the experimental program wasdesigned to simulate exural wind design loads in eld structures.It was impractical to apply a distributed wind loading, so each

Fig. 5. Longitudinal strengthening conguration of Monopoles HM-ST and I

Fig. 6. Testing conguractures 47 (2009) 10371047 1041monopole was tested as a cantilever with a single applied loadapproximately 5750mm from the pole base to generate equiva-lent moments and shear forces. The loading was applied withnylon straps or chains to allow free rotation of the pole shaft at thepoint of load application, as shown in Fig. 6.

Measurements were recorded of the deection and extremeber strains at the quarter-points of the monopoles. Displace-ments were measured using wire potentiometers, while thestrains were measured using strain-gauge type displacementtransducers with a gauge length of either 200 or 300mm.Additional electrical foil gauges, with a gauge length of 6mmwere applied to the surface of the CFRP. Deection was alsomeasured at the base plate to determine slip and rotation at thesupport. All instrumentation in addition to the applied load wasrecorded at a sample rate of 1.0Hz.

M-ST, with exploded view of CFRP strips (symmetric about mid depth).

tion for monopoles.

4. Test results and discussion

Results from the three tests indicate signicant additionalexural yield and ultimate strength and stiffness can be devel-oped due to the use of CFRP strengthening while the pole steelremains within elastic limits. Stiffness increases, which parallelelastic exural strength increases, varying between 13% and 64%were measured at various locations along the monopole shaftfrom the three tested specimens. This was determined bycomparing the applied lateral loads from the rst and secondloading cycles, shown in Table 3, as the second loading wasterminated once mid-span deection equivalent to the rstloading was achieved. Measured elastic longitudinal strains werealso reduced by 2050% along the monopole shaft from the threetested specimens. Table 4 lists the stiffness of the unstrengthenedand unstrengthened monopoles at the tip and mid-span for eachof the three tests. Fig. 7 illustrates the measured deection results

from the rst and second load cycles of the Monopole HM-ST test.Similar results were obtained for the tests of Monopoles HM-WL,and IM-ST. Fig. 8 shows the load deection behavior to ultimatefor the three tests.

Apart from some minor, localized debonding observed in thethird load cycle, the installed high-modulus CFRP sheets used forMonopole HM-WL remained intact throughout the second andthird load stages. This was the only test to be unloaded andreloaded past yield due to the large deections obtained. Fig. 9shows Monopole HM-WL just prior to the ultimate lateral loadof 95 kN. Failure was due to local buckling, which resulted in therupture of the transversely applied CFRP sheets. The ruptureoccurred, shown in Fig. 10, just outside the clip angles at a tipdeection of 353mm or L/17. Inspection of the remainder of themonopole shaft after failure found the adhesive bond in excellentcondition outside of the buckled region. No deterioration of theadhesive bond or high-modulus CFRP material was found underthe clip angles. As evident from the tests with the adhesivelybonded CFRP strips, the presence of the transversely appliedsheets delayed the onset of local buckling.

No deterioration of the high-modulus CFRP strip or adhesivebond was observed following the second load stage for MonopoleHM-ST. Crushing of the strips in compression at 230mm from thebase, at the end of the stiffeners, occurred as load increased from48 to 63kN, as shown in Fig. 11. Strain at strip crushing wasmeasured at 0.15%. Complete debonding and rupture of the stripsin tension, also at 230mm, occurred as load increased to 63kN.Rupture strain was measured at 0.18%. Beyond 63kN, the behaviorwas dominated by the steel cross-section properties, due to therupture of the strips on the tension side and the crushing of

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Table 3Summary of applied load (Equivalent to 60% of unstrengthened exural yield

capacity) resulting in equivalent mid-span deections.

Monopole test Loading cycle Applied load at L (kN)

Monopole HM-WL Unstrengthened 32.0

Strengthened 36.3

Monopole HM-ST Stiffeners alone 32.6

Strengthened 42.8

Monopole IM-ST Stiffeners alone 32.6

Strengthened 41.5

Table 4Summary of elastic stiffness increase (Maximum loading equivalent to 60% of unstreng

Monopole test Loading cycle

Monopole HM-WL Unstrengthened

Strengthened

Monopole HM-ST Stiffeners alone

Strengthened

Monopole IM-ST Stiffeners alone

Strengthened

B. Lanier et al. / Thin-Walled Structures 47 (2009) 103710471042Fig. 7. Comparison of load-deection behavior of Mthe strips on the compression side. Ultimate load capacity of themonopole, with the high-modulus CFRP strips providing no

thened exural capacity).

Stiffness at 0.5L (kN/mm) Stiffness at L (kN/mm)

1.18 0.38

1.48 0.44

1.33 0.40

1.91 0.57

1.29 0.39

2.11 0.57onopole HM-ST before and after strengthening.

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StruB. Lanier et al. / Thin-Walledresistance, was measured at 79kN. Local buckling of themonopole shaft occurred outside the ends of the stiffeners. Postfailure inspection of the adhesive bond found signicant voidsbetween the strips to shaft and strip to strip interfaces, due to slipduring installation. However, within the stiffener boundary, nodeterioration or delaminating of the strip matrix or adhesive bondwas found.

Fig. 8. Load-deection behavio

Fig. 9. Monopole HM-WL near the ultimate load.

Fig. 10. Local buckling of Monopole HM-WL showing rupture of transverselyapplied CFRP sheets.r to ultimate for each test.ctures 47 (2009) 10371047 1043Similar behavior was observed for Test IM-ST, although theinitial strength loss was due to debonding of the strips in tension.At loading of 55kN, almost all strips in tension debonded andruptured near the end of the stiffeners. The strips in compressionremained bonded until loading exceeded 85kN, which was theultimate load capacity of the monopole, when all remaining stripssimultaneously debonded or crushed just outside the stiffenerends. The compressive crushing strain was measured to be 0.25%.After loss of strength to 78kN, the monopole was loaded untilbuckling occurred outside the ends of the stiffeners at 83kN, asshown in Fig. 12. As with the test of Monopole HM-ST, theadhesive bond and strip matrix remained intact throughout theloading between the stiffeners.

Based on comparison of the ultimate load capacities from theHM-WL and HM-ST tests, a minimum ultimate load increase of16kN, or 20% exural strength increase, was found from the HM-WL test. The IM-ST found an ultimate load increase of 6 kN, or 8%exural strength increase. The HM-ST monopole buckled at theultimate load with the high-modulus CFRP strips debonded andruptured, thus the ultimate strength of this specimen may beconsidered as the ultimate strength of a control specimen.

5. Analytical model

An elastic stiffness model was designed to account for lateralexural stiffness and strength of the monopoles both with and

Fig. 11. Crushing of high-modulus CFRP strip bonded to compression side ofMonopole HM-ST.

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StruB. Lanier et al. / Thin-Walled1044without the sheets and strips. The model was limited to withinthe steel elastic range as the strip compressive strain would notallow development of the steel beyond its yield strain. This limitsultimate strength design as the model is limited to the crushingstrain of the CFRP or the yield strain of the monopole steel.However, the intent was to model the elastic behavior of themonopole as steel monopole shafts are typically designed toelastic exural limits. The model, therefore follows current designpractice [8,9].

5.1. Elastic exural stiffness model

The exural stiffness model used to predict shaft deection isbased on the moment-area method and the transformed-sectionmethod [10]. The moment-area method is especially effective as itallows for non-prismatic pole sections to easily be modeled usinga nite number of elements. The transformed-section methodallows for easy conversion of materials with un-similar modulusto be modeled together. Since the model was limited to within theelastic range of both materials, no special accommodations wereapplied to the model, aside from the strip crushing strain. Strainwas calculated based on transformed-section mechanical proper-ties and moments derived from both methods.

Several assumptions were included in the model to predict theload deection and strain behavior of the unstrengthened and

to

str

Fig. 12. Local buckling and debonding of CFRP strips for test of Monopole IM-ST.strengthening the IM-ST test revealed a calculated 55% decreasein strain from the base to the mid-span. Measured strainstraComredconat. 14 illustrates the measured versus calculated longitudinalins from the rst and second load cases of the HM-ST test.parison of the elastic monopole strain before and aftertoFigmeasured strain reduction of 20%. Average elastic calculatedain reduction from the HM-ST test was 39% from the basemid-span. This compares well to the measured 31% reduction.ovethestrains were reduced by an average of 31% from the basemid-span due to the strengthening system. This resulted inrestimation of the expected strain reductions as compared tooftheComparison of the calculated elastic longitudinal strain of thethe HM-WL monopole before and after strengthening showsnot particularly accurate in determining tip deection.

moicated stiffness increases of 64% and 44%. This indicates thedel is largely accurate in predicting mid-span deection, but iscalindstrengthened monopoles. These assumptions are:

1. Strains are assumed to vary linearly across the depth of thecross-section.

2. Perfect composite action was considered. No bond slip orfailure between the monopole shaft and the high-modulusCFRP was assumed to occur.

3. Clip angles were assumed to have no impact on the deectionor stress calculations, but the moment of inertia of thestiffeners was included in the analysis.

4. Both the steel and high-modulus CFRP are linear elasticmaterials.

5. Shear deformations are not included in the analysis.6. The boundary condition at the base is rigid, with no rotation or

slip.7. The adhesive thickness is ignored.

5.2. Model results

Based on the predicted load deection slope, the calculatedelastic exural stiffness values of the unstrengthened monopolemodel at mid-span and tip were 1.28 and 0.37 kN/mm, respec-tively. The exural stiffness values predicted from the HM-WLmodel resulted in calculated stiffness of 1.76 and 0.46 kN/mm atmid-span and tip, respectively. Comparison of the calculatedstiffness values at the respective locations reveals that thestrengthened monopole stiffness increased 38% and 26%. Thesepredicted stiffness increases are signicantly higher as comparedto measured stiffness values of 25% and 17%.

The predicted elastic exural stiffness values calculated at themid-span and tip before and strengthening of the HM-ST monopolewas 1.40 and 0.39kN/mm versus 2.08 and 0.53kN/mm, respec-tively. Calculation of the stiffness values from the HM-ST model atmid-span and tip resulted in stiffness increases of 48% and 37%,respectively. Like the results from the HM-ST model, these resultsconform very well with respect to the measured stiffnessincreases of 43% and 41%. Measured versus calculated deectionfrom the rst and second load cycles of monopole HM-ST areillustrated in Fig. 13.

Calculated elastic exural stiffness values of the IM-STunstrengthened monopole were identical from HM-ST as themonopoles had identical dimensions of shaft and stiffeners.Predicted stiffness values from the IM-ST monopole model werecalculated to be 2.21 and 0.59 kN/mm at the mid-span and tip,respectively. The resulting monopole stiffness increase was

culated as 68% and 52%. Measurements taken from IM-ST test

ctures 47 (2009) 10371047uction was found to be 52%, thus the calculated resultsformed very well to the measured values. Strains calculatedthe mid-span to the tip for all three models before and after the

ARTICLE IN PRESS

rst a

StruFig. 13. Experimental and calculated net deection proles for B. Lanier et al. / Thin-Walledmonopole was strengthened were equivalent in magnitude andconformed very well to the measured results.

Comparison of the tested results versus the calculated resultsindicates the calculated values are not effective for the HM-WLmodel and fairly accurate for the ST models. The reason for the HM-WL model being somewhat higher than observed is likely due tolack of full initial development of the ber modulus. The sheetmodulus in tension and compression is likely slightly less than thecoupon values due to out of straightness and any waviness of thebers installed on the monopole. This behavior would reducethe expected strengthened increases. Reasons for the good con-formity of the strip models are likely due to the exact compressiveand tensile modulus provided by the manufacturer. Also, thestiffeners may have helped create a more rigid, moment resistingbase, which more closely replicated the model assumption.

6. Design aspects

High-modulus strips are recommended for strengtheningstructurally decient monopoles as opposed to high-modulussheets due to their ease of installation and much betterconformance of tested compared to the modeled results. Also,lesser amounts of material are necessary to generate equivalentstrength and stiffness results when compared to the intermediate-modulus strips. Yield strength design of the monopole should

Fig. 14. Experimental and calculated strain proles for rst and snd second load cases of tested Monopole HM-ST at 60% of yield.

ctures 47 (2009) 10371047 1045consider the ultimate strain of the strips in compression, whichmay be more critical the rupture strength. Strengthening withhigh-modulus sheets, particularly in the transverse direction, maybe benecial in delaying the onset of local buckling, but additionalstudy is necessary before further recommendations can bedeveloped. The high-modulus sheets have also demonstrated thatsignicant ultimate exural strength capacity can be achievedthrough their use, but additional research is necessary to conrmthe extent of this strength increase. The design procedure forutilizing high-modulus strips for strengthening monopoles shouldconsider the following criteria; elastic exural design of the high-modulus strips, strength of the adhesive bond and development ofthe high-modulus strips, specically at the base of the structure. Aload and resistance factor design approach is presented, althoughthe allowable stress approach is still used in practice and remainsa viable design approach with appropriate factors of safety.

6.1. High-modulus CFRP strip design

Use of any set of reasonable assumptions can be used fordetermining the exural elastic stiffness of the monopole shaftstrengthened with high-modulus strips. The simplest approachfor modeling the combined cross-section of steel shaft and stripshapes is using the transformed area method. Use of this methodwill generate reliable results assuming the effective exuralstresses are limited to the ultimate compressive or rupture strain

econd load cases of tested Monopole HM-ST at 60% of yield.

manufacturer as well as tested results shown through published

through single or double lap shear coupon tests, assuming

ARTICLE IN PRESS

Struof the strips or yield strength of the steel shaft. The design shouldbe considered fully composite, with the adhesive thickness beingignored when calculating the section modulus. The compressivemodulus should be considered in addition to the tensile moduluswhen calculating the transformed section. The nominal momentcapacity should be limited lesser of the crushing or rupture strain(er) of the strips or the steel yield strain (ey), as follows:

Mn St;minFyRwhere Smin is the minimum transformed-section modulus, basedon steel elastic modulus, Fy is the effective steel yield strength, R isthe er/ey, not to exceed 1.0.

Flexural tower strength design is typically limited to the yieldstrength of the steel as standard tower design practice forpolygonal shaped tubular poles [8,9] is to limit the nominalmoment capacities to the steel yield strength. Local bucklingcriteria, which could potentially lead to inelastic behavior, canlimit the effective exural yield strength of the sections and areapplicable.

Plastic design is allowed for round tubular poles as per TIA-222-G and is expected to be a design alternative for polygonalshaped tubular poles with future editions of the TIA-222 standard.Assuming that both the tensile rupture and compressive crushingstrain of the strip material exceed a strain of 0.004, plastic designcan be an alternative. However, scaled testing is recommendedprior to utilizing plastic design. The 0.004 strain limit is basedon identical limits placed on tension-controlled exural strengthof reinforced concrete sections (ACI, 2005). Also, full developmentof the steel shaft at the extreme ber strain of 0.004 can beexpected and 0.004 will provide ductility of approximately twicethat of the steel yield.

Strengthening using high-modulus strips should be limited tothe exural nominal capacity of the structure to ensure that themonopole remains safe in case of possible loss of the strengthen-ing system. The exural nominal capacity of the strengthenedmonopole should be limited to the strength of the unstrengthenedmonopole as follows:

My; Strengthenedp1:6My; UnstrengthenedThe justication for allowing up to 60% increase in strength is

design wind loads are factored by 1.6, thus even during anextreme design wind event, signicant unused capacity of thesteel shaft is available. The monopole will still meet theunfactored wind design loading.

Although the intended strengthened tower design of this paperconsiders a ductile steel cross-section resisting exural loads, thepotential failure of the strengthened system is very similar toreinforced concrete or steel shear connection design. Failuremechanisms include failure of the strip in compression or tension,or by ductile yielding of the steel shaft. The recommended exuralstrength reduction factor (fb) for strip design is based similarfailure mechanisms for CFRP strengthened concrete or steelexure design criteria, per ACI440R-07 [11] and AISC LRFD Steel[12] manuals. If the CFRP tensile rupture strain (er) controls thedesign, then a factor of 0.85 may be used for fb, whereas if theyield strain (ey) controls, a factor of 0.90 may be used due tothe greater ductility of this failure mode. No existing guidelinesindicate appropriate factors for the CFRP material in a compres-sion controlled failure. It is anticipated that this factor would belower numerically, providing a higher factor of safety against thisundesirable failure mode. High-modulus CFRP strips should beinstalled though the decient section of the monopole into shaftsections with sufcient nominal exural capacity. The length of

B. Lanier et al. / Thin-Walled1046the high-modulus CFRP strips used will be dependent on thelength of shaft that does not have the required exural capacityand the adhesive bond design.identical materials, adhesive, surface preparation and applicationtechniques are followed. Maximum normal and shear stresses canbe calculated using established bond models [16], with maximumprinciple stress being derived from these values. Appropriatefactors of safety can then be applied to complete the adhesivedesign [17].

6.3. Development of high-modulus CFRP strips at monopole base

Development of the high-modulus CFRP strips at the baseof the monopole can be achieved through use of steel stiffeners.The intent of stiffener design is to limit the stress on the monopoleshaft surface such that gradual transfer of the design forces canimparted into high-modulus CFRP strips. Stiffener installation alsoreduces stresses in the base weld connecting the shaft to the baseplate that cannot be accounted for using strips alone unless thestrips can be effectively grouted through the base plate and intothe existing concrete foundation. A stiffener should be installed onboth sides of the strip, although strips installed on adjoining shaftats can share stiffeners. Stiffener thickness, height away frommonopole shaft and material grade should be designed to carrythe full design moment at the base without assistance from thehigh-modulus CFRP material. The stiffener height should betapered back to the monopole shaft thorough the requireddevelopment length of the adhesive bond.

7. Conclusions

Initial testing has shown that high-modulus CFRP materialsmay be used to provide exural stiffness and strength increaseswithin the elastic range of the monopole. Strengthening withhigh-modulus CFRP sheets in the transverse direction, may alsoprovide ultimate strength increases by delaying the onset of localresearch or data provided by the manufacturer. Detailing of thehigh-modulus CFRP strip ends can signicantly enhance mostadhesive connections by limiting localized stress concentrationsand should be implemented when using high-modulus CFRPstrips to strengthen monopoles. Tapering strip ends has proven tosignicantly relieve stress concentrations. Allan et al. [13]recommended detailing strip ends to a taper of 10:1 (5.71 fromhorizontal) to alleviate stress concentrations. Increasing adhesivethickness at the strip ends has also been shown by Wright et al.[14] to lower adhesive layer stiffness, which in turn reduces shearstress increase at the end of the strip. This ultimately has provenmore effective than tapering the strip ends alone. Combiningthese two effects has provided the best results (price and moulds[15]) and should be implemented in strengthened monopoledesign.

Maximum bond stress should also be limited to 2030% of thepublished ultimate strength of the adhesive as fatigue loadingbeyond the adhesive elastic strength can result in poor creepperformance [6]. Effective small scale testing can be accomplished6.2. Adhesive bond design

Specic design of the adhesive bond is dependent upon manyvariables such as surface preparation, specic epoxy, stripthickness, and environmental conditions that make generalrecommendations for adhesive bond design very difcult. Bonddesign should be based on recommendation by the adhesive

ctures 47 (2009) 10371047buckling. Additional research is required to conrm this observa-tion. Simple analytical tools can be use to determine the exuralbehavior of the strengthened material and to design the

strengthening for given loading conditions. To prevent debondingof the high-modulus CFRP material, it is important to consider theactual state of stress near the end of the CFRP strip including shearand peeling stress components. Proper installation of the high-modulus CFRP material is critical to ensure that the strengthenedmember behaves as intended by the designer. High-modulus CFRPmaterials are an effective alternative to conventional strengthen-ing techniques for steel monopole towers.

Acknowledgements

The authors acknowledge the National Science Foundation(NSF) Grant EEC-0225055, the NSF Industry/University Coopera-tive Research Center (I/UCRC) for the Repair of Buildings andBridges with Composites (RB2C), Mitsubishi Chemical FP America,Mr. David Brinker of Radian Corporation, and Simon Weisman ofWeisman Consultants for their contributions towards this re-search and paper.

References

[1] Morrison Hersheld Group, DualPole Monopole Reinforcing Solution. /http://www.dualpole.com/index.htmS. April 9, 2004.

[2] Westower Dywidag Monopole Reinforcing System. /http://www.dywidag-systems.com/docs/dsi_index.php#S. April 9, 2004.

[3] AeroSolutions, LLC. AeroForce Systems, Monopole and Tower Upgrades./http://www.aerosolutionsllc.com/index.htmlS. June 19, 2003. p. 18.

[4] Ferreira JG, Branco FA. Structural application of GRC in telecommunicationtowers. Construction and Building Materials 2007;21(1):1928.

[5] Dawood M. Fundamental behavior of steelconcrete composite beamsstrengthened with high modulus carbon ber reinforced polymer materials.Masters Thesis, North Carolina State University, 2005, p. 213.

[6] Cadei JMC, Stratford TJ, Hollaway LC, Duckett WG. Strengthening metallicstructures using externally bonded bre-reinforced polymers. PublicationC595, Construction Industry Research and Information Association (CIRIA),London, UK, 2004.

[7] Lanier B, Rizkalla S. Study in the improvement in strength and stiffness capacityof steel multi-sided monopole towers utilizing carbon ber reinforced polymersas a retrotting mechanism. North Carolina State University, 2005.

[8] TIA-222-F, Structural standards for steel antenna towers and antennasupporting structures. June 1996.

[9] TIA-222-G, Structural standard for antenna supporting structures andantennas. August 2005.

[10] Gere James M, Stephen P Timoshenko. Mechanics of materials. 4th ed. Boston,MA: PWS Publishing Company; 1997. p. 4003.

[11] American Concrete Institute. ACI440-07 Report on Fiber-Reinforced Polymer(FRP) Reinforcement for Concrete Structures, 2007. p. 100.

[12] American Institute of Steel Construction. Manual of Steel Construction, 13thed., 2005.

[13] Allan RC, Bird J, Clarke JD. Use of adhesives in repair of cracks in shipstructures. Materials Science and Technology 1988;4:8539.

[14] Wright PNH, Wu Y, Gibson AG. Fibre reinforced composite-steel connectionsfor transverse ship bulkheads. Plastics, Rubber and Composites 2000;29(10):54957.

[15] Price A, Moulds RJ. Repair and strengthening of structures using platebonding. Construction and Building Materials 1991;5(4):18992.

[16] Hart-Smith LJ. Further developments in the design and analysis of adhesive-bonded structural joints. ASTM Special Technical Publication 749. In: KedwardKT. editor, Joining of composite materials: a symposium, April 16, 1980, p. 331.

[17] Schnerch D, Dawood M, Rizkalla S, Sumner E, Stanford K. Bond behaviorof CFRP strengthened steel structures. Advances in Structural Engineering2006;9(6):80517.

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B. Lanier et al. / Thin-Walled Structures 47 (2009) 10371047 1047

Behavior of steel monopoles strengthened with high-modulus CFRP materialsIntroductionCurrent design and strengthening practice

Material propertiesExperimental programFabrication of scaled monopolesSurface preparationStrengthening configurationTesting configuration and instrumentation

Test results and discussionAnalytical modelElastic flexural stiffness modelModel results

Design aspectsHigh-modulus CFRP strip designAdhesive bond designDevelopment of high-modulus CFRP strips at monopole base

ConclusionsAcknowledgementsReferences