Fatigue Analysis Of Overhead Sign And Signal Structures
Fatigue Analysis ofOverhead Sign andSignal StructuresPhysical Research Report No. 115L-Time (see)Illinois Department of TransportationBureau of Materialsand Physical Research
TECNNICAL1. R-pwt2. rtSTANDARDTITLEPAGEs Cotoba No.FHWA/IL/PR-l 154.TitloandSubtitloFATIGUE ANALYSIS OF OVERHEAD SIGN ANDSIGNAL STRUCTURES7.,8.Adds)PerformingJeffrey M. South, P.E.9.Po.fermtnqOrg #zataonN spurposetheory,reportofthisAddros-be sifiedbothdifferedForm DOT F 1700.7 (8-69]20.Securttysafetystrainas ndduewereattotubea trafficusewithandanalyticallyreader.as a ntlvfrom t.h other18. oltsdiscusseda lablecollectedmeasuredgenerallywas cysignalandthanThedamageon signcoefficients,on gnalplatesitedetailsinwereestimatedusingstatic ol!,Linn This document is availNo restrictions.able to the public through the NationalTechnical Information Service, Springfield,Virginia 22161.Fatigue, wind forces, safety factors,strain gages, histogram-linear damagerule.19.No,No.andmay be much alcontrolled-speedandwere(N)signvibrationa estressesmechanicsas a functiondueReportof Transportation,literature.actuallycanby tedfracturewereUnits y odelsnumberina pertinentinducedcyclicand limitationsas a romas N–S equations,signalsimplemethodstoforcesThe amplificationmode sexperimentally.todocumentsreportand experimentalstructures.OrSmtxatton11.Contract or Grant No.IHR-31 913.Typo of Raport and P riodInterim Report:July 1990 throughMay 19947Illinois Department of TransportationBureau of Materials and Physical Research126 East Ash StreetSpringfield, Illinois 62704-4706IS.supP/omontory NO**SStudy conducted in cooperation with the U.S. DepartmentFederal Highway Administration.mainxot, on Cod*PRR-115Illinois Department of TransportationBureau of Materials and Physical Research126 East Ash StreetSpringfield, Illinois 62704-4706iz.DataMay 1994b. Porfom#n90rgentCla wi.(efUnclassifiedthispogo)21.No.ofpoges 22.Prtco118
FATIGUE ANALYSISOFOVERHEAD SIGNANDSIGNAL STRUCTURESJeffrey M. South, P. E.Engineer of Technical ServicesPhysical Research Report Number 115Illinois Department of TransportationBureau of Materials and Physical ResearchMay 1994
FORE140RDThis report shou d be of interest to engineersinvolved in structuraldesign, planning, ma ntenance and inspection; consultants, and other technicalpersonnel concerned with the fatigue life of structures.NOTICEThe contents of this report reflect the views of the author who isresponsible for the facts and the accuracy of the data presented herein.The contents do not necessarily reflect the official views or policy of theFederal Highway Administrationor the Illinois Department of Transportation.This report does not constitute a standard, specification, or regulation.Neither the United States Government nor the State of Illinois endorsesproducts or manufacturers.Trade or manufacturers’names appear herein solelybecause they are considered essential to the object of this report.
ACKNO14LEDGMENTSThe author gratefully acknowledgesthe kind assistance andsupport of:Mr. ChristopherHahin, Engineer of Bridge Investigations;Ms. Mary Milcic, Research Engineer; Mssrs. Dave Bernardin,Tom Courtney, Russ Gotschall,Brian Zimmerman.Harry Smith, Ken Wyatt, andThe typing efforts of Ms. Bev Buhrmesterare gratefully acknowledged.
TABLE OF CONTENTSPaueList of FiguresiList of Tablesv1.Introduction‘ 2.Wind-Induced1Forces and Vibrations43.Hind Speed Data194.Strain Gage and Frequency Data355.Determination766.Sample Fatigue Damage Analyses Using BothStrain Gage Methods and Analytical Methods887.Factor of Safety Equations for Fatigue Design998.Summary9.ConclusionsReferencesof Fatigue Damage to Components102and Recommendations105107
LIST OF FIGURESDescri tionFiqure No. Representation of the vortex sheddingprocess. U is the windspeed, D iscylinder diameter, Ay is transversedeflection due to vortex shedding.62A flow chart for estimating amplitudeand drag for vortex-induced vibration.173A flow chart for estimating lift forcesfor vortex-induced vibrati&.184Wind speed versus frequency of occurrencehistogram for signal structure at Illinois 54at White Oaks Drive in Springf eld, Illinosfor calendar year 1992.225Wind speedWhite Oakson Januaryone–minute2316data collected at Illinois 54 atDrive in Springfield, Illinois8, 1992. Data were collected atintervals.Nind speed data collected at Illinois 54 at“White Oaks Drive in Springfield, Illinoison February 26, 1992. Data were collected atone-minute intervals.247Wind speed data collected at Illinois 54 atWhite Oaks Drive in Springfield, Illinoison March 3, 1992. Data were collected atone-minute intervals.258Hind speed data collected at Illinois 54 atWhite Oaks Drive in Springfield, Illinoison April 29, 1992. Data were collected atone-minute intervals.269Wind speedWhite Oakson May 16,one-minutedata collected at Illinois 54 atDrive in Springfield, Illinois1992. Data were collected atintervals.2710Wind speedWhite Oakson June 2,one-minutedata collected at Illinois 54 atDrive in Springfield, Illinois1992. Data were collected atintervals.2811blind speed data collected at Illinois 54 atWhite Oaks Drive in Springfield, Illinoison July 27, 1992. Data were collected atone-minute intervals.29i
LIST OF FIGURES (CONTINUED)Fiqure No.12Descrl tion!b41eWind speed data collected at Illinois 54 atWhite Oaks Drive in Springfield, Illinoison August 18, 1992. Data were collected atone–minute intervals.30Wind speed data collected at Illinois 54 at14hite Oaks Drive in Springfield, Illinoison September 1, 1992. Data were collected atone-minute intervals.3114blind speedWhite Oakson Octoberone–minutedata collected at Illinois 54 atDrive in Springfield, Illinois30, 1992. Data were collected atintervals.3215Wind speed data collected at Illinois 54 atWhite Oaks Drive in Springfield, Illinoison November 7. 1992. Data were collected atone–minute intervals.3316Wind speed data collected at Illinois 54 aWhite Oaks Drive in Springfield, Illinoison December 22, 1992. Data were collected atone-minute intervals.3417Instrumented traffic signal mastarm instal’ edat Physical Research Laboratory in Springfield,Illinois.Strain gage locations are shown.3618Instrumented section of traffic signal structurefor controlled wind speed tests. Testing andinstrumentation were conducted at Smith-EmeryCompany, Los Angeles, California.Strain gageswere placed at 12, 3, 6, and 9 o’clock positionsnear the toe of the fillet weld.4519Data acquisition system used for controlledwind speed tests.4620Equipment and setup used to apply wind loads.Technician is checking wind speed with ahand-held anemometer.4721Eighty mile per hour wind load being applied totraffic signal structure.48ii
LIST OF FIGURES (CONTINUED)Fiaure No.Description!39-!222Lift stresses at top strain gage due to20 mph contro led wind application.5023Lift stresses at top strain gage due to40 mph contro led wind application.5124Lift stresses at top strain gage dueto 50 mph controlled wind application.5225Lift stresses at top strain gage due to60 mph controlled wind application.5326Lift stresses at top strain gage due’to70 mph controlled wind application.5427Lift stresses at top strain gage due to80 mph contro’ led wind application.5528Drag stresses at west strain gage(facing wind) due to 20 mph controlledwind applicat on.5629Drag stresses at west strain gage(facing wind) due to 40 mph controlledwind application.5730Drag stresses at west strain gage(facing wind) due to 50 mph controlledwind application.5831Drag stresses at west strain gage(facing wind) due to 60 mph controlledwind application.5932Drag stresses at west strain gage(facing wind) due to 70 mph controlledwind application.6033Drag stresses at west strain gage(facing wind) due to 80 mph, controlledwind application.6134Lift stresses at bottom strain gage due to20 mph controlled wind application.6235Lift stresses at bottom strain gage due to40 mph controlled wind application.63iii
LIST OF FIGURES (CONTINUED)Fiuure No.Description 36Lift stresses at bottom strain gage due to50 mph controlled wind application.6437Lift stresses at bottom strain gage due to60 mph controlled wind application.6538Lift stresses at bottom strain gage due to70 mph controlled wind application.6639Lift stresses at bottom strain gage due to80 mph contro led wind application.6740Drag stresses at east strain gage due to20 mph contro led wind application.6841Drag stresses at east strain gage due to40 mph controlled wind application.6942Drag stresses at east strain gage due to50 mph controlled wind application.7043Drag stresses at east strain gage due to60 mph controlled wind application.7144Drag stresses at east strain gage due to70 mph controlled wind application.7245Drag stresses at east strain gage due to80 mph controlled wind application.7346Stress concentration factor, Kt, for astepped, round bar with a shoulder filletUsed within bending, from Peterson.permission.8647Stress concentration factor, Kt, for agrooved shaft in tension, from Peterson.Used with permission.87iv
LIST OF TABLESTable No.Descri tion!?.a9E1Reynolds Number Regimes for VortexShedding from Smooth Circular Cylinders.72Dimensionless Mode Factors for SomeStructural Elements and Natural Frequencies.133Measured Wind Speeds and Frequency ofOccurrence.214aStress Range-Frequency Data for InstrumentedFillet Weld Connection on Traffic SignalMastarm.374bStress Range-Frequency Data for InstrumentedAnchor Bolts for Cantilevered Traffic SignalStructure.385Measured Dead Load Strains on Traffic SignalStructure.,396Measured Strain Versus Tip Deflection.417Statistics for Controlled Hind Speed TestData.498Apparent Drag Coefficient for In-Place TrafficSignal Using Average Strain Data.749Fat gue Strength Coefficients and Exponentsfor Various Welded Tubular Steel Details,AWS Data.79’10Fat gue Strength Coefficientsfor Some Steels.8211Expected Fatigue Damage Calculation forTraffic Signal Mastarm Using Strain Gage Data.8812Parameters Used for AnalyticalEstimation.9213Wind Speeds at Which SynchronizationExpected.14Calculated Drag Stresses on Mastarm for EachMeasured Wind Speed.9415Expected Fatigue Damage Estimation UsingMeasured Wind Speed Data.95vand ExponentsFatigue Lifeis92
11.INTRODUCTIONThe unexpected failure of an overhead sign or signal structure couldresult in serious injuries, property damage, and/or increased trafficcongestion and accidents.Overheadstructures intended to support signs or traffic signals aredesigned to resist dead loads, live loads, ice loads, and wind loads.’Dead loads include the weight of the member, signs, traffic signals, orother attachments.platforms.Live loads are defined as walkways and serviceIce loads are used in areas of the country where winter icebuildup is expected.The wind load is idealized as a maximum pressurebased on mean recurrence intervals of a maximum wind speed for thelocation of service, member shape, and height above ground of the memberbeing loaded.All loads are considered to be static for design purposes.However, overhead sign and signal structures are subjected to varyingwind loads every day.In addition, vortex shedding induces cyclic loads.The variability of these forces from day to day and even from instant toinstant implies that these structures are sustaining some amount ofcumulative fatigue damage.experienced,The cyclic nature of the actual stressesand therefore, the potential for fatigue damage and failureis not accounted for in the design process.A need exists for rationalmethods to evaluate the expected fatigue life of overhead sign and signalstructures and for methods to assess the fatigue susceptibility of newstructures while in the design phase.The number of overhead sign and signal structures in service issurprising.These structures occur at nearly every modern urban inter-section with three or more lanes.2Large overhead sign structures arefound both before and at nearly every interchange on the interstate system
2and on other divided highways.The large number of these structures andthe cost to build or replace them requires some study of their fatiguebehavior.There are many types of overhead structures in service.signs’ structures in particularshow wide variety.TrafficPopular cantileveredsigns’ structures include straight and arced tapered single arm and planeframe mastarms.Tubular cross-sectionsagonal (16-sided), dodecagonalelliptical.in use include circular, hexdec-(lZ-sided), octagonal,square, andMaterials used include steel and aluminum.include cantileveredSign structuresspace frames with rectangular gross cross-sectionsand simply supported space frames with both rectangular and triangularcross-sections.are circular tubes, although plane frame structures withapplicationsrectangularThe most common components used in overhead sign structuretube or I–shaped cross-sectionsare becoming popular for newconstruction.Fatigue is a failure mode that involves repeated loading at stresslevels which may be only a small fraction of the tensile strength of aparticular material.Because fatigue failures result from repeatedloadings, it is characterizedas a progressive failure mode that proceedsby the initiation and propagation of cracks to an unstable size.stress cycle auses a certain amount of damage.stress levels and material properties,EachDepending on appliedcomponent fatigue lives can rangefrom a few hundred to more than 108 cycles to failure.The most commonsites of fatigue failures in components are at areas of stressconcentrations.IWelds, notches, holes, and materialimpurities such as
3slag inclusions are examples of stress concentrations.Not surprisingly,common fatigue cracking areas in overhead sign and signal structures areat welds and anchor bolts (threads are sharp notches).The fatigue life of a component may be thought of in two phases.Crack initiation life is that portion of fatigue life which occurs beforea crack forms.Crack propagationlife is the portion of fatigue lifewhich occurs between crack formation and unstable crack growth.Typically, for the steels used in sign and signal structures, theinitiation life for a weld detail is far longer than the propagationlife.This implies that once a crack appears, especiallynonredundantin astructure, the fatigue life of that detail is effectivelyused up, and the component could rupture relatively quickly.The purposes of this report are to combine pertinent existing windloading and vibration theory, fatigue damage theory, and experimental datainto a usable fatigue analysis method for overheadsign and signalstructures and to outline factor of safety equations for estimation ofweld detail fatigue susceptibility.
42.WIND-INDUCEDFORCES AND VIBRATIONSThis chapter discusses theoretical aspects of vortex-inducedvibration.The purpose of this chapter is to illustrate the complexityof actual wind loading and to outline methods for estimating the forcescaused by winds.It is not intended to cover the subject in depth.Muchof the discussionin this chapter is condensed from the work of Blevins.2The study of wind-induced forces and vibrations begins with a lookIn aerodynamics,at aerodynamics.number.fluid flow is characterizedThe Mach number is a nondimensionalby the Machnumber which is defined asthe ratio of free stream fluid velocity to the local speed of sound.Thelocal speed of sound in air is a square-root function of the local airtemperature.The magnitude of the Mach number is a measure of thetendency of the fluid to compress as it approaches an object.speeds and air temperatures generally encounteredsignal structuresby overheadsign andin the United States the Mach number is less than 0.3,and compressibilityof the fluid does not influence vibrations.implies that the density of air is relatively constant.0.3 correspondsFor windThisA Mach number ofto a wind speed of 202 mph (325 km/hr) at an airtemperature of 70 degrees Fahrenheit (21.1 degrees Celsius).Observationof long traffic signal mastarms during windy periodsreveals that the general motion of the tip of the mastarm is fairlycomplex.Vortex shedding causes primarily vertical oscillations,drag forces move the tip horizontally.approximate figure eight shape.The combined motion describes anThe long axis of the figure eight rotatesunder influence of the direction of the wind.eight depends on wind speed.whileThe size of the figure
5Vortex SheddingVortex sheddingstructures.other.s a phenomenon which occurs in subsonic flow pastVortices are shed from one side of a member and then theAs this continues, oscillatingthe structure.vibrate.The oscillatingsurface pressures are imposed onpressures cause elastic structures toA description of the process of vortex shedding is given byBlevins.3“AS a fluid particle flows toward the leading edge of acylinder, the pressure in the fluid particle rises from thefree stream pressure to the stagnation pressure.The highfluid pressure near the leading edge impels flow about thecylinder as boundary layers develop about boths ides.However, the high pressure is not sufficient to force theflow about the back of the cylinder at high Reynoldsnumbers.Near the widest section of the cylinder, theboundary layers separate from each side of the cylindersurface and form two shear layers that trail aft in theflow and bound the wake. Since the innermost portion ofthe shear layers, which is in contact with the cylinder,moves much more slowly than the outermost portion, theshear layers roll into the near wake, where they foldon each other and coalesce into discrete swirlingvortices.A regular pattern of vortices, called a vortexstreet, trails aft in the wake. The vortices interact withthe cylinder and they are the source of the effects calledvortex–induced vibration.”A representationof the vortex shedding process is shown in Figure 1.Vortex shedding from smooth, circular cylinders with steady subsonic flowis a function of the Reynolds number.The Reynolds number is given by:(1)Re l whereU free stream velocity,D cylinder diameter,v kinematic viscosity of the fluid,. 1.564 x 10-4 ft2/sec (1.681 x 10-3m2/see) for air.The Reynolds number is the ratio of the inertia force and thefriction force on a body.The Reynolds number is a parameter used toindicate dynamic similarity.Two flows are dynamicallyReynolds number is equal for both flows.similar when the
6a9.F 1 gure 1. Representa t ion of the Vor tex sheed ng proce s
16. Abat,ocf This report documents methods of fatigue analysis for overhead sign and signal structures. The main purpose of this report is to combine pertinent wind loading and vibration theory, fatigue damage theory, and experimental data into a useable fatigue analysis method
Application of ASME Fatigue Code: 3-F.1 ASME Smooth Bar Fatigue Curves Alternative Effective Stress vs Fatigue Cycles (S-N Curve) With your weld quality level assessed and an accurate FEA stress number, one can use the ASME fatigue curve to calculate the number of Fatigue Cycles. In our prior example, the fatigue stress could be 25.5 ksi or as .
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