Fatigue Design - · PDF fileStructural Welding Code 01.1-Steel. Fatigue de ... Section 11:...

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Transcript of Fatigue Design - · PDF fileStructural Welding Code 01.1-Steel. Fatigue de ... Section 11:...

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    Section 11:

    Fatigue Design

    Constant-amplitude tatiguethresholeJ-also known as constant-amplitude fatigue limit (CAFL) or endur-ance limit, a stress range below which a fatigue life appears to be infinite.

    SPECIFICATIONS

    11.1 SCOPE

    1his section contains provisions for the fa-tigue design of cantilevered steel and aluminumstructural supports for highway signs, luminaires,and traffic signals.

    COMMENTARY

    This section focuses on fatigue, which is de-fined herein as the damage that may result infracture after a sufficient number of stress fluctua-tions. It is based on NCH RP Report 412, FatigueResistant Design of Cantilevered Signal, Sign andLight Supports (Kaczillski et al. 1998). The studyfocused on critical support structures that show

    A

    Fatigue-c-damage resulting in fracture caused by stress fluctuations.

    In-plane bending-bending in-plane for the main member (column). At the connection of an arm or arm'sbuilt-up box to a vertical column, the in-plane bending stress range in the column is a result of galloping ortruck-induced gust loads on the arm and/or arm's attachments.

    Limit state wind load effect-a specifically defined load criteria.

    Load bearing attachment-attachment to main member where there is a transverse load range in the at-tachment itself in addition to any primary stress range in the main member.

    Non-load bearing attachment-attachment to main member where the only significant stress range is theprimary stress in the main member.

    Out-ot-plane bending-bending out-of-plane for the main member (column). At the connection of an arm'or arm's built-up box to a vertical column, the out-of-plane bending stress range in the column is a result ofnatural wind gust loads on the arm and the arm's attachments.

    Pressure range-c-magnitude of force, in terms of pressure, of a limit state wind load effect.

    Stress range-c-magnitude of stress fluctuations.

    Yearly mean wind velocity-long-term average of the wind speed for a given area.

    11-1

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    Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals

    SPECIFICATIONS

    11.3 NOTATIONS

    COMMENTARY

    bCddDEtntn1(F)n9H

    Ilavg

    Itop

    Ibottom

    Ii:

    LLLLPGPNWPTG

    PvsrRSnSRttbtctpVcVmeanWwpa.a.t!..cr

    = flat-to-flat width of a multisided section (m, ft)= appropriate drag coefficient from Section 3, "Loads," for given attachment or member= diameter of a circular section (m, ft)= inside diameter of exposed end of female section for slip-joint splice (mm, in)= modulus of elasticity (MPa, ksi)

    first natural frequency of the structure (cps)= first modal frequency (cps)= fatigue strength (CAFL) (MPa, ksi)= acceleration of gravity (9810 mm/s2, 386 in/s2)= effective weld throat (mm, in)= moment of inertia (mm4, in4)

    average moment of inertia for a tapered pole (mm4, in4)= moment of inertia at top of tapered pole (mm4, in4)= moment of inertia at bottom of tapered pole (mm4, in4)= importance factors applied to limit state wind load effects to adjust for the desired level of

    structural reliabilitylength of the pole (Article 11.7.2) (mm, in)slip-splice overlap length (example 1 of Figure 11-1) (mm, in)length of reinforcement at handhole (example 13 of Figure 11-1) (mm, in)

    = length of longitudinal attachment (examples 12, 14 and 15 of Figure 11-1) (mm, in)= galloping-induced vertical shear pressure range (Pa, psf)

    ,= natural wind gust pre~sur,e rClnge(Pa, psf)= triJck-il1duc'ed gust, pressure range (Pa, psf) , _~~i"

    ~" .,~. - ~ ~ "

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    --

    Section 11: Fatigue Design

    SPECIFICATIONS

    11.5 DESIGN CRITERIA

    Cantilevered support structures shall be de-signed for fatigue to resist each of the applicableequivalent static wind load effects specified inArticle 11.7, and modified by the appropriate im-portance faCtors given in Article 11.6. Stressesdue to these loads on all components, mechani-cal fasteners, and weld details shall be designedto satisfy the requirements of their respectivedetail categories within the constant-amplitudefatigue thresholds provided in Table 11-3. Asummary of typical fatigue-sensitive cantileveredsupport structure connection details is presentedin Table 11-2 and illustrated in Figure 11-1.

    COMMENTARY

    hibited poor fatigue performance. Square cross-sections have been much more prone to fatigueproblems than round cross-sections. Cautionshould be exercised regarding the use of squarelighting poles even when a fatigue design is per-formed. The provisions of this section are not ap-plicable for the design of span wire (strain) poles.

    In general, overhead cantilevered sign andtraffic signal structures should be designed for fa-tigue due to individual loadings from galloping,natural wind' gusts, and truck-induced wind gusts.High-level lighting structures should be designedfor fatigue for loadings from natural wind gusts.Vortex shedding should be considered for single-member cantilevered members that have tapersless than 0.0117 m/m (0.14 in/ft) , such as lightingstructures or mast arms without attachments.

    NCH RP Report 412, Fatigue Resistant Designof Cantilevered Signal, Sign and Light Supports(Kaczinski et al. 1998) is the basis for the fatiguedesign provisions for cantilevered structures. Otherstructures, including overhead bridge support

    ","strlJctur(3s for signs and signals, are, also suscepti~'\:',ble tt)"',fati~j'ue'dariiage: SOmehofthe design pro\ii~~'

    sionsof this section can also be applicable to non-, ",_cafl,ti!E?V~J~q,,~tr~:~tur,e.s.;'.A' r'e.searc,t"1project isc:ur~ j;._.L .. ,:-" ' rentJy underWay to deverop"compretr'tatigu~de~"-:';;'-""~' ,"pign proyi~iolJs" J9r ,I,.or)ca,ntil,E;)y.ereq,$"'upporj"sJrlJc~ ..

    tiJres.'

    Accurate load spectra and life prediction tech-niques for defining fatigue loadings are generallynot available. The assessment of stress fluctua-tions and the corresponding number of cycles forall wind-induced events (lifetime loading histogram)is practically impossible. With this uncertainty, thedesign of cantilevered sign, luminaire, and trafficsignal supports for a finite fatigue life becomes im-practical. Therefore, an infinite life fatigue designapproach is recommended and considered soundpractice. It is generally based on the constant-amplitude fatigue limit (CAFL). The CAFL valuesprovided in Table 11-3 are approximately thesame as those given in Table 1O.3.1.A of the Stan-dard Specifications for Highway Bridges (AASHTO1996).

    An infinite-life fatigue approach was devel-oped in an experimental study that consideredseveral critical welded details (Fisher et al. 1993).

    11-3

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    Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals

    SPECIFICATIONS COMMENTARY

    The infinite-life fatigue approach can be used whenthe number of wind load cycles expected duringthe lifetime of the structures is greater than thenumber of cycles at the CAFL. This is particularlythe case for structural supports where the windload cycles in 25 years or greater lifetimes are ex-pected to exceed 100 million cycles, whereas typi-cal weld details reach the CAFL at 10 to 20 millioncycles.

    Fatigue critical details should be designedwith nominal stress ranges that are below the ap-propriate CAFL. To assist designers, a categoriza-tion of typical cantilevered support structure detailsto the existing AASHTO and American WeldingSociety (AWS) fatigue design categories is pro-vided in Table 11-2 and Figure 11-1. Based on areview of state departments of transportation stan-dard drawings and manufacturers' literature, theabove referenced list of typical cantilevered sup-port structure connection details was produced.This list should not be considered as a completeset of all possible connection details, but rather it isintended to remove the uncertainty associated withapplying the provisions of the Standard Specifica-

    tions for Highway Bridges to. the fatigue design ofcantilevered support structures .

    . This detailed categorization of fatigUe~sensitive connection 9E!tails..can be us~d ..by de-

    . 'signersand fabricators to produce more fatigue-resistant cantilevered support structures. Properdetailing will improve the fatigue resistance ofthese structures, and it can eliminate or reduceincreases in member size required for less fatigue-resistant details.

    The notes for Table 11-2 specify the use ofStress Category K2- This stress category corre-sponds to the category for cyclic punching shearstress in tubular members specified by the AWSStructural Welding Code 01.1-Steel. Fatigue de-sign for the column's wall under this condition mayrequire sizes of the built-up box connection or col-umn wall thicknesses that are excessive for practi-cal use. For this occurrence, an adequate fatigue-resistant connection other than the built-up boxshown in Figure 11-1 should be considered.

    Regarding full-penetration groove-weldedtube-to-transverse plate connections, NCHRP Re-port 412 did not fully investigate the effects fromthe possible use of additional reinforcing filletwelds. Additional research and testing of thesetypes of detail configurations are needed to sup-port future updates of this section.

    11-4

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    Section 11: Fatigue Design

    SPECIFICATIONS

    11.6 FATIGUE IMPORTANCE FACTORS

    An importance factor, IF, that accounts forthe degree of hazard to traffic and damage toproperty shall be applied to the limit state windload effects specified in Article 11.7. Importancefactors for cantilevered traffic signal, sign, andluminaire support structures exposed to the four