Fatigue steel vs. concrete

7/29/2019 Fatigue steel vs. concrete

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^ WJ U' ^F A T I G U E

C o n c r e t e v s . S t e e lby

W. Jack Wilkes, P.E.Senior Vice President/ManagerFigg and Muller Engineers, Inc.Austin, Texas

Catastrophic failures are the ultimatenightmare of structural engineers.The greatest number of bridge failuresoccur during construction, and the secondlargest cause of bridge failure is fromflood water either from scour or fromovertopping. However, the failures thatseem to attract the most media attentionare those that occur to aging bridges thathave been in service for many years, par-ticularly if there is loss of life involved. Inmany cases, the offending culprit, anoverweight truck on a load posted bridge,can be identified because it is trapped inthe wreckage. Unfortunately, it is oftenonly a lightly loaded vehicle that is on thebridge when it collapses.

It may be unfair to point out that, in-variably, the collapsed bridge or span is asteel beam or steel truss bridge; in yearspast, most long span structures in theUnited States were constructed with steel.Therefore, it is rare that the collapsedstructure is a concrete bridge.

When one searches for a reason for thecollapse of older bridges, the usual causeis a fatigue failure of the steel members.Since all concrete structures require steelelements, strands or bars, to take the im-posed tensile stresses in the structure,there needs to be an explanation of whyconcrete structures are tougher, moredurable and more tolerant of frequentoverloads.

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It is shown that the inherent high dead load to liveload ratio in concrete bridges allows such structuresto be particularly effective in resisting fatigue.

Perhaps the single most important fea-ture of concrete structures is the favorabledead load to live load ratio. Since con-crete structures have greater mass, theyare designed for greater dead load. Forequal span lengths, the total moments forconcrete spans are greater than those forsteel spans because of the additionalweight of the concrete. Also, concretestructures have greater total momentcapacity than steel structures', therefore,the same live load results in a lower stressrange in concrete structures.

Fatigue is a phenomenon that affectsall construction materials: wood, con-crete, steel and aluminum. The insidiousfeature of fatigue is that the load inducedstress effects are cumulative in thematerial for both magnitude and frequen-cy. The AASHTO Bridge Committee wasthe first code writing agency to developcomprehensive specifications for fatiguedesign.

Fatigue design is based upon the prin-ciple of range of stress. The upper fatiguelimit for structural steel design for redun-dant structures is a range of 24.000 psi(165 MPa) for the base metal at two mil-lion cycles. As defects are introduced intothe base metal through fabrication, the al-lowable stress range is markedly reducedto 18,000 psi (124 MPa) for welded plategirders and rolled beams with cover platesdown to only a few thousand pounds persquare inch for some very poor weldingdetails. For nonredundant or single loadpath structures, the allowable range ofstress is even lower. The metallurgicaltreatment of metals has improved thestrength and notch toughness (brittleness)of the material, but these changes have

had little or no effect upon the stress rangefatigue properties.To compare the effect of stress rangefor steel and concrete structures, analyseswere made of typical standard simplespan designs prepared by the FederalHighway Administration. The standardplans used were for HS 20-44 live loadsand generally comply with the AASHTOworking stress design specifications forboth rolled beam and welded plate girdersand a variety of concrete structures. Ananalysis of a 150 ft (45.7 m) concrete boxgirder span was included to provide acomparison for the long span weldedplate girder span.

The summaries and comparisons of thevarious spans are shown in Table 1. Inevery case, the ratio of live load momentto the total moment is smaller for the con-crete spans and is significantly smaller forthe concrete box spans. These live loadpercentages for the various spans areshown in Fig. 1.

Table 2 illustrates the effect that a liveload which produces the maximum al-lowed range of stress upon steel spanswould have upon concrete structures ofequal span lengths. Such a live load wouldexceed the design load, but all bridges areoccasionally subjected to overload condi-tions, particularly the older bridges thatwere designed for less than HS 20 load-ings. Table 2 illustrates the favorable totalmoment capacity of the concrete struc-tures and their smaller live load ratios, insome cases less than one-half of the steelcounterpart. These attributes of the con-crete bridges may partially explain whyconcrete structures are mo p e tolerant ofoverload conditions and why they pro-

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Table 1. Summary of moments.'Dead load,ive load plus Live load plus impact,Span, ft ft-kipsmpact, ft-kips Total total percentReinforced concrete girders40 38881 76 9 50.060 104668 1714 39.0Prestressed I-girders40 33 1 416 747 55.760 822 729 1551 47.080 1657 I031 2688 38.4100 3243 1608 4851 33.1120 4796 1958 6754 29.0130 5037 1739 6776 25.7

Concrete box systems80 2171 845 3016 28.0100 3659 1086 4745 22.9120 5642 1323 6965 19.0150 38,503 8775 47,278 18.6Steel beams

4050 416662.56072 7293016.0801040311358.3Welded girders

100 1662 1325 2987 44.4120 2473 1614 4087 39.5130 2960 1757 4717 37.2150 4062 2082 6144 33.9*All rnornents are for typical girders except for the special 151) N (45.7 m) concrete box girder wh ich is forentire span. Quantities and d etails for other designs determined from Standard Plans For Highway8 rirdges, Volumes 1 & II, prepared 6y I' [ I WA. Simple spans, 44 11 (13.4 m) roadway, HS 20-44 design.Table 2. Live load plus impact ratios and equivalent ranqe of stress.

Steel Reinforcedconcrete girders Prestressed1-girders Concretebox system sLive Ioad Range Live load Range Live load Range Live load Rangeplus of plus of plus of plus ofSpan, Im pact ratio, stress, Impact ratio, stress, Imp act ra tio , s tr ess, Im pact ratio, stress,ft percent ksi percent ksi percent ksi percent ksi

40 62.5 18 50.0 14.4 55.7 16 . 1 )60 56.0 18 39.0 12.5 47.0 15.180 48.3 18 38.4 14.3 28.0 104100 44.4 18 33.1 13.4 22.9 9.3120 39.5 18 29.0 13.2 19.0 8.7130 37.2 18 25.7 12.4150 33.9 11 18.6 9.9

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STE L BEAMS

REINFORCED \ E WELDED GIRDERSCONCRETEGIRDERS

a F

PRESTRESSED aEi GIRDERS

CONCRETE BOXSYSTEMS

40 60 d0 100 120 130SPAN, FEETFig. 1. Ratios of live load and impact to total moments.

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vide more strength and durability.If bridge owners selected only concrete

structures, can they forget about fatigue?Not entirely. Researchers have been ableto induce fatigue failures in a fewprestressed beams under laboratory con-ditions. To accomplish these failures.the y first had to crack the beams withgross overloads, then apply repeated loadsto the cracked beams until failure was in-duced. The solution to inhibit the failurewas to provide a small amount of mild

reinforcement throughout the bottomflange of the beam to distribute the crack-ing over a greater length of the beam.When this nominal change in the specifi-cation was proposed, the AASHTOBridge Committee elected not to makethe change for the simple reason thatthere had been no experience of fatiguefailures in prestressed beam s in actual ser-vice.

For durable, long lasting and econom-ical bridges, the clear choice is concrete.

PCI JOURNAL/July-August 1989 79

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Fatigue steel vs. concrete