Fatigue Evaluation for Reinforced Concrete Box Culverts

ACI Structural Journal/January-February 2010 13

ACI Structural Journal, V. 107, No. 1, January-February 2010.MS No. S-2008-098.R1 received October 11, 2008, and reviewed under Institute

publication policies. Copyright © 2010, American Concrete Institute. All rights reserved,including the making of copies unless permission is obtained from the copyright proprietors.Pertinent discussion including author’s closure, if any, will be published in the November-December 2010 ACI Structural Journal if the discussion is received by July 1, 2010.

ACI STRUCTURAL JOURNAL TECHNICAL PAPER

This paper summarizes the experimental program conducted bythe authors to evaluate the fatigue effects on reinforced concrete(RC) box culverts, and the resulting recommendations that weremade to the American Association of State Highway Officials(AASHTO). The study presented herein includes testing of two full-scale RC box culvert sections designed and manufactured accordingto ASTM C1577. The first specimen was 12 ft x 4 ft x 12 in. (3657.6 x1219.2 x 304.8 mm), and the second was 7 ft x 4 ft x 8 in. (2133.6 x1219.2 x 203.2 mm).

Test results show a good distribution of the load resistance betweenthe two reinforcement directions in box culvert sections. Fatigueeffect on the flexural capacity of the RC box culvert sections was foundto be minimal. As a result of the study, the authors proposed that thefatigue check for RC box culverts designed according to ASTM C1577be eliminated; this recommendation is accepted by AASHTO.

Keywords: box culverts; buried structures; fatigue; load; reinforced concrete.

INTRODUCTIONRecent research on welded wire reinforcement (WWR)

and fatigue resistance resulted in two proposed changes toAASHTO’s design of bridge superstructures for fatigueresistance. First, research by Amorn and Tadros1 on WWRhas indicated that when checking for fatigue, there may be aneed for a lower stress range limit if WWR is used instead ofreinforcing bars, and the crosswelds are in a high-stress zone,as is often the case with standard WWR mesh configurations.Second, a proposal was introduced to the AASHTO TechnicalCommittee for Concrete Structures (Committee T10) toincrease the load factor for fatigue from 0.75 to 1.5 in theAASHTO LRFD Standard Specifications.2 According tothis proposal, the 1.5 load factor should be applied alongwith the special fatigue truck, the dynamic allowance factor,and the distribution factor for the design of concrete stringersof a bridge superstructure. While this change allows forconsistency between provisions for fatigue design ofconcrete and steel members, neither the study nor theproposal made to the AASHTO committee included anyconsiderations on the impact of these revisions on boxculverts. In fact, although the proposed changes are appropriatefor bridge superstructures, they may be overly conservativefor box culverts. In response to this situation, a committee,comprising representatives from the American ConcretePipe Association (ACPA) and AASHTO Committee T13,along with University of Nebraska-Lincoln (UNL)researchers, was formed to discuss the need for additionalresearch in this area.

It is evident that a combination of a higher load factor anda lower allowable fatigue range would result in substantiallylarger steel areas, especially for shallow-depth box culverts.Current precast box culvert designs, however, haveperformed well in the past and have not shown any indication offatigue problems. Furthermore, other scholarly work showsthat AASHTO load factor resistance design (LRFD) provisions

result in increased design loads and reinforcement areas forbox culverts.3 Therefore, the requested increase in steelreinforcement should be reevaluated for the case of boxculverts. Specifically, box culverts buried at a shallowdepth (≤2 ft [609.6 mm]) should be studied because theyreceive more direct impact from fatigue in contrast to thoseunder thick layers of soil cover.

This paper summarizes the experimental programconducted by the authors to evaluate the fatigue effects onbox culverts, the recommendations made to AASHTO, andthe final decision made by AASHTO Committee T13.

RESEARCH SIGNIFICANCEPrior research conducted by the UNL on WWR alone, and

on the fatigue resistance of bridge superstructures reinforcedwith WWR, has resulted in changes in AASHTO that arepotentially overly conservative for reinforced concrete (RC)box culverts. Therefore, there was an immediate need forresearch to understand whether the changes recommendedfor fatigue resistance were valid for box culverts. This studyprovided such research. Based on the results of this study,recommendations were made to AASHTO Committee T13concerning the fatigue resistance guidelines for concrete boxculverts reinforced with WWR. These recommendationswere approved by AASHTO Committee T13 and thechanges will appear in the AASHTO LRFD revisions.

BACKGROUND AND RESEARCH METHODOLOGYFor the design of box culverts, two standard specifications

are available: ASTM C15774 and ASTM C14335 for LRFDand load factor design (LFD), respectively. In other words, ifthe designer chooses to use LRFD for the box culverts, thenthe boxes must be produced in accordance with ASTMC1577 for an earth cover of 0 to 2 ft (0 to 609.6 mm). Designaccording to LFD and ASTM C1433 is not relevant to thestudy, and therefore will not be discussed any further.

In ASTM C1577, the design of the steel reinforcementfollows the AASHTO LRFD Bridge Design Specificationsup to the 2005 Interim version.2 In this version, the loadfactor for fatigue is 0.75 and the stress range for fatigue is21,000 psi (144.8 MPa). With these limits, the required steelareas for box culverts are typically governed by flexure, andnot by fatigue.

As mentioned in the introduction, two independentchanges were proposed to the AASHTO LRFD 2005 InterimSpecifications: 1) based on WWR studies, fatigue stressrange should be changed to 16,000 ± fmin; and 2) load factor

Title no. 107-S02

Fatigue Evaluation for Reinforced Concrete Box Culvertsby Hany Maximos, Ece Erdogmus, and Maher K. Tadros

ACI Structural Journal/January-February 201014

should be increased to 1.5. As a result of these proposedchanges in the AASHTO LRFD Specifications,6 boxculverts would be inadvertently affected, as they fall underthe same umbrella. Because WWR reinforced box culvertshave been performing satisfactorily for decades, theproposed changes created a need for research to determinewhether the proposed AASHTO LRFD specificationchanges should be waived or altered for box culverts.

A committee of experts was formed and an experimentalresearch plan using full-scale specimens was developed. Torepresent commonly used short and long spans, tworepresentative sizes of box culverts were selected: 7 ft and12 ft. The specimens were designed using the old load factor(0.75) and the stress range; however, they were tested in sixstages of static and cyclic loading going beyond these limits(loads corresponding to load factors of 1.0 and the proposed1.5). After enduring these phases of static and cyclic loading,both specimens were loaded to failure. The results werestudied and recommendations were made to AASHTOCommittee T13.

EXPERIMENTAL INVESTIGATIONTest specimens

In previous research at UNL, hundreds of specimens weretested to determine the stress range at fatigue life of WWR.1 Inthe current study, two full-scale precast box culverts werecarefully chosen to be tested for fatigue evaluation: Specimen A,12 ft x 4 ft x 12 in. (3657.6 x 1219.2 x 304.8 mm); and SpecimenB, 7 ft x 4 ft x 8 in. (2133.6 x 1219.2 x 203.2 mm).

These two span lengths represent short and long spanscommonly used and currently found in ASTM4,5 andAASHTO Material Standards. All tested segments were 4 ft(1219.2 mm) long. The areas of reinforcement required by

ASTM C1577 (old requirements) are listed in Table 1 alongwith the actual wire sizes used.

Material propertiesSpecimens were constructed using normalweight concrete

with a specified characteristic compressive strength of5000 psi (34.47 MPa). Four x 8 in. (102 x 204 mm) concretecylinders cast for each specimen and cured under thesame conditions as the box culvert section were tested todetermine the average compressive strength for the concreteat the time of testing. The average concrete compressivestrengths for Specimens A and B were found to be 7800 and7600 psi (53.78 and 52.40 MPa), respectively. The reinforcementused in the specimens was deformed WWR of Grade 65.

Fatigue life determinationFatigue limit state is defined as the ability of the structure

to withstand the load for a certain number of load cycles. Inliterature, infinite life is considered after 1 million cycles.7

Previous work done at UNL concludes that 1 to 5 millioncycles (the long-life region) is more important for designpurposes based on the results of an extensive experimentalprogram.1 Other studies also adopt a range of 5 millioncycles for a testing procedure.8 Therefore, specimens weretested for 5 million cycles in this study.

Test setup and instrumentationA hydraulic actuator with an axial capacity of 110 kips

(489.3 kN) was used to apply a concentrated load at themidspan of the spigot end. A 10 x 20 in. (254 x 508 mm)footprint steel plate of 1 in. (25.4 mm) thickness was used totransfer the load from the actuator and to comply withAASHTO Specifications2 for the tire contact area. A 0.5 in.(13 mm) thick neoprene pad was placed underneath the steelplate to account for any surface roughness under the contactarea of the footprint. The specimen was rested on a plywood-rubber panel with 0.75 in. (19 mm) thick plywood and 0.5 in.(13 mm) thick rubber to simulate the elastic support underthe box culvert sections in a field installation. Figure 1illustrates the test setup for Specimen A.

A total of 25 channels of instrumentation were used on eachbox culvert; a load cell along with 24 electrical resistancestrain gauges 0.25 in. (6 mm) in gauge length were attached toselected reinforcement wires. Each of the specimens wasinstrumented with 24 strain gauges (Fig. 2). Strain gaugeswere mounted on the wire reinforcement at the locations ofexpected maximum positive and negative moments. Datawere electronically recorded by a data acquisition system.

Each specimen was tested in six different phases and afinal ultimate load test stage. A wheel load of 16 kips(71.2 kN) was used in calculating the load level at differentphases along with a 15% dynamic allowance factor. The loadlevel changed depending on the load factor. Before testingwith a load factor of 1.5, an intermediate load factor of 1.0was used. Table 2 summarizes the six different phases oftesting and the final ultimate load test stage.

EXPERIMENTAL RESULTS AND DISCUSSIONThe specimens were designed in accordance with ASTM

C1577. Reinforcement areas in ASTM C1577 are based onAASHTO LRFD 2005.4 The load factor used for checkingfatigue limit state was 0.75. This means that the test specimenswere designed for a fatigue load of 0.75 × 16 × 1.15 = 13.8 kips(61.39 kN) and tested under higher loads of 1.5 × 16 × 1.15 =

Hany Maximos is a PhD Candidate in the Department of Civil Engineering at theUniversity of Nebraska-Lincoln, Omaha, NE. He received his BSc and MSc fromAlexandria University, Alexandria, Egypt, in 1994 and 2000, respectively.

Ece Erdogmus is an Assistant Professor in the Department of ArchitecturalEngineering at the University of Nebraska-Lincoln. Her research interestsinclude the assessment and rehabilitation of existing concrete and masonrystructures, assessment and design of buried pipes and culverts, and concretemixture design and optimization.

Maher K. Tadros, FACI, is the Leslie D. Martin Professor of Civil Engineering in theDepartment of Civil Engineering at the University of Nebraska-Lincoln. He is a PastPresident of the ACI Nebraska Chapter and is a member of ACI Committee 546,Repair of Concrete; and Joint ACI-ASCE Committees 343, Concrete Bridge Design,and 423, Prestressed Concrete.

Fig. 1—Test setup for Specimen A before testing.


Table 2—Summary of different phases of testing program

Test phaseDescription

(load × load factor × dynamic allowance) Load type

Location of load

application Cycles Notes

1

16 × 1.0 × 1.15 = 18.4 kips (81.85 kN).Load was applied at rate of 1 kip/s (4.45 kM/s) starting from zero

and held at 18.4 kips (81.85 kN) for 15 seconds, and then unloaded at rate of 1 kip/s (4.45 kM/s)

Static point load using

footprint plate

Spigot end of box, at

midspanNA

To measure strain in different wires before fatigue load is applied. A LF of 1.0 is used to simulate service state conditions.

216 × 1.0 × 1.15 = 18.4 kips (81.85 kN).

Load was ranging from zero to 18.4 kips (81.85 kN) at frequency of 2 Hz.

Cyclic load using

footprint plate


midspan5 million cycles

To simulate fatigue from wheel load of 16 kips (71.17 kN) with dynamic allowance of 15%.

316 × 1.0 × 1.15 = 18.4 kips (81.85 kN).

Load was applied and unloaded at rate of 1 kip/s (4.45 kN/s). Load was held at mentioned level for 15 seconds.

Static point load using footprint

plate


midspanNot applicable

To compare strains inreinforcement to recorded measurements in Phase 3.

416 × 1.5 × 1.15 = 27.6 kips (122.77 kN).

Load was applied and unloaded at rate of 1 kip/s (4.45 kN/s).Load was held at mentioned level for 15 seconds.


plate


midspanNA

To measure strain in different wires before applying next 5 million cycles of fatigue loading.

516 × 1.5 × 1.15 = 27.6 kips (122.77 kN).

Load ranged from zero to 27.6 kips (122.77 kN) at frequency of 2 Hz.

Cyclic loadusing footprint

plate


midspan

5 million cycles or until test is

stopped if before that number.

Test stopped for Specimen A after 2 million cycles because deflection exceeded trigger value of 1/2 in. (12 mm).

616 × 1.5 × 1.15 = 27.6 kips (122.77 kN).

Load was applied and unloaded at rate of 1 kip/s (4.45 kN/s).Load was held at mentioned level for 15 seconds.


plate


midspanNA

To compare strains inreinforcement to recorded measurements in Phase 4.

Finalultimate load

test stageLoading monotonically until failure


plate

Center of top slab NA

To evaluate section capacity in flexure after being fatigued throughout different stages.

Note: NA = not available.

Table 1—Reinforcement areas required by ASTM C1577 versus actual areas provided

Reinforcement As1 As2 As3 As4 As5 As6 As7 As8

Specimen A*

ASTM C1577(req.), in.2/ft (mm2/mm)

0.380 (0.806)

0.310 (0.657)

0.290 (0.615)

0.290 (0.615)

0.290 (0.615)

0.290 (0.615)

0.290 (0.615)

0.290 (0.615)

As built Actual area,

in.2/ft (mm2/mm)0.420

(0.890)0.330

(0.699)0.330

(0.700)0.330

(0.700)0.300

(0.636)0.300

(0.636)0.300

(0.636)0.300

(0.636)

Wire size D10.0/D4.0 D5.5 D5.5 D5.5 D10.0 D10.0 D10.0 D10.0

Specimen A†

ASTM C1577(req.)

in.2/ft (mm2/mm)0.210

(0.445)0.340

(0.721)0.250

(0.530)0.190

(0.403)0.190

(0.403)0.190

(0.403)0.190

(0.403)0.190

(0.403)

As built Actual area,

in.2/ft (mm2/mm)0.210

(0.445)0.345

(0.731)0.255

(0.541)0.195

(0.413)0.195

(0.413)0.195

(0.413)0.210

(0.445)0.210

(0.445)

Wire size D7.0 D6.5/D5.0 D8.5 D6.5 D6.5 D6.5 D7.0 D7.0*Specimen A: Box 12 ft x 4 ft x 12 in. (3657.6 x 1219.2 x 304.8 mm).†Specimen B: Box 7 ft x 4 ft x 8 in. (2133.6 x 1219.2 x 203.2 mm).

27.6 kips (122.77 kN). An intermediate step correspondingto a load factor of 1.0 was also tested with a fatigue load of18.4 kips (81.85 kN). The experimental results of differenttest phases for both specimens are discussed in this section.

Experimental results and discussion for Specimen ASpecimen A endured 5 million cycles at 18.4 kips

(81.85 kN) with no signs of degradation. Figure 3 shows the

strain gauge readings for As2 with a static load of 18.4 kips(81.85 kN). There were no visible cracks during the first5 million cycles of fatigue testing using a load of 18.4 kips(81.85 kN). Figure 4 shows the strain gauge readings for As2after the fatigue loading. Comparing Fig. 4 to Fig. 3 showsthat the distribution of stresses among the reinforcementafter fatigue testing for 5 million cycles remains constant,while magnitudes of the stresses increase as anticipated.


The stress range in different reinforcement wires of As2was calculated using a modulus of elasticity E of 29,000 ksi(200.1 MPa). Figure 5 shows the stress range in differentreinforcement wires of As2 before and after the fatiguetesting in comparison to different fatigue ranges suggestedby AASHTO LRFD 20052 and AASHTO LRFD 2007.6 Ascan be seen, all of the strain gauge results are considerablylower than the stress ranges recommended in either of theAASHTO standards.

Figure 6 shows the strain gauge readings for As2 under thestatic load of 27.6 kips (122.77 kN) before commencing thesecond 5 million cycles. During the second 5 million cycles,which were carried out at a load of 27.6 kips (122.77 kN), acrack became visible after 1 million cycles. After the boxendured 2 million cycles, the deflection measured at themidspan section of the top slab at the same time was foundto be 0.5 in. (13 mm), causing the actuator program to trigger

a shutdown due to reaching the preprogrammed excessivedeflection limit.

Figure 7 shows strain gauge readings for As2 at the conclusionof the test. Figure 8 shows the stress range under the pointload of 27.6 kips (122.77 kN) in different wires for As2. Thefigure also shows the comparison with the fatigue rangesuggested by different AASHTO versions.

The specimen was then loaded monotonically to failure. Aspecial testing frame was prepared for this purpose. A pointload was applied using a footprint similar to the one used forfatigue testing acting at the midspan and midlength sectionof the top slab. The ultimate load recorded was 43.43 kips(193.19 kN). This load level was greater than the StrengthI load in AASHTO LRFD.2,6 The Strength I load wascalculated as per Section 3, using a load factor of 1.75 anda dynamic allowance of 33%: 16 × 1.33 × 1.75 = 37.2 kips(165.47 kN).

Fig. 2—Strain gauge distribution for Specimen A; box culvert 12 ft x 4 ft x 12 in.(3657.6 x 1219.2 x 304.8 mm).

Fig. 3—Strain gauge readings for positive reinforcement oftop slab (As2) for Specimen A, under static point load of18.4 kips (81.85 kN). (Note: Strain Gauge-2 was damagedduring casting of this box. As shown in Fig. 2, the locationof this strain gauge was, counting from the spigot end of thetop slab, on the fourth wire of As2.)

Fig. 4—Strain gauge readings for top slab positive momentreinforcement (As2) for Specimen A, under static point loadof 18.4 kips (81.85 kN) after first 5 million cycles. (Note:Strain Gauge-2 was damaged during casting of this box. Asshown in Fig. 2, the location of this strain gauge was, countingfrom the spigot end of the top slab, on the fourth wire of As2.)


Figure 9 shows the failure of the midspan section andnegative moment section of the top slab. The actual loadfactor (that is, the ratio between the ultimate load achievedby the specimen and the design ultimate load) was 2.04instead of 1.75. This result clearly demonstrates a high levelof conservatism in the design.

Experimental results and discussion for Specimen BFigure 10 shows the strain gauge readings for the top slab

positive moment reinforcement (As2) under a point load of18.4 kips (81.85 kN). The specimen endured 5 million cyclesat a load level of 18.4 kips (81.85 kN) without any visiblecracks. It was then loaded monotonically to 18.4 kips (81.85 kN)

Fig. 7—Strain gauge readings for top slab positive momentreinforcement (As2) for Specimen A, under static point loadof 27.6 kips (122.77 kN) at time of stopping fatigue test.(Note: Strain Gauge-2 was damaged during casting of thisbox. As shown in Fig. 2, the location of this strain gaugewas, counting from the spigot end of the top slab, on thefourth wire of As2.)

Fig. 5—Stress range in different wires of As2 under pointload of 18.4 kips (81.85 kN).

Fig. 6—Strain gauge readings for top slab positive momentreinforcement (As2) for Specimen A, under static point loadof 27.6 kips (122.77 kN) after first 5 million cycles. (Note:Strain Gauge-2 was damaged during casting of this box. Asshown in Fig. 2, the location of this strain gauge was, countingfrom the spigot end of the top slab, on the fourth wire of As2.)


18 ACI Structural Journal/January-February 2010

at a rate of 1 kip/s (4.45 kN/s), held for 15 seconds, andunloaded. Figure 11 shows the strain gauge readings forpositive moment reinforcement (As2) under the describedloading process. Figure 12 shows the stress range fordifferent wires for As2 under the load level of 18.4 kips(81.85 kN) before and after the first 5 million cycles. Thefigure also shows a comparison with the suggested valuesin different versions of AASHTO. It is clear that thestress range in the wires is much less than the suggestedstress range.

Figure 13 shows the strain gauge readings for As2 in thetop slab under the static point load of 27.6 kips (122.77 kN)acting at the spigot end, after concluding the first 5 millioncycles at a load level of 18.4 kips (81.85 kN).

During the second 5 million cycles at 27.6 kips (122.77 kN)load level, a crack became visible after 40,000 cycles. Thecrack was narrow and almost constant in width, but started topropagate from the spigot end toward the midlength of thebox segment as the number of cycles increased. The crackstopped extending further after 760,000 cycles and remainedconstant throughout the duration of the test (Fig. 14). Afterthe box endured the second 5 million cycles, static loadingwas performed using a point load of 18.4 kips (81.85 kN) toinvestigate if the fatigue test had any negative impact on thereinforcement behavior after 10 million cycles (Fig. 15). A

Fig. 9—Positive and negative moment sections at failure forSpecimen A: (a) midspan section of top slab; and (b) negativemoment section of top slab.

Fig. 10—Strain gauge readings for positive reinforcement (As2)of top slab for Specimen B, under static point load of18.4 kips (81.85 kN).

Fig. 11—Strain gauge readings for top slab positivereinforcement (As2) for Specimen B, under static point loadof 18.4 kips (81.85 kN) after first 5 million cycles.



point load of 27.6 kips (122.77 kN) was then applied to thebox (Fig. 16). Figures 15 and 16, respectively, show thestrain gauge readings for these two test phases.

After the fatigue test was concluded, the box was loadedmonotonically until failure by applying a point load at thecenter of the top slab using the footprint plate. The failuremechanism was initiated by cracks at the negative momentsections of the side walls at a load level of 55 kips (244.65 kN).Later, there was a crack at the bottom of the midspan sectionof the top slab, which increased in width until failure. Thiscrack was a new one along the edge of the footprint, andapproximately 5 in. (127 mm) away from the crack initiatedduring fatigue testing.

The ultimate load recorded was 115.86 kips (515.37 kN).Figure 17 shows cracks at the negative moment section ofthe side wall and midspan section of the top slab. This loadlevel was 3.12 times the Strength I load in AASHTO LRFD,which is 16 × 1.33 × 1.75 = 37.2 kips (165.47 kN).

Although the effect of fatigue was recorded as an increasein the strain in reinforcement under the same load level, theachieved ultimate load, after 10 million cycles, states that thesection capacity was not affected dramatically. Figure 12shows a stress range in the wires of As2 that is considerablyless than the suggested stress ranges in different versions ofthe AASHTO LRFD specifications. In addition, Fig. 17(b)shows that the cracks that led to failure of the midspansection of the top slab are different than the crack initiatedduring the fatigue testing.

The observed failure mechanism (Fig. 17) was due to theformation of a plastic hinge at the negative moment section.Once this hinge was formed, there was an increase of the

moment at the midspan section of the top slab because theslab started to act as a simple beam.

Based on this information, the load factor is calculated tobe 5.45 [115.86/(16 × 1.33) = 5.45], a value that is 211.5%greater than the load factor used for design as per theStrength I limit state in AASHTO LRFD.

Many other studies have been conducted on the behaviorof buried RC box culverts.9-14 Based on these studies’ findings, itis reasonable to consider that behavior is not just a beambehavior. Due to the arching action resulting from therestraint provided by the side walls and the supporting earthfill, box culverts present greater strength in the field ascompared to in-laboratory conditions. The presence of the

Fig. 14—Crack extension throughout the second 5 millioncycles at load level of 27.6 kips (122.77 kN) for Specimen B.

Fig. 16—Strain gauge readings for top slab positivereinforcement (As2) for Specimen B, under static point loadof 27.6 kips (122.77 kN) after finishing fatigue testing.

Fig. 15—Strain gauge readings for top slab positivereinforcement (As2) for Specimen B, under static point loadof 18.4 kips (81.85 kN) after finishing fatigue testing.

Fig. 17—Negative and positive moment sections at failurefor Specimen B.

Fig. 13—Strain gauge readings for top slab positivereinforcement (As2) for Specimen B, under static point loadof 27.6 kips (122.77 kN).


distribution reinforcement (the reinforcement that isperpendicular to the main reinforcement in the section) helpsto redistribute the load between the total reinforcement in thesection. This leads to less stress in each reinforcement wiredue to the acting wheel load than the stresses calculated bybeam theory and flexural analysis. This is proven by thedifferences between the achieved ultimate capacity—even afterfatigue testing—and the design ultimate load for both boxes.

Even in the absence of the additional restraint from lateralearth pressure behind the wall in the lab testing conditions,the capacity of the box culvert sections was greater thancalculated. In field installation conditions, the capacitywould be greater due to the arching action.

SUMMARY AND CONCLUSIONSTwo box culvert specimens (Specimens A and B) were

produced according to ASTM C1577 and tested in this study.Specimens were first tested using a static load of 18.4 kips(81.85 kN) corresponding to a load factor of 1.0. The sameload was then applied for 5 million cycles at a frequency of2 Hz. Both specimens endured the first 5 million cycles. Fivemore million cycles, at a frequency of 2 Hz, were appliedusing the load level of 27.6 kips (122.77 kN), correspondingto a load factor of 1.5. Specimen A endured 2 million cyclesunder this load level, whereas Specimen B endured 5 millioncycles. When loaded monotonically until failure, bothspecimens showed a higher load capacity than the Strength Idesign load in AASHTO LRFD, which corresponds to a loadfactor of 1.75.

Based on the results of this study, it is evident that, if bothrecently proposed changes regarding fatigue design wereadopted for the design of box culverts, highly conservative,and therefore uneconomical, designs will result. Therefore,the authors recommended to AASHTO Committee T13that one of the following options be adopted for boxculvert design:

1. Waive the fatigue requirements in box culvert design,similar to the requirements for bridge decks; or

2. Design according to AASHTO 2007 with a fatigue loadfactor of 0.75.

It must be noted that these two options do not present identicalresults; Option B is more conservative than Option A and isconsistent with AASHTO LFD (25% reduction in load and25% reduction in stress limit).

After the conclusion of the project, the committee selectedto waive the fatigue requirements in box culvert design(Option A), which will appear in the next version of theAASHTO LRFD standards.

ACKNOWLEDGMENTSThe authors would like to express their deepest appreciation to the American

Concrete Pipe Association (ACPA) and its technical committee for sponsoringthis study, and to Cretex for donating the box culvert sections. Specialthanks go to J. Beakley from ACPA for his sincere help in coordinating thedifferent parties involved in this study, and to D. Mertz, Chair of theAASHTO T13 Committee, in supporting the modification adopted byAASHTO based on this research. Authors are also grateful for the help ofMinnesota Department of Transportation bridge engineers D. Dorgan andK. Western, and appreciate the assistance of K. Lein in the experiments.

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14. Garg, A. K., and Abolmaali, A., “Finite Element Modeling andAnalysis of Reinforced Concrete Box Culverts,” Journal of TransportationEngineering, ASCE, V. 135, No. 3, 2009, pp. 121-128.

Fatigue Evaluation for Reinforced Concrete Box Culverts

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