limits of acceptable rail-and-post deﬂection in crash ...combined rail-and-post deflection...

be incurred only to the rail element, with minimal or no deflection ofthe supporting posts and soil. Impacts with a higher speed but shallowerangle can also cause more distributed rail-and-post deflection.

The amount of deflection that can be sustained by guardrail beforeits safety is compromised is a major concern. Maintenance crews andhighway agencies are often forced to balance the expense of contin-ual repairs against the potential liability if the damaged guardrail isstruck again. A recent survey of U.S. states and Canadian provincesrevealed that very few agencies have quantitative criteria underlyingthe decision of when to replace deflected guardrail (2, 3). Amongthose agencies that do, the threshold deflection was most commonlyset at 6 in. (152 mm) of deflection. This is also the recommendedrepair threshold for minor deflection specified by the FHWA (4). Indi-vidual agencies (2, 3) had thresholds as low as 3 in. (76 mm) or as highas 12 in. (305 mm). This study is intended to test the performanceof guardrail with rail-and-post deflection to support a unified repairthreshold for deflection on the basis of quantitative data.

Currently, all guardrail systems are thoroughly crash tested accord-ing to the testing procedure and thresholds specified in NCHRP Report350 (1). NCHRP Report 350 provides a variety of measurable criteriawith which the performance of the guardrail can be assessed. These cri-teria were divided into three categories: structural adequacy, occupantrisk, and post impact vehicle trajectory. Guardrail meeting the require-ments of all three categories should be accepted by the FHWA forinstallation on the national highway system. However, guardrail sys-tems that have suffered minor deflection damage may no longer meetthe same criteria and would require repair to restore functionality.

METHODS

The ideal method to test the safety of strong-post W-beam guardrailwould be to perform crash tests with varying levels of prior damage.However, the cost of doing so for the range of conditions under con-sideration would be prohibitive. In addition, no existing literature onthe crash performance of deflected guardrail was found that mightbe used to reduce the number of tests needed. To conduct a thoroughevaluation while containing costs, a two-part approach was used,consisting of (a) a full-scale crash test with a level of prior damagethat might reasonably be expected to fail, and (b) finite elementmodeling to predict the outcome if crash tests had been performedwith lower levels of damage.

Full-Scale Crash Tests

Two crash tests were performed by the MGA Research Corporationfor NCHRP Project 22-23 “Criteria for Restoration of Longitudinal

Limits of Acceptable Rail-and-PostDeflection in Crash-Damaged Strong-Post W-Beam Guardrail

Carolyn E. Hampton, Douglas J. Gabauer, and Hampton C. Gabler

95

The guidelines for the testing of strong-post W-beam guardrail, intendedto ensure the safety of errant vehicles, are specified in NCHRP Report 350.A limitation of these tests is that they are always performed on new,undamaged guardrail, whereas guardrail along highways is in a continu-ous cycle of damage and repair. No tests have ever evaluated the perfor-mance of deflection-damaged guardrail. A full-scale, two-part crash testwas conducted to evaluate the performance of a guardrail with 14.5 in.(368 mm) of prior crash damage. When this guardrail was struck by a4,409-lb (2000-kg) pickup truck traveling at 62 mph (100 km/h), the vehi-cle vaulted and came to rest upright behind the barrier. A critical factorwas the failure of a post to separate from the rails. Finite element modelswere used to evaluate the guardrail performance at lower levels of deflec-tion. The crash tests and finite element models demonstrated that rail-and-post deflection of 11 in. (279 mm) or higher resulted in vaulting or rollover.Repair was recommended for strong-post W-beam guardrail withcombined rail-and-post deflection exceeding 6 in. (152 mm). This limitallows a margin of safety for variations in soil strength and vehicleheight. The existence of rail-only deflection had a minimal effect on thecrash performance up to the maximum tested value of 6 in.

Strong-post W-beam guardrail is widely used as a roadside barrierthroughout the United States and other countries. Guardrail is testedto ensure that it is capable of safely containing and redirecting errantvehicles in accordance with NCHRP Report 350: Recommended Pro-cedures for the Safety Evaluation of Highway Features before beingapproved for use along roadways (1). However, in the act of redirect-ing a vehicle, the guardrail itself will inevitably sustain some amountof damage that will remain until the guardrail can be repaired. No testshave ever been performed to show that guardrail with minor damagecan safely redirect vehicles. Although there are many different typesof minor damage, this study is concerned with the examination ofimpacts into guardrail with prior deflection of the rails and posts.

Rail-and-post deflection is one of the most prevalent types of dam-age in guardrail, most often caused by a lower severity crash. An exam-ple of this damage type is shown in Figure 1. Impacts in which thevehicle speed or angle of impact are lower may result in localizedminor deflection. Depending on the impact angle, the deflection may

C. E. Hampton and D. J. Gabauer, 440 ICTAS Building, and H. C. Gabler, 445 ICTASBuilding, School of Biomedical Engineering and Sciences, Virginia Polytechnic Insti-tute and State University, Stanger Street (MC 0194), Blacksburg, VA 24061.Corresponding author: C. E. Hampton, [email protected].

Transportation Research Record: Journal of the Transportation Research Board,No. 2195, Transportation Research Board of the National Academies, Washington,D.C., 2010, pp. 95–105.DOI: 10.3141/2195-10

Barriers” to evaluate the performance of guardrail with rail-and-postdeflection (5, 6). These two crash tests represented successiveimpacts to one point on a guardrail. In accordance with NCHRPReport 350 Test Level 3 (TL3) guidelines, the guardrail was installedat a 25° angle relative to the incoming vehicle trajectory. Becauseneither the towing system nor the guardrail could be easily reorientedbetween tests, the first crash test was conducted at 25° as well.

In the first test, a 1997 Chevrolet 2500 pickup truck impacted theguardrail at 30 mph (47 km/h) and 25°. The purpose of this low-speedimpact was not to evaluate the performance of the guardrail, but ratherto create some minor deflection damage in the guardrail in prepara-tion for the second crash test. No repairs or alterations were made tothe guardrail in between this and the second crash test.

The second crash test was performed according to NCHRP Report350 TL3 standards. A second 1997 Chevrolet 2500 pickup truckimpacted the guardrail at 62 mph (100 km/h) and 25°. The vehicle fol-lowed the same trajectory as the first crash test and impacted theguardrail where the damage was located. The results of this crash testprovided both evidence of the guardrail performance when damagedand data against which the finite element models could be validated.

Finite Element Model

A full-scale finite element model was created from two parts: (a) amodel of a 176-ft (53.6-m) length of strong-post W-beam guardrailand (b) a model of a Chevrolet 2500 pickup truck. Each model isdescribed in more detail here. All of the initial conditions for the fullscale model were adjusted to match the values specified by NCHRPReport 350 TL3, that is, the vehicle was given an initial velocity of62 mph (100 km/h) and angle of impact was set to 25°.

Rail-and-post deflection is typically produced by a low severityimpact. However, “low severity” encompasses a wide range of initialconditions ranging from high-speed, low-angle impacts to low-speed,high-angle impacts. Ideally, the initial conditions for the low severityimpacts in the finite element simulations would be chosen to producethe greatest risk to the vehicle occupants in a second impact. An impactangle of 25°, with varying initial speeds, was selected for the initialconditions for two reasons. The first, and perhaps most important, rea-son was that the higher impact angle would maximize the potential forpocket formation. Second, the impact angle matched the impact angleof the MGA crash tests. This facilitated a more straightforwardcomparison between the MGA and finite element results.

Having selected the initial approach angle of the vehicle, gettingthe desired amount of rail deflection required adjustments to the ini-tial speed of the vehicle. Low-speed impacts in the range of 18.6 to

96 Transportation Research Record 2195

37.3 mph (30 to 60 km/h) were sufficient to cause 3, 6, 9, and 11 in.of deflection in the rails. Post deflection was also observed, particularlyfor the higher deflection levels. An example of a completed full scalemodel with 6 in. (152 mm) of rail deflection and 1.6 in. (41 mm)of post deflection is shown in Figure 2. In some models, artificial con-straints were introduced to prevent post motion so that the effects ofthe rail deflection could be studied in isolation.

Strong-Post W-Beam Guardrail Model

Strong-post W-beam guardrail is the most widely used of the steelroadside barriers on the national level. It comes in two varieties, thewood post and steel post system. This study focused on the type ofguardrail that uses steel posts with plastic blockouts, called the mod-ified G4(1S). A guardrail model with steel posts was selected becausethe steel posts represent the worst-case scenario for snagging of thevehicle tires during impact. Although the results using a steel post sys-tem will be conservative, it was felt to better to err on the side of cau-tion than to allow a borderline hazardous condition to be consideredan acceptable amount of deflection.

The basic modified steel strong-post W-beam guardrail model wasa publicly available model from the National Crash Analysis Center(NCAC) finite element library (7 ). The model was designed to beused with the LS-DYNA finite element simulation software (8). Theguardrail system was 176 ft (53.6 m) in length from end to end with29 posts. The midline of the guardrail was 21.65 in (550 mm) high.Routed plastic blockouts were used instead of wood blockouts. Thesoil supporting the guardrail system was modeled as individual buck-ets around each post rather than as a continuum body. Each steel postwas embedded in a cylindrical volume of soil 6.9 ft (2.1 m) deep and5.25 ft (1.6 m) in diameter. The soil model was representative of astrong, compacted soil using the material parameters provided withthe NCAC guardrail model.

Only one modification was made to the NCAC guardrail modelbecause the model had been previously validated (9). The stiffness ofthe springs holding the splice bolts together was increased from 15 kipto 540 kip (66.5 to 2,400 kN) to keep the splice bolts from unrealisti-cally separating during impact. The increase in stiffness reflected thebolt strength used in a model developed for a Texas TransportationInstitute (TTI) study on guardrails encased in paved mow strips (10).

Pickup Truck Model

To simulate a crash test, a model of a test vehicle matching the NCHRPReport 350 test criteria was also needed. The detailed model of a 1994

FIGURE 1 Guardrail with rail-and-post deflection. FIGURE 2 Simulated guardrail with rail-and-post deflection.

Chevrolet 2500 pickup truck was also available from the NCAClibrary. This model was Version 0.7 that was published to the onlineNCAC library on Nov. 3, 2008 (11). Like the guardrail model, thisvehicle model was designed to be used with the LS-DYNA finiteelement solver.

The success or failure of a crash test can depend greatly on the rel-ative height of the vehicle and guardrail. Marzougui et al. found thatlowering the height of the guardrail by 2.5 in (60 mm) could causethe vehicle to vault over the guardrail (12). Although the height ofthe guardrail was not changed in this study, bumper heights of theChevrolet 2500 pickup truck, the test vehicle that is frequently usedin NCHRP Report 350 crash tests, have been observed to vary fromtest to test.

Vehicles with higher bumper heights have higher centers of gravityand are more prone to vaulting and rolling when striking a guardrail.The finite element vehicle model should match the recorded dimen-sions of the real test vehicles to maximize accuracy. However, the testvehicles of the three crash tests used as validation cases for this study,the TTI 405421-1 (13) and MGA crash tests, had drastically differentbumper heights, as shown in Table 1. This necessitated the develop-ment of alternative vehicle models to match the dimensions in all ofthe crash tests.

The original NCAC vehicle model dimensions matched the TTI405421–1 test vehicle dimensions, meaning that only one alternativevehicle model was needed to represent all of the crash test vehicles.A modified version of the original NCAC vehicle was developed tomatch the different dimensions of the MGA crash tests. However,the remaining finite element simulations were conducted with theoriginal vehicle model because it more closely represented the vehicledimensions in most crash test reports.

Planned Simulations

A series of simulations was planned to determine how much deflec-tion could be permitted in a strong-post W-beam guardrail with-out compromising the safety of the system. All simulations wereconducted with the LS-DYNA 971 finite element solver. Simula-tions with combined rail-and-post deflection were conducted for3, 6, 9, and 11 in. (76, 152, 229, and 279 mm) of deflection. Thesecombined rail-and-post deflection simulations were run twice,once with post separation allowed and again with a critical postprevented from separating from the rail. These constraints wereapplied only in the simulations of the second impacts because postseparation did not occur in the lower severity first impacts. A

Hampton, Gabauer, and Gabler 97

small number of simulations in which only rail deflection wasallowed were also conducted for 3 and 6 in. of deflection. Largerrail deflections would not occur without also deflecting the posts.

RESULTS

Full-Scale Crash Tests

In the first crash test, the vehicle struck the guardrail at a speed of 30 mph (48.3 km/h) at 26.0°. The critical impact point waslocated 1.94 ft (591 mm) before Post 11 with the direction oftravel toward the higher numbered posts. This impact resulted indamage to 36 ft (11 m) of barrier length and a maximum permanentrail-and-post deflection of approximately 14.5 in. (368.3 mm). Thebarrier successfully contained the vehicle. The vehicle came torest alongside the barrier because of the low initial speed of thevehicle.

The day after the low severity impact, a second high-speed testwas run. A second pickup truck impacted the guardrail at the sameinitial impact point and area damaged by the previous vehicle. Theimpact conditions were 62.1 mph (99.9 km/h) at 25.5°. Because ofthe damage that was already incurred to the guardrail, the vehiclefailed to redirect and overrode the guardrail. The vehicle returned toground on the opposite side of the guardrail and continued to travelat 43.2 mph (69.5 km/h) and an angle of 18.7° from the guardrail.Post 13 failed to separate from the guardrail despite the significantamount of rail-and-post deflection during the test. Figure 3 presentsa series of photographs showing the vehicle vaulting over theguardrail. The pickup truck vaulted over the barrier and came to restupright behind the test installation.

The outcome of these crash tests demonstrated that there are limitsto the amount of damage that can be sustained by guardrails while stillmaintaining the functional capacity. This test showed that 14.5 in.(368.3 mm) of deflection damage in a guardrail represented an unac-ceptable condition that warrants high priority repair. However, theexact amount of deflection delineating acceptable and unacceptableperformance was still unknown. The performance of guardrail withlower amounts of deflection was evaluated, as described in the follow-ing sections, with finite element models to determine the threshold ofallowable deflection.

Validation of Finite Element Model

Before running the deflection simulations, it was important toshow that the finite element model was both able to reproduce theresults of a documented crash test and applicable to conditions out-side those of the validation test. To demonstrate this capability, thesame finite element model was used to predict the outcome ofthree crash tests. In TTI Test 405421-1, a Chevrolet 2500 pickuptruck impacted an undamaged guardrail and was successfully redi-rected (13). The remaining two tests were the previously discussedMGA C08C3–027.1 (5) and MGA C08C3–027.2 crash tests (6). Byvalidating against multiple crash tests, the acceptability of using thefinite element approach to model a wide range of crash conditionscould be assured.

A series of photos from the TTI and second MGA crash tests andsimulations is shown in Figure 4. For both tests, there was goodvisual agreement between the real crash test and the finite element

TABLE 1 Dimensions of Finite Element Models of the Chevrolet 2500 Pickup Truck

Modified Dimension NCAC Chevrolet 2500 Chevrolet 2500

Width 76.9 in. 195.4 cm 77.0 in. 195.5 cm

Length 222.6 in. 565.5 cm 222.6 in. 565.5 cm

Height 70.6 in. 179.2 cm 73.0 in. 185.4 cm

Front bumper height 25.0 in. 63.6 cm 26.8 in. 68.1 cm

Rear bumper height 27.8 in. 70.6 cm 30.1 in. 76.5 cm

Tire diameter 28.7 in. 73.0 cm 28.7 in. 73.0 cm

Weight 4,438 lb 2,013 kg 4,440 lb 2,014 kg

model. The ability of the model to reproduce the NCHRP Report350 criteria values observed in the crash tests is shown in Table 2.

The simulation of the TTI crash test agreed well with the reporteddata from the test report. The exit speed and angle, occupant impactvelocities, and maximum vehicle rotations for the simulation wereall similar. The greatest deviation was observed in the maximumobserved dynamic guardrail deflection, which was 1 ft (0.3 m) lowerin the simulation than in the crash test. The lower deflection of thesimulation was related to the higher stiffness of the soil in the finiteelement model relative to the crash test.

The first MGA crash test, a low-speed collision intended to causea minor amount of deflection, was successfully reproduced. A sim-ulation speed of 32 mph (52 km/h) was required to reproduce the


14.5 in. (368 mm) of deflection observed in the 30 mph (48.3 km/h)crash test. For the second MGA crash test, initial attempts atreproducing the results were unsuccessful. A critical factor in theoutcome of the crash test was found to be the failure of Post 13,located roughly 12.8 ft (3.9 m) downstream of the impact point,to separate from the rail during both the first and second impacts.The addition of a constraint on the same post in the simulationschanged the outcome of the simulation from a successful redirec-tion to failure by the vehicle vaulting over the guardrail. Occu-pant impact velocities and ridedown accelerations were below theNCHRP Report 350 limits in all the crash tests and simulations.The roll and pitch in the simulation matched well with the TTI405421-1 crash test.

(a) (b)

(c) (d)

(e) (f)

FIGURE 3 Time series for second impact into damaged guardrail section: (a) 0 ms, (b) 200 ms, (c) 300 ms, (d) 500 ms, (e) 700 ms, and(f ) 900 ms.


TTI Crash Test 405421-1[13]

Simulation ofTTI Crash Test

MGA Crash TestMGA C08C3-027.2 [6]

Simulation ofMGA Crash Test

t = 0 ms t = 0 ms t = 0 ms t = 0 ms

t = 120 ms t = 120 ms t = 120 ms t = 125 ms

t = 242 ms t = 240 ms t = 242 ms t = 250 ms

t = 359 ms t = 360 ms t = 360 ms t = 350 ms

t = 491 ms t = 490 ms t = 490 ms t = 500 ms

t = 691 ms t = 690 ms t = 690 ms t = 700 ms

FIGURE 4 Validation of finite element simulations against TTI and MGA crash tests.

Rail-and-Post Deflection Simulations

The MGA tests demonstrated that the separation of posts from the railscould radically change the crash performance of strong-post W-beamguardrail. Finite element modeling may not be able to accurately pre-dict which behavior will occur in a real crash when relevant factorssuch as soil strength or bolt position are not known. The approach wasto bracket the crash performance by conducting two series of simula-tions. In the first series, the rails and posts were allowed to sepa-rate. In the second series, a single post was prevented from separating.


This constraint was applied to Post 17, located 12.5 ft (3.8 m) down-stream of the impact point, which maximized the effect on vehicleperformance.

In the first set of simulations, guardrail with combined rail-and-post deflection of deflection of 3, 6, 9, and 11 in. (76, 152, 229, and279 mm) was tested. The NCHRP Report 350 test values recordedfor each simulation are shown in Table 3. Despite the huge differ-ence in performance between the MGA test simulation and theundamaged simulation, there was very little variation in perfor-mance between the simulations of lesser deflection. Even the simu-

TABLE 2 Validation of Finite Element Simulations Against TTI and MGA Crash Tests

TTI Crash Test TTI Validation MGA Crash Test MGA Crash Test405421-1 Simulation C08C3-027.2 Simulation

Impact conditionsSpeed (km/h) 101.5 100.0 99.9 100.0Angle (degree) 25.5 25.0 26.4 26.4

Exit conditionsSpeed (km/h) 55.0 53.0 69.5 57.0Angle (degree) 16.0 14.5 18.7 5.7

OccupantImpact velocity X (m/s) 7.1 7.5 6.1 9.3Impact velocity Y (m/s) 4.4 5.5 3.7 5.4Ridedown X (G) −7.9 −11.8 −6.1 −10.4Ridedown Y (G) 8.4 −12.3 −5.6 −5.450 ms average X (G) −5.3 −6.7 −5.5 −10.050 ms average Y (G) 4.3 −6.8 −3.1 −6.350 ms average Z (G) −4.8 −3.8 −4.1 −6.5

Guardrail deflectionsDynamic (m) 1.0 0.69 2.2 1.00Static (m) 0.7 0.55 1.0 0.80

Vehicle rotationsMax. roll (degree) −10 −14.4 30 7.1Max. pitch (degree) −4 −9.9 12 11.5Max. yaw (degree) 42 40.3 −10.2 −21.3

TABLE 3 Simulation Results for Rail-and-Post Deflection with No Separation Constraints

Undamaged 3-in. Rail-and- 6-in. Rail-and- 9-in. Rail-and- 11-in. Rail-and-Model Post Deflection Post Deflection Post Deflection Post Deflection

Impact conditionsSpeed (km/h) 100 100 100 100 100Angle (degree) 25 25 25 25 25

Exit conditionsSpeed (km/h) 53 53 52 56 50Angle (degree) 14.5 13.2 13.8 15.6 15.0

OccupantImpact velocity X (m/s) 7.5 8.0 8.0 8.6 8.3Impact velocity Y (m/s) 5.5 5.6 5.5 5.5 5.9Ridedown X (G) −11.8 −12.0 −12.2 −10.7 −12.8Ridedown Y (G) −12.3 −13.0 −10.1 −12.0 −10.450 ms average X (G) −6.7 −6.7 −6.8 −7.9 −7.150 ms average Y (G) −6.8 −6.7 −6.5 −6.5 −6.850 ms average Z (G) −3.8 −3.9 −3.0 −4.2 −4.6

Guardrail deflectionMax. dynamic (m) 0.69 0.72 0.74 0.76 0.78Static deflection (m) 0.55 0.62 0.55 0.60 0.64Preexisting deflection (m) 0.00 0.07 0.15 0.22 0.28

Vehicle rotationMax. roll (degree) −14.40 −12.9 −13 −16.6 −13.2Max. pitch (degree) −9.9 −10 −6.6 −5.6 10Max. yaw (degree) 40.3 40 40 41 40.5

lation with 11 in. of deflection yielded virtually the same crashresults and test values as the undamaged simulation.

In the second series of simulations, the models were set up inan identical manner, except that a constraint was added to a postlocated 12.5 ft (3.8 m) downstream of the impact point to preventthe rail and post from separating. NCHRP Report 350 results areshown in Table 4. The outcomes of these simulations are shown inFigure 5. The vehicle began to move upward and roll with increasingamounts of prior deflection damage. The vehicle eventually rolledonto its side when the deflection damage reached 11 in. (279 mm).However, even at 6 in. (152 mm) of deflection, the roll was veryhigh and reached more than 35° before the vehicle began to recover.

Figure 6 shows the local vehicle velocity at the center of gravity asa function of time for both the separation-constrained and uncon-strained simulations. There was almost no difference in the velocitybetween the undamaged simulation and the unconstrained rail-and-post deflection simulations. All of the exit speeds were in the range of31 to 35 mph (50 to 56 km/h). The velocities for the simulations witha fixed post were a little more varied. The vehicle in the 11-in. simu-lation retained the most speed because of rolling on its side, whichlimited the amount of interaction with the guardrail. The 3-in. simu-lation vehicle showed the lowest amount of roll and lost more speedbecause of more opportunities to interact with the posts.

There were increases in the maximum deflection of the guardrailwith increasing extent of rail-and-post deflection for both sets of sim-ulations, as shown in Figure 7. However, for both sets, each addi-tional 3 in. (75 mm) in preexisting deflection yielded only 0.8 to1.6 in. (20 to 40 mm) of extra dynamic deflection. The limited effectof the preexisting deflection was attributed to the narrow range overwhich the damage was incurred on the rail.

Rail Deflection-Only Simulations

Two simulations in which only rail deflection was allowed were cre-ated to determine the relative contributions of the rails versus those of


the posts. No constraints were added to the posts in the second impactsof these simulations. Because larger rail deflections do not occur with-out also deflecting the posts, these simulations were limited to 3 and6 in. (76 and 152 mm) of rail-only deflection.

The NCHRP Report 350 test criteria were almost entirelyunchanged from the values recorded for the undamaged simulation.Between the undamaged and 6 in. rail-only deflection simulation, theroll and pitch decreased by less than 4° and the maximum dynamicdeflection increased by less than 3%. The longitudinal occupantimpact velocity showed the greatest increase, rising to 27 ft/s (8.2 m/s)from 24.6 ft/s (7.5 m/s), but was still within the recommended limit.The lack of change in crash test outcome for rail-only deflection sup-port the earlier theory the contributions of the posts may be moreimportant in predicting the outcome of a crash.

DISCUSSION OF RESULTS

Importance of Rail-and-Post Separation

A critical contribution to the vaulting of the vehicle in the MGA crashtest was believed to be the failure of some of the posts to detach fromthe guardrail. In the second MGA crash test, a post failed to separatefrom the rail during impact. In a preliminary simulation of this crash,the post did separate, and the vehicle was successfully redirected.When a constraint was added to prevent the rail from separating fromthe post, the vehicle vaulted over the guardrail. The deflection of thispost during impact was believed to have pulled the rail downward asthe post deflected back, enhancing the chance of the vehicle vaultingover the guardrail.

Simulations of Rail-and-Post Deflection

In the simulations of the 3, 6, 9, and 11 in. (76, 152, 229, and 279 mm)of rail-and-post deflection with no separation constraints, minor

TABLE 4 Simulation Results for Rail-and-Post Deflection with One Post Separation Constraint

Undamaged 3-in. Rail-and- 6-in. Rail-and- 9-in. Rail-and- 11-in. Rail-and-Model Post Deflection Post Deflection Post Deflection Post Deflection

Impact conditionsSpeed (km/h) 100 100 100 100 100Angle (degree) 25 25 25 25 25

Exit conditionsSpeed (km/h) 53 46 55 55 64Angle (degree) 14.5 19.1 12.3 15.6 3.4

OccupantImpact velocity X (m/s) 7.5 8.4 7.9 8.1 7.9Impact velocity Y (m/s) 5.5 5.5 5.1 5.2 5.4Ridedown X (G) −11.8 −11.7 −13.2 −14.5 −8.8Ridedown Y (G) −12.3 −10.4 −11.9 −11.7 −8.750 ms average X (G) −6.7 −8.8 −8.2 −8.3 −7.050 ms average Y (G) −6.8 −6.5 −6.2 −6.2 −6.050 ms average Z (G) −3.8 −3.9 −3.7 4.6 5.2

Guardrail deflectionMax. dynamic (m) 0.69 0.82 0.86 0.86 0.90Static deflection (m) 0.55 0.64 0.66 0.67 0.77Preexisting deflection (m) 0.00 0.07 0.15 0.22 0.28

Vehicle rotationMax. roll (degree) −14.4 32.1 35.5 39.7 RollMax. pitch (degree) −9.9 −14.6 −19.8 22.7 28.2Max. yaw (degree) 40.3 46.2 35.8 39.0 23.3

rail-and-post deflection had very little effect on the simulationresults. The occupant impact velocity (a measure of the speed withwhich the occupant will strike the inside compartment of the vehi-cle), ridedown, and 50 ms average accelerations were satisfactory,and the increases in maximum deflection were less than the increasein prior deflection.

When the simulations were altered to prevent a post from separat-ing from the rail, different outcomes were observed. The vehicle rollincreased with increasing preexisting deflection. The vehicle over-turned during impact with a guardrail having 11 in. (279 mm) of pre-existing rail deflection. Even for as little as 6 in. of rail deflection,substantial rolling was observed.

By failing to separate, two different hazardous conditions can becreated. If the post remains mostly upright, the vehicle may be atgreater risk of snagging. Another possible outcome was reflected inthe results of the MGA crash test. If an unseparated post was deflectedbackward and downward, as in the simulations with greater than 6 in.(152 mm) of deflection, the rail is pulled downward as well and therisk of vaulting is increased.

The vehicle behaviors for both 3 and 6 in. (76 and 152 mm) of raildeflection without post deflection were a little different from that ofthe undamaged simulation. The static and dynamic guardrail deflec-tions were almost unchanged. These results provide further support


for the theory that the behavior of the posts in strong-post guardrailsystems can strongly influence the outcome of a crash test.

Effects of Prior Damage on Rail Height

Existing literature has suggested that rail height can be a major con-tributor to vaulting (12). The rails in the finite element simulationswere examined to determine whether the minor rail deflectionincurred in the first impact resulted in changes in the rail height thatcould be correlated to the outcome of the simulated second impact.The hypothesis was that the preexisting damage would lower the railheight and lead to the vehicle vaulting.

Figure 8 presents the minimum height of the rail bottom, maximumheight of the rail top, and the length of preexisting deflection after thefirst impact but before the second impact. All of the measurementswere made from the simulations with a separation constraint added.This situation represented the worst case scenario for vaulting becausethe deflection of the post would pull the rail downward as it deflected.

Figure 8 shows that one consequence of an impact is that the rail flat-tens. The bottom of the rail moved downward from 15.3 in. (388.6 mm)to 12.6 in. (320 mm) above the ground surface. The top of the railmoved upward from 27.9 in. (709 mm) to 31.8 in. (808 mm). The max-

(a) (b)

(c) (d)

(e)

FIGURE 5 Rail-and-post deflection simulations with separation constraint after impact (t � 700 ms): (a) undamaged rail, (b) rail with 3-in.prior deflection, (c) rail with 6-in. prior deflection, (d) rail with 9-in. prior deflection, and (e) rail with 11-in. prior deflection.

imum height of the guardrail increased with increasing deflection, indi-cating that the guardrail was becoming increasingly flattened. Thelength of deflection also increased with increasing magnitude of deflec-tion. These results indicate that the initial hypothesis was not cor-rect and that the height of the bottom of the rail or the damage lengthmay have been larger contributors to the crash outcome in these sim-ulations. These findings do not disprove the significance of rail heightbut rather imply that there can be multiple factors, such as flattening,that contribute to a vehicle vaulting over a barrier.

Evaluation of Rail Rupture Potential

Localized tearing is possible in impacts of this type, but ourmodel was not configured to accurately compute element tearingresulting from localized stress concentrations and did not includefailure criteria for the steel components. The model was meshedusing large element sizes 0.4 to 0.6 in (10 to 40 mm), which wereappropriate for determining vehicle dynamics but were too coarseto realistically model the initiation and propagation of tears. As analternative, the tension carried by the rails was used to determine therelative risk of rail rupture.


Ray et al. conducted a study on rail rupture in crash tests thatshowed that rails can carry up to 92.2 kip (410 kN) under quasistatic loading (14). To assess whether rail rupture was a concernwhen the guardrail had sustained prior damage, the maximum ten-sion was measured for all simulations. The maximum tension car-ried by the guardrail in the undamaged TTI 405421-1 simulationwas 53.4 kip (237.5 kN). For the deflected guardrail, the maxi-mum rail tension increased by less than 10% for the guardrailswith a post constrained from separating and less than 25% for thefreely separating guardrails, even with the prior deflection as highas 11 in. (279.4 mm) (2).

Other Variables Affecting Crash Performance

Marzougui et al. concluded that lowering the guardrail resulted inunacceptable crash performance (12). However, the examination ofrail height in a previous section showed that changes in the vehicleheight were not exactly analogous to changes in the rail height.Although the height of the guardrail was not changed in this study,it was noted that the Chevrolet 2500 pickup truck does vary in heightand center of gravity. A test vehicle with a higher height than finite

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element model would then be expected to have a greater risk ofvaulting and rollover, necessitating a lower deflection threshold.

Another factor that can influence a crash test outcome is thestrength of the soil around the posts. The soil used in this model wasa strong, compacted soil, which minimized post deflection and max-imized the chance of snagging. However, if the soil was weaker orthe guardrail was installed on a backslope, the lateral stiffness of theguardrail would decrease. This would increase both the deflectionof the guardrail and the risk of vaulting.

CONCLUSIONS

This study examined the crash performance of strong-post W-beamguardrail with rail-and-post deflection from a previous impact.Crash tests and finite element simulations of second impacts intodamaged guardrail have shown that the combination of rail-and-postdeflection can negatively affect the crash performance. These resultswere supported by the following:

• Crash tests demonstrated that 14.5 in. (368 mm) of rail-and-postdeflection with a damage length of 36 ft (11 m) was a damage levelrequiring high priority repair. Two full-scale crash tests were con-ducted to evaluate the limits of acceptable rail-and-post deflection


in crash-damaged strong-post W-beam guardrail. The damaged bar-rier failed to contain the Chevrolet 2500 pickup truck that impactedat 62 mph (100 km/h) and 26.4°. The vehicle vaulted over theguardrail and came to rest upright behind the barrier. A critical fac-tor in the outcome of the test was the failure of a post near the areaof impact to separate from the rails during impact.

• Finite element simulations were used to investigate the accept-ability of damage levels less than 14.5 in. (368 mm) of rail-and-postdeflection. Simulations were conducted for rail-and-post deflectionvarying from 3 to 11 in. (76 to 279 mm). A series of simulationswas run in which a single post was prevented from separating. Thevehicle experienced significant roll beginning at 6 in. (152 mm) ofdeflection and eventually rolled over when the deflection reached11 in. (279 mm).

• A set of simulations for which only rail deflection was allowedwere run for 3 and 6 in. of prior damage. The vehicle and guardrailperformance in these simulations were almost unchanged from theundamaged simulation. These results illustrate the importance of postdeflection on the crash outcome.

• The tension carried by the guardrail when a post was preventedfrom separating increased for 3 in. (76 mm) of deflection but wasunchanged for all other simulations. However, when the posts couldfreely separate, the tension increased along with increasing deflec-tion. The largest observed increase was 23.3% over the tension of theundamaged simulation because of 9 in. (229 mm) of deflection. Thepeak rail tension in the 6 in. (152 mm) simulation was 19.2% higherthan the undamaged simulation. These results indicated that rupturerisk modestly increased with increasing magnitude of rail-and-postdeflection.

• Both the maximum rail height and length of deflection increasedwith increasing amounts of preexisting deflection. The minimumheight was roughly constant amount for any amount of deflection.Each of these factors could be an important contributor to crashincome, but the significance of each could not be isolated. Furtherstudy will be needed to better understand these factors.

Repair of damaged guardrail with combined rail-and-post deflectionexceeding 6 in. (152 mm) is recommended. For strong soils, the crashperformance of guardrails with deflection up to 9 in. (229 mm) wasadequate, but higher amounts of deflection were not. Adjusting for amargin of safety, that is, to account for softer soils or vehicles withhigher centers of gravity, the limit of acceptable rail-and-post deflec-tion was set to 6 in. (152 mm). The presence of any amount of deflec-tion in the guardrail was found to increase the maximum dynamicdeflection. Damaged guardrails with hazardous objects directly behindthe guardrail should be repaired. The repair of strong-post W-beamguardrail with deflection damage exceeding 9 in. should also be ahigh priority repair. Guardrail with deflection between 6 to 9 in.(152 mm to 229 mm) should be a moderate priority repair sincedeflection in this range had a lesser effect on the crash performance.

ACKNOWLEDGMENTS

The authors thank Charles Niessner, NCHRP senior program offi-cer, and the NCHRP Project 22–23 panel for their contributions tothe success of this project. The authors also gratefully acknowledgeTrinity Industries and Gregory Industries for providing the guardrailmaterials used in the MGA crash tests. The authors thank LivermoreSoftware Technology Corporation and Altair Engineering for pro-viding academic licenses for the software to develop and run the

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finite element models. They also thank MGA Research and TTI forproviding crash test data.

REFERENCES

1. Ross, H. E., Jr., D. L. Sicking, R. A. Zimmer, and J. D. Michie. NCHRPReport 350: Recommended Procedures for the Safety PerformanceEvaluation of Highway Features. TRB, National Research Council,Washington, D.C., 1993.

2. Gabler, H. C., D. J. Gabauer, and C. E. Hampton. NCHRP Report 656:Criteria for Restoration of Longitudinal Barriers. Transportation ResearchBoard of the National Academies, Washington, D.C., 2009.

3. Gabauer, D. J., and H. C. Gabler. Evaluation of Current Repair Criteriafor Longitudinal Barrier with Crash Damage. Journal of TransportationEngineering, Vol. 135, No. 4, 2009, pp. 225–234.

4. W-Beam Guardrail Repair and Maintenance: A Guide for Street andHighway Maintenance Personnel. FHWA-RT-90-001. FHWA, U.S.Department of Transportation, 1990.

5. MGA Research Corporation. 1997 Chevrolet 2500 Pickup Impact withthe Strong Steel Post W-Beam Guardrail—Part 1. MGA ReferenceNo. C08C3-027.1. 2008.

6. MGA Research Corporation, 1997 Chevrolet 2500 Pickup Impact withthe Strong Steel Post W-Beam Guardrail—Part 2. MGA ReferenceNo. C08C3-027.2. 2008.


7. National Crash Analysis Center. NCAC Finite Element Archive.http://www.ncac.gwu.edu/vml/models.html. Accessed Feb. 12, 2009.

8. LS-DYNA Keyword User’s Manual Version 970. Livermore SoftwareTechnology Corporation, Livermore, Calif., 2003.

9. Whitworth, H. A., R. Bendidi, D. Marzougui, and R. Reiss. Finite ElementModeling of the Crash Performance of Roadside Barriers. InternationalJournal of Crashworthiness, Vol. 9, No. 1, 2003, pp. 35–43.

10. Bligh, R. P., N. R. Seckinger, A. Y. Abu-Odeh, P. N. Roschke, W. L.Menges, and R. R. Haug. Dynamic Response of Guardrail SystemsEncased in Pavement Mow Strips. FHWA/TX-04/0-4162-2. TexasTransportation Institute, College Station, 2004.

11. National Crash Analysis Center. Finite Element Model of C2500 PickupTruck. http://www.ncac.gwu.edu/vml/archive/ncac/vehicle/c2500pickup-0.7.pdf. Accessed March 2, 2009.

12. Marzougui, D., P. Mohan, and C. Kan. Evaluation of Rail Height Effectson the Safety Performance of W-Beam Barriers. 6th European LS-DYNAUser’s Conference, Gothenburg, Sweden, 2007.

13. Bullard, D. L., W. L. Menges, and D. C. Alberson. NCHRP 350: Compli-ance Test 3-11 of the Modified G4(1S) Guardrail with Timber Blockouts.TTI 405421-1, FHWA-RD-96-175. Texas Transportation Institute,College Station, 1996.

14. Ray, M. H., C. A. Plaxico, and K. Engstrand. Performance of W-BeamSplices. In Transportation Research Record: Journal of the Transporta-tion Research Board, No. 1743, TRB, National Research Council,Washington, D.C., 2001, pp. 120–125.

The Roadside Safety Design Committee peer-reviewed this paper.

limits of acceptable rail-and-post deﬂection in crash ...combined rail-and-post deflection...

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