National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 3 - Research Approach

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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
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Suggested Citation:"Chapter 3 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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11 3.1 Research Plan The goal of this research program is to develop guide- lines to assist highway personnel in identifying levels of minor barrier damage and deterioration that require repairs to restore operational performance. The guidelines are to be based upon objective and quantitative threshold values for which barrier repair is recommended. This chapter describes the research approach used to develop these quantitative underpinnings. The research team’s approach was to evaluate the more com- mon damage types with a combination of controlled experi- ments and computational modeling to develop the repair guidelines. The experiments were used both to directly eval- uate barrier performance and to validate the computational models. The research program was conducted in the following three phases: • Phase I: Candidate Repair Guidelines • Phase II: Evaluation of Candidate Repair Guidelines • Phase III: Recommendations for Improved Repair Guide- lines. 3.1.1 Phase I: Candidate Repair Guidelines The first phase of the research program was to develop a candidate set of repair guidelines needed to address commonly observed modes of minor damage. Selection of the damage modes for which guidelines were needed was conducted based upon a survey of U.S. and Canadian transportation agencies presented in Chapter 2. Additional damage modes were added as needed based upon analysis of a photographic catalog of minor barrier damage categories developed through field inspection of damaged barrier sites, and consultation with the project panel. The next step was to propose objective and quantifiable repair criteria for each damage mode. The guidelines must fur- ther be based upon straightforward metrics which are practi- cal for maintenance crews to use for quantifying minor barrier damage. The candidate set of guidelines and quantitative crite- ria to gauge minor barrier damage were based on current met- rics employed by transportation agencies and supplemented as needed to address specific damage modes. The final set of candidate repair guidelines to be evaluated was presented to and approved by the project Panel prior to commencement of Phase II: Evaluation of Candidate Repair Guidelines. Table 8 presents the candidate list of damage modes for non- proprietary w-beam barriers, including strong and weak post w-beam barriers which were to be evaluated in this program. Table 8 also presents the objective repair criteria which were sought for each damage mode. The objective of Phase II was to analytically determine these repair criteria. The Project Panel also requested that the final report spec- ify repair criteria for generic end terminals. The intent was to specify guidance which was applicable to all end terminal types (except as noted). Manufacturers of proprietary end ter- minal systems may recommend additional repair thresholds specific to an individual terminal. Note that this guidance was based solely on engineering judgment; no finite element simulations or pendulum tests evaluating these end terminal damage modes were conducted. These guidelines, shown in Table 9, were based primarily on an End Terminal Routine Maintenance Checklist developed for use by the Ohio DOT. 3.1.2 Phase II: Evaluation of Candidate Repair Guidelines The candidate guidelines were evaluated through a combi- nation of controlled experiments and computational modeling. Both pendulum tests and full scale barrier crash tests were conducted on longitudinal barrier into which a flaw, e.g., a vertical tear, had been purposely introduced. The tests were supplemented by a suite of computational simulations to fur- ther evaluate the crash performance of longitudinal barrier C H A P T E R 3 Research Approach

with minor damage. The physical experiments were used both to directly evaluate barrier performance and to validate the computational models. The results of each damaged barrier impact experiment or simulation were evaluated using criteria based heavily on NCHRP Report 350. Pendulum tests were evaluated based on the ability of the barrier to contain the pendulum, i.e., no pen- dulum penetration, underride, or override. For the full-scale crash tests and computational simulations of full-scale crash tests, the criteria shown in Table 10 were used to evaluate crash performance. The full-scale crash tests and simulations were also assessed for vehicle instability resulting from impact in- cluding roll, pitch and yaw, wheel snagging, and the presence/ absence of vaulting. In these cases, the baseline case (for the qualitative comparison) was the respective vehicle impacting an undamaged barrier. The results of each evaluation were used to set the threshold for repair. Repair priorities were assigned to the barrier damage evalu- ated to provide maintenance personnel with guidance regard- ing the relative importance of barrier damage types. This was accomplished by qualitatively ranking each damage type based on the scheme presented in Table 11. The priority rankings were based on the results of the finite element simulations and pendulum tests. 12 Component Damage Type Damage Description Quantitative Repair Criterion (to be determined) Rail Deflection Deflection from as-built Rail Flattening (thickness) Percent Flattened Deflection Rail Flattening (height) Percent Flattened Non-Manufacturer hole in Rail Diameter of hole Non-Manufacturer holes in Rail Number of holes in single section Vertical Tear Length of tear Tearing/Breaks / Punctures Horizontal Tearing Length of tear Rail Element Deterioration Any structural corrosion Amount of Section Loss Post/Rail Deflection Deflection from as-built Deflection Steel Post torsion Number of damaged posts Tearing/Breaks Broken Posts Number of broken posts Rotten Wood Posts (any visible rotting) Number of rotted posts Posts Deterioration Any Structural Corrosion (hole or section loss) Amount of section loss Deflection Twisted/Misaligned Blockouts Number of affected blockouts Missing Missing Blockouts Number of missing blockouts Blockouts Deterioration Rotten Wood Blockouts (any visible rotting) Number of rotted blockouts Splice Damage Amount of rail material left between splice and bolt hole Missing, Loose, or Damaged Splice Bolts Number of affected bolts Connections Integrity Loss Post Separated from Rail (any) Number of posts separated from rail Table 8. Repair thresholds to be determined for w-beam barriers. Component Damage Description Rail Element Rail Element not Aligned Properly in Impactor Head* Posts Post Number 1 is Broken or Missing Blockouts Any Twisted / Misaligned Blockouts > 1 in. of Slack in Anchor Cable or Missing Anchor Cable Bearing Plate Rotated or Missing Connections Any Failed Lag Screws Securing Impactor Head * * Applies only to Energy Absorbing End Terminals Table 9. Preliminary proposed repair thresholds for generic end terminals.

3.1.3 Phase III: Recommendations for Improved Repair Guidelines In the third and final phase of the project, the results of the impact tests and simulations were used to develop a recom- mended set of repair guidelines in a form suitable for mainte- nance personnel in the field. The end customer for these repair guidelines are highway maintenance personnel. In addition to being based upon a strong analytical foundation, the guide- lines must be easily understood and implemented. The repair threshold guidelines were presented in a graphical format that clarified how damage to w-beam barriers should be measured and repair priority assessed. A workshop on the new guidelines was presented to an Iowa DOT maintenance group to obtain the feedback from actual maintenance practitioners in Mason City, IA, in May 2009. Comments from the workshop participants were invaluable and were used to fine-tune the guidelines for improved read- ability and practicality. 3.1.4 Damage Evaluation Techniques The remainder of this chapter describes the methods used to evaluate the crash performance of longitudinal barrier with each damage mode. The following sections describe the fol- lowing techniques: • Pendulum Testing Plan • Full Systems Crash Test Plan • Finite Element Modeling Plan • Validation of Finite Elements Models 3.2 Pendulum Testing Plan Pendulum tests were used in this research program for two purposes: (1) as tests of structural integrity and (2) to provide test data for validation of computational models. Pendulum tests are a better method than finite element modeling to check for structural integrity under impact conditions which might result in tearing or fracture. Finite element modeling using the LS-DYNA code is a less than ideal method of model- ing this type of damage. Examples would include vertical tears, horizontal tears, holes, and splice damage. The pendulum tests were conducted at the Federal Outdoor Impact Laboratory (FOIL) in conjunction with the FHWA. 3.2.1 Experimental Design Development and Test Methodology Pendulum Apparatus and Impactor Face. The pendulum tests used the pendulum device currently located at the FHWA FOIL in McLean, VA. The FOIL pendulum consists of a sup- port structure, a 2000-kg (4500 lb) pendulum mass (center image in Figure 3), and two rigid posts (left image in Figure 3) located on either side of the suspended pendulum mass. A rounded triangular pendulum impactor face was fabricated for the tests (right image in Figure 3). The radius of chamfer at the impactor face center was 152 mm (6 inches), which was based on measurements of a 2006 Chevrolet 1500 pickup truck. The impactor face is 420 mm (16.5 inches) tall and is capable of 13 Criterion Required Performance 1. Barrier contains and redirects the vehicle Structural Adequacy 2. No vehicle penetration, underride, or override 3. Vehicle should remain upright during and after the collision; moderate pitch and roll are acceptable 4. Lateral and longitudinal occupant impact velocity < 12 m/s (as computed by the flail space model) Occupant Risk 5. Lateral and longitudinal occupant ridedown acceleration < 20 G (as computed by the flail space model) 6. Vehicle intrusion into adjacent traffic lanes is limited or does not occur Vehicle Trajectory 7. Vehicle exit angle should preferably be less than 60 percent of the impact angle Table 10. Barrier crash performance requirements. Priority Level Description High A second impact results in unacceptable safety performance including barrier penetration and/or vehicle rollover. Medium A second impact results in degraded but not unacceptable safety performance. Low A second impact results in no discernible difference in performance from an undamaged barrier. Table 11. Preliminary proposed repair priority scheme.

14 Figure 3. Existing rigid posts (left), FOIL pendulum mass (center), and new impactor face (right). Figure 4. Overall pendulum test setup for an undamaged section. engaging the full w-beam cross section. The combined mass of pendulum and the impactor face was 2061.5 kg (4,545 lbs) to represent the mass of the NCHRP Report 350 2000-kg pickup truck (2000P) test vehicle. Note that the pendulum mass is slightly higher than the 2045 kg recommended mass limit spec- ified by NCHRP Report 350 for the 2000P test vehicle. W-Beam Test Section, Anchorage, and Embedment. A two-post section of modified G4(1S) strong-post w-beam barrier with wood blockouts was selected for testing. The bar- rier test section length was constrained by the available span (approximately 5.5 meters) between the existing rigid posts on either side of the FOIL pendulum. Using standard 1905 mm (6.25 feet) post spacing and 3810 mm (12.5 feet) rail lengths, this allowed for one post to be located at a rail splice and the other post a non-splice location. As this section represents the smallest repeating unit for the strong-post w-beam barrier, this configuration was thought to be most representative of a typical full-length installation. Note that this two post section is roughly one tenth the length of a barrier in a full-scale crash test, which typically has 29 posts. The w-beam section was oriented such that the impact was mid-span between the two posts. Figure 4 is a schematic of the overall test setup. The overall rail length is approximately 5 meters (198 inches) and the posts were W150 × 13.5 steel posts, 1830 mm (6 feet) in total length. Developing an appropriate method to anchor each end of the test section to the rigid posts proved to be the most chal- lenging portion of the test setup. The goal was to replicate a two post section as if it was within a full length barrier section, which requires each end of the test section some freedom to both translate and rotate. Due to the close proximity of the rigid posts on either side of the pendulum, the primary focus was on designing the end fixture to allow rotation of each end of the w-beam test section. Also, an effort was made to use as much standard guardrail hardware as possible in the end fix- ture design. The original end fixture design selected consisted of 3 standard cable anchor brackets and a 910 mm (3 feet) ver- sion of the standard 1830 mm (6 feet) swaged cable typically used to anchor w-beam terminals (left image of Figure 5). This configuration was originally selected to ensure w-beam rail rupture would occur before failure of the anchorage.

Later, an alternative 2-cable end fixture design was developed (center image in Figure 5). A comparison of two undamaged section pendulum tests showed no discernable difference in deflection. The 2-cable end fixture proved to be robust and was used in the remainder of the tests to simplify the test setup and reduce costs. As experience was gained in conducting these tests, several minor modifications were made to the 2-cable end fixture, primarily to prevent tearing and bending failures within the fixture. Larger 82.6 mm (3.25 inches) outside diam- eter washers were used inside the rigid posts to prevent pullout of the cables from the rigid posts. The length of swaged cables was increased by 102 mm (4 inches) so the cable would bend instead of the swage. To prevent tearing in the fixture, the typ- ical washers used in conjunction with the anchor brackets were replaced by an anchor plate. Results from pendulum tests con- ducted using both the 2-cable and 3-cable end fixture schemes will be presented later in this report. As the anchor points on the existing rigid posts were higher than the standard w-beam rail height, a soil box was used to raise the ground level around the posts by 7 inches (178 mm) as shown in the right image in Figure 5. The soil box was con- structed of four 38 mm × 235 mm × 2.44 m (2 in. × 10 in. × 8 in. long) pine boards and supported on each side by steel rebar to provide the soil restraining force such that proper com- paction could be attained. As specified by NCHRP Report 350, the soil used in the test conformed to AASHTO M-147-65. A mechanical tamper was used to compact the soil surrounding each W150 × 13.5 steel post in 6-inch lifts. A nuclear density gauge (Troxler Model 3440) was used to determine the com- paction and soil properties of each soil lift for each post. For each lift, the preferred compaction level was 95 percent. Instrumentation. Instrumentation for all tests included two accelerometers located at the rear of the pendulum mass. Both accelerometers were in-line with the pendulum center of gravity and were aligned in the pendulum direction of travel. Tri-axial accelerometers were placed on each rigid post to quantify the motion of the rigid posts during the pendulum 15 Soil BoxCable Anchor Bracket Figure 5. Three cable (left) and two cable (center) w-beam end fixture and soil box (right). Figure 6. Analogous NCHRP Report 350 and pendulum impact scenarios. impact. Four high speed cameras were used in all tests to cap- ture the behavior of the w-beam section during the impact. Each test had a minimum of two common camera views: (1) a top view of the middle of the w-beam section and (2) a perpen- dicular rear view of the entire w-beam section. The other two high speed camera views varied between tests depending on the location of the minor damage. In addition to the high speed cameras, one real time camera was used to capture a perspec- tive view of the test in real time. Impact Conditions and Relevance to Full-Scale Crash Testing. As the FOIL pendulum is not capable of reproduc- ing an oblique impact characteristic of NCHRP Report 350 longitudinal barrier test procedures, the tests were designed to mimic the lateral forces experienced in a NCHRP Report 350 redirectional test (Figure 6). Pendulum tests were conducted at two impact speeds: 32.2 km/hr (20 mph) and 28.2 km/hr (17.5 mph). A 32.2 km/hr (20 mph) impact speed was originally selected to approxi- mate the lateral forces that would result from a 2000 kg test vehicle impacting at 100 km/hr (62 mph) and 20 degrees. As- suming that all the impact energy is absorbed in a two post sec- tion of a full-scale test barrier, these conditions represent a lat- eral impact speed approximately 75 percent that of an NCHRP

Report 350 Test 3-11 impact (100 km/hr and 25 degrees). The constraining factor was the maximum speed of the FOIL pen- dulum, which is 32.2 km/hr (20 mph). The speed is limited by the maximum height to which the pendulum can be raised. The pendulum impacts, however, are more severe than a full-scale crash test for two primary reasons: (1) the pendulum test section is a more rigid system and (2) the impact energy is distributed over a smaller area. The end fixtures attaching each end of the w-beam test section to the rigid posts allow only minimal longitudinal translation of the rail section in contrast to the full-scale test where the posts surrounding the impact area deflect, reducing the tension in the rail and splices. Sec- ond, in the pendulum test, all the impact energy is absorbed by a single two post (1,905 mm) barrier section. In full-scale tests, however, the lateral energy is primarily distributed over two to four of these 1,905 mm (6 foot) barrier sections. To account for this distributed loading, the pendulum impact speed was reduced to 28.2 km/hr. This impact speed conser- vatively assumes that the lateral impact energy in a full-scale test is absorbed by two 1,905 mm barrier sections, with each section absorbing half the vehicle lateral kinetic energy. The pendulum tests were primarily intended to test the structural adequacy of barrier and damaged barrier sections based on representative lateral forces induced by a perpendi- cular impact to the barrier section. Other relevant barrier per- formance factors, such as wheel snagging, vehicle rollover, and occupant risk, cannot be evaluated using this test methodology. 3.2.2 Test Plan and Barrier Damage Modes A total of 3 pendulum tests were conducted of undamaged two-post barrier section to serve as a baseline against which to compare the impact performance test sections with minor damage. Eleven tests of damaged barriers were conducted to test five different barrier damage modes. In each test, a flaw was artificially introduced into the test article prior to the pendu- lum impact. Table 12 presents a field example of each damage mode and the analogous pendulum test setup. Pendulum tests were conducted either at 32.2 km/hr (20 mph) or 28.2 km/hr (17.5 mph). 3.3 Full-Scale Crash Test Plan This section describes the configuration of a crash test series to evaluate the crash performance of a damaged longitu- dinal barrier. The crash performance of the deflected post/rail, the most common damage mode, was evaluated in a full-scale crash test. The plan was to conduct a full-scale crash test of a large pickup truck (2000P) into a damaged strong-post w-beam barrier at NCHRP Report 350 test level 3 conditions (Test 3-11). Both tests were conducted by MGA Research in Burlington, WI. As shown in Figure 7, this task actually conducted two crash tests. In the first test, a length of guard rail was purpose- fully damaged in a low-severity crash, i.e., a low-speed angled impact. This test produced a realistic profile of minor damage to the barrier before the second test. Finite element modeling predicted that minor deflection could be produced through an impact speed of 47 km/hr (30 mph). In the second test, a Chevy C-2500 pickup truck was impacted into the dam- aged section of the barrier at NCHRP Report 350 conditions (100 km/hr and an impact angle of 25 degrees). The result of the crash performance was a laboratory assessment of the performance of a barrier with minor post rail deflection damage. Instrumentation for these tests included a tri-axial ac- celerometer at the vehicle center of gravity, yaw, roll, and pitch sensors, as well as high-speed photography of the tests. Detailed pre-test and post-test photographs were taken of both the guardrail system and the pickup trucks (Figures 8 and 9). The first lower severity test was documented by a total of six high-speed cameras recording at 500 frames per second and one real-time camera. High-speed cameras were placed alongside the guardrail to obtain both a front and rear over- all view and a single camera was suspended over the impact site to collect an overhead overall view. There were also three high- speed cameras mounted behind the right side of the guardrail at varying distance to record the guardrail behavior. The final real-time camera was located behind the left side of the guardrail and panned to capture the full impact. The second full-scale test was documented with an almost identical setup, except that the front overall high-speed camera was removed. The remaining high-speed cameras and the real-time camera were placed in the same location and recorded at the same rates as for the first crash. This test also served as a validation case for the finite ele- ment model of vehicle-to-guardrail impact. An LS-DYNA simulation of both crash tests was conducted of both tests prior to the tests themselves. The accuracy of the LS-DYNA model was assessed by comparing the results of the simula- tion with the results of the crash tests. Parameters which were compared included vehicle acceleration at the vehicle center of gravity (x, y, and z-axes), vehicle yaw rate, vehicle depar- ture angle from the barrier, and vehicle stability. 3.4 Finite Element Modeling Approach The ideal method to test the safety of strong-post w-beam guardrail with minor damage would be to perform crash tests. However, the cost of evaluating large numbers of different damage modes would be prohibitive. As an alternative ap- proach, finite element modeling was used to evaluate the 16

17 Damage Mode Field Example Pendulum Test Setup Vertical Tear Horizontal Tear Splice Damage Twisted Blockout Missing Blockout Hole in Rail Table 12. Barrier damage modes evaluated in pendulum tests.

crashworthiness of damaged guardrail. Four damage modes were evaluated using finite element modeling: (1) post and rail deflection, (2) missing or damaged posts, (3) post and rail separation, and (4) rail flattening. This chapter describes the development and validation of the model used to evalu- ate each of these damage modes. The LS-DYNA code was used to develop a finite element model of the damaged longitudinal barrier systems. LS-DYNA is used extensively by the roadside safety community to study the impact performance of roadside safety features, and by the automotive industry to study the crashworthiness of passenger vehicles. LS-DYNA is well suited to model the large deformations and high strain rates which are char- acteristic of vehicle crashes into roadside features. It is a general-purpose, explicit finite element program used to ana- lyze the nonlinear dynamic response of three-dimensional structures (LSTC, 2003). All of the LS-DYNA finite element models were run on a SGI Altix parallel system with 120 processors and 512 GB of memory. Each simulation was run using four processors, with multiple simulations being run in parallel to decrease the time needed to complete the study. Each of the finite element models was built using roughly 172,000 elements. Running the simulations on the system described previously, each sim- ulation took approximately one day of real time to calculate 1,000 ms of simulated time. 3.4.1 The Vehicle-Guardrail Model A full-scale finite element model was created from two parts: (1) a model of a 175.8-foot (53.6 meters) length of strong-post w-beam guardrail and (2) a model of a Chevrolet 2500 pickup truck. Each model is described in more detail below. All of the initial conditions for the full scale model were adjusted to match the values specified by NCHRP Report 350, i.e., the vehicle was given an initial velocity of 62.1 mph (100 km/hr) and angle of impact was set to 25 degrees. An example of a com- pleted full-scale model with 6 inches of rail and post deflec- tion is shown in Figure 10. 3.4.2 Strong-Post W-Beam Guardrail Model This research program focused on the modified G4 (1S) strong-post w-beam guardrail that uses steel posts with plastic 18 Crash Test 1 (30 mph) “dents” the rail Crash Test 2 (62 mph) - second impact to the “dent” 25° 2000 kg Pickup 60 mph 25° 2000 kg Pickup 30 mph Figure 7. Full-scale crash testing plan of minor post/rail deflection. Figure 8. Vehicle orientation prior to first MGA crash test. Figure 9. Guardrail for MGA crash tests. Figure 10. Simulated guardrail with rail and post deflection.

blockouts. A guardrail model with steel posts was selected because the steel posts represent the worst case scenario for both snagging of the vehicle tires during impact and the devel- opment of localized stress concentrations on the edges of the post flanges. While the results using a steel post system will be conservative, it was felt better to err on the side of caution than to allow a borderline hazardous condition to be consid- ered an acceptable amount of damage. The basic modified steel strong-post w-beam guardrail model was a publicly available model from the National Crash Analysis Center (NCAC) finite element library (NCAC, 2009a). The basic guardrail model used for this study is shown in Fig- ure 11. The model was designed to be used with the LS-DYNA finite element simulation software (LSTC, 2003). The guardrail system was 53.6 meters (175.8 feet) in length from end to end with 29 posts. Routed plastic blockouts were used instead of wood blockouts. The soil supporting the guardrail system was modeled as individual buckets around each post, rather than as a continuum body. Each steel post was embedded in a cylin- drical volume of soil 2.1 meters (6.9 feet) deep and 1.6 meters (5.25 feet) in diameter. Since the vehicle and guardrail models selected for use in this research were validated against test data, there was little need to make changes to the models. The only alteration to the guardrail model was an increase in the stiffness of the springs holding the splice bolts together. The increase in stiff- ness from 66.5 to 2,400 kN (15 to 540 kip) was needed to keep the splice bolts from unrealistically separating during impact. The increase in stiffness reflected the bolt strength used in a model developed for a study on guardrails encased in paved strips (Bligh et al., 2004). 3.4.3 Pickup Truck Model As a test vehicle, the finite element simulations used the detailed model of a 1994 Chevrolet 2500 pickup truck avail- able from the NCAC library. Specifically, the simulations used Version 0.7, published by the NCAC finite element library on November 3, 2008 (NCAC, 2009b). Like the guardrail model, this vehicle model was designed to be used with the LS-DYNA finite element solver. The vehicle is shown below in Figure 12. The detailed Chevrolet 2500 pickup model was selected for a number of reasons. First, the model was already subjected to a thorough validation effort to ensure the fidelity of the suspen- sion of structural stiffness (NCAC, 2009a). The detailed model also incorporates many interior parts that would not be pres- ent in a reduced model, such as the seating, steering column, bearings, fuel tank, and battery. The higher mesh density for the detailed pickup model also improved the accuracy and contact stability during simulation. A limitation of the finite element model of the Chevrolet 2500 was that the dimensions of the vehicle were fixed. Most real pickup trucks have adjustable suspensions, which allow the front and rear bumper height of the vehicle to vary by as much as 100 mm (3.9 inches). However, even changes of a few centimeters in the relative height of the vehicle and guardrail have had been shown to have dramatic effects on the crash test results (Marzougui et al., 2007). The success or failure of a crash test can depend greatly on the relative height of the vehicle and guardrail (Marzougui et al., 2007). It was critical that the finite element vehicle model match the recorded dimensions of the real test vehicles as closely as possible. The three crash tests that were used for this study had drastically different bumper heights, as shown in Table 13. A modified version of the original finite element vehicle was developed to match these alternate dimensions. The majority of the simulations in this study were performed with the vehicle matching the dimensions for the Texas Trans- portation Institute (TTI) crash test. 3.4.4 Matrix of Finite Element Simulations Table 14 shows the finite element simulation matrix. In each case, a flaw was artificially introduced into the basic guardrail model prior to impact. 19 Figure 11. The NCAC strong-post w-beam guardrail model. Figure 12. The NCAC finite element model of a 1994 Chevrolet 2500 pickup truck.

20 Dimension Original Chevrolet 2500 for TTI Test Chevrolet 2500 for UNL Test Chevrolet 2500 for MGA Test Width 195.4 cm 195.4 cm 195.5 cm Length 565.5 cm 565.4 cm 565.5 cm Height 179.2 cm 182.9 cm 185.4 cm Front Bumper height 63.6 cm 60.3 cm 68.1 cm Rear Bumper height 70.6 cm 79.9 cm 76.5 cm Tire Diameter 73.0 cm 73.0 cm 73.0 cm Weight 2013 kg 2011 kg 2014 kg Table 13. Dimensions of finite element models of the Chevrolet 2500 pickup truck. Damage Mode Field Example FE Model Rail and Post Deflection Missing Post Separated Rail / Post Rail Flattening Table 14. Barrier damage modes evaluated through finite element modeling. 3.5 Validation of the Finite Element Models Proper validation of the finite element models is crucial to the accuracy of the simulations. The validation plan for the finite element simulations was as follows: (1) validation of cou- pled vehicle-roadside hardware models, (2) validation against full-scale crash tests involving damaged barrier, and (3) vali- dation against component tests conducted with a pendulum impact rig. In each case, the finite element models were able to faithfully reproduce the corresponding impact experiment. Details of the validation studies are provided in the appendices.

3.5.1 Undamaged Barrier Full-Scale Crash Test Validation The most crucial form of validation was the validation of LS-DYNA models of the coupled vehicle-longitudinal barrier systems. To validate the model, the research team constructed an LS-DYNA model of an NCHRP Report 350 crash test of the subject barrier under impact loading. The crash test that was selected for this purpose was a test performed by the TTI to demonstrate the crash performance of the modified G4 (1S) guardrail. The crash test was a success, with the vehicle being redirected away from the guardrail. The occupant impact velocity and ridedown acceleration were well below the rec- ommended values of 9 m/s (20.1 mph) and 15 G, respectively. The damage to the guardrail was considered to be moderate, with approximately 1 meter (3.3 feet) of dynamic deflection and 0.7 meters (2.3 feet) of static deflection recorded. Because this was a validation simulation, there was no need to induce any pre-existing damage in the guardrail. Thus, the finite element model of the vehicle and guardrail was unmodified. The finite element vehicle was given ini- tial conditions to match the test level 3 criteria i.e., an ini- tial velocity of 100 kph (62.1 mph) and an impact angle of 25 degrees. This varied slightly from the real test, for which the initial speed was 101.5 kph (63 mph) at 25.5 degrees. The model was run as is for 1,000 ms and compared to the documented crash test results. The crash test results were compared with the structural impact response of the simulated vehicle-barrier system. The model was able to reproduce maximum dynamic and per- manent rail deflection, vehicle exit conditions (exit speed and angle of the test vehicle), and the occupant injury pa- rameter response (impact velocity and occupant ridedown acceleration as prescribed by NCHRP Report 350). Figure 13 shows the good qualitative comparison between the crash test and simulation. Detailed validation results are contained in the appendices. 3.5.2 Damaged Barrier Full-Scale Crash Test Validation This research program also conducted an NCHRP Report 350-type crash test of a vehicle colliding with a damaged sec- tion of strong-post w-beam barrier. In parallel, an LS-DYNA model of this scenario was constructed and executed. The experimentally measured structural impact response of the vehicle/barrier was compared with the corresponding response from the simulation using the validation methodology used to validate the models against standard NCHRP Report 350 crash tests involving undamaged barrier sections. In each case, the finite element models were able to faithfully reproduce the corresponding impact experiment. 3.5.3 Pendulum Component Test Validation Pendulum tests were conducted to provide additional vali- dation data for the finite element longitudinal barrier models. Damaged two-post sections of barrier were impacted with a 2000 kg concrete impactor. The presumption is that if the finite element model can replicate a pendulum test, this is a neces- sary (but not necessarily sufficient) test of a 100-foot long rail section. The models were able to reproduce barrier response (maximum dynamic deflection and post position vs. time) and pendulum acceleration response. 3.6 Extensions to Other Damage Modes and Barrier Types Comparison of the minor damage catalog in the appendices with the proposed repair guidelines also shows that it was not possible to test all proposed repair guidelines. However, the tests and simulations that were conducted allow us to infer the per- formance of several other damage modes under crash loading. 21 TTI Crash Test 405421-1 Simulation of TTI Crash Test t = 0 ms t = 0 ms t = 120 ms t = 120 ms t = 242 ms t = 240 ms t = 359 ms t = 360 ms t = 491 ms t = 490 ms t = 691 ms t = 690 ms Figure 13. Comparison of undamaged guardrail crash test and simulation.

The following paragraphs provide a summary of those dam- age modes: • Rotten Wood Posts/Blockouts—The proposed guide- lines recommended replacement of rotted wood posts or blockouts. Although the research team did not simulate or test this directly, the effect of a rotted post or blockout would be the same as a missing post or blockout—a con- dition which was evaluated. • Steel Post Torsion—The proposed guidelines recommend repair of barrier systems with posts that have been severely twisted. Although the research team did not simulate or test this directly, the effect of a severely twisted steel post would be similar to a missing post—a condition which was evaluated. • Any Structural Corrosion (hole or section loss)—The proposed guidelines recommend repair of barrier systems which have suffered structural corrosion as opposed to sur- face corrosion of the galvanizing treatment. Although the research team did not simulate or test the effect of structural corrosion directly, the response of a seriously corroded rail was expected to be similar to a rail hole or tear—a condi- tion which was evaluated. • Rail Flattening (vertical dent)—The matrix evaluated length-wise flattening of the rail. This was observed to be a more common occurrence than height-wise flattening or vertical denting of rail. The research team’s recom- mendation was based upon the consensus of current state guidelines. No simulations or tests were planned. • Missing or Loose Bolts—The guidelines proposed that problems with bolts should be corrected. No simulations or tests were planned. • Weak Post W-Beam Systems—Weak post w-beam guide- lines were not evaluated independently of strong-post w-beam guidelines. Crash tests have shown that a primary failure mechanism of weak post w-beam barrier is rail rup- ture (Ray et al., 2001a; 2001b). The results of strong post pendulum tests of vertical tears, horizontal tears, and holes were assumed to apply to weak post systems. Because of the crucial function of the splice in weak-post systems, the pro- posed guidelines do not allow any splice damage or absence of splice bolts. 22

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 656: Criteria for Restoration of Longitudinal Barriers explores the identification of levels of damage and deterioration to longitudinal barriers that require repairs to restore operational performance.

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