Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide (2022)

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11   The project team used extensive simulation analysis to develop the concepts for an improved barrier design. Appendix C in NCHRP Web-Only Document 334 provides a complete descrip- tion of the simulation analysis and results. This chapter provides a summary of this effort. 2.1 Design Concept Process The project team developed concepts for improving barrier design for short-radius situations by discussing objectives to serve as a common means to compare concepts, reviewing existing designs, and then conceptualizing new designs that capitalize on integrating current designs. The following section describes this process. 1. Set design objectives. The ideal system for short-radius situations must – Effectively contain or redirect errant vehicles approaching at various angles from the main roadway at speeds of 60 mph; – Accommodate large and small vehicles; – Have a reasonable cost, as agencies face many of these situations on low-volume roads; – Be relatively simple to install; – Be relatively easy to maintain; – Be adaptable to varying distances from the end of the bridge rail and angle and location of intersecting road; and – Perform effectively when used to shield any steep slopes behind the rail, as typical of terrain adjacent to bridges. 2. Review existing designs. The project team undertook a comprehensive review to determine the types of designs that have been developed and deployed to enhance safety for situations where there is restricted space to provide adequate shielding for the rigid bridge end and the area adjacent to it. At the outset of this research, only one short-radius design had been suc- cessfully tested to MASH TL-3. 3. Design conceptualization. The project team proposed ideas for new short-radius barrier designs in the third step. Each concept had to meet as many design objectives as possible. These concepts were then reviewed to determine the likelihood that the design would meet the requirements and address shortcomings of existing designs. The team used first principles analysis to evaluate each, which involved applying the basic laws of physics to examine the potential for a design concept to be feasible. This type of analysis allowed the team to determine preliminary costs and estimate the feasibility for each design alternative. These estimates were then used to filter out the more costly and less feasible alternatives. Successive reviews added further details, allowing the designs to achieve a level of detail that could be modeled. These steps led to the formulation of a few preliminary design concepts, as well as others that could be developed later. These design concepts were subsequently subjected to iterations of C H A P T E R 2 Conceptual Design for Improved Short-Radius Barrier

12 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide simulation to understand the device behavior for various types of impacts and to get a sense of crashworthiness for each concept. Those designs having the greatest merit were examined further to consider other modifications that optimized the design or allowed variations in deployment conditions. The following two concepts, Midwest Guardrail System (MGS) and hybrid crash cushion, appeared most promising. • MGS-based concept. The first design is similar to the curved beam and post designs previously tested and deemed acceptable at TL-2 impact speeds approximating 45 mph. The project team reviewed these designs and analyzed the failure modes to identify design changes likely to result in satisfactory MASH TL-3 performance. Basic changes from previous designs included using 10-gauge W-beam rail and the MGS barrier design, supplemented with steel cables to provide needed tensile strength and small-car containment. The advantage of this design is that it can be fabricated using generic, standard barrier hardware. • Hybrid crash cushion concept. The second design consists of a TL-3 crash cushion con- nected directly to a bridge end. This crash cushion would be a currently accepted proprietary product (to expedite development of the short-radius barrier) that would effectively shield the bridge end and some portion of the feature crossed by the bridge. A set of four wire ropes would connect to the back (field) side of the attenuator from the bridge end to its nose, and then parallel to the intersecting side road, to shield the remainder of the feature under the structure that lies within the minimum design clear zone. The hybrid design incorporates two different types of barriers: an energy-absorbing crash cushion connected to the bridge rail and a cable barrier system that acts as a net to shield the feature under a structure (e.g., a waterway or road). This design would incorporate an existing, narrow, crashworthy attenuator (all of which were proprietary) and a generic cable system. The cable system would provide a lower-cost barrier that could be adapted by adding cables and cable connectors to capture or redirect errant vehicles. However, concerns were raised about the use of a propriety device under an NCHRP project; as a result, this concept was dropped from further consideration. 2.2 Finite Element Models The project team used vehicle FE models, developed by the Center for Collision Safety and Analysis staff under an FHWA contract, to conduct simulations. These models met the require- ments for test vehicles under MASH. These models had been extensively verified and validated, as reported in previous studies. They had been proven stable in a large number of simulation efforts by the staff and others. Table 2.1 summarizes details of these models. The project team used FE models to develop the various design concepts. These designs were based upon integrating various standard roadside hardware elements. FE models of these com- ponents exist and have been used extensively in other roadside safety studies, so there was a high level of confidence that they were accurate representations of these elements. Integrating FE models was carefully undertaken and their functionality was continually scrutinized by com- parisons of metrics, as well the graphic representations of the simulated events, to ensure that they were providing consistent results. These barrier design concepts were modeled as a system with minimum test lengths and configurations to meet requirements. 2.3 Impact Performance There are many ways and degrees of detail that could be used to assess barrier performance. The primary impact evaluation was based upon the MASH requirements—under MASH, there was no specific crash-test matrix for a short-radius barrier design, but previous evaluations used

Conceptual Design for Improved Short-Radius Barrier 13   some of the tests specified for non-gating crash cushions. This was appropriate, as the most critical function of the short-radius barrier is to address the most critical type of crash—an end-on impact into the bridge rail—and a crash into the radius portion of the design. Therefore, it was assumed that the previous tests would be considered acceptable for these efforts. The primary MASH critical tests for non-gating crash cushions included the following: • Test 3-31. Head-on impact by the 2270P vehicle at 62 mph and 0 degrees; • Test 3-32. 15 degree impact into the curved section of rail by the 1100C vehicle at 62 mph; • Test 3-33. 15 degree impact into the curved section of rail by the 2270P vehicle at 62 mph; and • Test 3-35. 25 degree impact at length of need (LON)/critical impact point (CIP) by the 2270P vehicle at 62 mph. These were considered the most difficult tests to pass and thus were the focus of efforts to develop conceptual designs. The primary test vehicles under MASH included • 2270P pickup truck, the 2007 Chevrolet Silverado model; and • 1100C small car, the 2010 Toyota Yaris model. Table 2.2 summarizes evaluation criteria for these MASH tests. These criteria were used to evaluate barrier performance on the basis of structural adequacy and occupant risk factors. 2.4 Deeper Assessment of MGS-Based Concepts The following section summarizes the simulation analysis conducted on the MGS-based concept, which uses a higher W-beam rail (31 in. rail height) and splices between posts. These features have been found to improve performance but have not been tried in short-radius barriers. Simulations were undertaken to evaluate this conceptual design, and the results were found to be promising. Table 2.3 summarizes results from five simulation iterations of this conceptual design. 2.4.1 MGS-Based Concept—Simulation Iteration 5 Based on feedback from the project review panel, the project team made additional changes to the design, including using standard 6 ft long steel posts instead of wood posts. Also, this design Table 2.1. Vehicle FE models representing MASH test vehicles. Description Vehicle Image Legends 2010 Toyota Yaris • Weight: 1,100 kg (2,420 lb) • CG: 1004 mm rear, 569 mm high • Model Parameters: Parts 771, Nodes 998,218, Elements 974,348 • Features: FD, CD, SD, IM • Validations: FF, OF, MDB, SI, IP, SP, SC, ST Validations Legend • FF: Full Frontal • OF: Offset Frontal • MDB: Modified Deformable Barrier • SI: Side Impact • IP: Inertial Parameters • SP: Spring Response • SC: Suspension Calibration • ST: Suspension Tests (full-scale) Features Legend • FD: Fine Detail version • CD: Coarse Detail version • SD: Suspension Details • IM: Interior Modeled 2007 Chevrolet Silverado Pickup Truck • Weight: 2,270 kg (5,000 lb) • CG: 736 mm (28.8 in.) • Model Parameters: Parts-606, Nodes-261,892, Elements-251,241 • Features: FD, CD, SD, IM • Validations: FF, IP, SP, SC, ST

14 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide Simulation Iteration No. System MASH TL-3 Criteria Test 3-33 Test 3-32 Test 3-31 Test 3-35 1 • MGS rail with 12 in. block • PennDOT Transition, W-beam and rub rail Did not meet Met Did not meet Did not meet 2 • MGS rail with 12 in. block • Missouri Transition, 10-gauge Thrie-beam • Cable anchor at LON point Met Did not meet Did not meet Did not meet 3 • Use 10-gauge for entire system • Change to wood post • Add a post downstream from radius • Attach cable to a separate ground anchor in the radius • Run a cable beneath W-beam Met Did not meet Did not meet Met 4 • Add posts in curved section • Lower cable placed beneath W- beam Met Marginal Met Met 5 • Replace wood post with 6 ft long steel post. • Use 8 in. blockouts • Add cable in center valley of rail Met Met Met Met Table 2.3. Summary of simulation iterations of MGS-based concept. Table 2.2. MASH crashworthiness evaluation criteria for simulation analyses. Evaluation Category Requirement (MASH Tests 3-31, 3-32, 3-33, and 3-35) Structural Adequacy A. Test article should contain and redirect the vehicle or bring the vehicle to a controlled stop; the vehicle should not penetrate, underride, or override the installation, although controlled lateral deflection of the test article is acceptable. Occupant Risk D. Detached elements, fragments, or other debris from the test article should not penetrate or show potential for penetrating the occupant compartment, or present an undue hazard to other traffic, pedestrians, or personnel in a work zone. Deformation of, or intrusions into, the occupant compartment should not exceed limits set forth in Section 5.3 and Appendix E of MASH (roof ≤4.0 in.; windshield = ≤3.0 in.; side windows = no shattering by test article structural member; wheel/foot well/toe pan ≤9.0 in.; forward of A-pillar ≤12.0 in.; front side door area above seat ≤9.0 in.; front side door below seat ≤12.0 in.; floor pan/transmission tunnel area ≤12.0 in.). F. The vehicle should remain upright during and after collision. The maximum roll and pitch angles are not to exceed 75 degrees. H. Occupant impact velocities should satisfy the following: • Longitudinal and Lateral Occupant Impact Velocity • Preferred: 30 ft/s (9.1 m/s) Maximum: 40 ft/s (12.2 m/s) I. Occupant ridedown accelerations should satisfy the following: • Longitudinal and Lateral Occupant Ridedown Accelerations • Preferred: 15.0 G Maximum: 20.49 G Vehicle Trajectory For redirective devices, document that the vehicle was smoothly redirected and exited the barrier within the “exit box” criteria (not less than 32.8 ft). Also report vehicle-rebound distance and velocity for crash cushions.

Conceptual Design for Improved Short-Radius Barrier 15   incorporates 8 in. deep blockouts instead of the 12 in. depth and adds a cable along the center of the W-beam, with one end connected to the back of the W-beam near a secondary road anchor and the other end connected to the back of the Thrie-beam near the bridge end. Figure 2.1 shows details from the updated design. This fifth design concept was again assessed using simulations. The model simulated MASH Tests 3-31, 3-32, 3-33, and 3-35. The following sections summarize results from these simulations. Figure 2.1. Views of MGS-based short-radius barrier concept—Iteration 5.

16 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide MASH Test 3-31 For MASH test designation 3-31, a 2270P pickup truck impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 0 degree angle, with the vehicle centerline aligned with the front face of the bridge end. Figure 2.2 presents sequential pictures from the simulation. The simulation results indicate that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-31. MASH Test 3-32 For MASH test designation 3-32, an 1100C small car impacted the short-radius barrier at a speed of 62 mph. The vehicle struck the barrier at a 15 degree angle at the center post of the curved section of the barrier. Figure 2.3 presents sequential pictures from the simulation. The simulation results indicate that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-32. MASH Test 3-33 For MASH test designation 3-33, a 2270P pickup truck impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 15 degree angle at the center post of the curved section of the barrier. Figure 2.4 presents sequential pictures from the simulation. The simulation results indicate that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-33. MASH Test 3-35 For MASH test designation 3-35, a 2270P pickup truck impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 25 degree angle. The vehicle impacted the barrier at the LON (at the end of the curved section). Figure 2.5 presents sequential pictures t = 0 ms t = 240 ms t = 160 ms t = 320 ms Figure 2.2. Time sequence of simulation results—Test 3-31, Iteration 5.

Conceptual Design for Improved Short-Radius Barrier 17   from the simulation. The simulation results indicate that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-35. Note: For the short-radius design, the CIP and the LON are essentially at the same point. 2.4.2 Simulation Analyses with 12 in. Wood Blockouts Simulations were conducted to assess the performance of the Iteration 5 design using 12 in. deep blockouts instead of the 8 in. deep ones. The barrier was modified so all blockouts, includ- ing the ones used at the Thrie-beam rails, were changed to 12 in. in depth. The height and width of the blockouts were kept the same. No other modifications were made to the design. Figure 2.6 shows details of the model. The project team conducted simulations of the four critical impacts (Tests 3-31, 3-32, 3-33, and 3-35) and analyzed the results. The next four sections summarize results from the evaluations of this design. MASH Test 3-31 For MASH test designation 3-31, a 2270P pickup truck impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 0 degree angle, with the vehicle centerline aligned with the front face of the bridge end. Figure 2.7 provides sequential pictures t = 0 ms t = 160 ms t = 80 ms t = 320 ms Figure 2.3. Time sequence of simulation results—Test 3-32, Iteration 5.

18 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide from the simulation. The simulation results indicate that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-31. MASH Test 3-32 For MASH test designation 3-32, a 1100C small car impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 15 degree angle at the center post of the curved barrier section. Figure 2.8 provides sequential pictures from the simulation. Simula- tion results indicate that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-32. MASH Test 3-33 For MASH test designation 3-33, a 2270P pickup truck impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 15 degree angle at the center post of the curved barrier section. Figure 2.9 presents sequential pictures from the simulation. The simulation results indicated that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-33. MASH Test 3-35 For MASH test designation 3-35, a 2270P pickup truck impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 25 degree angle. The vehicle impacted t = 0 ms t = 400 ms t = 200 ms t = 800 ms Figure 2.4. Time sequence of simulation results—Test 3-33, Iteration 5.

Conceptual Design for Improved Short-Radius Barrier 19   t = 0 ms t = 100 ms t = 150 ms t = 250 ms Figure 2.5. Time sequence of simulation results—Test 3-35, Iteration 5. Figure 2.6. Views of the MGS-based short-radius barrier concept with 12 in. blockouts.

20 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide t = 0 ms t = 80 ms t = 160 ms t = 240 ms t = 320 ms t = 400 ms Figure 2.7. Time sequence of simulation results—Test 3-31, with 12 in. wood blockouts. t = 0 ms t = 80 ms t = 160 ms t = 240 ms t = 320 ms t = 400 ms Figure 2.8. Time sequence of simulation results—Test 3-32, with 12 in. wood blockouts.

Conceptual Design for Improved Short-Radius Barrier 21   the barrier at the LON point (at the end of the curved section). Figure 2.10 presents sequential pictures from the simulation. The simulation results indicate that the short-radius barrier met all MASH evaluation criteria for impact configuration 3-35. 2.4.3 Simulation Analyses on Slopes The project team conducted simulations to assess the performance of the short-radius bar- rier with a sloped section directly behind the curved portion of the barrier. The team created models with different slopes behind the barrier, including 6H:1V, 4H:1V, 3H:1V, and 2H:1V. Figure 2.11 shows the models with these sloped profiles. The slopes were started 2 ft behind the short-radius barrier, which is the minimum allowable distance for strong post barriers. The team also conducted simulations with slopes starting at 3 ft and 4 ft behind the posts. The simulations were carried out using two vehicle models: the 1100C vehicle and the 2270P pickup truck. The impact configurations selected for these simulations were those of Test 3-32 (1100C vehicle at 15 degrees into the center of the curved section of the barrier) and Test 3-33 (2270P vehicle at 15 degrees into the center of the curved section of the barrier). The team considered these two test configurations most affected by the slope behind the barrier. Table 2.4 presents results from the simulations. The next section summarizes select simulations with slope. MASH Test 3-33—6H:1V Slope, 2 ft Flat Section For MASH test designation 3-33, a 2270P pickup truck impacted the short-radius barrier at a target speed of 62 mph. The vehicle struck the barrier at a 15 degree angle at the center post of the curved barrier section. The back slope in this simulation was 6H:1V and started 2 ft behind the short-radius barrier. Figure 2.12 presents sequential pictures from the simulation. Simulation results indicate that the short-radius barrier met all MASH evaluation criteria for this impact configuration. t = 0 ms t = 150 ms t = 300 ms t = 450 ms t = 600 ms t = 750 ms Figure 2.9. Time sequence of simulation results—Test 3-33, with 12 in. wood blockouts.

22 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide t = 0 ms t = 80 ms t = 160 ms t = 240 ms t = 320 ms t = 400 ms Figure 2.10. Time sequence of simulation results—Test 3-35, with 12 in. wood blockouts. 6H:1V Slope, 2 ft Flat Section 2H:1V Slope, 2 ft Flat Section Figure 2.11. Short-radius model with varied back slope profiles.

Conceptual Design for Improved Short-Radius Barrier 23   Figure 2.12. Time sequence of simulation results—Test 3-33, with 6H:1V slope, 2 ft flat section. t = 0 ms t = 600 ms t = 300 ms t = 900 ms Flat Section 1100C Small Car 2270P Pickup Truck 2 ft 3 ft 4 ft 2 ft 3 ft 4 ft Back Slope 6H:1V Pass Pass Pass Pass Pass Pass 4H:1V Pass Pass Pass Fail Marginal Marginal 3H:1V Pass Pass Pass Fail Fail Marginal 2H:1V Pass Pass Pass Fail Fail Fail Table 2.4. Evaluation summary based on simulation results.

24 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide 2.4.4 Simulation Analyses on 2H:1V Slope at Lower Speed At the request of the review panel, the project team conducted further analysis with a 2H:1V slope behind the barrier. The goal in this analysis was to determine the maximum speed at which the 2270P truck can impact the final design system on a 2H:1V slope and still meet all MASH criteria. The sloped section was placed 2 ft behind the barrier posts. Simulation setup consisted of a 2270P vehicle impacting the short-radius barrier at the nose of the curved railing, equivalent to MASH Test 3-33 impact, at both impact angles of 15 degrees and 25 degrees relative to the primary road. Simulations at different speeds (70, 80, 90, and 100 km/hr) were conducted to estimate the maximum vehicle speed that could be used while still meeting the criteria recom- mended by MASH. Figure 2.13 shows sequential images from three simulations carried out at a 15 degree impact angle; Figure 2.14 shows the simulations at a 25 degree impact angle. Overall, the simulations show no significant difference between the 15 degree and 25 degree cases. The 25 degree cases had a slightly higher penetration but that did not have a significant effect on the barrier performance. Based on this finding, the project team decided to use 15 degrees for the actual test, which was similar to the flat terrain test and conformed to the 15 degree impact angle depicted in MASH for Test 3-33. In comparing the different speed cases, the project team observed that the short-radius barrier met all criteria at speeds of 70 km/hr and 80 km/hr. At the higher speeds, 90 km/hr and 100 km/hr, the vehicle was less stable, and significant roll and pitch were observed. Based on these results, the team decided to run this test at an impact speed of 80 km/hr. 2.5 Simulation Conclusions The simulation process allowed the project team to incrementally assess a complex barrier system, leading to a candidate design that seemed likely to meet physical performance require- ments and crashworthiness metrics. The final design was deemed ready for the final develop- ment stage—full-scale crash testing—which Chapter 3 describes.

Conceptual Design for Improved Short-Radius Barrier 25   70 km/hr Simulation 80 km/hr Simulation 90 km/hr Simulation 0.00 s 0.00 s 0.00 s 0.25 s 0.25 s 0.25 s 0.50 s 0.50 s 0.50 s 0.75 s 0.75 s 0.75 s 1.00 s 1.00 s 1.00 s Figure 2.13. Time sequence of simulation results—2H:1V slope, 2 ft flat section, 15 degrees.

26 Roadside Barrier Designs near Bridge Rail Ends with Restricted Rights-of-Way: A Guide 70 km/hr Simulation 80 km/hr Simulation 90 km/hr Simulation 0.00 s 0.00 s 0.00 s 0.25 s 0.25 s 0.25 s 0.50 s 0.50 s 0.50 s 0.75 s 0.75 s 0.75 s 1.00 s 1.00 s 1.00 s Figure 2.14. Time sequence of simulation results—2H:1V slope, 2 ft flat section, 25 degrees.