National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 13 - Evaluation of Rail Flattening

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Page 66
Suggested Citation:"Chapter 13 - Evaluation of Rail Flattening." 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 13 - Evaluation of Rail Flattening." 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 13 - Evaluation of Rail Flattening." 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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Page 68
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Suggested Citation:"Chapter 13 - Evaluation of Rail Flattening." 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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Page 69
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Suggested Citation:"Chapter 13 - Evaluation of Rail Flattening." 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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Page 70
Page 71
Suggested Citation:"Chapter 13 - Evaluation of Rail Flattening." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 71
Page 72
Suggested Citation:"Chapter 13 - Evaluation of Rail Flattening." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 72
Page 73
Suggested Citation:"Chapter 13 - Evaluation of Rail Flattening." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 73
Page 74
Suggested Citation:"Chapter 13 - Evaluation of Rail Flattening." 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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Page 74

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66 Rail flattening in strong-post w-beam guardrail was a damage mode of concern to many state agencies, ranking just below rail and post deflection. In the field, rail deflection is often associated with collisions at shallow angles or caused by a snowplow rubbing against the rail. Rail flattening was char- acterized by loss of depth in the w-beam rail element, which was often accompanied by rail deflection and post deflection. Concurrent with the loss of depth was an increase in the height of the guardrail, i.e., the upper edge of the guardrail extended higher while the lower edge moved closer to the ground. Fig- ure 55 shows an example of rail flattening caused by a snow- plow, and a finite element model of this damage mode. Rail flattening was of concern for two reasons. First, the loss of depth in the rail reduced the spacing between the strik- ing vehicle and the posts. Thus, rail flattening may increase the risk of vehicle snagging on the posts. Second, the flatten- ing of the rail increases the maximum height and lowers the minimum height of the guardrail, changing the way in which the vehicle interacts with the guardrail system. 13.1 Approach A series of simulations of impacts into flattened strong- post w-beam guardrail were run and compared to the perfor- mance of the undamaged guardrail simulation. The flattening in these simulations varied from 25 to 100 percent. This type of damage commonly occurs in combination with minor rail deflection, but in this study it was considered in isolation. The detailed procedure for inducing rail flattening in the finite ele- ment model is described in the appendices. The complete set of finite element models covered all degrees of flattening between 25 and 100 percent, in increments of 25 percent. These simula- tions are shown in Figure 56. 13.2 Results Each of the flattening simulations was run on the Inferno2 computer system using four processors. Each run required roughly 26 hours per simulation to complete. The results of the simulations at 700 ms are shown in Figure 57. The vehi- cle exit behavior became increasingly unstable as the rail was flattened by greater amounts. Both roll and pitch increased with the amount of flattening. However, the yaw and exit angle decreased with increasing flatness. At 100% flattening, the vehicle was unable to remain upright and rolled to the right after exiting the guardrail. In Table 27, the NCHRP Report 350 test criteria are shown for both the undamaged simulation and all of the flattening simulations. As observed in Figure 57, the roll and pitch were higher and the yaw was lower for all of the flattening simula- tions. The degree of flattening in the guardrail had a strong effect on the exit speed and angle of the vehicle. The increase in exit speed was particularly pronounced at the highest lev- els of flatness, with a 13 kph (8.1 mph) increase in exit speed between 75 and 100 percent flattening. Exit angle showed the opposite behavior, i.e., it decreased with increasing flat- ness, from 14.5 degrees for the undamaged simulation to only 10 degrees for the 100 percent flattened simulation. The deflec- tion of the guardrail, particularly the maximum dynamic deflection, increased along with flattening. The maximum deflection increased by 15.5 percent for a completely flattened rail. All occupant injury metrics, i.e., occupant ridedown accel- eration and occupant impact velocities, were well below the NCHRP Report 350 limits. Figure 58 shows the roll, pitch, and yaw vs. time curves for the undamaged simulation and all of the flattening sim- ulations. As expected, the roll for the 100 percent flattening simulation was the largest. The yaw for all of the flattening simulations peaked in the range of 400–500 ms, after which it started to decline. As the yaw was directly related to the heading of the vehicle, this implied that the vehicle was turn- ing back toward the guardrail after exiting. The opposite sign on the pitch for the 100 percent flattening simulation implied a possibility of vaulting. Figure 59 shows the local vehicle CG velocities for the undamaged and all flattening simulations. All of the simu- C H A P T E R 1 3 Evaluation of Rail Flattening

67 lations showed stable exit velocities. The 50 percent and 75 percent simulations also showed a relatively large amount of lateral skidding and upward motion as the vehicle was exiting the guardrail. This skidding motion was caused by the edge of the vehicle bed catching on a fold in the guardrail near a post, which also contributed to the decrease in yaw. Figure 60 presents approximate damage contours for the guardrail. All of the damage contours were measured starting at post 9 (position = 0) up to post 21 (position = 22860 mm). For all simulations, except the 100 percent flattening simula- tion, the maximum dynamic deflection occurred at 165 ms. At this time, the vehicle was still moving into the guardrail and was just starting to be redirected. The static deflection contours for 25–75 percent flattening were very uneven. This was due to vibrations induced in the rail when the pickup truck bed slapped the guardrail. 13.3 Discussion A full series of simulations, with flattening ranging from 25–100 percent, were run to determine whether rail flatten- ing posed a risk to vehicle and occupant safety. It was found that the vehicle became unstable above 75 percent flattening. Figure 55. Rail flattening—field example vs. finite element model. Field Example (courtesy of Ontario Ministry of Transportation) FE Model Figure 56. Rail flattening simulations before impact: 25% flattening (top left), 50% flattening (top right), 75% flattening (bottom left), and 100% flattening (bottom right).

68 Figure 57. Flattening simulation results at t = 0.7s. 25% flattening (top left), 50% flattening (top right), 75% flattening (bottom left), and 100% flattening (bottom right). Table 27. Results for rail flattening simulations. Un- damaged 25% Flattening 50% Flattening 75% Flattening 100% Flattening Impact Conditions Speed (kph) 100 100 100 100 100 Angle (deg) 25 25 25 25 25 Exit Conditions Speed (kph) 53 56 59 60 73 Angle (deg) 14.5 12.1 9.1 10.7 10.0 Occupant Impact Velocity X (m/s) 7.51 7.3 7.5 6.8 5.9 Impact Velocity Y (m/s) 5.54 5.5 5.7 5.7 5.7 Ridedown X (G) -11.77 -14.7 -10.9 -14.1 -7.4 Ridedown Y (G) -12.27 11.4 -11.6 -12.3 -11.4 50 ms Average X (G) -6.68 -5.6 -6.0 -6.1 -5.4 50 ms Average Y (G) -6.82 -6.6 -7.1 -6.9 -7.2 50 ms Average Z (G) -3.85 3.3 -3.7 -4.1 2.6 Guardrail Deflections Dynamic (m) 0.69 0.74 0.75 0.75 0.80 Static (m) 0.55 0.57 0.44 0.43 0.62 Vehicle Rotations Max Roll (deg) -14.4 -15.8 -16.7 15.2 Roll Max Pitch (deg) -9.9 -12.3 20.2 -20.7 > 18 Max Yaw (deg) 40.3 38.3 38.0 38.0 33.5 At 100 percent flattening, the vehicle rolled over as it exited the guardrail. A key factor in the exit behavior of the vehicle was the mo- tion of the front left tire. Figure 61 shows the vertical displace- ment of the center of the front left and rear left tires over time, relative to each tire’s original position at the start of the simu- lation. The simulation of 100% flattening showed the greatest displacement of the tire, reaching over 1600 mm (63 inches) by the end of the simulation. Such a large change in the vertical position of the vehicle can be an indicator of vaulting. However, in this case the vehicle was redirected before this could occur. The undamaged simulation showed the lowest amount of ver- tical tire motion, which was an indicator of vehicle stability. In the plot of the front left tire displacement for the undamaged simulation, the time at which the wheel struck and rolled over a post can be easily discerned by the peaks in the displacement.

-50 -30 -10 10 30 50 70 90 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) A ng ul ar D is pl ac em en t ( de gr ee s) X - Roll Y - Pitch Z - Yaw -50 -30 -10 10 30 50 70 90 Time (s) A ng ul ar D is pl ac em en t ( de gr ee s) X - Roll Y - Pitch Z - Yaw -50 -30 -10 10 30 50 70 90 Time (s) A ng ul ar D is pl ac em en t ( de gr ee s) X - Roll Y - Pitch Z - Yaw 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -50 -30 -10 10 30 50 70 90 Time (s) A ng ul ar D is pl ac em en t ( de gr ee s) X - Roll Y - Pitch Z - Yaw -50 -30 -10 10 30 50 70 90 Time (s) A ng ul ar D is pl ac em en t ( de gr ee s) X - Roll Y - Pitch Z - Yaw 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Figure 58. Roll, pitch, and yaw curves for flattening simulations: undamaged (top), 25% flattening (middle left), 50% flattening (middle right), 75% flattening (lower left), and 100% flattening simulations (lower right). 69

70 Figure 59. Velocity curves for flattening simulations: undamaged (top), 25% flattening (middle left), 50% flattening (middle right), 75% flattening (lower left), and 100% flattening simulations (lower right). -20 0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity -20 0 20 40 60 80 100 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity -20 0 20 40 60 80 100 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -20 0 20 40 60 80 100 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity -20 0 20 40 60 80 100 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

71 Figure 60. Guardrail damage contours for flattening simulations: undamaged (top), 25% flattening (middle left), 50% flattening (middle right), 75% flattening (lower left), and 100% flattening simulations (lower right). 0 200 400 600 800 1000 1200 0 5000 10000 15000 20000 25000 0 5000 10000 15000 20000 25000 0 5000 10000 15000 20000 25000 Guardrail Lengthwise Position (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.165s) 0 200 400 600 800 1000 1200 Guardrail Lengthwise Position (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.165s) 0 200 400 600 800 1000 1200 Guardrail Lengthwise Position (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.165s) 0 5000 10000 15000 20000 25000 0 200 400 600 800 1000 1200 Guardrail Lengthwise Position (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.165s) 0 200 400 600 800 1000 1200 05 0001 0000 150002 0000 25000 Guardrail Lengthwise Position (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.295s)

72 Figure 61. Displacement of vehicle tires for the flattening simulations: front left tire (left) and rear left tire (right). -200 0 200 400 600 800 1000 1200 1400 1600 1800 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) Ve rt ic al D is pl ac em en t ( mm ) Undamaged 25% Flattening 50% Flattening 75% Flattening 100% Flattening 0 1.20.2 0.4 0.6 0.8 1 -200 0 200 400 600 800 1000 1200 1400 1600 1800 Time (s) Ve rt ic al D is pl ac em en t ( mm ) Undamaged 25% Flattening 50% Flattening 75% Flattening 100% Flattening Figure 62. Height of the rails relative to the vehicle: undamaged (left) and 100% flattening (right). The vehicle instability at greater than 75 percent flattening was caused by the vehicle riding up the flattened rail. Both the flatness of the rail and the lower bottom height of the rail were contributors to the rollover. As shown in Figure 62, a maximally flattened rail extends both higher and lower than an undeformed rail would and also presented a much smoother surface. In the undamaged simulation, because of the height of the rails, the collision force was concentrated on the front of the fender, leading to extensive crush on the front left corner of the vehicle. This deformation allowed the top half of the rail to penetrate the space above the front left tire. The presence of the rail above the tire provided a downward force that pre- vented the tire from moving upward. The upward motion caused by the left tires hitting the post bases was counteracted by the downward force exerted by the rail. When the rail was 100% flattened, a different behavior was observed. Because of the higher top height of the rail and the flatness of the surface, the force of the collision was distrib- uted over a larger portion of the fender. These factors pre- vented the rail from penetrating the space above the tire. The lower bottom height of the rails also presented a problem. As the tire was forced upward by contact with the posts, the ele- vation of the tire increased so that the majority of the tire was on or above the rails. This, combined with a slight outward slope in the rail caused by the crash damage, provided a ramp for the tire to ride up. The increase in rail height, which was concentrated on the left side of the vehicle, imparted a rolling motion that the vehicle was unable to recover from. The vehicle exit speed also varied by the degree of flatten- ing. For the undamaged simulation the exit speed was 53 kph, whereas for the 100 percent flattening simulation the exit

Figure 63. Simulation of 75% flattened guardrail leaning back by 10 degrees. Vehicle before impact on the left, and after the impact (t = 0.7s) on the right. speed was 73 kph. It was believed that by flattening the rails before impact, the ability of the guardrail to absorb kinetic energy was being reduced. To check if this was the case, the energy absorbed by the vehicle and guardrail was broken down by component. In the 100 percent flattening simulation, the guardrail absorbed roughly 40 kJ less than, or 83 percent of, the energy that was absorbed by the undamaged simulation. When bro- ken down even further, it was found that the rails actually absorbed about 45 kJ less energy, but the other components of the guardrail absorbed 5 kJ more energy, resulting in the net drop in energy absorption of 40 kJ. The components of the guardrail that absorbed more energy were the posts and block- outs, as the flattening of the guardrail allowed the vehicle to engage these components more easily. Since it was believed that the flattened rails created a ramp- like surface, a supplementary simulation was performed to examine how the angle of the guardrail would affect the per- formance. The finite element model of 75 percent flattened rail was modified by bending the posts and rails in the area of contact backwards. This resulted in an 80-degree angle between the post and ground line rather than the standard 90 degrees, as shown in Figure 63. The slight incline in the rails was sufficient to cause the vehicle to both vault and roll. From these results, it was evident that the angle of the guard- rail, whether caused by damage or pre-existing because the ground was sloped, could drastically alter the outcome of a crash. However, the same incline in an otherwise undamaged guardrail had little effect on the outcome of the simulation. In the future, the full effect of combined incline and flatten- ing should be examined in more detail. 13.4 Recommendation A series of finite element simulations of impacts into flattened strong-post w-beam guardrail were run and com- pared to the performance of the undamaged guardrail sim- ulation. The flattening in these simulations varied from 25 percent to 100 percent. The following observations were noted: • Vehicle roll and pitch increased with increasing degrees of flatness. The vehicle became unstable once the flatten- ing reached 75 percent. At 100 percent flattening, the vehi- cle rolled as it exited from the guardrail. Note that these simulations were conducted for perfectly upright posts. Based on field inspections of damaged barriers, the research team has observed that rail flattening almost always occurs in tandem with some degree off post and rail deflection. Any incline in the post would exacerbate the tendency for vehicle rollover or instability. Therefore it is recom- mended that all guardrails for which there is 50 percent or greater flattening be repaired as soon as possible due to a greatly increased risk of vaulting and rollover. • In any situations where there is a hazardous object directly behind the guardrail, the damage should be repaired imme- diately because even a small amount of rail flattening increased the maximum deflection of the guardrail by roughly 10 percent. One observation from the field inspections of damaged bar- riers was that it was difficult to quantify the amount of rail flat- tening by direct measurement of the w-beam cross section. As an alternative method of determining the amount of rail flat- tening, the research team is proposing a method where the maintenance personnel can measure the maximum section width of the flattened w-beam cross section, a much easier measurement to obtain. Based on finite element simulations of flattened w-beam barriers, the research team has correlated the maximum deformed cross section height to the approx- imate portion of rail flattening. Fifty percent flattening cor- responds to a growth in section width from 12 inches for undamaged rail to 18 inches. In the guidelines, the 50 percent flattening limit is prescribed as a section width of 18 inches or greater. See Exhibit 10.0 for recommendations for rail fatten- ing repair. 73

74 Exhibit 10.0. Recommendation for rail flattening repair. Damage Mode Repair Threshold Priority for damage above the threshold Rail cross-section height more than 17 in. (such as may occur if rail is flattened) Rail Flattening Rail cross-section height less than 9 in. (such as a dent to top edge) Medium

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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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