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66 CHAPTER 13 Evaluation of Rail Flattening Rail flattening in strong-post w-beam guardrail was a roughly 26 hours per simulation to complete. The results of damage mode of concern to many state agencies, ranking the simulations at 700 ms are shown in Figure 57. The vehi- just below rail and post deflection. In the field, rail deflection is cle exit behavior became increasingly unstable as the rail was often associated with collisions at shallow angles or caused by flattened by greater amounts. Both roll and pitch increased a snowplow rubbing against the rail. Rail flattening was char- with the amount of flattening. However, the yaw and exit acterized by loss of depth in the w-beam rail element, which angle decreased with increasing flatness. At 100% flattening, was often accompanied by rail deflection and post deflection. the vehicle was unable to remain upright and rolled to the right Concurrent with the loss of depth was an increase in the height after exiting the guardrail. of the guardrail, i.e., the upper edge of the guardrail extended In Table 27, the NCHRP Report 350 test criteria are shown higher while the lower edge moved closer to the ground. Fig- for both the undamaged simulation and all of the flattening ure 55 shows an example of rail flattening caused by a snow- simulations. As observed in Figure 57, the roll and pitch were plow, and a finite element model of this damage mode. higher and the yaw was lower for all of the flattening simula- Rail flattening was of concern for two reasons. First, the tions. The degree of flattening in the guardrail had a strong loss of depth in the rail reduced the spacing between the strik- effect on the exit speed and angle of the vehicle. The increase ing vehicle and the posts. Thus, rail flattening may increase in exit speed was particularly pronounced at the highest lev- the risk of vehicle snagging on the posts. Second, the flatten- els of flatness, with a 13 kph (8.1 mph) increase in exit speed ing of the rail increases the maximum height and lowers the between 75 and 100 percent flattening. Exit angle showed minimum height of the guardrail, changing the way in which the opposite behavior, i.e., it decreased with increasing flat- the vehicle interacts with the guardrail system. 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 13.1 Approach deflection, increased along with flattening. The maximum A series of simulations of impacts into flattened strong- deflection increased by 15.5 percent for a completely flattened post w-beam guardrail were run and compared to the perfor- rail. All occupant injury metrics, i.e., occupant ridedown accel- mance of the undamaged guardrail simulation. The flattening eration and occupant impact velocities, were well below the in these simulations varied from 25 to 100 percent. This type NCHRP Report 350 limits. of damage commonly occurs in combination with minor rail Figure 58 shows the roll, pitch, and yaw vs. time curves deflection, but in this study it was considered in isolation. The for the undamaged simulation and all of the flattening sim- detailed procedure for inducing rail flattening in the finite ele- ulations. As expected, the roll for the 100 percent flattening ment model is described in the appendices. The complete set of simulation was the largest. The yaw for all of the flattening finite element models covered all degrees of flattening between simulations peaked in the range of 400500 ms, after which 25 and 100 percent, in increments of 25 percent. These simula- it started to decline. As the yaw was directly related to the tions are shown in Figure 56. 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 13.2 Results a possibility of vaulting. Each of the flattening simulations was run on the Inferno2 Figure 59 shows the local vehicle CG velocities for the computer system using four processors. Each run required undamaged and all flattening simulations. All of the simu-