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KDOT
is planning to simplify our Inertial Barrier Standard Drawing (current drawing
attached) to have only two options for IBS layouts; one for low speed (less
than or equal to 45 mph TL-2) and one for high speed (greater than 45 mph
TL-3). Through conversations with Scott, he indicated MwRSF may have a program
or spreadsheet to evaluate different configurations. Can you take a look at the
current layouts we have shown for 45 mph, 65 mph, and greater than or equal to
70 mph and let me know if the current configurations are sufficient for a TL-2
(45 mph) or TL-3 (65 or greater than or equal to 70 mph) type hit? If you have
any questions or want to discuss further let me know or give me a call.

We have done an analysis on the sand barrel array configurations that you sent us. The analysis consisted of the following NCHRP Report 350 configurations.

1.       We analyzed each system at its design speed shown in the following cases. The 40 mph array was evaluated at 40mph, the 45 mph array was evaluated at 45 mph, and so on… We ran all of the analyses with the barrels oriented parallel to the roadway rather than the 0-10 degree orientation shown as an option in the detail. It was believed that the 0 degree orientation parallel to the roadway was more critical.

a.       Test no. 3-40 using an 820C vehicle centered on the end of the array rather than the ¼ point offset  as the center impact would maximize the decelerations.

b.      Test no. 3-41 with the 2000P vehicle

c.       Test no. 3-42 using an 820C vehicle

d.      Test no. 3-43 with the 2000P vehicle

e.      Test no. 3-44 with the 2000P vehicle

2.       We also ran the reverse direction impacts along the barrier with the 820C and 2000P vehicle to evaluate its performance for that type of impact.

3.       We also analyzed the 65 mph configuration under the TL-3 impact speed – 62.1 mph.

From these analyses, we found the following.

1.       Almost all of the arrays were acceptable under the required NCHRP 350 impact tests for the design speeds listed.

2.       For the 65 mph array, our analysis found that the array was not quite long enough to bring the 2000P vehicle below a critical velocity prior to the end of the concrete parapet for test no. 3-41. Typically we design the arrays to drop the vehicle velocity below 10 mph and then place a final row of barrels beyond that point to ensure safe vehicle deceleration. In the case of the 65 mph array, the vehicle velocity was still slightly above the 10 mph cutoff when it reached the last row of two 2100 lb barrels. Thus, you may consider modifying this array slightly to alleviate this issue by changing rows 4 and 5 from 200 lbs and 400 lbs to 400 lbs and 700 lbs, respectively. See attached. The addition of one additional row of barrels past the 10 mph velocity point is not necessarily required, but is mentioned as an option in the RDG and is recommended by some of the manufactures.

3.       Analysis of the 65 mph at the TL-3 impact speed of 62.1 mph found that the array was acceptable under the required NCHRP 350 impact tests.

4.       Analysis of the 60 mph array under the 60 mph design found a similar issue to the above 65 mph case in that the array was not quite long enough to bring the 2000P vehicle below a critical velocity prior to the end of the concrete parapet for test no. 3-41. Depending on your preference, we could attempt to adjust that array as well.

5.       The 55 mph array analysis found that the acceleration limits were exceeded for test no. 3-44 with the 2000P vehicle. This is a function of the length of the array changing the impact point for the test to a more critical location. The impact point for test no. 3-44 is the length of the array divided by two. Thus, the shorter array used here places the impact point further down the array. Again, we can look into adjustment of that array if you desire.

Take a look at this information and let me know what you think. I also have some additional thoughts that we can discuss on the phone if you want to give me a call.