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When you have time could you please give me a call to
|Date||July 7, 2015|
I reviewed the schematic you sent regarding a curb offset behind a guardrail system to help control erosion. You had asked about our thoughts on the offset of the curb behind the barrier and the height of the curb. We have several comments on this setup for your consideration.
1. We would assume that this setup would be made with the MGS system rather than the previous metric height G4(1S) system. Thus, all of the remaining comments are made with respect to the MGS.
2. As you noted, the guardrail posts will be installed in a 4” thick concrete pad with leave outs filled with grout. I assume that this is based on the TTI research on mow strips and leave outs. We would recommend using that research for the leave out details.
3. The main concern with this type of system is the interaction of the vehicle with the curb during deflection of the guardrail system. For an MGS system, that deflection is typically in the 4’ range for TL-3. We have seen that vehicle interaction with curbs can increase rail loads and increase the potential for vehicle instability. However, we believe that the curb can be placed in the later portion of the barrier deflection and still maintain safe performance as the overall barrier stiffness will be lower at that point in the impact.
4. With regards to the barrier offset, the MGS with an 8” or a 12” blockout is approximately 18” to 22” deep, respectively. A 24” offset from the back of the post to the curb would make the offset to the curb 42”-46” depending on the blockout size. This would be at the latter region of the MGS deflection. Thus, placement of the curb at the 24” offset should have minimal effect on the barrier performance.
5. As far as curb height, we believe that a 4” curb would be a more forgiving setup in terms of its effect on barrier performance. However, we believe a 6” curb will work given the 24” offset and the limited vehicle interaction with the curb.
Let me know if that answers your questions or if you need anything else.
|Date||July 9, 2015|
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
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