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MGS Long Span with a Flared End Terminal

Question
State IA
Description Text

With regards to the MGS Long Span, the TRB paper recommends installing tangent guardrail for 62.5' from the unsupported rail or 50' from the last CRT prior to flaring the guardrail. In addition, the overall system length is recommended to be 175' which makes for a minimum of 75' of guardrail from the end of the unsupported span or 62.5' from the last CRT.

If we want to install a FLEAT terminal on the system, following the flare guidelines forces the system length to be greater than 175'. Is this necessary?

 

Keywords
  • Guardrail
Other Keywords End Terminal
Date June 18, 2007


Response
Response

The FLEAT has been tested in the flared region, so we know that the system works when impacted under NCHRP 350 conditions in the flared region. Our recommendation in the report was based on general flares for tangent end terminals that we had no real test data for. Because the FLEAT has been tested to 350 in the flared region, we believe that you can count it as part of the "tangent" length. The same would not be true for tangent terminals installed on flares.

That said, the second factor for this question deals with the overall system length needed for anchorage that you had touched on before. We cannot recommend system lengths less than 175' at this time. Thus you must have 75' outside the unsupported span or 62.5' outside of the CRT post for anchorage purposes. This is 12.5' longer than our flare recommendation. However, as mentioned above, the FLEAT is special as it was designed and tested as a flared system, and thus, we don't believe that the flare starting distance applies to the FLEAT. The anchorage length limit still applies however. Thus, the setup for an MGS long span installation with the FLEAT would require 37.5' or tangent rail adjacent to the unsupported span or 25' of tangent rail adjacent to the end of the CRT posts and then the 37.5' FLEAT terminal. This would yield the anchorage length required, but would waive the flare recommendation due to the use of the FLEAT.

 

Use of standard tangent terminals installed on recommended flares would require that the flare criteria be met, because these systems have not been tested on a flare. Also, these recommendations only apply with respect to the MGS long span and not the old version with nested guardrail. The old version with nested rail would require that both the flare and anchorage requirements be met due to the use of the nested guardrail, even for the FLEAT. 

Date June 18, 2007


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