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MGS Long Span Length

Question
State IA
Description Text I have a question regarding proper anchorage for the long span system.  We are working on a project that will use the long span design along an interstate (traffic approaching from one end only).  We will be using the FLEAT-MGS at the approach end, but I'm unsure how to lay out the trailing end anchorage.  Can we start our anchorage immediately following the third CRT post after the culvert?  And if we use our standard design (attached), it appears that we will end up with an extra post as part of the transition from MGS to our end anchor.  Is this what we'll have to do, or do you have any other recommendations?
Keywords
  • Guardrail
Other Keywords none
Date March 27, 2007


Response
Response

I have some comments regarding you MGS Long Span questions. The question you raise is a good one. Because you have traffic from only one side, you won't need a terminal on the downstream end as you have suggested, but the length of the downstream end and the anchorage are critical to proper performance of the system.

The first question address should be is there a minimum length of guardrail that is required to ensure that the guardrail system adequately contains and redirects the impacting vehicles?

Most of the strong-post, W-beam guardrail systems have been crash tested using a system length of approximately 175 ft. For these lengths, it has been demonstrated that the barrier system will meet impact safety standards and allow the designer/researcher to gain knowledge on dynamic barrier performance. Whether or not the system's performance or deflection is adversely affected by an installed length shorter than the tested length is unknown. For an impact closer to the barrier system ends, dynamic barrier deflection may actually increase when impacted at the same 25-degree angle. However, the LON test on the terminal is currently conducted at 20 degrees instead of 25 degrees. In the Update to NCHRP Report No. 350, this LON test will become 25 degrees, potentially requiring modifications to be made to existing terminal anchors.

Flared systems or systems such as the long span system can actually further increase the loading of the barrier system and create higher anchor loads and affect the length of the system and the anchorage. Although it is likely that guardrail lengths shorter than 175-ft can redirect 2270P vehicles impacting at the TL-3 conditions, there is no crash test data to support or recommend the use of shorter lengths at this time.

In addition, trailing-end guardrail treatments are typically used to anchor the downstream end of strong-post, W-beam guardrail systems when vehicular impacts are not expected on the system end. These trailing-end designs consist of varying configurations using blunt ends or spoons, turned-down terminals, tension rods with concrete anchors, etc. In addition, these downstream anchorage devices are often located longitudinally near to the hazard that is shielded by the roadside barrier system. To date, no trailing-end (downstream) terminals have been evaluated according to the NCHRP Report No. 350 guidelines. There are concerns that vehicle impacts slightly upstream of the trailing-end terminals may induce rollover or severe snagging on the anchor system.  Further, if the downstream anchor proves to release too quickly, vehicles impacting a short distance upstream of the terminal may be allowed to penetrate through the guardrail and strike the shielded hazard.

There exists a need to standardize the trailing-end, guardrail anchorage systems that are capable of meeting current impact safety standards.

There will be proposals to better address these issues at the upcoming Pooled Fund Meeting.

That said, we do not believe that you can install the downstream anchorage immediately following the third CRT post on the downstream end. Due to the concerns listed above, we would recommend that the downstream length of the installation including the end anchorage be no less than 62.5 ft beginning at the third CRT post. This length is based on the 175 ft system length that was tested. We believe that we may be able to reduce this distance based on the proposed pooled fund studies mentioned above.

I have a couple of additional notes regarding you end anchorage that was attached. The end anchorage shown is not appropriate for use with the MGS Long Span system due to the high anchor loads developed in this type of system. I have attached details of the anchorage used in our testing, and I would recommend that you use this anchorage on the downstream end. As for the end spacing issues raised by the MGS, you are free to add another post if you like. You could also hang the additional guardrail length off the end of the anchorage along with your end shoe if that is easier.

Date March 28, 2007


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