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MGS Omitted Post Questions

Question
State
Description Text I have several questions regarding the MGS when posts are skipped or omitted.

1. When using the MGS and a post is needed to be skipped due to an underground feature that is in conflict, what happens if just one or two post need to be skipped? Can you place less than the 3 CRT post? If we skip one post can we place just 1 CRT post? If we skip 2 post can we place just 2 CRT post? If we skip 3 post then we need to place all 3 CRT post?

2. If you are on a one way roadway or a wide roadway so that you are not concerned about apposing traffic, do we need to place CRT post on the near side of the missing post or just on the far side (downstream)? Is the number of nearside CRT post the same as the downstream need?

3. Does the steepness of the slope behind the guardrail matter? (Max 3:1 or 2:1 or 1.5:1 or ? or vertical culvert headwall???)

4. If you are skipping a post due to an underground conflict, but there is another fixed object behind the guardrail just downstream, say 12' downstream, but just 5' behind the face of the rail. Should we increase our CRZ behind the rail for some distance downstream and further from the guardrail from where the post needs to be skipped?

5. If you are missing 3 post I understand that the guardrail should remain inline before any flaring of terminals or redirection of the guardrail for an additional 62' but if you are just skipping 1 or 2 post can this be reduced to 50' or 38' or 25' or ???

6. If you have to skip a post or two and then 50' latter need to do this again is there concern or limitation to how often this can repeat?
Keywords
  • Guardrail
Other Keywords Omitted Post
Date July 18, 2014


Response
Response

I have some feedback for you regarding the MGS with omitted post questions you gave me at the meeting.

 

1.       When using the MGS and a post is needed to be skipped due to an underground feature that is in conflict, what happens if just one or two post need to be skipped? Can you place less than the 3 CRT post? If we skip one post can we place just 1 CRT post? If we skip 2 post can we place just 2 CRT post? If we skip 3 post then we need to place all 3 CRT post?

 

The MGS Long-Span system was developed for use to span transverse culverts measuring 24 ft wide or less. In this circumstance, three posts would be removed from the system. This system also utilizes three CRT posts on each side of the culvert structure. For culverts measuring less than 24 ft wide and where one or two posts are omitted, it still would be necessary to utilize the CRTs on each side of the unsupported segment of rail.

 

Although it may be possible for the MGS to work with one post removed and without CRTs adjacent to the long span, it should be noted that crash testing has not been performed on this MGS system nor to verify that acceptable performance would result. As such and in the absence of test data, we recommend that the CRTs be installed in systems where one, two, or three posts are removed.

 

2.       If you are on a one way roadway or a wide roadway so that you are not concerned about apposing traffic, do we need to place CRT post on the near side of the missing post or just on the far side (downstream)? Is the number of nearside CRT post the same as the downstream need?

 

Similar to the comment above, we believe that the CRT’s are needed on both the upstream and downstream end without further analysis. The CRT’s on the downstream side are more critical in terms of rail pocketing and snag, but review of the barrier performance in testing found that the CRT’s on the upstream end often fracture as well, which may make the barrier system more forgiving and reduce rail loads and pocketing angles. Thus, we cannot recommend removing the upstream CRT’s without further analysis and/or testing.

 

3.       Does the steepness of the slope behind the guardrail matter? (Max 3:1 or 2:1 or 1.5:1 or ? or vertical culvert headwall???)

 

We recommend providing 2 ft of level, or mostly level, soil grading behind the wood CRT posts. However, we understand that this can be difficult. As such, there is potential that the wood CRT posts could be lengthened to account for the reduction in soil resistance resulting from an increased soil grade behind these six posts, especially when placed at the slope break point of a 2:1 fill slope.

 

Recently, MwRSF performed limited research to determine an acceptable MGS post length for a 6-in. x 8-in. solid wood post installed at the slope break point of a 2:1 fill slope. MwRSF determined that 7.5-ft long wood posts are an acceptable alternative when considering the 31-in. tall MGS placed at the slope break point of a 2:1 fill slope using 6-ft 3-in. post spacing.

 

The MGS Long Span system utilizes six CRT wood posts. A CRT post's moment capacity about its strong axis of bending is approximately 81 percent of that provided by the standard wood post. In the absence of dynamic component test results, it is believed that the six CRT wood posts could also be fabricated with the 7.5-ft length when used in the MGS Long Span system. If the steep fill slopes continue beyond the location of the CRT posts, then the guardrail would transition to the MGS for 2:1 Fill Slopes using either 6-in. x 8-in. by 7.5-ft long wood posts or W6x9 by 9-ft long steel posts.

 

For general installations at slope breakpoints or offsets less than 2’ from the slope breakpoint of 2:1 to 6:1 slopes, we would recommend using the 7.5’ long CRT posts. For slopes steeper than 2:1, we have little test data or analysis to guide us. Thus, we would recommend maintaining the 2’ minimum offset in those locations.

 

4.       If you are skipping a post due to an underground conflict, but there is another fixed object behind the guardrail just downstream, say 12' downstream, but just 5' behind the face of the rail. Should we increase our CRZ behind the rail for some distance downstream and further from the guardrail from where the post needs to be skipped?

 

In locations where posts are left out, dynamic barrier deflections and working widths would be expected to increase. Test results are available for the case with three posts removed from the MGS. However, data is not available for cases with one or two posts removed. BARRIER VII computer simulations could be performed to estimate barrier deflections and working widths. A small modeling study would be necessary to validate the model for the MGS long-span system and then predict barrier performance with fewer posts removed.

 

That said, attempting to account for the expected increase in deflection would be a step in the right direction. Full-scale crash testing of the MGS Long Span had dynamic deflections of 92.25”. Based on this level of increased deflection over the 60” you note above, the a reasonable approach may be to assume a 12” increase in dynamic deflection for every omitted post. For example, one omitted post = 72” and two omitted posts = 84”. However, this guidance is only based on rough approximations on a limited number of tests and it would be better to derive more accurate values through modeling as noted above.

 

We currently have a project to investigate the omission of a single post from the MGS without the use of the CRT’s through full-scale crash testing that may provide further information.

 

5.       If you are missing 3 post I understand that the guardrail should remain inline before any flaring of terminals or redirection of the guardrail for an additional 62' but if you are just skipping 1 or 2 post can this be reduced to 50' or 38' or 25' or ???

 

The MGS Long-Span Guardrail System was successfully crash tested and evaluated according to the Test Level 3 (TL-3) safety performance criteria found in MASH. For this testing program, the overall system length was 175 ft, including 75 ft of tangent rail upstream from the long span, a 25-ft long unsupported length, and 75 ft of tangent rail downstream from the long span. As part of the final recommendations, MwRSF had noted to provide a minimum “tangent” guardrail length adjacent to the unsupported length of 62.5 ft.

 

In lieu of a recent MASH crash testing program on a 75-ft long version of the MGS (unpublished at this time), there may reason to consider potentially reducing the 75-ft total guardrail length on the upstream and downstream ends of MGS Long-Span Guardrail System. For example and based on the MASH 2270P test into the MGS Minimum Length System, we believe that the MGS Long-Span Guardrail System would likely have performed in an acceptable manner with 62.5 ft of rail on the upstream and downstream ends, thus resulting in an overall system length of 150 ft. A 62.5-ft long tangent length adjacent to the unsupported length would still provide adequate space to incorporate a 37.5 ft or 50 ft long energy-absorbing guardrail end terminal.

 

For unsupported lengths of 18.75 ft and 12.5 ft, it would seem reasonable to consider a reduction in the required guardrail length both upstream and downstream from the unsupported length using the test information and arguments noted above. For two missing posts or an unsupported length of 18.75 ft, we believe that the upstream and downstream guardrail lengths likely could be 56.25 ft each with a minimum overall system length of 131.25 ft. For one missing post or an unsupported length of 12.5 ft, we believe that the upstream and downstream guardrail lengths likely could be 50 ft each with a minimum overall system length of 112.5 ft. However, we believe that the three CRT posts still would be required on the upstream and downstream ends of the 18.75 ft and 12.5 ft long unsupported lengths. In addition, one would need to discuss with and likely obtain approval from the manufacturers as to whether they would allow three CRTs to be used within the last 12.5 ft of a 50-ft long guardrail terminal.

 

If one were to follow the logic used above and consider the situation of no missing posts (i.e., 6.25 ft post spacing throughout), the upstream and downstream ends would be reduced by 6.25 ft each and include the interior 6.25 ft long span in the middle of the system. As a result, the overall system length would be 43.25 ft + 6.25 ft + 43.25 ft for a total of 92.75 ft. As noted above, MwRSF recently crash tested a 75-ft long version of the MGS with satisfactory results, effectively configured with two 37.5-ft long guardrail segments with tensile anchorage devices and placed end-to-end.

 

Of course, it should be noted that the design modifications for the 25 ft, 18.75 ft, and 12.5 ft long unsupported lengths were based on engineering judgment combined with the unpublished results from the MGS Minimum Length System crash testing program. In addition, the opinions noted above are based on the assumption that the currently-available proprietary guardrail end terminals would provide comparable tensile anchorage for the MGS as provided by the common tensile anchorage system using in the MwRSF crash testing program (i.e., two steel foundation tubes, one channel strut, one cable anchor with bearing plate, and BCT posts at positions 1 and 2 on each end). Although we are confident that the modifications noted above would provide acceptable performance, the only sure means to fully determine the safety performance of a barrier system is through the use of full-scale vehicle crash testing. We are hopeful that these design modifications can be evaluated in the near future and as part of a continued R&D Pooled Fund program involving the MGS Long-Span Guardrail System.

 

 

6.       If you have to skip a post or two and then 50' latter need to do this again is there concern or limitation to how often this can repeat?

 

This question has not been answered to date. There are concerns about how close these post omissions can occur. We currently have a project to investigate the omission of a single post from the MGS without the use of the CRT’s that may shed some light on this issue. We plan to test the MGS with the omitted post with the 2270P vehicle. Based on the outcome of that test we plan to give guidance on what the minimum allowable offset between omitted posts should be. 

Date July 21, 2014


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