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MGS Working Width

Question
State VA
Description Text

We are in the process of finalizing our MASH MGS standard
details and would like a quick review for the content. 



 



One of the remaining details is the minimum distance either
behind the post or from the face of rail to a hazard.  Do you have a
detail of the distance and how it is measured for the MASH testing or a
description similar to page 225 of the NCHRP 350 report?



 



Thanks

Keywords
  • Guardrail
Other Keywords none
Date November 30, 2016


Response
Response

The following text is located in MASH with respect to the deflections and working width. They may help serve as basic definitions for you.

 

“Test article deflections—Report the permanent and dynamic deflections of the test article plus the working width during impact. These measurements normally apply to longitudinal barriers, terminals, crash cushions, and TMAs. Permanent deflection is the residual lateral displacement of the test article remaining after the impact. Dynamic deflection is the maximum lateral displacement of the test article on the traffic side that occurs during the impact. The working width is the maximum dynamic lateral position of any major part of the system or vehicle. These measurements are all relative to the pre-impact traffic face of the test article. For the working width, the height of the maximum working width should also be documented and reported.”

 

working width—The distance between the traffic face of the test article before the impact and the maximum lateral position of any major part of the system or vehicle after the impact”

 

Working width would define the distance from the face of the rail to the hazard.

 

We have fielded questions regarding the variation of the MGS dynamic deflection and working width in the past for several states. Related to that, we have compiled charts of the working width and deflections for the standard system. These can be seen below. A chart with similar values is located in the Roadside Design Guide.

 

Table 1. Guardrail Testing under Test Designation 3-11.

Testing Agency

Test Number

Testing Criteria

Dynamic Deflection

in. (mm)

Working Width

in. (mm)

MwRSF

NPG-4

350

43.1 (1,094)

49.6 (1,260)

MwRSF

2214MG-1

MASH

57.0 (1,447)

57.4 (1,457)

MwRSF

2214MG-2

MASH

43.9 (1,114)

48.6 (1,234)

MwRSF

MGSMIN-1

MASH

42.2 (1,072)

48.8 (1,240)

MwRSF

MGSDF-1*

350

60.2 (1,529)

60.3 (1,530)

MwRSF

MGSPP-1*

350

37.6 (956)

48.6 (1,234)

MwRSF

MGSWP-1*

MASH

46.3 (1,176)

58.4 (1,483)

MwRSF

MGSSYP-1*

MASH

40.0 (1,016)

53.8 (1,367)

MwRSF

MGSRF-1*

MASH

55.8 (1,417)

57.4 (1,458)

MwRSF

MGSNB-1**

MASH

34.1 (867)

43.2 (1,097)

TTI

220570-2**

MASH

40.9 (1,040)

44.0 (1,119)

SwRI

GMS-1**

MASH

35.0 (890)

NA

TTI

400001-TGS1**

MASH

38.4 (975)

40.8 (1,036)

Holmes Solutions

057073112**

MASH

41.3 (1,050)

NA

*Guardrail with alternate posts and/or blockouts.

**Guardrail with no blockouts.

Table 2. Guardrail Testing under Test Designation 3-10.

Testing Agency

Test Number

Testing Criteria

Dynamic Deflection

in. (mm)

Working Width

in. (mm)

MwRSF

NPG-1

350

17.4 (441)

40.3 (1,022)

MwRSF

2214MG-3

MASH

35.9 (913)

48.3 (1,227)

MwRSF

MGSSYP-2*

MASH

22.2 (564)

39.7 (1,008)

MwRSF

MGSRF-3*

MASH

NA

38.4 (975)

MwRSF

MGSNB-2**

MASH

29.1 (740)

34.5 (877)

*Guardrail with alternate posts and/or blockouts.

**Guardrail with no blockouts.

 

The deflections and working widths listed for the MGS do fluctuate, even for the steel post version with standard 6’-3” post spacing. This fluctuation in the working widths is a reflection of several factors.

 

  1. First, there has been a transition in the soil resistive forces that we use in our full-scale crash tests under MASH. Thus, the original crash testing of the MGS with the 2270P vehicle under 22-14 would have likely used a soil foundation that was less stiff than the soil recommendations that were eventually incorporated into MASH. Thus, there will be some variation of deflection and working width based on the change in the foundation conditions.
  2. Second, the table presents tests with both the 2000P and 2270P vehicle types. Again the MGS was developed and tested during the transition between NCHRP 350 and MASH. Thus, the change in pickup truck vehicles represents an approximately 13.5% increase in kinetic energy. This change in impact conditions also accounts for some of the variation you are observing between the working widths and deflections in the full-scale testing.
  3. Third, the table here and others in the Roadside Design Guide show deflections for a wide range of MGS systems, including wood and steel post versions as well as several special applications. Thus, the use of different post types, post spacing, slopes, flares, etc… affect the working width numbers.
  4. Finally, full-scale crash tests are not an exact science. We have tried over the years to develop test procedures to make crash test results more consistent and repeatable. The current soil standard in MASH is one part of that effort. However, even with these efforts, there is a certain degree of variation from test-to-test that is difficult to avoid. Thus, full-scale crash tests of two identical MGS systems may result in deflections that vary. This is simply difficult to avoid given all of the potential variation in materials, environmental conditions, soils, and other factors.

 

While it is clear that deflection and working width data taken from full-scale crash tests can vary for several reasons, we have still not answered the question regarding what values you need to consider for your installations. Our advice here would be to review the available data from the crash tests of most similar systems and error on the side of being conservative. For example, if you have an MGS system installed on a 2:1 slope, then we would recommend using the working width guidance from the full-scale crash test of the 2:1 slope. For standard, steel post installations, we may suggest considering a working width of 60 in. The 60-in. working width corresponds with the upper end of the values observed in the full-scale testing and also allows for some tolerance if the soil for your real world installations in not as stiff as the soil currently specified in MASH. For the wood post versions of the standard MGS system, we would recommend that you refer to the crash tests of the specific wood post system and use those working widths if they are increased over the 60-in. For the ½ post and ¼; post spacing versions of the system, we would recommend using the tested working widths listed in the RDG.

 

 

Let me know if this addresses your concerns and if you have further questions.

 

Date February 5, 2017


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