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Deflection for MGS - Distance from the Front Face of Rail to a Fixed Object Hazard

Question
State WY
Description Text To all Pooled Fund Members and MwRSF: We are completing our details to implement MGS and I have a question about the distance to fixed object hazards which is very confusing in the roadside design guide. In the old days prior to the definition of working width, we specified the distance from the back of the guardrail system to the hazard as at least the dynamic deflection distance. The new Roadside Design Guide has several values for deflection of MGS. It also has values for working width, which is typically the width of a system (in the case of MGS, approximately 23 inches from the rail face to the back of the posts) plus the dynamic deflection. However the working widths fluctuate all over the board. What I would like to as, is what should we be using as the clear distance from the front face of the rail to a fixed object hazard behind the rail so the fixed object hazard is not struck. I would like to have values for the standard post spacing, half post spacing and quarter post spacing. I don\'t recall if we talked about deflection in our past review of various state standards for MGS. I would have normally posed the question on the Pooled Fund Website, but I am interested in both MwRSF take on this as well as what other states are using.
Keywords
  • Guardrail
Other Keywords none
Date January 30, 2013


Response
Response

The deflections and working widths listed in the RDG 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 RDG 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 RDG shows 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 your questions on 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 January 31, 2013


Response
Response

Ohio introduced MGS to our standards just a couple of weeks ago.  This is the table for the deflection values for our designers to accommodate.  The minimum barrier clearance values are from face of rail to the hazard.

Links to our drawings if anyone wants to see what we’ve got so far:

“MGS” drawings http://www.dot.state.oh.us/Divisions/Engineering/Roadway/DesignStandards/roadway/Pages/StandardConstructionDrawing.aspx

Date February 1, 2013
Attachment Ohio deflection table.jpg


Response
Response

Wisconsin uses working with.  For MGS we measure from face of rail.

 

Most designers just draw a line on the plan for beam guard.  So they don’t have a good idea where the post are.

 

Date February 1, 2013


Response
Response

So what values do you use then for each post spacing I mention?

Date February 1, 2013


Response
Response Because we use weaker wood I had to use bigger working with values than what MwRSF recommended.  The chart below is on the backside guidance of our standard drawings.
Date February 2, 2013
Attachment Wisdot deflection table.jpg


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