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Questions on MGS Implementation

Question
State NE
Description Text

NDOR asked a series of questions regarding the MGS system.

 

(1) The thrie-beam bridge approach sections:

 

- Is the mounting height for thrie-beam moving down from 31 5/8" to 31"?

 

- Do we add just the 12.5' nested Thrie-beam to the bridge, or is 6'-6"

enough?

 

- Is the bridge approach acceptable @ 27 5/8" too?

 

(2) 1/2 post spacing: placing a post on the splice or gap the splice?

 

(3) 1/4 post spacing in photo appears to show the post on a splice.

 

(4) What is the post spacing when I want to transition from 75" MGS to 1/4 post spacing? Phase through 1/2 post for 12'?

 

(5) What is the radius to 7:1 tapers, posts are moved to the 1/4 and 3/4 points?

 

(6) I assume the 6" curb is acceptable closer to the face and behind the face of the MGS?

 

(7) Are the end treatments acceptable behind curbs? Or what is the transition to 4"? Or no curb?

 

(8) "First & only NCHRP 350 system to be successfully tested 6" behind a 6"

tall curb"?

 

- I thought MinnDOT had a test pass(fall 2002) with 6" curb face directly under regular w-beam.

 

(9) MGS performance summary:

 

What is the working width? back of posts? or vehicle extension beyond the rail?

 

(10) Future testing concerns:

 

- Long Span Testing: If this uses a drop-off at a given distance and passes is the drop-off only allowed at that distance?

 

- 8:1 slopes: Can the MGS start on a 10:1 and transition to the 8:1?

 

(11) Please supply the pooled fund states with dimensions of the Y-shape transition. Or submit it to manufacturers to work out any problems with manufacturing.

 

*****

 

Additional items:

 

(12) "40 in. embedment depth"

 

- Embedment depth varies as curb height/non-curbed varies. The 6' post covers the hardware used.

 

(13) Can I use the same attachment to concrete box culvert with the MGS?

 

(14) 7:1 flare rate: End Terminals for this?

Keywords
  • Guardrail
Other Keywords none
Date January 25, 2006


Response
Response

(1) The thrie-beam bridge approach sections:

 

- Is the mounting height for thrie-beam moving down from 31 5/8" to 31"?

 

      Over the last decade, several thrie beam approach guardrail transition systems have been successfully developed according to the NCHRP Report No. 350 criteria. For these systems, the top mounting height has utilized either the 31 in. or 31.65 in. upper rail dimension. This range occurred due to two reasons. First, early corrugated steel beam guardrail systems used a 20.875 in. or 21 in. (when rounded up) center rail height, thus resulting in top of rail thrie beam heights of 31 in. Second, the more recent corrugated steel beam guardrail systems utilized a 550 mm or 21.65 in. center height which occurred during the metrication of roadside safety hardware in the late 1980's to early 1990's. As a result, the top of rail thrie beam height for these systems became 31.65 in. Since both small car and pickup truck crash tests have been successful performed into thrie beam approach guardrail transitions using both mounting heights, we are not concerned with the downward shift of 0.65 in. Even though it will require changing the height of the transition system, reducing the height to 31 in. might provide the simplest overall design. Alternatively, you could raise the height of the MGS to 31 5/8 in. over the last 2 or 3 post spans.

 

- Do we add just the 12.5' nested Thrie-beam to the bridge, or is 6'-6" enough?

 

      Many of the existing thrie beam approach guardrail transition systems have utilized a 12-ft 6-in. long, nested section of 12-gauge thrie beam rails, followed by the symmetrical, W-beam to thrie beam transition section. The new approach guardrail transition system has been configured with an additional 6-ft 3-in. long, single (non-nested) 12-gauge thrie beam rail, followed by the new asymmetrical, 10-gauge W-beam to thrie beam transition section. It is our current opinion that all of the prior nested thrie beam transition systems should be modified to include the additional 6-ft 3-in. thrie beam section as well as the asymmetrical transition section. In addition, the existing approach guardrail transition systems will require a longer transition length for which the support posts are installed and using a reduced post spacing. As part of another change, W6x12 steel posts were implemented in order to provide a more gradual change in lateral stiffness of the overall guardrail system. Final design details for this new, longer transition system will be included in a future research report, or in electronic draft form at your request.

 

- Is the bridge approach acceptable @ 27 5/8" too?

 

      No.

 

(2) 1/2 post spacing: placing a post on the splice or gap the splice?

 

      The MGS was designed using three different post spacing - standard (6 ft - 3 in.), half (3 ft - 1 1/2 in.), and quarter (1 ft - 6 3/4 in.). In the standard MGS, guardrail splices are not to be placed at the post locations. However, guardrail splices will be located at post locations in the two reduced post spacing configurations. Note that stiffening the guardrail by adding posts reduces the stress on the rail and allows the barrier to function adequately, even though there is a post at the splice.

 

(3) 1/4 post spacing in photo appears to show the post on a splice.

 

      As noted previously, the MGS in combination with either half or quarter post spacing will require that guardrail splices be located at post locations.

 

(4) What is the post spacing when I want to transition from 75" MGS to 1/4 post spacing? Phase through 1/2 post for 12'?

 

      At this time, a stiffness transition has not been developed nor tested for use in switching between the standard MGS and the quarter-post spacing version of MGS. Until a transition is determined with the aid of an engineering and numerical analysis, or possibly crash testing, it is recommended that four half-post spaces be utilized in order to allow for a more gradual transition in lateral stiffness between barrier systems. As such, this stiffness transition would likely occur over approximately 12 ft - 6 in.

 

(5) What is the radius to 7:1 tapers, posts are moved to the 1/4 and 3/4 points?

 

      First, we have assumed that you have asked what radius is used make the gradual bend from the tangent roadside guardrail to the guardrail flared away from the roadway at a 7:1 rate. Based on this question, we measured the actual radius that was used in the field installation via an overhead film analysis. From this analysis, we have estiamted the radius at the knee to range between 88 and 91 ft. Posts remained at the full post spacing at the location of the knee.

 

(6) I assume the 6" curb is acceptable closer to the face and behind the face of the MGS?

 

      Yes.

 

(7) Are the end treatments acceptable behind curbs? Or, what is the transition to 4"? Or, no curb?

 

      To date, no full-scale vehicle crash tests have been performed on guardrail end terminals placed behind curbs. As such, it is not recommended that existing guardrail end terminals be installed behind curbs until satisfactory safety performance has been demonstrated according to the NCHRP Report No. 350 guidelines. Terminal manufacturers might be able to provide guidelines for use of their systems behind a curb. General guidance on the transitioning of curbs behind guardrail end terminals is provided in a MwRSF research report no. TRP-03-127-03 entitled, "Bridge Rails and Transitions for Pedestrian Protection." Other options may also exist that are based on the engineering judgment but are not provided herein.

 

(8) "First & only NCHRP 350 system to be successfully tested 6" behind a 6" tall curb"?

 

- I thought MinnDOT had a test pass(fall 2002) with 6" curb face directly under regular W-beam.

 

      The MinnDOT design had the face of the curb flush with the face of the guardrail.

 

(9) MGS performance summary:

 

What is the working width? back of posts? or vehicle extension beyond the rail?

 

      Working width is defined as the distance measured from the original front face of the longitudinal barrier system to either the rearmost part of the barrier at the maximum dynamic barrier deformation (i.e., back side of rail, post, etc.) or the farthest vehicle extend beyond the barrier system, whichever is greater. For the MGS, this dimension was normally measured to the back of one of the posts, such that a 17 in. barrier deflection produces a 17 in. + 3 in. rail depth + 12 in. blockout + 6 in. post = a working width of 38 in.

 

(10) Future testing concerns:

 

- Long Span Testing: If this uses a drop-off at a given distance and passes is the drop-off only allowed at that distance?

 

      The vertical drop-off could be no closer than in the test. The drop-off could be farther from the rail.

 

- 8:1 slopes: Can the MGS start on a 10:1 and transition to the 8:1?

 

      Future crash testing is planned for the MGS using an 8:1 approach slope. Approach slopes of 10:1 or flatter are deemed acceptable and do not require crash testing. If the 8:1 slope test passes, starting on a 10:1 and transitioning to an 8:1 would be acceptable.

 

(11) Please supply the pooled fund states with dimensions of the Y-shape transition. Or submit it to manufacturers to work out any problems with manufacturing.

 

      Detailed drawings have been supplied to the manufacturers. MwRSF will send them to the Pooled Fund States.

 

Additional items:

 

(12) "40 in. embedment depth"

 

- Embedment depth varies as curb height/non-curbed varies. The 6' post covers the hardware used.

 

      For the MGS curb combination, it is understood that an additional post embedment depth would result due to the 6-in. curb height. Recall that when used over a curb, stiffer guardrails perform better. Hence the posts are designed to be embedded farther.

 

(13) Can I use the same attachment to concrete box culvert with the MGS?

 

      Although we believe that the MGS attached to the top of concrete box culverts would provide adequate safety performance. Future full-scale vehicle crash testing would be required to demonstrate that the system would perform in an acceptable manner. In addition, further research may allow designers to either optimize the existing design or move it closer to the front face of the concrete headwall.

 

(14) 7:1 flare rate: End Terminals for this?

 

      For now, it is appropriate to use standard guardrail end terminals in this flared configuration.

Date February 17, 2006


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