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Questions on Thrie beam transitions, nesting of beam guard and thrie beam barrier

Question
State WI
Description Text

I have some questions about transitions to concrete barrier, nesting of rails, and thrie beam barrier.

 

Transitions to Concrete Barrier

 

When I look at our detail drawings for thrie beam transitions, I notice that we are requiring a minimum 4' embedment.  In TRP-03-69-98 "Two Approach Guardrail Transitions for Concrete Safety Shape Barriers" posts 1-7 have 4'-4" of embedment. 

 

I'm planning on updating our details to match TRP-03-69-98 for posts 1-7.  However, there is note 2 on the detail sheet 1 indicates that if the grading behind the thrie beam transition cannot be provided that the field staff is to use post that are 4'-6" or longer. 

 

I believe using longer post would be warranted in reduced grading situations, but I do not believe that what is in the detail is sufficiently long enough to provide the needed soil resistance to rotation.  What would be MwRSF's recommendation for post length for reduced grading for a structural transition?

 

I'm planning on adding the 4" tall by 8" wide dike that is below the rail between post 1 to 7.  Discussions with staff have indicated that it may be problematic having the post of the transition flush with the dike. They are concerned that it may not be possible to drive the post flush.  Is there some tolerance to how close the post can be to the back of the dike?

 

In some existing installation, staff has omitted a post or posts between posts 1 and 7 to accommodate drainage structures.  I will be writing some guidance to discourage the practices, but there could existing installations or unique site-specific situations that may require post or posts to be omitted.  In these situations, does MwRSF have a recommendation on what to do?

 

On page 2 and 3 of the SDD, we currently allow the use of beam guard on the down stream end of a concrete barrier installation. I'm have some concerns that transitioning from concrete barrier to beam guard may be too abrupt. Does MwRSF have any recommendations on what to do on downstream ends of concrete barrier?

 

Nesting of rails for Beam Guard

 

I'm currently writing up design guidance requiring the uses of nested beam guard when using 4" mountable curb.  One of the questions that has come up is deals with the structural transition.   Would using nested beam guard change the overall design of the thrie beam transition?

 

The second question is: Would using nested beam guard effect how a beam guard end treatment would be attached to the nest beam guard system?

The third question would be: Would a special transition be needed between standard beam guard and nested beam guard?

 

My first answer to the three question proposed was to say no.  However, if MwRSF would provide comment it would be appreciated.

 

Thrie beam

 

If a situation developed where 2' of grading could not be provided behind a Thrie beam barrier run (i.e. not a structural thrie beam transition just regular thrie beam), could the guidance for beam guard with reduce grading (i.e. longer posts at ½ post spacing) be and equal alternative to providing the 2' of grading behind the post?

Keywords
  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date April 10, 2009
Attachment 20090410092452147.pdf


Response
Response

See my comments below in red on the various topics.

 

Transitions to Concrete Barrier

 

When I look at our detail drawings for thrie beam transitions, I notice that we are requiring a minimum 4' embedment.  In TRP-03-69-98 "Two Approach Guardrail Transitions for Concrete Safety Shape Barriers" posts 1-7 have 4'-4" of embedment. 

 

I'm planning on updating our details to match TRP-03-69-98 for posts 1-7.  However, there is note 2 on the detail sheet 1 indicates that if the grading behind the thrie beam transition cannot be provided that the field staff is to use post that are 4'-6" or longer. 

 

I believe using longer post would be warranted in reduced grading situations, but I do not believe that what is in the detail is sufficiently long enough to provide the needed soil resistance to rotation.  What would be MwRSF's recommendation for post length for reduced grading for a structural transition?

                Currently, MwRSF is exploring the necessary embedment depth of a 6"x8" post on a 2:1 slope to match the force-deflection characteristics of a standard MGS post (40 in. embedment).  From recent component testing for the MGS on 2:1 slope project, MwRSF had concluded that a 9 ft long W6x9 was adequate to replace the standard 6 ft long post used in the MGS on flat terrain.  However, a test impacting a 9-ft long 6"x8" wood posts on the 2:1 slope resulted in the post breaking early on in the test " (The same result was observed during a test with an 8-ft long wood post on a 3:1 slope).  Therefore, the post embedment and/or the post size may need to be altered to achieve the required force-deflection characteristics.  Until the data is fully analyzed and a solution is prepared, the bogie testing had been put on hold. This problem should be answered in the near future.

 

I'm planning on adding the 4" tall by 8" wide dike that is below the rail between post 1 to 7.  Discussions with staff have indicated that it may be problematic having the post of the transition flush with the dike. They are concerned that it may not be possible to drive the post flush.  Is there some tolerance to how close the post can be to the back of the dike?

                I would recommend installing the posts before pouring the curb " that was how the test installation was constructed.  In doing so there may be a ½" " 1" gap between the posts and curb from the concrete form.  This size of gap would be acceptable.  Anything larger could affect the response of the system.

 

In some existing installation, staff has omitted a post or posts between posts 1 and 7 to accommodate drainage structures.  I will be writing some guidance to discourage the practices, but there could existing installations or unique site-specific situations that may require post or posts to be omitted.  In these situations, does MwRSF have a recommendation on what to do?

                MwRSF would also strongly discourage this practice.  Eliminating a post from a transition can result in drastically changing the stiffness characteristics of the transition and can lead to the creation of hazardous pocketing locations.  I would try to avoid these situations by moving either: (1) the drainage structure, or (2) extending the concrete barrier to the drainage structure so that the transition does not span over the obstacle.

 

On page 2 and 3 of the SDD, we currently allow the use of beam guard on the down stream end of a concrete barrier installation. I'm have some concerns that transitioning from concrete barrier to beam guard may be too abrupt. Does MwRSF have any recommendations on what to do on downstream ends of concrete barrier?

                The transition of concrete to w-beam rail is not a problem unless there is a possibility of vehicle's traveling in the opposite direction impacting this transition (thus, making it a w-beam to concrete transition).  This is detailed on page 1 of these drawings in which an undivided roadway should contain thrie beam transitions at all 4 locations .

 

Nesting of rails for Beam Guard

 

I'm currently writing up design guidance requiring the uses of nested beam guard when using 4" mountable curb.  One of the questions that has come up is deals with the structural transition.   Would using nested beam guard change the overall design of the thrie beam transition?

                The thrie beam transition drawings you have sent already prescribe nested thrie beam, so I am unclear on your question.  If you are asking if you can use only a single rail instead of nesting the rail... this would have to be evaluated/tested for the particular system in question.

 

The second question is: Would using nested beam guard effect how a beam guard end treatment would be attached to the nest beam guard system?

                YES.  End treatments are specifically design for a particular rail.  Thus, unless the end treatment was designed for nested rail, only single rail segments should be hung adjacent to the end terminal

 

The third question would be: Would a special transition be needed between standard beam guard and nested beam guard?

                If you are asking if special treatment is needed when going from w-beam to nested w-beam (or thrie to nested thrie), this has not been done previously.  However, if this is going to change a transition system design, I would not recommend it without further analysis/testing.

 

My first answer to the three question proposed was to say no.  However, if MwRSF would provide comment it would be appreciated.

 

Thrie beam

 

If a situation developed where 2' of grading could not be provided behind a Thrie beam barrier run (i.e. not a structural thrie beam transition just regular thrie beam), could the guidance for beam guard with reduce grading (i.e. longer posts at ½ post spacing) be and equal alternative to providing the 2' of grading behind the post?

                See response above about current work for posts on slope break points.

 

Date April 13, 2009


Response
Response

With regards to the nested rail question, the question is: If I have nested beam guard (i.e. I'm not using standard beam guard) going into a "normal" thrie beam transition (i.e. as per the detail), do I need to do something different with the "normal" thrie beam transition because the nested beam guard is stiffer than normal beam guard.

Date April 13, 2009


Response
Response

Going from nested guardrail into a transition system with only a single thrie rail should not be a problem. Transitions are sensitive because they go from a relatively "weak" rail section to stiff or "strong" rail section. However, having a nested rail on the front end, or the "weak" section, reduces the difference in stiffness or strength between the two sections, making the transition easier.

Date April 14, 2009


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