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MGS Bridge Approach Transition

Question
State NE
Description Text

We are attempting to switch to MGS/ 31" w-beam guardrail height.

Re.  In light of the MGS 31" guardrail using 12" blockouts & changing post spacing away from the joints...

Should the bridge approach section use 12" Blockouts?

If it was tested with 8" blockouts & these should stay 8", where should the transition to 12" blockouts take place?

 

The original design tested did not include posts 10 & 12.

With the post spacing changing away from the joint, are Posts 10 & 12 required?

Or should 10 & 12 be placed to start the transition to stiffen the area?

 

What end treatments are available for 31" guardrail?

Keywords
  • Approach Guardrail Transitions
  • Bridge Rails
Other Keywords none
Date October 5, 2009
Attachment 74000e00.pdf


Response
Response

We have successfully crash tested two steel post, thrie beam approach guardrail transition alternatives for the Midwest States Pooled Fund Program. These two transition designs were based on starting with a thrie beam system that was supported by W6x15 steel posts near the bridge end and spaced on 3 ft " 1.5 in. centers. See the attached for the two approach transitions successfully crash tested by MwRSF.


File "MWT FD R0" shows the design which utilized 3 W6x15's (7 ft long), 7 W6x12's (7 ft long), and 2 W6x9's (6 ft long). The transition length of this system " as defined in your drawing as the distance from the bridge rail to the DS end of standard MGS rail segment (additional post at this location) " is 37.5 ft.


File "MWT-SP R4" shows the design which utilized 3 W6x15's (7 ft long) and 7 W6x9's (6 ft long). The transition length of this system " as defined in your drawing as the distance from the bridge rail to the DS end of standard MGS rail segment (additional post at this location) " is 25 ft.


The system that you have sent (file "7400e00") have the same rail elements and post spacings / locations as MwRSF's first approach transition design (file "MWT FD R0"). However, the posts used in the 2 designs are very different. The transition you are working with was designed with long heavy posts near the bridge rail (designed to accommodate the circumstance of Post 1 being omitted) and did not consider the upstream end " or the approach transition. Thus, NDOT's transition utilizes three 8.5 ft long W6x25's followed by seven 7.5 ft long W6x20's. (In comparison were the MwRSF transition has three 7 ft long W6x15's followed by seven 7.5 ft long W6x12's.) Although we see no problem with the transition from 8.5 ft long W6x25's to 7.5 ft long W6x20's, major problems are likely at the beginning of the transition where the 7.5 ft W6x20's are adjacent to 6 ft W6x9's. This large difference in stiffness would likely create critical pocketing and snagging. Therefore, I recommend softening the front end of the approach transition.


In order to soften the approach transition but still keep the large W6x25's, the transition length will have to be increased. As stated previously, I like the transition between the W6x25's and the W6x20's, so nothing will change on the DS end of the transition. To attach either MwRSF designed approach transition, the W6x20's will be used to represent the W6x15's of the tested MwRSF system. Both W6x20's and W6x15's have 6 in. wide flanges and the embedment depth of the two posts is only changed by 0.5 ft. The additional stiffness provided to the W6x15's by rail cap used in the MwRSF system should make the total stiffness similar to the W6x20's with a slightly deeper embedment.


Therefore, using the relation described in the previous paragraph, the 2 approach transitions designed by MwRSF can be attached to the DS end of NDOR's current transition. These attachments will result in the transition being extended for an additional 6 ft " 3 in. (the length of the segment supported by W6x25's), as shown in the attached sketches titled "Nebraska Transition Design Adaptations". What do you think of these 2 designs?


One note: MwRSF has previously adapted the approach transition to Iowa's transition. Although the posts are very different, the result was the same in that the total length of the transition was extended 6 ft " 3 in. see attached PDF.


To answer your blockout question: I would keep the blockout depth at 12 in. for the W6x25's and W6x20's (I'm assuming that was the blockout depths of the system as tested). Thus, the DS end remains the same as tested and accepted. Additionally, the increased blockout depth of 4 in. (12 in. deep blockouts on the W6x20's are replacing the 8 in. deep blockouts on the W6x15's used in the MwRSF system) can be thought of as an extra safeguard to prevent snagging between the untested transition between the W6x20's and the approach guardrail transitions.

Date October 6, 2009
Attachment MWT-SP_R4.pdf
Attachment MWT_FD_R0.pdf
Attachment Nebraska Transition Design adaptations.pdf
Attachment Iowa Transition Design Adaptation.pdf


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