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We’re looking to make some adjustments to our standard guardrail drawings, specifically where MGS transitions into a concrete barrier end section as we are having trouble aligning as-designed with actual field conditions. In comparing recent MGS approach tests (TRP-03-210-10 and TRP-03-291-14), it appears the space between the end of the concrete barrier and the first post (or between posts 19 and 18 in the reports) is 37.5”. Our current transition design (BA-201) connects to one of the three typical end sections by BA-202 (Type A, B, or C). Type A is new construction, Type B is an older design with a slight flare, and Type C is either the flared end or a catch-all for those not fitting A or B.

My question to you is, what is the maximum spacing between the concrete barrier end section and the center of the first post before we would be concerned about a vehicle’s ability to contact the leading edge of the end section? As it relates to BA-202, this would be the 11.5” dimension currently shown in the plan view at the top of page 1. Type A is less of a concern as we can adjust the bolt hole locations as part of construction, but for Types B and C, we’re frequently at the mercy of what was there previously and are concerned that we’re leaving too much of a gap. The designs in the two reports would suggest a dimension less than 37.5”, as some of that would overlap the end section, but I was curious if recent testing for a generic end section provided a better value.

The gap between the concrete buttress and the first post of a transition system is dependent upon the specific transition system you are using.  The Iowa transition was initially developed with the first post offset 11.5” from the buttress, as you noted in your email and have in your details.  The thrie beam transition in which the upstream w-to-thrie beam transition (TRP-03-210-10 and TRP-03-291-14) was tested with utilizes larger posts at 37.5” spacing and a 37.5” offset.  However, this offset/gap would not directly apply to the Iowa transition.  So, the Iowa transition’s nominal offset between the first post and the concrete buttress should remain at 11.5”.

 It’s important to note that this distance is measured from the center of the first post to where the rail contacts the face of the buttress (i.e., it’s the unsupported span length of the of the thrie beam).  Changing the shape of the buttress will alter the unsupported span length if  the taper, chamfer, of flair is increased without shifting the location of the first post.  An increased unsupported span length may lead to increased deflections and vehicle snag on the buttress.  Further, utilizing a different buttress shape from the as-tested system can have significant effects on the performance of the transition.  We generally don’t recommend altering the buttress on a transition without first doing some sort of evaluation on the new combination.

 I understand that there will be situations where you have to attach to existing buttresses.  In these situations, I would recommend that you try to keep the unsupported span length at 11.5”, or minimize this distance as much as possible.  I don’t have good feeling on what the maximum allowable safe distance would be, but I would not be comfortable with increasing the distance to 37.5”, or over 3 x’s the nominal distance. We have previously conducted a study on retrofits for transitions to existing buttress where the first post could not be installed as intended.  A couple of horizontal beams that attach to the backside of the buttress and support a blockout at the appropriate location were designed as part of this study.  See report no. TRP-03-266-12 on the website for more details.

 The current project to develop a standardized buttress aims to allow for singular buttress design for all thrie beam transitions. However, the nominal offset to the first post (unsupported span length) will remain the same.  If you need a distance near 37.5”, you may want to consider using a secondary transition system which incorporates larger posts at 37.5” spacing as these transitions often utilize a larger offset to the first post.  If you are interested in another transition option, let me know and I can help identify potential systems (Nebraska uses one that may work for you).