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Guardrail Connector Plate

Question
State MN
Description Text MnDOT, is in the final phases of developing a new AGT standard. It’s the thrie-beam version with the first three larger posts (sized at 84” – W6 X 15) before the concrete end parapet connection. See the attached proposed standard plan (694_AGT_type31_SingleSlope.pdf).
Most newer bridge designs will have an integral abutment with the approach panel. The expansion/contraction joint will be as the end of the approach panel and the parapet.
A concern has been brought to our attention concerning the thrie-beam anchorage plate (see attached standard 8350A). The concern is that the 1 1/8” slotted holes will not allow enough room for expansion/contraction and that the guardrail will be push back and forth, which would move the posts out of vertical. While we do not know if this is a valid concern, we do have an existing standard for a w-beam rail anchorage plate with 3” slots (see attached standard 8318C).
Our question; would a modification of our 8350A standard, to include a 3” (or less) slot, be acceptable for our proposed 694_AGT_type31_SingleSlope.pdf design?
I look forward to your response. Please feel free to give me a call if you have any questions or need additional information.
Keywords
  • Guardrail
Other Keywords End shoe attachment
Date September 16, 2015
Attachment 8350A.pdf
Attachment rte01b.pdf


Response
Response

I have accumulated some feedback from my colleagues. Here are a few thoughts.

 

Karla Summary:

In the hardware guide, RWE02a-b is the W-beam Terminal Connector. When comparing the RWE02a-b slots to the slots in MNDOT’s standard 8318C, they are similar. The hole diameter in the MNDOT drawing is a little smaller. Slot dimensions for RWE02a-b is 31/32” x 3”, while slot dimension for MN 8318C is 29/32” x 3”.

 

RTE01b is the Thrie-Beam Terminal Connector. When comparing the RTE01b slots to the slots in MNDOT’s standard 8350A, they are different. The RTE01b shows the slots at a 50-degree angle, and they are 31/32” x 1¾”.  However, it is noted that the slots could be oriented parallel to the longitudinal axes rather than at the 50-degrees. MNDOT’s standard has a smaller diameter and a shorter length, 13/16” x 1â…›”.

 

Therefore, I would say MNDOT could at least increase the slot size to what the hardware guide shows without any concerns. I know that this doesn’t get to 3”.

 

Scott Summary:

First thoughts – a 3” slot is large. During impacts, it may take quite a while for the rail to shift enough, develop tensile loads, and create the membrane action typically associated with guardrail redirections. This shift could lead to pocketing/snag issues. On the other side of the argument, transitions rely more on lateral force loads provided by posts and rail bending. So maybe enlarged slots don’t cause any issues. Without tests, I’m unsure what effect this has on performance.

 

An idea to strengthen their transition with 3” slots may include shortening the unsupported length of the thrie beam between the concrete parapet and Post 1 (shift the guardrail system DS). Reduce this length from 29” to say 15”-18”. I would feel better about reduced snag concerns on the concrete end with a reduced distance.

 

I would be more comfortable if the expansion/contraction joint had concrete on both sides of it – which I believe is more common amongst our Pooled Fund States. Ideally, the 7-ft long standardized buttress would be on the upstream side of the joint. However, they may be trying to shorten the length of the transition system and thus, have spanned the thrie beam across the joint. Not sure they want to add an additional 7 ft.

 

Last thought, I hope that any potential enlarged slots would be punched into the end shoe during fabrication, not cut by hand just prior to installation. Are they working with a manufacturer to produce the part?

 

*****************

 

Based on the above feedback, there are some concerns with extending slots more than used in historical crash-tested systems. Increased slot length could lead to increased lateral deflection in advance of the rigid end. With a longer slot at the end shoe splice location, the rail tension will not develop as quickly. I concur with Scott on this issue. Instead, the rail may more easily move back even though resisted by posts and rail bending capacity due to overlap on parapet. At this time, we unfortunately do not know how much longitudinal shift is acceptable before rail tension is developed. I am aware from prior BARRIER VII modeling efforts that rail tension can be greater than some expect and near the buttress end, say up to 80 to 90 kips during high-energy impact events. Excess rail deflections can lead to pocketing and/or wheel snag as well as increased propensity for vehicle instabilities.

 

Unfortunately, I cannot easily determine the exact slot lengths, and bolt positions within those slots, that were used in prior crash test efforts of AGTs. What we can say is that standard thrie beam end shoes were used with the commercially-available, industry-accepted slot patterns and sizes, including some variations denoted in hardware guide.

 

Finally, if I had a choice, I would rather locate the expansion gap between the concrete rail end and the buttress end. However, it may be possible that the 3-in. slot length will not cause any problems in a crash testing program. Unfortunately, I am less certain regarding the system’s safety performance with a change under MASH.

 

Please let me know if you want to further discuss this issue! Also, I need to look at a few other dimensions in the near future. Thanks!

Date September 17, 2015


Response
Response

We appreciate your (Karla and Scott, also) information and insight.

 

We are looking into using the TF-13 design RTE01b, and have a few questions.

 

Was the RTE01b version used in your research outlined in the Transition Report TRP-03-180-06?   We have this as one of our standard options, but could not tell from the report.

 

I have seen both versions on other state standards, one similar to ours and the RTE01b.

 

Also, I am interested in the number of attachment holes for attachment to the barriers on the end.  Ours has five 1” diameter holes and RTE01b has 9 (two ¾” and seven 1”).

Why is there a difference.  When would we ever need more than the five 1” holes?

 

You help is appreciated.

Date September 23, 2015


Response
Response

Hello Mike!

 

I will respond to your questions below. I have also provided a photograph from the noted reported to inform you of what bolt hole pattern was used in this test series. The end show have 5 bolt holes versus nine.

 

Photo from test 2, page 71, figure 46 of TRP-03-180-06.

 

Ronald K. Faller, Ph.D., P.E.

Director and Research Associate Professor

 

Midwest Roadside Safety Facility (MwRSF)

Nebraska Transportation Center

University of Nebraska-Lincoln

130 Whittier Research Center

2200 Vine Street

Lincoln, Nebraska  68583-0853

 

(402) 472-6864 (phone)

(402) 472-2022 (fax)

rfaller1@unl.edu

 

From: Elle, Michael (DOT) [mailto:michael.elle@state.mn.us]
Sent: Wednesday, September 23, 2015 11:39 AM
To: rfaller1@unl.edu
Cc: Brown, Timothy (DOT) <timothy.j.brown@state.mn.us>
Subject: RE: Guardrail Connector Plate

 

Rom,

 

We appreciate your (Karla and Scott, also) information and insight.

 

We are looking into using the TF-13 design RTE01b, and have a few questions.

 

Was the RTE01b version used in your research outlined in the Transition Report TRP-03-180-06?   We have this as one of our standard options, but could not tell from the report.

**As shown above, the as-tested end shoe part used 5 mounting holes. It does appear as though the CAD details in figure 45, page 70, shows extra holes in the end shoe.

 

I have seen both versions on other state standards, one similar to ours and the RTE01b.

**The previously-provided TTI report revealed static testing on multiple versions of end shoes.

 

Also, I am interested in the number of attachment holes for attachment to the barriers on the end.  Ours has five 1” diameter holes and RTE01b has 9 (two ¾” and seven 1”).

Why is there a difference.  When would we ever need more than the five 1” holes?

 

**Thrie beam end shoes are anchored with 5 bolts – three in column 1 and two in column 2 located 8 in. away from vertical centerline of column 1. Unfortunately, I do not know the history as to why some end shoes have 9 holes. I would suspect that a greater number of holes spaced close to one another could potentially lead to part fracture at slightly lower loads. At any rate, we use only 5 anchor bolts for these parts.

 

Let me know if you have any further questions regarding this information.

Date October 1, 2015


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