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I’m looking for
guidance for the following situation regarding the length of 4” curb placed
through the end of the nested w-beam of a typical 37.5’ Barrier Transition
System (BA-201).

As you can see
on BA-201, we show possible curb running through the end of the BTS as report 03-291-14
Dynamic Evaluation of MGS Stiffness Transition with Curb suggests in the
second full paragraph on page 136. We denote this 4” curb via circle note 2,
which states to see project plans. Project plans in this case typically refer
to our Bridge Approach Standards (BR
Series), and therein lies the problem. On the Bridge Approach Standards
that show abutting pavement (BR-102, 103, 104, 105, 106, 107, 112, 205, 211,
212, 231), we have a circle note that states something to effect of “Build 4
inch Sloped Curb to end of Double Reinforced Section.” This note has been shown
on our approach standards since April 1999 when we switched to the 4” curb. A
typical double reinforced section extends 20’ out from the center of the
roadway (see image below). I would say that in almost no case would the double
reinforced section extend 37.5’ and meet the report’s suggestion. What I would
like to know is which of the following is the real issue with curb location:

1)      
If
curb is present along any length of the thrie-beam portion, asymmetric
transition portion, or nested w-beam, it must be run from the bridge end out to
at least 37.5’, and thus typically ending at the nested-to-single w-beam
splice, or

2)      
If
curb is present under the 7’ (approx.) asymmetric transition, it must be run
out to the nested-to-single w-beam splice.

The first
option forces a 37.5’ minimum install length of curb. The second option says to
end the curb before the asymmetric transition (somewhere in the first 18’ or
so) OR carry it out to the nested-to-sing w-beam splice
(at least 37.5’).

I ask because
both our “old” Barrier Transition Section standard (older
BA-201) and “new” Barrier Transition System (BA-201)
have the asymmetric transition ending 25’ away from the bridge end. With a
typical bridge end section of 7’ (BA-107),
that placed the asymmetric transition 25’ to 32’ from the edge of the
deck/beginning of double reinforced section. Perhaps on the long side of a
strong skew we could have seen the edge of the double reinforced be in line
with the asymmetrical transition, but it was likely a rare occurrence, which is
great if #2 above is what was meant. If #1 is the real issue, then I’ll have a
follow up email about the immediacy of change for our current standards. 

Your option 2 is more accurate.  If you wish to have curb extend from the bridge rail/buttress and terminate prior to the asymmetric rail section, this would be acceptable.  In fact, prior to the success of the full-scale test you are referencing, we had recommended terminating the 4” curb within the thrie beam section of the approach guardrail transition.   I apologize for this not being clearer in the report.

Given that clarification, here is what I plan to propose to designers. Please let me know if this is incorrect.

Using BA-201 as an assumed Barrier Transition Section and a standard 5’ curb transition as shown on page 2 of PV-102 (which meets your 3’ min curb transition recommendation on page 136 of TRP-03-291-14):

A)      If any part of the 4” sloped curb (including the curb transition) extends into the asymmetrical transition section, the minimum curb length must be extended to 37.5’ and they are to use the new 37.5’ BA-201 showing the nested w-beam and also meet the general layout requirements as shown on page 137 of TRP-03-291-14 Dynamic Evaluation of MGS Stiffness Transition with Curb:

B)       If the 4” sloped curb (including the curb transition) is terminated prior to the asymmetrical transition section, the older BA-201 (25’ instead of 37.5’) may be used but must also meet the general layout requirements as shown on page 154 of TRP-03-210-10 Development of the MGS Approach Guardrail Transition using Standardized Steel Posts:

I ask for confirmation because the potential installation length differences between the two are significant. Assuming all other things equal, the first has a minimum install of (37.5 BTS+50.0 w-beam+3.125 connection+50.0 terminal) 140.625’. The second has a minimum install of (25.0 BTS+12.5 w-beam+3.125 connection+50’ terminal) 90.625’, or a 50’ installation length increase for not terminating the curb early. This may not matter on the interstate or higher volume roadways where the runout length (according to Roadside Design Guide) is well beyond that, but certainly would come into play on our lower volume roadways and especially our county/local roadways. It may also cause us to rethink requiring the curb to extend to the end of the double reinforced section (discussed below) as this would essentially guarantee the longer installation.

Another situation we run into frequently is having to drop down from a TL-3 Barrier Transition Section to a TL-2 where we essentially eliminate the thrie-beam downstream of the asymmetrical transition and attach the asymmetrical transition piece to the Bolted End Anchor (BA-202). This TL-2 system was tested at TTI (available here with detail attached). This typically comes into play when we have an entrance/side road within the normal guardrail installation. Since I wouldn’t expect for you to speak for another research facility, TTI in this case, I will make the following statement and ask that you agree or disagree based on the general principles at play.

A)      Since the failure in TRP-03-291-14 was due to the existence of curb in the asymmetrical piece, any curb coming off of the bridge end would extend into that section and thus a 12.5’ section of nested w-beam should be added to the upstream end of the transition, along with w-beam equal to the length of the end terminal (25’ TL-2 typically).

B)       If there is no curb, no nested w-beam is needed but a length of w-beam equal to the length of the end terminal should be included, OR

C)       If there is no curb, no nested w-beam is needed but a 12.5’ section of w-beam should be included before the end terminal.

Again, please confirm my understanding on both points. And as always, your assistance is appreciated.

I have commented below in RED

Given that clarification, here is what I plan to propose to designers. Please let me know if this is incorrect.

Using BA-201 as an assumed Barrier Transition Section and a standard 5’ curb transition as shown on page 2 of PV-102 (which meets your 3’ min curb transition recommendation on page 136 of TRP-03-291-14):

A)      If any part of the 4” sloped curb (including the curb transition) extends into the asymmetrical transition section, the minimum curb length must be extended to 37.5’ and they are to use the new 37.5’ BA-201 showing the nested w-beam and also meet the general layout requirements as shown on page 137 of TRP-03-291-14 Dynamic Evaluation of MGS Stiffness Transition with Curb:

B)       If the 4” sloped curb (including the curb transition) is terminated prior to the asymmetrical transition section, the older BA-201 (25’ instead of 37.5’) may be used but must also meet the general layout requirements as shown on page 154 of TRP-03-210-10 Development of the MGS Approach Guardrail Transition using Standardized Steel Posts:

I ask for confirmation because the potential installation length differences between the two are significant. Assuming all other things equal, the first has a minimum install of (37.5 BTS+50.0 w-beam+3.125 connection+50.0 terminal) 140.625’. The second has a minimum install of (25.0 BTS+12.5 w-beam+3.125 connection+50’ terminal) 90.625’, or a 50’ installation length increase for not terminating the curb early. This may not matter on the interstate or higher volume roadways where the runout length (according to Roadside Design Guide) is well beyond that, but certainly would come into play on our lower volume roadways and especially our county/local roadways. It may also cause us to rethink requiring the curb to extend to the end of the double reinforced section (discussed below) as this would essentially guarantee the longer installation.

I agree with the installation options A) and B) above.  However, the minimum total lengths of the systems should be identical.  The only difference between the installation length recommendations was the reference point.  The Option A reference point is the upstream end of the W-beam section, while the reference point for Option B is the upstream end of the asymmetrical segment.  Since the nested region is 12.5 ft long, the lengths should add up:

                Requirement #1

                                A: 37.5 ft (end shoe through nested w-beam) + Terminal  =  37.5 ft + Terminal length

                                B: 25 ft (end shoe through w-to-thrie segment) + 12.5 ft (standard MGS) + Terminal = 37.5 ft + Terminal length

                Requirement #2

                                A: 37.5 ft (end shoe through nested w-beam) + 34.38 ft (w-beam)  =  71.88 ft

                                B: 25 ft (end shoe through w-to-thrie segment) + 46.88  ft (standard MGS) = 71.88 ft

                Requirement #3

                                A: 37.5 ft (end shoe through nested w-beam) + 12.5 ft (MGS) + Flared Terminal  =  50 ft + Flared Terminal

                                B: 25 ft (end shoe through w-to-thrie segment) + 25 ft (MGS) + Flared Terminal = 50 ft + Flared Terminal

Another situation we run into frequently is having to drop down from a TL-3 Barrier Transition Section to a TL-2 where we essentially eliminate the thrie-beam downstream of the asymmetrical transition and attach the asymmetrical transition piece to the Bolted End Anchor (BA-202) There should be 37.5” of either 10 ga. thrie beam (as tested) or nested 12 ga. thrie beam between the asymmetrical segment and the end shoe . This TL-2 system was tested at TTI (available here with detail attached). This typically comes into play when we have an entrance/side road within the normal guardrail installation. Since I wouldn’t expect for you to speak for another research facility, TTI in this case, I will make the following statement and ask that you agree or disagree based on the general principles at play.

A)      Since the failure in TRP-03-291-14 was due to the existence of curb in the asymmetrical piece, any curb coming off of the bridge end would extend into that section and thus a 12.5’ section of nested w-beam should be added to the upstream end of the transition, along with w-beam equal to the length of the end terminal (25’ TL-2 typically).

B)       If there is no curb, no nested w-beam is needed but a length of w-beam equal to the length of the end terminal should be included, OR

C)       If there is no curb, no nested w-beam is needed but a 12.5’ section of w-beam should be included before the end terminal.

Since we do not have testing of the TL-2 system with curb, it would be conservative to add the nested section of W-beam upstream of the asymmetrical segment to prevent possible rail tearing when a curb is present. As such, I agree with these installation configurations and  lengths.

Two additional questions for you then.

I agree that requirements 2 and 3 would produce installations that are identical in length. Makes sense that you’re simply turning a piece of standard w-beam into nested. What I’m perhaps more confused with now is how requirement 1 is bringing me to the same length for each situation.

Part A

                BTS = 37.5’

                W-beam upstream of nested to be greater than or equal to end terminal = 50’ (or 37.5’ for cable connection)

                End terminal = 50’ (or 37.5’ for cable connection)

Total install = BTS + 2 times end terminal length = 37.5’ + 2(50’) = 137.5’

Part B

                BTS = 25’

                w-beam = 12.5’

End terminal = 50’ (or 37.5’ or 25’)

Total install = 87.5’

The other question is in regards to the 12.5’ section of standard w-beam between the asymmetrical transition piece and the end terminal (requirement B,1 below). What was the underlying concern that introduced this section? We have plenty of existing installations where it is simply a BTS to End Terminal.

Sorry for all the questions, I’m just trying to understand the underlying principles in order to make correct modifications as needed.

For requirement 1, the length upstream of the nested w-beam section is to include the terminal itself.  Thus, you only need your BTS length (37.5 ft) plus the length of the terminal (50 ft), for a total of 87.5 ft – same as option B.  Sorry for the mix up there, we probably could have worded that better.

Concerning the 12.5 ft for requirement B-1:

The overlying reasoning is to separate any transition elements from the end terminal so as not to affect the performance of the end terminal.  A different rail segment could easily affect performance, so we need to keep the stroke length of the terminal upstream of the 10 ga. w-to-thrie segment. Additionally, there is a post at ½ post spacing (37.5”) upstream of the w-to-thrie segment.  Depending on the specific terminal, utilizing a different post spacing (or different post altogether) may also affect the performance of the terminal.  Thus, we wanted to stay upstream of the ½ post spacing portion of the transition as well.  We could have used a 6’-3” distance in this recommendation (B-1), but many states don’t stock 6’-3” segments of w-beam guardrail.  Therefore, the length was conservatively extended to 12’-6”.  If you desire to use the shorter 6’-3” distance for plans, I couldn’t argue against it.