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Tall median barriers and appropriate levels of dynamic load

Question
State VA
Description Text

Gentlemen:



 



As part of our pooled fund tech support from MWRSF, I have
some questions related to median barriers, but I want to have the new research out
of TTI related to TL-4/TL-5 moment slabs be considered so I have sent this to
both TTI and MWRSF to get a more broad perspective… I hope that is OK.



 



Our roadway folks would like to have a median barrier on our
bridges that performs as both a bridge barrier and a glare screen. They want it
to be a minimum of 50” high and be suitable for a TL-4 impact.  We do not
have bridge barrier details for barriers taller than 42” except for our 54”
TL-5 pier protection barrier (nobody wants to build that barrier because of the
high levels of reinforcing we have required details in the attachment). 
They want a much lighter level of reinforcing because TL-4 has a much smaller
horizontal load. They have proposed 2 alternatives from another state (details
in the attachment). There is such a great difference between the two barriers
that I think I need some questions answered.



 



So…



 



I have attached a file that collects some of our details in
coherent fashion (I hope).  And I am asking a few questions related to how
we might accomplish everyone’s deepest desires for this project.  Some of
these questions may be more of a best practices approach than what is specifically
required in the AASHTO LRFD Spec in Chapter 13.



 



1.       Is
there a problem with having an F shape barrier transition to a vertical face
above 32” (or  36” for MASH) as is shown on page 1 of the attached.



2.       If
we call something a “split median barrier” shouldn’t it still be treated as 2
barriers?  I don’t have a combination barrier do I? The 2 separate
barriers each needs to meet all crash testing and design criteria.  In
other words, I can’t treat the pair of disconnected barriers as a single
element and I can’t just design the top of the barrier to fail and be propped
up by the barrier in the other direction.



3.       From
a best practices perspective (i.e. Justified by testing but not in the LRFD
Specification), if I increase the height of a TL-4 F-shape barrier from a MASH
minimum of 36” (required by testing) to 50” (or more) for the purposes of
glare, it will absorb a much bigger load when hit by the design vehicle. And to
perform as intended when hit by a TL4 sized box truck, it should be designed
for a higher load (based on the new TTI research).



4.       To
be safe, and not to potentially snag the design vehicle, and to not to leave a
big failed region of concrete at the top of the tall barrier, I should increase
the dynamic force being applied to the barrier and detail reinforcing to resist
that increase dynamic force.  (In other words, there is no free pass to
arbitrarily increase the height of any barrier; increases in height are
followed by commensurate increases in load. The increases are currently
identified in stepwise fashion in the yet to be published NCHRP report and are
proposed for inclusion in the LRFD specification but are not currently part of
the specification.)



5.       Under
the existing LRFD Specification ( including direction outside of Chapter 13) is
there any language requiring an increase in force as the height of the barrier
is increased? I don’t see it, but I don’t want to miss it if it is there.



6.       Is
there a recommended minimum thickness of concrete for either TL-4 or
TL-5?  (what thickness should the top of the barrier be, is there a number
below which we should not go)? One of the issues that I am concerned about is
that the top just fails due to shear as the thickness gets smaller and smaller,
especially if the FDOT style U shaped bar is used with the straight tips at the
top of the barrier where there is limited development length and a thin section
as is proposed on pages 6 and 7.



7.       Is
a single layer of rebar appropriate at the top of a barrier or should the top
always have enough thickness to have 2 layers of bars? (this is related to how
thin is too thin above). 



 



Thank you for your time in advance. I think the answers are
mostly straight forward but I can’t be 100% sure.  I hope that you can
help me out.  

Keywords
  • Permanent Concrete Barriers
Other Keywords none
Date August 25, 2017
Attachment median railing questions with reference documents-2.pdf


Response
Response

Please see my comments below!

.      Is there a problem with having an F shape barrier transition to a vertical face above 32” (or  36” for MASH) as is shown on page 1 of the attached.

**There have been a few transitions between F-shape and vertical barriers. Often, a bridge rail or roadside barrier are transitioned to a buttress where an approach guardrail transition connects to the a vertical-face barrier.[andy.zickler]   so this detail will provide safe and smooth redirection as long as the steel detailing and concrete mass are sufficient to hold the barrier together?

2.      If we call something a “split median barrier” shouldn’t it still be treated as 2 barriers?  I don’t have a combination barrier do I? The 2 separate barriers each needs to meet all crash testing and design criteria.  In other words, I can’t treat the pair of disconnected barriers as a single element and I can’t just design the top of the barrier to fail and be propped up by the barrier in the other direction.

**In general, both barriers would need to be treated separately unless you ensured that they behave as one barrier throughout the length.[andy.zickler]  I am not sure how I could get them to “behave as one barrier”.  I would like to discuss further.

3.      From a best practices perspective (i.e. Justified by testing but not in the LRFD Specification), if I increase the height of a TL-4 F-shape barrier from a MASH minimum of 36” (required by testing) to 50” (or more) for the purposes of glare, it will absorb a much bigger load when hit by the design vehicle. And to perform as intended when hit by a TL4 sized box truck, it should be designed for a higher load (based on the new TTI research).

**At 39 to 42”, we would design the barrier for 80 kips using the new NCHRP 22-20(2) loads. If at 50 to 54”, our TL-4 design loads would be higher, say 93.3 kips.[andy.zickler]  so say 95kips as a load to consider for the yield line analysis.  As these numbers go up I get concerned about punching through, especially since failures in the field tend to look more like punching shear issues than flexural failures.

4.      To be safe, and not to potentially snag the design vehicle, and to not to leave a big failed region of concrete at the top of the tall barrier, I should increase the dynamic force being applied to the barrier and detail reinforcing to resist that increase dynamic force.  (In other words, there is no free pass to arbitrarily increase the height of any barrier; increases in height are followed by commensurate increases in load. The increases are currently identified in stepwise fashion in the yet to be published NCHRP report and are proposed for inclusion in the LRFD specification but are not currently part of the specification.)

**As noted above, the taller MASH TL-4 barrier would resist an 93.3-kip load but at 39 – 42” resist 80 kips. Punching shear near the top of the barrier may be the only other minor concern. However, I am not overly concerned with it if you use 93.3 kips on a tall barrier.[andy.zickler]  following up on your last sentence about not being overly concerned… is that because the “d” required to exceed the moment requirements would result in a section that is thick enough to withstand a punching shear failure?

5.      Under the existing LRFD Specification ( including direction outside of Chapter 13) is there any language requiring an increase in force as the height of the barrier is increased? I don’t see it, but I don’t want to miss it if it is there.

**NCHRP 22-20(2) provided this guidance, as noted in prior emails from TTI and attached.[andy.zickler]  I am looking for direction inside of the LRFD specification.  I have design build contracts that require the designs to meet the specification but they don’t have to exceed it. I am looking for spec language that would help us to require parapet designs with forces greater than 54kips when the parapets are very tall.

6.      Is there a recommended minimum thickness of concrete for either TL-4 or TL-5?  (what thickness should the top of the barrier be, is there a number below which we should not go)? One of the issues that I am concerned about is that the top just fails due to shear as the thickness gets smaller and smaller, especially if the FDOT style U shaped bar is used with the straight tips at the top of the barrier where there is limited development length and a thin section as is proposed on pages 6 and 7.

**For the top of a TL-4 barrier, I might say that a 7 to 8” top thickness may be close to a minimum width to fit bent and straight bars in region with adequate cover. Slightly thicker that 7 to 8” would be preferred. We are working on a similar effort and will be showing preliminary results soon. I will ask for Jeremiah Dixon to add any comments based on his current research. This is good news.

7.      Is a single layer of rebar appropriate at the top of a barrier or should the top always have enough thickness to have 2 layers of bars? (this is related to how thin is too thin above).

**We have worked toward always providing a dual layer of vertical reinforcement near the top of the barrier. However, there could be scenarios where only one layer would work with the load imparted lower on the barrier face.[andy.zickler]  you seem to be leading to something like “but when the load is applied higher two layers of rebar will be required to be most effective or efficient.

**Once you review the comments, we may want to further discuss over the phone, if desired.

Date September 7, 2017


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