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TL-2 minimum installation length

Question
State IA
Description Text

We are working on putting together standard drawings for our lower speed (45mph or under) as well as our lower volume (400 vpd or under) roadways. Doing so potentially involves aspects of both TTI and MwRSF research, so I’d like to address this as a shared conversation.

For the bridge connection, we would like to go with TTI’s MASH 31” TL-2 transition (Report 9-1002-8).

For the tangent w-beam section, we would like to go with MwRSF’s nested w-beam recommendation (borrowed from their TL-3 test 03-291-14) to cover all cases regardless if there is a curb or not, as we would rather nest when it wasn’t needed than not nest when it was.

For the end terminal, we are looking at the MASH TL-2 Softstop (38’-3.5” long) as well as NCHRP 350 TL-2 ET-Plus and SKT (25’) options.

My questions to both research groups are: What is the minimum length needed for a TL-2 system to function when attached to a bridge? What would it be if it was a free standing installation, say protecting a point hazard?

My question to MwRSF research group is: Does a TL-2 situation pose the same rupture risk with a curb under the transition/w-beam connection?

Potential installations would then be the following:
1. Assuming curb is not present at all, ends before transition/w-beam connection, or does not pose rupture risk:
a. System length of approximately 35’ (10’ transition + X’ of w-beam to meet minimum length + 25’ terminal)
b. System length of approximately 48’ (10’ transition + X’ of w-beam to meet minimum length + 38’ terminal)
2. Assuming curb is under transition/w-beam connection and does pose a risk:
a. System length of approximately 48’ (10’ transition + 12.5’ nested w-beam + X’ of w-beam to meet minimum length + 25’ terminal)
b. System length of approximately 60’ (10’ transition + 12.5’ nested w-beam + X’ of w-beam to meet minimum length + 38’ terminal)

Please let me know if you have any questions. Thank you for your assistance.

Keywords
  • Approach Guardrail Transitions
  • Guardrail
Other Keywords Minimum Length
Date February 25, 2016


Response
Response

You have several good questions below. I will try to give you my best response based on our current knowledge.

 

1.       Your first question seems to be what is the minimum system length for a the TL-2 approach guardrail transition system. For the TL-3 system, we made several recommendations related to the upstream system length based on our testing and interaction with terminals for both the transition with and without the curb. The placement of the upstream end anchorage too close to the stiffness transition may negatively affect system performance, thus potentially resulting in excessive barrier deflections, vehicle pocketing, wheel snagging on posts, vehicle-to barrier override, or other vehicle instabilities. For the transition without the curb, we recommended:

 

A recommended minimum length of 12 ft – 6 in. for standard MGS is to be installed between the upstream end of the asymmetrical W-beam to thrie beam transition section and the interior end of an acceptable TL-3 guardrail end terminal.

 

Or

 

A recommended minimum barrier length of 46 ft – 10½ in. is to be installed beyond the upstream end of the asymmetrical W-beam to thrie beam transition section, which includes standard MGS, a crashworthy guardrail end terminal, and an acceptable anchorage system.

 

For the transition without the curb, we made similar recommendations:

 

The length of W-beam guardrail installed upstream of the nested W-beam section is recommended to be greater than or equal to the total system length of an acceptable TL-3 guardrail end terminal. Thus, the guardrail terminal’s interior end (identified by stoke length) should not intrude into the nested W-beam section of the modified MGS stiffness transition.

 

A recommended minimum barrier length of 34 ft – 4½ in. is to be installed beyond the upstream end of the nested W-beam section, which includes standard MGS, a crashworthy guardrail end terminal, and an acceptable anchorage system.

 

With respect to the TL-2 transition tested by TTI, the system used the same 46 ft – 10½ in. length upstream of the asymmetrical W-beam to thrie beam transition section. Thus, while the overall system length was shorter due to the simplified transition section, the amount of guardrail upstream of the asymmetrical W-beam to thrie beam transition section and the corresponding distance to the upstream anchor was identical.

 

While we would expect that anchor loads are lower for the TL-2 system, we do not have data that confirms the anchor loads and the effect of a shorter system length at this time. Thus, we cannot shorten these length recommendations at this time. It seems reasonable that one could reduce the distance between the asymmetrical W-beam to thrie beam transition section and the end anchorage by 6 ft – 3 in. or 12 ft – 6 in. and still have acceptable system performance. However, that cannot be verified without further analysis and/or testing.

 

2.       Your second question is what is the minimum system length for a standard TL-2 MGS system. We have conducted previous research into minimum system lengths for the MGS under TL-3 (http://mwrsf.unl.edu/researchhub/files/Report281/MGSLENGTH_R8.pdf). In that study we successfully evaluated a 75’ long MGS system. Although the 75-ft MGS performed successfully, several factors, including Lateral Extent of the Area of Concern and the Guardrail Runout Length, must be considered when determining the overall barrier length for shielding a roadside hazard. Only a few roadside hazards can be properly shielded by short guardrail installations. Thus, longer guardrail installations are still required for shielding many hazards.

 

BARRIER VII simulations were conducted to investigate system lengths of 62 ft – 6 in. and 50 ft. The 62-ft 6-in. model showed promising results with rail forces, barrier deflections, vehicle behavior, cable anchor forces, and anchor displacements similar to those observed in the validated 75-ft MGS model. Thus, a 62-ft 6-in. MGS showed potential for successfully meeting MASH TL-3 standards. BARRIER VII simulations of the 50-ft system produced erratic results and model instabilities once the vehicle contacted end anchorage posts. It was concluded that the simplified BARRIER VII models of the end anchorages were limited in their ability to accurately simulate BCT posts during vehicle contact.

 

The 50-ft MGS was further investigated with LS-DYNA simulations. The LS-DYNA simulations provided more realistic wood post fracture behavior and insight into vehicle roll and pitch tendencies. The simulations showed successful redirection of the 2270P vehicle for impacts between post nos. 3 and 4, while the system gated for impacts at post nos. 5 through 8. The 62-ft 6-in. and 50-ft models both exhibited the potential for successfully redirecting an errant vehicle at the MASH TL-3 test conditions. However, these reduced-length systems would have a narrow window for redirecting vehicles and would only be able to shield limited size hazards. Due to limitations associated with the computer simulations, full-scale crash testing is recommended before these shorter systems are installed.

 

The scope of the research did not include evaluation of the performance of end terminals on a reduced-length guardrail system. Further study may be needed to evaluate reduced system length in conjunction with guardrail end terminals in redirective impacts as well as end-on terminal impacts. Guardrail end terminals may have different post sections and/or anchorage than what was utilized in test no. MGSMIN-1. Thus, shorter guardrail lengths may not have the same redirection envelope found in this study. Additionally, for compression based terminals, the system post must develop the compressive forces required for the terminal to function. Very short systems may not provide sufficient resistance to the rail forces in end-on impacts.

 

Thus, our current recommendation for TL-2 MGS lengths would still be the 75 ft length until further research can be conducted.

 

3.       Your third question was whether a TL-2 situation pose the same rupture risk with a curb under the transition/w-beam connection. We believe that the rail rupture potential would still exist if a curb was used with the transition. During the testing of the MGS transition system with a curb to TL-3, the rail rupture appeared to occur due to a combination of to heavy upward and lateral forces on the lower region of the guardrail in advance of the splice between the W-beam and asymmetrical transition segments. While a TL-2 impact would result in lower lateral guardrail forces, there is concern that the upward forces produced by the vehicle wedging between the curb and the W-beam could be very similar in a TL-2 impact. In fact, if you look at the TL-2 transition test conducted at TTI, it appears that the 1100C  vehicle did extend underneath the W-beam and lift up on the rail, and similar behavior would thus be expected when the curb was present.

 

As a side note, we have had several states that have simply incorporated the nested rail in the transition when the curb is present or not as you noted above. This, may be prudent as a means to ensure the transition always is within the tested limits.  

 

 

Thus, from your email and based on the comments above, I would get the following installation lengths (shown in red). I should note that here and in the discussion above we are referring to maximum of the tested end terminal stroke lengths or the paid system length quoted by the manufacturer. We are concerned with interaction of the terminal with the transition and paid lengths may be shorter than the actual head travel. Thus, the lengths below are assuming the 25 ft and 38 ft terminal lengths you mention reflect the maximum of the head travel or terminal length. Thus, the lengths may change slightly based on the terminal lengths.  Again, there may be potential to shorten these somewhat as noted above, but these values are based on our current guidance.

 

Potential installations would then be the following:

1.       Assuming curb is not present at all, ends before transition/w-beam connection, or does not pose rupture risk:

a.       System length of approximately 35’ (10’ transition + X’ of w-beam to meet minimum length + 25’ terminal)

                                                               i.      For a 25’ terminal length (8.67 ft from end of bridge to end of W-thrie section + 46.875 ft including 25 ft terminal = 55.545 ft)

b.      System length of approximately 48’ (10’ transition + X’ of w-beam to meet minimum length + 38’ terminal)

                                                               i.      For a 38’ terminal length (8.67 ft from end of bridge to end of W-thrie section + 12.5 ft of standard MGS + 38 ft terminal = 59.17 ft)

2.       Assuming curb is under transition/w-beam connection and does pose a risk:

a.       System length of approximately 48’ (10’ transition + 12.5’ nested w-beam + X’ of w-beam to meet minimum length + 25’ terminal)

                                                               i.      For a 25’ terminal length (8.67 ft from end of bridge to end of W-thrie section + 46.875 ft including 25 ft terminal = 55.545 ft)

b.      System length of approximately 60’ (10’ transition + 12.5’ nested w-beam + X’ of w-beam to meet minimum length + 38’ terminal)

                                                               i.      For a 38’ terminal length (8.67 ft from end of bridge to end of W-thrie section + 12.5 ft of nested MGS + 38 ft terminal = 59.17 ft)

 

Lance, if you have further thoughts on this, feel free to chime in. We are being conservative based on lack of more knowledge on minimum length systems, and you may have arguments for revising what I have here. 

Date February 26, 2016


Response
Response

In case I forgot to send it along, thank you for that response.

 

Attached are three of our initial markups based on that information.

1.       els-tl2-250 will detail a situation where we are protecting a point hazard with minimal to no concern regarding a secondary hazard.

2.       eba-tl2-250 will detail a situation where there is concern for a secondary hazard.

3.       eba-tl2 transition will detail the Barrier Transition Section shown in both layouts. This includes the short thrie-beam, asymmetrical section, and nested w-beam.

 

My question today is related to eba-tl2-250. In report TRP-03-291-14 (page 137 – statement 3) it mentions that when a TL-3 installation is flared, there needs to be an additional 12.5’ section of single w-beam added upstream of the nested w-beam before the flare (as stated in our current BA-250 circle note 5). What I’m wondering is whether or not that remains true for our TL-2 situations.

 

Date February 27, 2016
Attachment eba-tl2 transition.pdf
Attachment eba-tl2-250.pdf
Attachment els-tl2-250.pdf


Response
Response

We would recommend keeping that 12.5’ tangent section upstream of the nested section for TL-2. The two concerns are:

1.       Having a flared section directly adjacent to a stiffened region of the barrier may increase pocketing, rail loading, and vehicle instability.

2.       We are somewhat concerned that small cars impacting in the flared region upstream of the  transition could extend further under the stiffened transition. This may cause increased underride, vehicle snag on posts, increased vehicle decelerations, and increased rail loads.

 

While we would agree that this potential should be reduced for TL-2, without further analysis or testing, we want to take a more conservative approach.

 

Thanks 

Date February 28, 2016


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