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Good afternoon!

We have been
approached by our Rail office to review available designs for putting chain
link fence on top of barriers to prevent materials from falling onto the tracks
below. Since the fence is adjacent to vehicular traffic, as opposed to
pedestrian traffic, there is some concern on my part regarding what testing
needs to be done to support this application for MASH.

In 1997, FHWA
published Crash Testing and Evaluation of Retrofit Bridge Railings and
Transition – FHWA-RD-96-032 (attached) that discussed a PL-2 testing of a
32” New Jersey shaped fence/barrier combination rail. Reviewing the PL-2
testing criteria suggests it would be equivalent to a point somewhere between
TL-2 and TL-3. Iowa has used this design in urban areas where a TL-2 speed is
present but there is a growing need to have something available at TL-3 speeds.

A recent search
for available designs yielded the following:

1. color:#595959">       Iowa - attached is an Iowa example from
a recent project

2. color:#595959">       Nebraska - http://www.roads.nebraska.gov/media/2912/bopp-manual.pdf
 (pdf page 437)

3. color:#595959">       Minnesota - http://www.dot.state.mn.us/bridge/pdf/lrfdmanual/section13.pdf 
(pdf page 6 lists as TL-2 for Design 5-397.212)

http://www.dot.state.mn.us/bridge/pdf/cadd/files/bdetailspart2/pdf/fig7119e.pdf
for drawing

I’m curious to
get your take on the following:

1. color:#595959">       What test level a PL-2 may be considered
equivalent to for NCHRP 350 and/or MASH.

2. color:#595959">       How high the concrete barrier would need
to be in order to not need the chain link attachment tested for TL-3 conditions
because we would not expect the TL-3 vehicle to interact with the fence (44
inches, 54 inches,?)

3. color:#595959">       What MASH tests would need to be
considered to verify the combination barrier met TL-3 conditions, as well as
estimated project costs and timeframe. This presumes building the resulting
barrier from question 2 is undesirable or infeasible given design or
construction constraints.

Thank you!

There are concerns with mounting fence structure on concrete barriers. First, the vehicle may interact with the fence structure causing snag. This may pull the fence down on the vehicle or cause deceleration or instability of the vehicle that is undesirable. it is essentially a zone of intrusion issue with the vehicle interacting with the fence.

We looked at this several years ago in a Pooled Fund proposal for Illinois, but the project was never funded. We also commented on a fence for 32” barrier for Illinois based on a FLDOT design tested at TTI under PL-2 as you mentioned in your email. See  link - http://mwrsf-qa.unl.edu/view.php?id=174

As to your questions:

1.                   What test level a PL-2 may be considered equivalent to for NCHRP 350 and/or MASH.

a.       AASHTO PL-2 is a lower speed and angle than the NCHRP 350 TL-3 testing requirements. Thus, we would consider PL-2 somewhere between TL-2 and TL-3. With the increased speed and angle, we would expect that the ZOI and potential for interaction with fence structures would increase with the angle having the largest effect. While arguments have been made in the past regarding PL-2 barriers equivalency with TL-3 based on test results and comparisons of barrier capacity and geometry, that argument may be more difficult here due to the concerns for increased interaction with the attached fence.

2.                   How high the concrete barrier would need to be in order to not need the chain link attachment tested for TL-3 conditions because we would not expect the TL-3 vehicle to interact with the fence (44 inches, 54 inches,?)

a.       We have not formally determined this, but we have investigated it in the past for a couple of states. We looked at ZOI values for single slope and F-shape barriers at various heights for Wisconsin and Florida. In this we recommended that the ZOI for a single slope and safety-shape barriers under NCHRP 350 TL-3 were zero for 42” tall barriers. Again, these were estimates based on our best knowledge at that time and do not consider the MASH vehicles. Additionally, TL-4 ZOI values would NOT be zero for the 42” tall  single slope and safety-shape barriers.

b.       For vertical barriers, our best guidance has been based on a TTI TL-3 test of a 42” tall, vertical shape aesthetic barrier. This testing had ZOI values of 1.4 and 1.7 ft. We have noted in the past that ZOI for vertical barriers may be higher than those for single slope and safety shapes due to differences in the vehicle roll. Top of hood heights for the 2000P vehicles tended to be around 42”-43” while for the 2270P vehicle the hood heights are more in the 45-48” range. Based on this, we would anticipate that the height of the barrier may need to be above 48” in order to eliminate ZOI concerns. However, this has not been verified to date.

c.       TTI has tested a 42”, vertical, open concrete bridge rail to MASH with the 2270P vehicle. (https://www.roadsidepooledfund.org/wp-content/uploads/2017/01/9-1002-15-5-T224Submittal.pdf)  Video and sequential photos from that test suggest that the vehicle extended over the top of the rail approximately 8-10”. This would seem to further suggest that vertical barrier heights to eliminate ZOI may be 48” or more.

3.                   What MASH tests would need to be considered to verify the combination barrier met TL-3 conditions, as well as estimated project costs and timeframe. This presumes building the resulting barrier from question 2 is undesirable or infeasible given design or construction constraints.

a.       Evaluation of a vertical parapet fence combination would likely require a single test, MASH test 3-11 to evaluate vehicle interaction with the fence and evaluate the barrier to TL-3. Small car testing would not be required.

Let me know if that answers you questions or if you need more information/discussion.