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Report TRP-03-162-07 Traffic/Bicycle Bridge Railing

Question
State IA
Description Text

Q1. The concrete barrier in the report is called a "standard single slope concrete barrier". Is this the barrier found in "Figure B7.65 Single Slope Concrete Bridge Rail (94, 36, 95, 96)" at the FHWA site, http://safety.fhwa.dot.gov/roadway_dept/docs/appendixb7g.pdf, ... or is it something else? I'm curious because there are several standards out there (California, Florida, Missouri, etc.), and they have different widths at the top of the barrier. This is important because at no point in the report is the setback dimension given for the steel railing from the top traffic face corner, and it isn't drawn up in either Figure 4 or Figure 37. Increasing the setback is mentioned as one strategy for improving the barrier's performance (Part 10 Recommendations).

 

Q2. See the photo labeled "0.060 sec" in Figure 21. Did your team make an effort to estimate the total intrusion of the test vehicle beyond/through the railing system? Did the steel railing actually prevent some intrusion that would have occurred with a plain SS barrier w/o attachments?

 

Q3. See photos "0.086 sec" in Figure 22, and "0.058 sec" and "0.096 sec" in Figure 46. Would you conclude that it was primarily the middle rail of Test MOBR-1 and the third rail up (from top of barrier) of Test MOBR-2 that were preventing vehicle ride-up on the barrier? Those rails exhibit significant upward deflection. It appears from the photos that the rails below those that I mentioned were actually deflected downward, and so probably did not apply a downward force to the vehicle. So, it seems that a steel rail with its bottom surface as low as 1136 MM (middle rail of first test) will potentially apply a downward force on the pickup hood, but a rail with its bottom surface as high as 1045 MM (2nd rail up on second test) may not have that potential.

 

This is important to us here in Iowa because we want to develop a new standard separation barrier that features a steel bicycle railing attachment up to the AASHTO minimum 1070 MM (3'-6) height for such an application (see attachment "SepBarrierStudy1.pdf"). I think our 1070 MM steel railing, with its bottom surface at 1020 MM high maximum, may keep the rail BELOW the critical zone where it would negatively affect vehicle trajectory if contacted. This is, of course, predicated on the assumption that the trail is not raised relative to roadway gutter elevation.

 

The separator we develop may use exclusively vertical-face concrete barrier, making less critical some of the issues of vehicle climb and potential rollover. We may want to retain an option to use a safety shape, however. We also don't build pedestrian/bicycle facilities on vehicular bridges that have design speeds over 45 mph, which we assume means we wouldn't see quite as significant zones of intrusion beyond the barrier face as your 62 mph tests showed. Thanks for your help with these questions. We're also wondering if you ever considered any testing of our 44-inch vertical face with setback (see "SetbackBarr1.pdf")? Any input on that design would be appreciated

Keywords
  • Bridge Rails
Other Keywords none
Date March 8, 2007
Attachment SepBarrierStudy1.pdf
Attachment SetbackBarr1.pdf


Response
Response

See my comments in red.

 

Q1. The concrete barrier in the report is called a "standard single

slope concrete barrier". Is this the barrier found in "Figure B7.65

Single Slope Concrete Bridge Rail (94, 36, 95, 96)" at the FHWA site,

http://safety.fhwa.dot.gov/roadway_dept/docs/appendixb7g.pdf,

... or is it something else? I'm curious because there are several

standards out there (California, Florida, Missouri, etc.), and they have

different widths at the top of the barrier. This is important because at

no point in the report is the setback dimension given for the steel

railing from the top traffic face corner, and it isn't drawn up in

either Figure 4 or Figure 37. Increasing the setback is mentioned as one

strategy for improving the barrier's performance (Part 10

Recommendations).

**I must apologize for the oversight of omitting the concrete barrier details. The single slope concrete barrier used in this study was also used in two other MwRSF studies. For all of this work, we selected the use of the same single slope barrier that was used in the Washington State DOT bridge railing study performed by TTI and published in Transportation Research Record No. 1468. For this barrier, a 32-in. top barrier height was used, in conjunction with top and bottom base widths of 9.5 in. and 15.625 in., respectively. This geometry resulted in a front slope of 10.84 degrees off of the vertical.

**In terms of the relative distance between the front of the tubes and the top corner of the barrier, I will acquire a detail from the existing CAD that will show those dimensions. I am glad you caught this error now so that we can send out a PDF correction, but I am surprised that no one caught it when the draft was submitted to the States for review and comment. Even we missed this detail in our internal reviews!

Q2. See the photo labeled "0.060 sec" in Figure 21. Did your team make

an effort to estimate the total intrusion of the test vehicle

beyond/through the railing system? Did the steel railing actually

prevent some intrusion that would have occurred with a plain SS barrier

w/o attachments?

**Yes, we track the lateral extent of the vehicle over the barrier's top surface. With regard to question 2, TTI researchers did not determine working width in their testing years ago. In addition, a TTI simulated median barrier, using temporary barrier segments anchored down, was used to represent the bridge railing system. As such, direct comparisons are difficult to make. At this time, I am trying to obtain working width data from the CALTRANS testing programs on a single slope shape with a 9.1 degree slope off of vertical.

**As denoted in Figure 17, the working width was reported as 507 mm, as measured from the front toe of the barrier to the maximum lateral extent from the toe to either the vehicle components or the total barrier width. The total barrier width was 511 mm (20.125 in.). From film analysis, the front corner of the engine hood provided a working width of 507 mm, while the barrier geometry resulted in a working width of 511 mm. As such, we should have reported a 511 mm working width in lieu of the 507 mm value that was reported for the first test.

**As denoted in Figure 42, the working width was reported as 333 mm, as measured from the front toe of the barrier to the maximum lateral extent from the toe to either the vehicle components or the total barrier width. The total barrier width was 511 mm (20.125 in.). From film analysis, the front corner of the engine hood provided a working width of 333 mm, while the barrier geometry resulted in a working width of 511 mm. As such, we should have reported a 511 mm working width in lieu of the 333 mm value that was reported for the first test.

Q3. See photos "0.086 sec" in Figure 22, and "0.058 sec" and "0.096 sec"

in Figure 46. Would you conclude that it was primarily the middle rail

of Test MOBR-1 and the third rail up (from top of barrier) of Test

MOBR-2 that were preventing vehicle ride-up on the barrier? Those rails

exhibit significant upward deflection. It appears from the photos that

the rails below those that I mentioned were actually deflected downward,

and so probably did not apply a downward force to the vehicle. So, it

seems that a steel rail with its bottom surface as low as 1136 MM

(middle rail of first test) will potentially apply a downward force on

the pickup hood, but a rail with its bottom surface as high as 1045 MM

(2nd rail up on second test) may not have that potential.

**From my re-review of the crash testing videos, I have the same opinion as from years ago following the tests whereby the middle rail of the three rail system (test no. 1) and the third rail of the four rail system (test no. 2) provided the greatest vertical restraint for the right-front corner of the engine hood and vehicle. As such, other railing systems with these general geometries and offsets would provide similar behaviors when attached to sloped face concrete barriers. However, it is believed that similar railing systems attached to vertical shaped barriers would provide improved safety performance.

**Consequently, a review of the crash tests performed on a smooth, steel single slope barrier without upper railings resulted in similar propensities for vehicle roll but with slightly lower roll angles and prevention of rolling onto the vehicle's side. This truck behavior demonstrated that vertical restraint near the corner is believed to be similar to that observed with pickup truck test climb inhibited by a smooth, low-friction, steel front face. In the future, bicycle railing heights may be reduced to the pedestrian height of 42 in. (1,067 mm). Using this lower height for future system may alleviate some of the propensity to get under the rails and restrain the truck corner region.

This is important to us here in Iowa because we want to develop a new

standard separation barrier that features a steel bicycle railing

attachment up to the AASHTO minimum 1070 MM (3'-6) height for such an

application (see attachment "SepBarrierStudy1.pdf"). I think our 1070 MM

steel railing, with its bottom surface at 1020 MM high maximum, may keep

the rail BELOW the critical zone where it would negatively affect

vehicle trajectory if contacted. This is, of course, predicated on the

assumption that the trail is not raised relative to roadway gutter

elevation.

**Please note that the AASHTO bicycle railing height requirements may be changing from 54 in. to 42 in. in the near future. Second, we developed a different bicycle/pedestrian railing system for MnDOT several years ago and which uses two longitudinal rails with vertical spindles tightly spaced. This system was attached to a NJ shape bridge railing and tested to TL-4.

The separator we develop may use exclusively vertical-face concrete

barrier, making less critical some of the issues of vehicle climb and

potential rollover. We may want to retain an option to use a safety

shape, however. We also don't build pedestrian/bicycle facilities on

vehicular bridges that have design speeds over 45 mph, which we assume

means we wouldn't see quite as significant zones of intrusion beyond the

barrier face as your 62 mph tests showed.

Thanks for your help with these questions. We're also wondering if you

ever considered any testing of our 44-inch vertical face with setback

(see "SetbackBarr1.pdf")? Any input on that design would be appreciated

-  thanks.

**MwRSF has previously developed a 42-in. high, TL-5 aesthetic, open concrete bridge railing, with other design variations, and one that incorporates a setback to account for head ejection of passengers out the side windows and against the face of taller, rigid concrete parapets. In addition, we have developed and are constructing another near-vertical (small slope), concrete median barrier with a similar setback near the upper region for the same reasons. Vertical barriers reduce vehicle climb and rollover propensity. However, we need to also begin to consider head ejection and contact with barrier hardware. Please email me if you have further questions and comments regarding the material contained herein. Also, let me know if you what details for any of the barrier systems noted above.

Date March 8, 2007


Response
Response As denoted in my email last week, we prepared CAD details that depict the bicycle/pedestrian railing system attached to the actual single-slope concrete bridge railing system. Those new CAD details are attached.
As shown therein and as you requested, dimensions are given which indicate the offset from the upper front-face corner of the parapet and the face of the tubes and posts.
Date March 13, 2007
Attachment Combination Traffic-Bicycle Rail Detail_Page_4.jpg
Attachment Combination Traffic-Bicycle Rail Detail_Page_3.jpg
Attachment Combination Traffic-Bicycle Rail Detail_Page_2.jpg
Attachment Combination Traffic-Bicycle Rail Detail_Page_1.jpg


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