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NDOR is planning 2” overlays on Bridge structures & your concurrence is requested:

NDOR has existing concrete bridge railings at 29” across the deck of approx. 1000 bridges, these raise at the end of the bridge railing to 32” for the guardrail to attach at 31”. We plan to overlay these with a thin water proof membrane and a 2” asphalt overlay across the bridge.

The FHWA has a list of acceptable bridge railings

http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/barriers/bridgerailings/

Under Vertical Concrete Parapet (Open or Closed) / General [PDF
1.08 MB]; the Concrete 27” railing similar to NDORs is here. Do you concur with the below statements?

1. NDORs concrete bridge railing (open) @ 27” meets NCHRP 350 testing which is confirmed by FHWA. 
2. The thrie-beam bridge approach at 29” meeting the 30” end of bridge rail, is NCHRP 350 approved.
3. The 29” thrie-beam bridge approach section will be raised to 31” over the first 75’ of the guardrail run.
4. The end of the 31” w-beam will use an approved end treatment.

Hello Phil,

MwRSF has examined the proposed resurfacing you identified. As we understand it, the roadway will be resurfaced with a water-repelling membrane and an additional 2" of asphalt overlay. The pavement overlay would reduce the effective heights of:

• The open concrete bridge rail from 29 in. to 27 in.;
• The approach guardrail transition (AGT) from 31 in. to 29 in.; and
• The guardrail leading up to the AGT from 31 to 29 in.

In reviewing relevant research, MwRSF researchers felt that the flared distance over which the effective rail height changes is acceptable, and the 31-in. tall W-beam (e.g., MGS) end treatments are acceptable. We have some concerns with other aspects of the proposed resurfacing project. Please note below.

1. No thrie beam AGT  has yet been approved at NCHRP Report No. 350 for use with a 29 in. top mounting height. All approved thrie beam transitions were designed to connect to the end of the parapet / end buttress of the concrete barrier with a top mounting height of 31 in. Researchers are unaware of any successful or unsuccessful tests with a thrie beam top mounting height of 29 in. Crash testing is recommended to confirm the adequacy of the 29-in. thrie beam AGT top mounting height according to NCHRP Report No. 350 TL-3 specifications. It should be noted that successful crash testing has been observed for some stacked W-beam with W-beam rubrail/curb approach guardrail transitions using a 29-in. top rail height.
2. Testing at TTI in the early 2000s indicated that there may be an increased propensity for rollover associated with an open concrete bridge rail with a top mounting height of 27 in.; please refer to the attached report "TTI_Report_GFRP_Bars_Concrete_Bridge_Rail_0-4138-3.pdf". As such, an 27-in. tall open concrete rail has demonstrated unacceptable behavior according to TL-3 of NCHRP Report No. 350.

These concerns may be resolved using one of the following approaches, in order of recommendation.

1. Modify the bridge rail height to be similar to what was shown in "TTI_Report_GFRP_Bars_Concrete_Bridge_Rail_0-4138-3.pdf" with a 3-in. tall tube mounted to the top of the bridge rail. Of course, other practical methods for increasing height are acceptable. Raise the height of the thrie beam transition by using modified blockouts and field-drilled holes in the posts, and re-attach the thrie beam to the end of the concrete parapet / end buttress per the original design drawing specs.
2. Modify the bridge rail height to be similar to what was shown in "TTI_Report_GFRP_Bars_Concrete_Bridge_Rail_0-4138-3.pdf" with a 3-in. tall tube mounted to the top of the bridge rail. Also, modify the AGT to be consistent with one of the stacked W-beam designs in FHWA approval letters b-65, b-77, or b-83; crash testing details may be found in the attached report "NCHRP_350_Testing_404211-F.pdf". These designs permitted the use of lower-height, W-beam transitions to the bridge rail with additional rub rails and/or curbs installed.These systems may require the use of alternative posts and structural elements in the transition region, but will not require new holes to be drilled in the end of the existing concrete parapet / end buttress.
3. Limit resurfacing projects that result in 29-in. tall AGTs and 27-in. tall vertical parapet bridge rails to TL-2 applications only.