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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-207

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U.S. Department of Transportation
Federal Highway Administration

1200 New Jersey Ave. S.E.
Washington, D.C. 20590

November 10, 2010

In Reply Refer To:
HSSI/B-207

Mr. Michael S. Bline, P.E.
Standards & Geometrics Engineer
Office of Roadway Engineering
Ohio Department of Transportation
1980 W. Broad Street, 2nd Floor
Columbus, OH 43223

Dear Mr. Bline:

This letter supersedes the Federal Acceptance Letter Number HSSD/B-207 dated July 7, 2010, regarding the Modified Ohio DOT (ODOT) Deep Beam Bridge Railing retrofit design of an existing ODOT Standard Bridge Railing for use on the National Highway System (NHS).

Name of system: Modified ODOT Deep Beam Bridge Railing (Guardrail Barrier)
Type of system: Post and Tube with W-Beam Permanent Barrier
Test Level: NCHRP Report 350 Test Level 3 (TL-3)
Testing conducted by: Acceptance based on Equivalence via Strength Analysis and nonlinear finite element simulation using LS-DYNA as conducted by Texas Transportation Institute
System Designator: SBB08c
Date of request: May 15, 2010
Date of Additional Information: August 20, 2010

Requirements
Roadside safety systems should meet the guidelines contained in the National Cooperative Highway Research Program (NCHRP) Report 350, "Recommended Procedures for the Safety Performance Evaluation of Highway Features" (NCHRP Report 350). The Federal Highway Administration (FHWA) memorandum "ACTION: Identifying Acceptable Highway Safety Features" of July 25, 1997, provides further guidance on crash testing requirements of longitudinal barriers. In addition, roadside safety system structural analysis of bridge railings for crashworthiness is also permissible as per the May 16, 2000, FHWA memorandum entitled, “Bridge Rail Analysis”.

Description
The ODOT Deep Beam Bridge Guardrail Barrier is a post and tube with w-beam panel permanent barrier system measuring 30 inches high. This bridge barrier is listed as the Ohio Box Beam Rail in the FHWA memorandum for Bridge Rails dated August 28, 1986, and was successfully crash tested under NCHRP Report 230 as a Performance Level 1 (PL-1) bridge barrier. The PL-1 railing has an equivalency of TL-2 per the FHWA memorandum, “Crash Testing of Bridge Railings.” dated May 30, 1997. A combination of analytical study and computer simulation was utilized to evaluate the performance of the ODOT Deep Beam Bridge Guardrail Barrier. The final product is a modified design of the ODOT Deep Beam Bridge Guardrail Barrier system (retrofit) to bring this modified system into compliance with the NCHRP Report 350 performance criteria for TL-3.

The existing bridge rail design was reviewed to investigate the performance aspects of all similar railing systems successfully crash tested as per NCHRP Report 350. The investigation revealed railing systems that share some of the characteristics of the ODOT Deep Beam bridge rail including the Texas Department of Transportation (TxDOT) Type T101, the Illinois side-mount rail and the Oregon side-mount bridge rail. The Illinois side-mount bridge rail design was chosen for the analysis comparison. The Illinois Side-Mount barrier consists of W6 x 25 (W 150 x 37.1) posts spaced at 6 ft-3 in. (1.905 m) with two tubular rails, TS 8 x 4 x 5/16 in. (203 x 102 x 8 mm) for the top rail and TS 6 x 4 x /4 in. (152 x 102 x 6 mm) for the lower rail. The height of the top rail above the asphalt surface is 32 inches (813 mm). The railing was mounted to the side of a concrete deck using four (4) AASHTO Ml64 anchors bolts. The Illinois side mounted bridge rail was successfully tested to AASHTO PL-2 including the single unit truck.

After review and analysis of the existing ODOT Deep Beam Bridge Railing (Guardrail Barrier) system, the suggested retrofit is the addition of two (2) additional tubular members to help improve the performance of the ODOT Deep Beam bridge rail. The following two rail members have been added to in such a way as to utilize the current bridge rail hardware and minimize retrofitting the existing bridge rail post.

  1. A. One tubular member (rub rail) added at 8 in. (203 mm) above the pavement surface to improve the crash performance for the small car (820C) in NCHRP Report 350 TL-3 conditions.
  2. One tubular member added to top of existing tubular block out to increase the overall height to 31 in. (787.4 mm) above the pavement surface. Increasing the height of the bridge rail is considered to be an improvement in crash performance by the design team particularly for impact conditions that involve the pick-up truck (2000P).

Additional test information provided to FHWA on August 20, 2010, is as follows. It should also be noted pavement overlay shall not be more than 3 in. (76 mm). Hence, the lower tubular member (rub rail) will be at 11 in. (280 mm) above the deck surface if no pavement overlay is used.

Strength analyses were then conducted to determine the strength of the retrofit rail design with respect to AASHTO Bridge Design Specification. Developed details for analysis of the 16 in. (406.4 mm) concrete deck were approved by ODOT and incorporated into the analyses for the retrofit bridge rail design. The 2004 AASHTO LRFD Bridge Design Specifications, table 13.7.2-1 and table A13.2-1 were used to calculate the strength of the modified ODOT Deep Beam Bridge Guardrail Barrier system, and is included as an attachment to this correspondence.

A detailed LS-DYNA finite element model was built for the ODOT deep beam post assembly. The assembly includes the W6 x 25 (Wl50 x 37.1) post, the stiffening plates, the 1-1/4 in. (31.75 mm) diameter A325 anchor bolt, the 16 in. (406.4 mm) deck and the detailed reinforcement per the system drawings enclosed with this correspondence.

Based on the LS-DYNA numerical analysis and engineering analysis, it is expected the post-deck assembly would have the capacity to withstand the 54kips (240 kN) load imparted by the 2000P test vehicle (per NCHRP Report 350 TL-3) without significant damage. A full model of a representative installation of the modified ODOT Deep Beam bridge rail per NCHRP Report 350 test requirements for rigid barrier was built. The model consists of a 75 ft (22.86 m) long rail that includes six w-beam rail segments and 13 (thirteen) post assemblies.

The system was able to contain and redirect the vehicle per the finite element simulation. The vehicle had a moderate roll angle (18 degrees) around 0.52 seconds (sec) but it became upright late in the simulation. The simulation calculated the maximum tensile force in the deck anchors to be 88.91 kips (395 kN). This is below the yield rating of these anchors of 99 kips (440.4 kN) as presented in strength analysis. The summary of results of TL-3-11 simulation is also enclosed with this correspondence.

Findings
We concur that based upon equivalence to the Illinois Side-Mount barrier and computational analysis, the modified ODOT Deep Beam Bridge Guardrail Barrier meets all barrier structural adequacy and vehicle trajectory criteria as outlined in NCHRP Report 350 and is acceptable for use on the NHS as a TL-3 barrier when allowed by the highway agency. Please note the following standard provisions that apply to FHWA letters of acceptance:

 

Sincerely yours,

/* Signature of Michael S. Griffith */

Michael S. Griffith
Director, Office of Safety Technologies
Office of Safety

Attachments

cc:    Carl Eugene Buth, Ph.D., P.E.
Assistant Agency Director
Texas Transportation Institute
Texas A&M University System
3135 TAMU
College Station, TX 77843

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