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                Safety Eligibility Letter B-191
  
  **Hardware Type:**
            Longitudinal Barriers and Bridge Rails
                          **Code:**
            B-191
                          **Date:**
            03/18/2010
                          **Testing Criteria:**
            NCHRP 350
                          **Manufacturer:**
            MWRSF
                          **Device Description:**
            Modified 3 cable adjacent to steep slope
                          **View PDF:**
            b191.pdf (4.42 MB)
    
    Safety Eligibility Letter B-19

			
						
			DOT logoU.S. Department of Transportation**Federal Highway Administration**
			1200 New Jersey Ave. S.E.Washington, D.C. 20590
			March 18, 2010
            In Reply Refer To:HSSD/B-191
			
			Ronald K. Faller, Ph.D., P.E.
            	Research Assistant Professor
                UNL - Midwest Roadside  Safety Facility
                527 Nebraska  Hall
                P.O. Box 880529
                Lincoln, Nebraska 68588-0529
             
            Dear Dr.  Faller:
            
            This letter is in response to your request for the Federal  Highway Administration (FHWA) acceptance of a roadside safety system and end  anchors for use on the National Highway System (NHS).
            Name of  system:
                	Low Tension, Three-Cable  Guardrail System placedadjacent  to a steep slope (1.5H:1V)and  Alternative End Anchor SystemsType of  system:
                	Cable Barrier with  4-foot post spacingTest Level:
                	NCHRP Report 350 Test Level 3Testing  conducted by:
                	Midwest  Roadside Safety FacilityDate of  request:
                	October 24, 2008Date of  Completed Package:
                	May 12, 2009
            You requested that we find this system and anchors  acceptable for use on the NHS under the provisions of the National Cooperative  Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the  Safety Performance Evaluation of Highway Features.”
            **Requirements**Roadside safety systems should meet the guidelines contained  in the NCHRP Report 350 or the AASHTO Manual for Assessing Safety Hardware. The FHWA Memorandum “Identifying Acceptable  Highway Safety Features” of July 25, 1997, provides further guidance on crash  testing requirements of longitudinal barriers.
            **Cable Barrier Description**The system’s overall length was 150.57-m (494-ft) and  consisted of four major structural components: (1) wire rope; (2) posts; (3) cable  compensator end assemblies, and (4) tangent anchor assemblies. Design details  are shown in the enclosure for reference.  Three 19-mm (3/4-in) diameter cables comprised of 3x7 wire rope were  used for the rail elements. The cable rails were supported by ninety-two  guardrail posts with an uppermost mounting height of 762 mm (30 in) and with 76-mm (3-in) incremental spacing for  the two lower cables. The cables were tightened with the use of cable  compensators. The ends of the cables were threaded rods that terminated in the  cable anchor. The threaded rods were attached to the cable anchor with three  51-mm (2-in) diameter washers and two 19-mm (3/4-in) diameter Grade 5 nuts.
            The line posts consisted of 1,600-mm (63-in) long S76x8.5  (S3x5.7) rolled steel sections with a 203-mm x 610-mm x 6-mm (8-in. x 24-in x 1/4-in) soil  plate welded along the back flange of the post.  The line posts were spaced 1,219 mm (48 in) on center with a soil embedment  depth of 762 mm (30 in). The line posts  were set 1,219 mm (48 in) back from the slope breakpoint.
            Post numbers 1 through 7 and 86 through 92 were part of the  tangent cable end anchor system as developed as part of a previous study. The anchor bracket post, post numbers 1 and  92, were W152x37.2 (W6x25) steel sections with a 610-mm x 610-mm x 13-mm (24-in  x 24 in x 1/2-in) soil plate. The anchor  post was embedded to a depth of 2,438 mm (96 in). A 368-mm x 229 mm x 13 mm (14 1⁄2 in x 9 in x 1⁄2 in) plate welded to the  top of the anchor post to which the cable anchor bracket was bolted with four  19-mm diameter x 64-mm long (3/4-in. x 2.5-in.) Grade 5 hex head bolts.
            Post numbers 2 and 91 were configured with S76x8.5 (S3x5.7)  sections measuring 838 mm (33 in) long for the slip posts with W152x13.4 (W6x9)  sections measuring 1,829-mm (72-in) long for the foundation posts. The foundation post was embedded to a depth  1,778 mm (70 in). A slip base plate was welded to the bottom of the slip post  and the top of the foundation post. Four  13-mm diameter x 51-mm (1/2-in x 2-in) long ASTM A307 bolts with nuts and  washers to form the slip base configuration. Post numbers 3 through 7 and 86 through 90  were also slip posts that were configured with S76x8.5 (S3x5.7) sections and  W152x13.4 (W6x9) sections. These posts were identical to post numbers 2 and 91,  except the cable bracket was replaced with three cable hooks as shown in  Figures 16 and 17. The top cable hook  was located 90 mm (3 1⁄2 in) down from the top of the post with the middle and  lower cable 166 mm and 242 mm (6 1⁄2 in and 9 1⁄2 in) from the top of the post,  respectively.
            A 48.7-m long x 6.1-m wide (160-ft x 20-ft) pit was  excavated behind the cable system. In  order to develop a 1.5H:1V slope, the pit’s profile was identified by  horizontal and vertical components of 6.1 m (240 in.) and 4.1 m (160 in.),  respectively.
            **Cable End Anchor  Alternatives Descriptions**Three anchor design options were considered to simplify  construction of the three-cable guardrail system. These include a reinforced concrete block, a  reinforced concrete shaft, and a driven steel post. The concrete block option mimics that of the  existing design, but utilizes a smaller concrete block that can be lifted with existing  equipment. The reinforced concrete shaft  provides a simplified concrete design alternative, but still relies on the use  of cast-in-place concrete. The steel  post design incorporates a large steel beam that is driven into the ground.
            **Concrete Block**The revised concrete block design is 1,524 mm long x 1,015 mm  deep x 610 mm wide (60-in x 40 in x 24 in) and weighs approximately 2,268 kg  (5,000 lbs). The block fabricated with 27.58-MPa (4,000-psi) minimum compressive strength concrete  contains Grade 60 Number 4 reinforcement bars.
            **Reinforced Concrete  Shaft**The drilled shaft concrete anchor is 457 mm (18 in) in  diameter and is 1,829 mm (72 in) deep. The anchor is reinforced with a spiral  rebar cage fabricated with Grade 60 steel. The spiral reinforcement was designed with a  38-mm (1.5-in) clear cover over Number 3 rebar and ten Number 4 vertical bars  equally spaced around the interior circumference. A concrete mix with compressive strength of  27.58 MPa (4,000-psi) was also specified.  Anchor rods embedded 305 mm (12 in) into the structure were used to secure the  cable anchor bracket.
            **Steel Post**A W152x37.2 (W6x25) steel section with a 248-MPa (36-ksi)  yield strength and a 2,438-mm (96-in) overall length was selected for the  design. The addition of a 610-mm x  610-mm x 13-mm (24-in x 24-in x 1/2-in.) soil bearing plate provides  further resistance against lateral forces.  In this case, the anchor bracket is bolted to a steel plate welded to  the top of the post.
            **Crash Testing**Longitudinal  barriers are typically subjected to testing with both a small car and a pickup  truck. The higher impact energy  associated with the pickup truck test produces larger barrier deflections and  greatly increases the likelihood of vehicle rollover as compared to the small  car test. The impact of the reduced post  spacing on the small car was considered and judged to have no adverse affect on  the barrier performance. Therefore, the  2,000-kg (4,409-lb) pickup truck test was selected as sufficient to evaluate  the performance of the cable guardrail adjacent to steep slopes, and the 820-kg  (1,808-lb) small car test was considered unnecessary for this project.
            **Developmental Test**A  developmental test of low tension 3-cable barrier was conducted with the posts  spaced at 4.88-m  (16-ft) and set back 12-in from the slope break point. This test failed when the test vehicle rolled  over the barrier and down the ditch slope. It was concluded that the barrier stiffness  and the offset from the hinge point both needed to increase in order for the  system to pass NCHRP Report 350 criteria.
            **Test on Final Design**A  subsequent crash test was conducted on the final barrier design as discussed in  the section above titled “Cable Barrier Description.” The line posts were spaced 1,219 mm (48 in)  on center with a soil embedment depth of 762 mm (30 in). The line posts were set 1,219 mm (48 in)  back from the slope breakpoint. The  2,032-kg (4,481-lb) pickup truck impacted this three cable guardrail system at  a speed of 99.1 km/h (61.6 mph) and at an angle of 23.6 degrees.
            The  vehicle was contained and redirected by the barrier. The longitudinal and  lateral occupant impact velocities were determined to be -4.16 m/s (-13.64 ft/s)  and 3.42 m/s (11.22 ft/s), respectively. The maximum 0.010-sec average occupant  ridedown deceleration in the longitudinal and lateral directions were -5.73 g’s  and 6.96 g’s, respectively. Both the  occupant impact  velocities (OIVs) and occupant ridedown decelerations (ORDs) were within the  suggested limits provided in NCHRP Report 350.  The maximum lateral permanent set post deflection was 578 mm (22.75 in)  at the centerline of post number 36 as measured in the field. The maximum lateral dynamic cable deflection  was 3,163 mm (124.5 in), as determined from high-speed  digital video analysis. The working  width of the system was found to be 3,318 mm (130.6 in).
            **Anchor Testing**A 2,223-kg (4,900-lb) wheeled bogie was attached to each  anchor and accelerated to provide a dynamic load of 178-kN (40,000-lbs) The reinforced concrete block option  represented the strongest design with a 254-kN (57,000-lbs) peak resisting  force. The drilled shaft produced a  resisting force of 205 kN (46,000 lbs) and the driven steel post sustained 187 kN  (42,000 lbs.) Although the driven steel  post anchor is considered a viable anchor design and performed satisfactorily  under full scale test, its structural performance was slightly out of the acceptable  performance range. This makesthe  reuse of this anchor undesirable with the strong likelihood it would have to be  replaced after each impact event.
            **Findings**Therefore, the low-tension 3-cable barrier system with posts  spaced 1,219-mm (48 in) on center set 1,219 mm (48 in) back from the slope  breakpoint of the steep slope described above and detailed in the enclosed  drawings is acceptable for use on the NHS under the range of conditions tested,  when such use is acceptable to a highway agency.  The three alternative cable anchor  systems are similarly acceptable as long as they are designed to work properly  with local soil conditions.
            Please note the following standard provisions that apply to FHWA letters of  acceptance:
            This acceptance is limited to the  crashworthiness characteristics of the systems and does not cover their  structural features, nor conformity with the Manual on Uniform Traffic Control  Devices.
            	Any changes that may adversely influence the  crashworthiness of the system will require a new acceptance letter.
                Should the FHWA discover that the  qualification testing was flawed, that in-service performance reveals  unacceptable safety problems, or that the system being marketed is  significantly different from the version that was crash tested, we reserve the  right to modify or revoke our acceptance.
                You will be expected to supply potential  users with sufficient information on design and installation requirements to  ensure proper performance.
                You will be expected to certify to potential  users that the hardware furnished has essentially the same chemistry,  mechanical properties, and geometry as that submitted for acceptance, and that  it will meet the crashworthiness requirements of the FHWA and the NCHRP Report 350.
                To prevent misunderstanding by others, this  letter of acceptance is designated as number B-191 and shall not be reproduced except in  full. This letter and the test  documentation upon which it is based are public information. All such letters and documentation may be  reviewed at our office upon request.
                This  acceptance letter shall not be construed as authorization or consent by the  FHWA to use, manufacture, or sell any patented system for which the applicant  is not the patent holder. The acceptance  letter is limited to the crashworthiness characteristics of the candidate  system, and the FHWA is neither prepared nor required to become involved in  issues concerning patent law. Patent  issues, if any, are to be resolved by the applicant.Sincerely yours,
						Signature of David A. Nicol
						David A. Nicol, P.E.
							Director, Office of Safety Design
							Office of SafetyEnclosure U.S. DEPARTMENT OF TRANSPORTATION

**Federal Highway Administration**

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WASHINGTON, DC 20590

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