National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 14 - Generic End Terminal Guidance

« Previous: Chapter 13 - Evaluation of Rail Flattening
Page 75
Suggested Citation:"Chapter 14 - Generic End Terminal Guidance." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Page 76
Suggested Citation:"Chapter 14 - Generic End Terminal Guidance." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Page 76
Page 77
Suggested Citation:"Chapter 14 - Generic End Terminal Guidance." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Page 77

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75 This chapter specifies the rationale for repair criteria for generic end terminals. The intent was to specify guidance which was applicable to all end terminal types (except as noted). Manufacturers of proprietary end terminal systems may recommend additional repair thresholds specific to an individual terminal. Note that this guidance was based solely on engineering judgment; no finite element simulations or pendulum tests evaluating these end terminal damage modes were conducted. The guidelines which follow were based on an Ohio Department of Transportation Energy Absorbing End Terminal Maintenance Checklist (Focke, 2007). 14.1 Generic End Terminal Damage Modes Damaged end post. The first post of a w-beam end termi- nal provides crucial anchorage of the w-beam section to the ground, typically through the use of a swaged anchor cable. If the first post is broken or missing, the redirective capabilities of the barrier downstream of the end terminal are likely com- promised, depending on the proximity of the second impact to the damaged terminal. To be conservative, the research team recommends the replacement of any terminal end post which is sufficiently damaged that it cannot fulfill its functions. Posts in this category include broken, severely cracked, missing, or rotted terminal end posts. An example is shown in Figure 64. The relative priority assigned to this damage type is high due to the likelihood of a loss of redirective capability of the barrier. Missing/slack anchor cable. Similar to the terminal end post, the anchor cable allows the w-beam section to develop adequate tension to redirect the vehicle. As with a broken end post, a missing anchor cable will impair the capability of the barrier to redirect an impacting vehicle. Based on this ratio- nale, a missing anchor cable was assigned a high-priority repair. A loose anchor cable or cable bracket was assigned a medium priority as the w-beam may still be able to develop a large portion of tension through the anchor cable. Examples of missing or incorrectly aligned cable bracket bearing plates are shown in Figure 65. A slack anchor cable has been defined as a cable that can be pushed down by hand by more than 1 inch based on the Ohio DOT end terminal checklist. Improper Stub Height. In some guardrail end terminal installations, the steel tubes or hinged posts may be too high for proper activation of the breakaway mechanism of the end terminal (Figure 66). Stub heights above the ground level should not exceed 4 inches. Stub heights have been observed to exceed this limit due both to incorrect installation and, in some areas, due to frost heave. A medium-priority repair has been assigned to this damage mode because an incorrect stub height will not cause the end terminal to fail but nevertheless may lead to small vehicle snagging or less than optimal oper- ation of the end terminal. Missing/Failed Lag Bolts on Impact head. Energy ab- sorbing terminals such as the ET-2000 and SKT-350 use an im- C H A P T E R 1 4 Generic End Terminal Guidance Figure 64. Wood post with cross-grain cracking (photo: courtesy of South Dakota DOT).

pact head to deform the w-beam rail during a head-on impact with the end terminal. The deformation of the w-beam absorbs the kinetic energy of the impacting vehicle to bring it to a safe and controlled stop. As such, the alignment of the impact head is crucial to the functioning of this terminal in a head-on con- figuration. This can result from missing or failed lag screws at the end post or a w-beam that is not properly seated in or is outside of the impact head. An example is shown in Figure 67. Figure 68 shows a failed attachment between lag screws and the end post. A high-priority repair has been assigned to this dam- age due to the potential for vehicle spearing, especially in the case where the end of the rail is outside the impact head. 14.2 Recommendation The criteria for repair of generic end terminals are sum- marized in Table 28. These guidelines have been based on an Ohio Department of Transportation Energy Absorbing End Terminal Maintenance Checklist (Focke, 2007). 76 Figure 65. Misaligned or missing bearing plates (photo: courtesy of South Dakota DOT). Figure 66. Incorrect stub height (photo: courtesy of South Dakota DOT). Figure 67. Misaligned impact head because of missing lag bolts (photo: courtesy of Ohio DOT).

77 Figure 68. Failed attachment of lag bolts to end post. Table 28. Summary of generic end terminal repair guidance. Damage Repair Threshold Relative Priority Damaged end post Not functional (sheared, rotted, severely cracked) High Missing HighAnchor cable Loose—more than 1 inch of movement when pushed down by hand Medium Cable Anchor Bracket Loose or not firmly seated in rail Medium Stub height of steel tube or hinged post Height which exceeds 4 inches Medium Lag bolts on impact head (Energy Absorbing Terminals Only) Missing or failed lag bolts High Loose or Misaligned Medium Bearing Plate Missing High

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 656: Criteria for Restoration of Longitudinal Barriers explores the identification of levels of damage and deterioration to longitudinal barriers that require repairs to restore operational performance.

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