National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 15 - Conclusions

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Page 78
Suggested Citation:"Chapter 15 - Conclusions." 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 79
Suggested Citation:"Chapter 15 - Conclusions." 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 80
Suggested Citation:"Chapter 15 - Conclusions." 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 80
Page 81
Suggested Citation:"Chapter 15 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 81
Page 82
Suggested Citation:"Chapter 15 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 82
Page 83
Suggested Citation:"Chapter 15 - Conclusions." 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 83

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

78 Damage to a longitudinal roadside barrier is not always characterized by the large rail deformations and sheared posts characteristic of a high severity crash. Much more common is minor damage such as a shallow dent which is a result of a low-speed collision or sideswipe. Unfortunately, the effect of this minor damage on the performance of the barrier in sub- sequent impacts is not well understood, and there is little bar- rier repair guidance available for highway personnel tasked with maintaining these systems. The goal of this research project was to develop guidelines to assist highway personnel in identifying levels of minor bar- rier damage and deterioration that require repairs to restore operational performance. The focus of the project was on the length of need of w-beam barriers. This chapter presents the conclusions of the project including (1) current practices for repairing damage to longitudinal barriers, (2) the approach to develop objective criteria for measuring damage to longitu- dinal barriers, (3) the approach for quantitatively evaluating these guidelines, and (4) the recommended guidelines with threshold values for which barrier repair is recommended. 15.1 Summary of Current Practices Current practices and research needs were determined based upon the results of a literature review and a survey of State and Canadian Provincial transportation agencies. A literature review on the repair and maintenance of longitudinal barriers revealed the following: • There appears to be no scientific basis for the existing national guardrail repair guidelines. Current guidelines appear to be based exclusively on engineering judgment. • With the exception of a select few state agencies, a majority of state agencies with published barrier maintenance guide- lines or rating schemes lack quantitative descriptions and examples of deficient barriers. • Five tort cases and one documented crash found in the literature show that impacts with previously damaged bar- riers do occur in the field. A survey was distributed to the State and Canadian Provin- cial Transportation agencies to ascertain current practices with respect to the repair and maintenance of longitudinal barriers. Based on the responses of 39 agencies, the research team has the following conclusions: • Two-thirds of responding agencies indicated more quan- titative guidelines for the repair of guardrails would be beneficial. • Approximately 60 percent of responding agencies reported specific guidelines for when to repair damaged guardrail. Less than one-third of these agencies indicated tangible and numeric criteria to identify when barrier repair is necessary. • Half of the top 10 damaged barrier modes deal exclusively with the w-beam rail element. • Post and rail deflection in excess of 6 inches is found to be the most prevalent type of guardrail damage; however, this damage is generally classified as moderate damage. Of the minor damage modes, rail deflection only, post and rail de- flection (< 6 inches), and rail flattening are most common. • Repair priority is found to be high for post and rail damage greater than 6 inches, rail tears, cable damage, w-beam splice damage, and loss of tension in the cable barrier. 15.2 Method of Evaluation of Guidelines A set of target damage modes was selected for evaluation based upon the results of the survey of U.S. and Canadian transportation agencies, and inspection of a catalog of minor barrier damage categories. Target damage modes were selected based on their frequency of occurrence and perceived threat to C H A P T E R 1 5 Conclusions

the motorist. The objective was to evaluate the crash perfor- mance of barriers with each of these minor damage modes, and to develop quantitative criteria for when repair of this minor barrier damage is warranted. To develop a strong technical foundation for the guidelines, the research team’s approach was to evaluate the more common damage types with a com- bination of controlled experiments and computational model- ing and adjust the preliminary proposed guidelines as neces- sary. The generic end terminal repair guidelines were based upon a procedure developed by the Ohio Department of Trans- portation. Table 29 presents the target damage modes and the method(s) used to evaluate each of these damage modes. The results of each damaged barrier impact experiment or simulation were evaluated using criteria based heavily on NCHRP Report 350. Pendulum tests were evaluated based on the ability of the barrier to contain the pendulum, i.e., no pendulum penetration, underride, or override. For the full- scale crash tests and computational simulations of full scale crash tests, the criteria shown in Table 30 were used to evalu- ate crash performance. The full scale crash tests and simula- tions were also assessed for vehicle instability resulting from impact including roll, pitch and yaw, wheel snagging, and the presence/absence of vaulting. The results of each evaluation were used to set the threshold for repair. 15.3 Recommended Criteria for Restoration of Longitudinal Barriers This section presents guidelines for repairs to damaged longitudinal barriers in order to restore them to operational performance. Included are guidelines for repair of minor dam- age to w-beam, generic end-terminals, and guidance for repair of more severe barrier damage. The following guidelines are based on the assumption that a damaged barrier will be subjected to a second collision with impact conditions similar to NCHRP Report 350 test level 3 (TL-3) conditions. The crash performance of barriers with different types of damage was assessed under the same impact conditions. Depending on individual conditions at a specific site, however, the probability of a second impact to a previ- ously damaged longitudinal barrier will vary considerably. The determination of the risk of a second collision is beyond the scope of this document but should be another factor that 79 Damage Mode Pendulum Tests Full-Scale Crash Tests Finite Element Modeling Rail / Post Deflection x x x Vertical Tear x Horizontal Tear x Twisted Blockout x Missing Blockout x Splice Damage x Hole in Rail x Missing Posts x Post-Rail Separation x Rail Flattening x Generic End Terminal Damage Table 29. Methods of evaluating each damage mode. Criterion Required Performance 1. Barrier contains and redirects the vehicle Structural Adequacy 2. No vehicle penetration, underride, or override 3. Vehicle should remain upright during and after the collision; moderate pitch and roll are acceptable 4. Lateral and longitudinal occupant impact velocity < 12 m/s (as computed by the flail space model) Occupant Risk 5. Lateral and longitudinal occupant ridedown acceleration < 20 G (as computed by the flail space model) 6. Vehicle intrusion into adjacent traffic lanes is limited or does not occur Vehicle Trajectory 7. Vehicle exit angle should preferably be less than 60 percent of the impact angle Table 30. Barrier crash performance requirements.

is considered when determining the repair priority of a dam- aged barrier section. 15.3.1 W-Beam Repair Guidelines The w-beam barrier repair guidelines are summarized in Table 31. These guidelines are intended to mark the level of bar- rier damage that begins to significantly affect the crash perfor- mance of the barrier and are intended to be the base thresholds to which barrier repair is recommended. The rationale for each of the guidelines has been presented throughout the report. Barriers with damage less than the threshold values shown in Table 31 have a crash performance that is indistinguishable from new barriers. For damage extent above the threshold, Table 31 provides a relative repair priority for each damage mode using a three category scale: high, medium, and low. While the priority assignments are not intended to dictate an exact time frame in which to repair a damaged barrier, they do provide maintenance personnel with general guidance on how differing damage modes are expected to affect the crash performance of the barrier relative to one another. A brief de- scription of each priority level is provided below: • High Priority: Indicates damage where the crash perfor- mance of the barrier has been compromised to such a degree that a second impact to the damaged barrier would result in unacceptable vehicle and/or barrier performance. This would include vehicle penetration of the barrier (via rail rupture, vehicle override, or vehicle underride) and vehi- cle rollover. • Medium Priority: Indicates damage where the crash per- formance of the barrier has likely been compromised to some degree but the damage is less likely to result in unacceptable vehicle and/or barrier performance than high-priority damage. • Low Priority: Indicates damage where the crash per- formance of the barrier is indistinguishable from the un- damaged condition. 15.3.2 Generic End Terminal Guidance The criteria for repair of generic end terminals are sum- marized in Table 32. These guidelines have been based on an Ohio Department of Transportation Energy Absorbing End Terminal Maintenance Checklist (Focke, 2007). 15.3.3 Guidance for Substantial Barrier Damage National guidance regarding the repair of w-beam barriers is provided by the Federal Highway Administration (FHWA) in a 2008 report entitled “W-Beam Guardrail Repair” (FHWA, 2008). Based on this document, a damaged barrier is classified into one of three categories: (1) Non-Functional, (2) Dam- aged but should function adequately under the majority of impacts, and (3) Damaged, but should not impair the guard- rail’s ability to perform. The intent of this section is to provide improved guidance for classification of barriers into categories 1 or 2 based on the sim- ulation and testing conducted under NCHRP Project 22-23. Table 33 summarizes the details of categories 1 and 2 from the original w-beam guardrail repair guide. Table 34 summarizes the proposed changes to the criteria for categories 1 and 2 based on the findings of the research team to date. The full-scale crash test conducted at MGA Research con- sisted of a TL-3 impact of a 2000P test vehicle into a strong-post w-beam barrier with approximately 14 inches of rail deflection. The vehicle impacted the barrier, climbed the deflected posts, and subsequently vaulted the barrier. Marzougui et al. (2007) showed that if rail height declines by 2 inches, vehicles can rollover or vault over the rail. A 2-inch decrease in rail height corresponds to a 10.5 inch deflection. Based on the results of the full-scale test and the findings of the Marzougui study, the research team recommends reducing the category 1 deflection from 18 inches to 10 inches. Finite element simulations have shown that a 2000P pickup truck striking a section missing even a single post will become unstable. Hence the guidelines have been revised to recom- mend the repair of a rail with any missing posts. 15.3.4 Barrier Locations with More than One Damage Mode Note that the thresholds and corresponding repair prior- ities above have been based on the presence of a single dam- age mode. Often, longitudinal barrier damage consists of more than one damage mode. For instance, rail flattening almost always occurs in tandem with post and rail deflection. A majority of the current analysis has focused on individual damage modes in an effort to more fully understand the ef- fect of a single damage mode on the crash performance of the barrier. The research team, however, does recognize the need for guidance regarding barriers with multiple damage modes. Until a more thorough analysis of barrier damage combi- nations can be conducted, the research team proposes that barrier repair be based on the highest priority level of the com- bined damage modes. For instance, if a barrier has both a twisted blockout along with rail flattening, the repair priority would be “medium” based on the rail flattening. 15.3.5 Limitations This study had several limitations. The approach was to evaluate longitudinal barriers with damage under the worst practical conditions. However, there are a number of other conditions, not examined in this study, to which these barriers may be subjected to potentially adverse consequences. The 80

81 2 or more posts with blockout attached with post- rail separation less than 3 in. 1 or more posts with post-rail separation which exceeds 3 in. Note: If the blockout is not firmly attached to the post, use the missing blockout guidelines. Note: If separation over 3 in., use deflected post/rail guidelines. Medium Posts Separated from Rail 1 post, with blockout attached, with post-rail separation less than 3 in. Low Missing/Broken Posts 1 or more posts Missing Cracked across the grain Broken Rotted With metal tears High Missing Blockout Any blockouts Missing Cracked across the grain Cracked from top or bottom of blockout through post bolt hole Rotted Medium Twisted Blockouts Any misaligned blockouts, top edge of block 6 in. or more from bottom edge Note: Repairs of twisted blockout are relatively quick and inexpensive. Low Damage at a rail splice More than 1 splice bolt: Missing Damaged Visibly missing any underlying rail Torn through rail High 1 splice bolt: Missing Damaged Visibly missing any underlying rail Torn through rail Medium Damage Mode Repair Threshold Priority for Damage above the Threshold One or more of the following thresholds: More than 9 in. of lateral deflection anywhere over a 25-ft length of rail Top of rail height 2 or more in. lower than original top of rail height High 6-9 in. lateral deflection anywhere over a 25-ft length of rail Medium Post and Rail Deflection Less than 6 in. of lateral deflection over a 25-ft length of rail Low 6-9 in. of lateral deflection between any two adjacent posts Note: For deflection over 9 in., use post/rail deflection guidelines. Medium Rail Deflection Only Less than 6 in. of lateral deflection between any two adjacent posts Low One or more of the following thresholds: Rail cross-section height more than 17 in. (such as may occur if rail is flattened) Rail cross-section height less than 9 in. (such as a dent to top edge) Medium Rail Flattening Rail cross-section height between 9 and 17 in. Low Table 31. Summary of W-Beam barrier repair threshold guidelines. (continued on next page)

82 Damage Repair Threshold Relative Priority Damaged end post Not functional (sheared, rotted, cracked across the grain) High Missing HighAnchor cable Loose—more than 1 in. 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 in. Medium Lag bolts on impact head (Energy Absorbing Terminals Only) Missing or failed lag bolts High Loose or Misaligned Medium Bearing Plate Missing High Table 32. Summary of proposed generic end terminal repair guidance. Damage Category Damage Attributes (1) Non-Functional A. Rail element is no longer continuous. B. 3 or more posts are broken off or no longer attached to the rail. C. Deflection of rail element is more than 18 in. D. Rail element is torn. E. Top of rail is less than 24 in. (2) Damaged but should function adequately under majority of impacts A. Rail element is continuous (can be bent or crushed significantly). B. 2 or fewer posts are broken or separated from the rail element. C. Deflection of the rail element is less than 12 in. Table 33. FHWA W-beam damage classification details (FHWA, 2008). More than 2 holes less than 1 in. in height in a 12.5’ length of rail Any holes greater than 1 in. in height Any hole which intersects either the top or bottom edge of the rail High Non-Manufacture d holes (such as crash- induced holes , lug-nut damage, or holes rusted - through the rail) 1-2 holes less than 1 in. in height in a 12.5-ft. length of rail Medium Vertical Tear Any length vertical (transverse) tear High Horizontal Tear Horizontal (longitudinal) greater than 12 in. long and greater than 0.5 in. wide Note : for horizontal tears less than 12 in. in length or less than 0.5 in. in height, use the non-manufactured holes guidelines. Medium Damage Mode Repair Threshold Priority for Damage above the Threshold Table 31. (Continued).

research team recommends that the items below be consid- ered in future evaluations of repair guidelines: • Conduct deeper sensitivity analyses for other impact con- ditions. Examples would be alternate impact speeds, impact angles, and impact points. The finite element modeling was applied to only some of the damage types, and if extended to other damage modes could yield further insight into the crash performance of damaged barriers. • Conduct additional full-scale crash tests of damaged longitu- dinal barriers both as a means to evaluate the crash perfor- mance of these systems, and to provide an additional source of finite element model validation data. • Assess the implications of “damage” under MASH criteria. The MASH criteria use a larger pickup truck than the stan- dard 2000P vehicle used in NCHRP Report 350. • The approach in this project to focus on worst case scenarios led to the decision to evaluate impacts with larger vehicles. However, there can also be significant issues associated with impacts with smaller vehicles. A follow-up project should assess the risk of smaller vehicle impacts with damaged lon- gitudinal barriers. • Rail tensions in the finite element simulations were examined to determine the risk of rupture occurring in the guardrail system as a whole. While localized tearing is possible in vehicle-guardrail impacts, this guardrail model did not in- clude failure criteria for the steel components and was not configured to look for element tearing due to localized stress concentrations. 15.3.6 Recommendations for Additional Damaged Barrier Repair Guidelines This project has evaluated the crash performance of a num- ber of the most commonly encountered damage modes incurred by longitudinal barriers. The research team’s eval- uation has not however been exhaustive. There is a continu- ing need for development of repair guidelines for a number of additional damage modes beyond those which could be evaluated under this contract. The research team recommends that these additional damage modes be given first priority for evaluation in a future follow-up phase to NCHRP Proj- ect 22-23. Additional damage modes for which repair guide- lines should be developed in a follow-up phase to the current project include the following: • Wood post systems: This project has evaluated steel post systems—the most common variety of strong-post w-beam barrier systems. Wood post systems are heavily used in many installations however, and because these systems fail in a very different manner than steel post systems, there is a need to determine repair criteria which are unique to wood posts. • Overlapping damage modes: Longitudinal barrier dam- age often consists of overlapping damage modes, e.g., rail deflection with flattening. There is a need to better under- stand the interaction between overlapping damage types. • Generic end treatments: The current guidelines are based on engineering judgment and would benefit from quanti- tative assessment. It would also be useful to extend the guidelines for generic end treatments to proprietary end treatments. • Damage to barriers near end terminals: A previously deflected rail element within the first 50 feet of rail in an energy absorbing terminal may not properly activate the end terminal in a head-on crash. If these rails are not straight, the rail may lose column strength and be unable to resist buck- ling when impacted end-on. • Transitions: It would be useful to broaden the guidance to full systems by addressing damage to transitions. 15.4 Guideline Format for Maintenance Personnel The end customer for these repair guidelines are highway maintenance personnel. In addition to being based upon a strong analytical foundation, the guidelines must be easily understood and implemented. Chapter 16 presents the repair threshold guidelines in a graphical format that clarifies how damage to w-beam barriers should be measured and repair priority assessed. 83 Damage Category Damage Attributes (1) Non-Functional A. Rail element is no longer continuous. B. 1 or more posts are broken off or severely bent. C. Deflection of rail element is more than 10 in. E. Rail element is torn. D. Top of rail is less than 26 in. (2) Damaged but may still work A. Rail element is continuous (can be bent or crushed significantly). B. Deflection of the rail element is less than 10 in. Table 34. Proposed revisions to original FHWA W-beam damage classification.

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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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