Criteria for Restoration of Longitudinal Barriers, Phase II (2021)

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437 CHAPTER 17 – CONCLUSIONS One of the oft-touted advantages of strong-post w-beam guardrails is that they can retain some effectiveness even after having been struck and damaged. It is not always easy to discern, however, when the extent of damage warrants repair and when the damaged barrier is still largely functional. Occasionally, the damage is severe and the need to repair is evident. More frequently though the damage is the result of low-speed collisions and side-swipes which cause only minor or moderate damage for which the need to repair is not obvious. Guardrails also suffer deterioration from environmental factors and frequently incur minor impact damage during roadway maintenance operations such as snow plowing and mowing. Unfortunately, very little information has been available to quantify degradation of barrier performance that these seemingly minor damages cause. Assessing guardrail damage is ultimately the responsibility of maintenance and repair personnel within each state’s DOT. Making proper damage assessments maximizes the overall safety of the roadway by ensuring that the limited resources of the DOT are applied only to those guardrails that are no longer deemed effective. The result of this research study was a Field Guide that provides quantitative criteria for assessing guardrail damage and repair priority based on measureable damage metrics for use by maintenance personnel. Phase I of this study was performed by Gabler et. al at Virginia Tech and was focused on damage assessments for the modified G4(1S) guardrail; those results were published in NCHRP Report 656. The work performed in the current study expands on Report 656 by including assessment criteria for the G4(2W) guardrail as well as additional damage assessment criteria for the modified G4(1S). The overall result is a relatively comprehensive field guide for assessing damages for these two very common strong-post guardrail systems. Further, the assessment criteria for the G4(2W) is considered applicable for all wood-post w-beam guardrails that are of the same basic design and that use guardrail posts of similar dimensions. For example, the G4(1W), G4(2W), and the ODOT Type 5 guardrail all use slightly different shaped wooden posts, but otherwise share the same system components. Although the posts are different (e.g., square, rectangular, and round, respectively), they are similar in that they are all wood and are of similar dimensions (i.e., 8-10 inches in width/diameter and 6 ft in length). Damage Modes Investigated The damage modes evaluated in this study included:  Guardrail post deterioration (e.g., rot and insect damage),  End-terminal damages that may affect anchor strength,  Combination damage modes of rail deflection and rail-to-post connection,  Soil erosion around guardrail posts, and  Additional splice damages that were not investigated in Phase I. The basic research approach was similar to that used in Report 656 and involved a combination of pendulum testing and computational analyses to assess performance degradation of the G4(2W) guardrail for various damage modes. Pendulum testing was used to quantify strength degradation of various guardrail components subjected to varying types and levels of damage, to evaluate failure modes that are not well suited for finite element analysis; and also to validate the accuracy of the finite element models and gain confidence in their results. Finite

438 element analysis was then used to investigate the effects of damage modes on overall system performance. For example, pendulum tests were used to quantify reduction in post strength as a function of deterioration (e.g., rot or insect damage); the results of the pendulum tests were used to validate/calibrate the finite element model of the post-soil system(s); FEA was then used to evaluate the effects of the various levels of damage on system performance. All crash performance evaluations of the damaged systems were carried out based on the crash testing procedures used in evaluation of the original undamaged systems, so that performance degradation could be directly assessed. For the systems evaluated in this study, the impact conditions corresponded to NCHRP Report 350 Test Level 3. The recommended procedures and assessment criteria for evaluating each damage mode are presented at the end of each corresponding chapter and are also assembled in Appendix A in the format of a “field guide.” The evaluation procedures in Appendix A are presented in a graphical format to facilitate the assessment process. For each damage mode, a commentary is also provided to support the evaluation criteria. The guidance presented herein is based solely on the effectiveness of the damaged guardrail to safely contain and redirect errant vehicles. Three classifications are used to denote the relative priority for repair – High, Medium and Low. These were adopted from NCHRP Report 656 and are defined as follows:  High Priority: Indicates damage where the crash performance 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 vehicle rollover.  Medium Priority: Indicates damage where the crash performance of the barrier has likely been compromised to some degree but the system should perform effectively for a majority of impact conditions.  Low Priority: Indicates that the damaged guardrail is expected to remain fully functional. The probability of a second impact on a previously damaged guardrail is dependent on many factors related to traffic exposure, including: crash history, ADT, operating speeds, roadway type, road curvature, roadway grade and pavement conditions to name a few. Determining the risk of a second impact was beyond the scope of this study, but is an important factor to consider when determining priority for repair. Recommendations for Future Work In developing these guidelines several areas for improvement were identified that should be considered in future research:  A more simplified in situ procedure for quantifying the level of deterioration of wood guardrail posts needs to be developed. The procedure developed herein appears to be reasonably accurate for use in research studies, but due to equipment cost and complexity of the method it may not be feasible for use in routine maintenance assessments.  Additional analyses or full-scale testing should be performed to evaluate the effects of anchor strength using an impact point located nearer to the upstream anchor system. Additionally, an investigation similar to the one conducted herein for assessing effects of anchor strength on the performance of the G4(2W) should be conducted to assess effects of anchor strength on the performance of the G4(1S). It is expected that the G4(1S) will have a greater sensitivity

439 to anchor strength due to the low torsional rigidity of the W6x9 steel posts of the G4(1S) guardrail.  The impact conditions used herein to generate pre-existing crash-induced rail deflections involved low speeds and a high impact angle which resulted in damage to a relatively localized section of the guardrail; i.e., the damaged area generally spanned only 3 to 4 posts, which increased the potential for pocketing. It is not known how the guardrail will respond to subsequent impacts when the rail deflections are spread over a longer length of the guardrail. It is recommended that future studies on the effects of pre-existing crash-induced rail deflections include higher impact speeds and smaller impact angles to create initial guardrail damage with similar magnitudes of rail deflection spread over a longer length of the guardrail.  Additional analyses or full-scale testing should be conducted to reevaluate the performance of the G4(1S), in which a standard anchor system (i.e., two foundation tubes and groundline strut) is installed on both the upstream and downstream end and a soil pit with adequate width to accommodate the full range of guardrail post deflections in the impact area.  A more comprehensive study of damaged w-beam splice connections should be considered in future studies. The small number of damaged test articles available for the study was not sufficient to represent the complete range of damage levels for each damage mode case.  Also, future impact studies for evaluating the capacity of rail splices should include boundary conditions that more accurately emulate the longitudinal and lateral stiffness behavior of the extended guardrail. Recommendations for Additional Barrier Types The guidance presented in Appendix A (i.e., the Field Guide) addresses a number of the most frequently encountered damage modes for the two most common guardrail systems (i.e., G4(1S) and G4(2W)). In Chapter 5 several additional guardrail systems were identified with relatively high rating for inclusion in the Field Guide, but had to be excluded from this study due to funding limitations. These systems include the G2 weak post guardrail, the G9 thrie-beam guardrail and the MGS. The G2 weak-post guardrail was the highest rated system for future study, followed very closely by the thrie-beam guardrail. The thrie-beam is not as popular as the strong-post w-beam, but almost all states have a number of thrie-beam installations. Although the G2 weak-post guardrail received the second highest rating from the survey respondents for inclusion in the study, the rating for the thrie-beam guardrail was very similar (see Appendix A), thus a case could be made for selecting either system. Consideration should also be given to the fact that the thrie-beam guardrail will require fewer damage mode evaluations for developing a comprehensive set of assessment criteria, compared to the weak-post system. As of this reporting date, there are currently not as many MGS guardrail installations compared to other strong-post guardrail systems; however, many states are now requiring that the MGS system be used in all new guardrail installations. Therefore, the MGS is also considered to have a relatively high rating for inclusion in future updates to the field guide.