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

Chapter: Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)

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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 9. Effects of Anchor Strength on Performance of the G4(2W)." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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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.

230 CHAPTER 9 – EFFECTS OF ANCHOR STRENGTH ON PERFORMANCE OF THE G4(2W) Finite element analysis was used to simulate NCHRP Report 350 Test 3-11 impact conditions on the G4(2W) guardrail system. The analyses were performed using various levels of anchor strength in order to quantify the effects on anchor strength on guardrail performance. Although the end-terminal of a guardrail serves many purposes, one of its primary functions is to “anchor” the ends of the rail so that the resulting tension in the rail can help to limit lateral deflection of the system during impacts. Once the effect of anchor strength on guardrail performance was quantified, the various damage modes for end-terminals were then defined simply in terms of their effects on anchor strength (see Chapter 13). Most guardrail end-terminals use the same basic arrangement that is illustrated in Figure 55. When the guardrail is struck, the anchor cable transfers the load to the foundation tubes to resist axial movement of the rail; the resulting tension in the rail helps to limit lateral deflections of the guardrail in the impact zone. Figure 56 shows a photo of the generic end-anchor for the modified G4(1S) with wood blockouts that was used in full-scale crash test 2214- WB1.[Polivka06a] The effectiveness of the anchor is not dependent on the terminal type (i.e., a FLEAT, ET-PLUS, SKT, REGENT, SRT, etc.) since all these terminals include essentially the same anchorage details. So, for example, if the groundline strut is missing, broken or otherwise nonfunctional, regardless of specific end-terminal type, then that damage mode would be associated with a corresponding anchor stiffness and strength. Likewise, eroded soil around an anchor post or an anchor post installed with inappropriate embedment depth would be associated with a corresponding anchor stiffness and strength. Figure 174. Sketch of typical guardrail anchor system. Foundation Tubes BCT PostBCT Post Anchor Cable Soil Bearing Plate Soil Bearing Plate Groundline Strut Cable Bearing Plate Cable Anchor to Rail

231 Figure 175. Generic end-terminal used in full-scale crash test 2214-WB1 of a strong-post guardrail system.[Polivka06a] Finite element analysis was used to quantify the effects of various levels of anchor strength degradation on the crash performance of the G4(2W) guardrail system. Two series of analyses were conducted. In the first series, an undamaged G4(2W) with various levels of end- anchor strength was evaluated. In the second series, the anchor strength was reduced by 47 percent (e.g., end-terminal with single foundation-tube anchor) and the posts were modeled with deteriorated strength properties. Research Approach The baseline anchor strength was determined through physical testing by measuring the force-deflection response of a standard two-post end-terminal anchor subjected to tensile loading, as illustrated in Figure 176. Ideally, the loading on the end of the rail would be applied dynamically with a loading rate similar to real-world vehicular collisions. However, there are many difficulties involved in designing a pendulum test experiment to “pull” on the end of the system. In general pendulums (and bogie vehicles) are used to strike and “push” an object rather than “pull”. Although it should be possible to construct a fixture (e.g., using pulleys or levers) to convert the compressive load of the pendulum to a tensile load on the rail, it would probably be difficult to separate out the dynamic effects of the mass, stiffness, energy absorption, etc. of the fixture from the test results. Also, the rail tension, which results from the lateral deflection of the guardrail during vehicular collisions, develops at a relatively low rate. For example, the maximum deflection of a guardrail in a TL3 event typically occurs over a period of approximately 0.2 to 0.3 seconds. If the resulting maximum rail deflection at the anchor is, say, 3 inches and assuming that the rail displaces at a constant rate, then the resulting deflection rate would be in the range of 10 in/s (0.6 mph ) to15 in/s (0.9 mph). To use a pendulum (or bogie) to achieve sufficient energy to displace the anchor system at such a low rate would require a very large mass and/or a fixture with significant mechanical advantage. Based on these facts and issues, it was decided that a more appropriate approach would be to perform a quasi-static test using a cable and winch system. The loading rate for the quasi-static tests was 1.0 to 1.5 in/s.

232 Figure 176. Example – finite element model for computing force-deflection response of the standard two-post guardrail anchor system.[Plaxico03] Once the baseline anchor response was determined, finite element analysis was then used to quantify the effects of various levels of anchor strength degradation on the crash performance of the G4(2W) guardrail system. The end-anchor response was modeled using non-linear springs attached to the ends of the w-beam rail. The baseline end-anchor condition corresponded to the two-post cable-and-strut anchor system and was characterized by a force-deflection response measured directly from physical testing. To evaluate the effects of anchor strength, the stiffness and strength of the anchor was incrementally reduced to represent various levels of anchor damage. The incremental reduction in strength was achieved by simply scaling the baseline anchor stiffness. The crash performance of the guardrail was then evaluated for each damage level. Table 55 shows the analysis matrix used in this study. Table 55. Analysis matrix for Task 4A-2. Post Strength Anchor Strength 167% 133% Baseline 67% 47% DL0 x x ** x x DL1 ** DL2 ** DL3 ** ** Analyses conducted in Task 4A-3 (See Chapter 8) Physical Testing Quasi-Static Pull-Test on End-Anchor A quasi-static test was performed to measure the force-deflection response of a standard two-post guardrail anchor system, shown in Figure 177. The components of the test article were donated to the study by three guardrail distributors: Trinity Industries, Gregory Industries, Inc. and Road Systems, Inc. The test was performed on December 16, 2013 by the staff of the Federal Outdoor Impact Laboratory (FOIL) at the Federal Highway Administration’s Turner Fairbank Highway Research Center in McLean, Virginia. Since the purpose of the test was to measure the

233 response of the anchor, only those components directly related to the anchor system were included in the test assembly. Figure 177. Test set-up for measuring force-deflection response of a standard two-post guardrail end-terminal anchor. Figure 178 shows a photo of the two foundation tubes with soil plates taken during the installation process. Because of the limited size of the soil pit at the test site, the test article was placed as far back as possible in the pit in order to minimize the influence of the rigid steel wall at the front of the soil pit. The distance from the front of the foundation tube at Post 2 to the front wall was approximately 30 inches; and the distance from the back of the foundation tube at Post 1 to the back of the soil pit was approximately 6 inches. It was not clear from the test results if the response of the system was influenced by the size limitation of the soil pit area. The load was applied to the end of the test article using a winch and a cable-pulley system with a 2:1 mechanical advantage, as shown in Figure 179. The winch system used for the quasi-static test was an existing component of the pendulum test device – used primarily to hoist the pendulum into position for dynamic tests. For the test, the winch cable was run through a stationary pulley bracket aligned with the test article, then around another pulley that was attached to the end of the test article, and then back to a fixed anchor point near the stationary pulley bracket. This arrangement resulted in a 2:1 mechanical advantage for the winch. For the pulley located at the end of the test article, a 1-inch diameter steel rod was attached to the pulley and then fastened onto the end of the w-beam rail using a standard cable-anchor-bracket (i.e., RWE02), as shown in Figure 179.

234 Figure 178. Photograph of anchor tubes with soil plates during installation. An additional wood post was fastened onto the rail near the load-point in order to maintain the proper height of the rail during the test. Recall that for most end-terminals a CRT post is fastened to the rail at this location. As a further precaution, additional materials were stacked underneath the end of the w-beam near the load point to ensure that the rail height did not drop during the test. The three vertical steel posts shown in the test set-up in Figure 177 were used for mounting (anchoring) the string-pot displacement transducers and were not part of the test article. Figure 179. Cable and pulley system used to apply tensile loading on end-terminal anchor. Winch 25 kip Force Transducer Pulley System Loading Bracket

235 Equipment and Instrumentation Force Transducer The load on the cable was measured using an Interface Model 1220 standard load cell, rated at 25-kip. With the 2:1 mechanical advantage of the cable-pulley system, the load on the test article was two times the load measured by the load cell. Displacement Transducers SpaceAge Control, Inc. Series 162 Miniature Position Transducers (i.e., string-pots) were used to measure displacements at three key locations on the test article during the test. One was used to measure displacement at the end of the w-beam rail at the load point, another was used to measure the groundline displacement of the foundation tube at Post 1 (i.e., end post), and the third was used to measure the groundline displacement of the foundation tube at Post 2. Photography The tests were also recorded using three high-definition digital video cameras with operating speed set to 60 frames per second. Figure 180 provides the specifications and the general placement of the cameras for the test. The pre-test setup and the post-test results were also documented with photographs. Figure 180. Video camera specifications and placement. Y Axis X Axis 2 NO. CAMERA LENS LENS (MM) ZOOM (MM) RESOLUTION (PIXELS) SPEED (FPS) LOCATION 1 GX-1 Nikon 24-85 30 1280 X 1024 60 Right Perp Post #1 2 Go Pro 1 Go Pro 24 24 Hi-Definition 60 Real Time Post #2 3 Go Pro Go Pro 24 24 Hi-Definition 60 Real Time Guardrail End 1 Pendulu m 3

236 Results The loading on the anchor system was applied in two steps. In the first step, a displacement of 1 inch was applied to the end of the rail at a displacement rate of approximately 1 inch per second. The displacement was then held constant for 2.8 seconds. The test then proceeded at a displacement rate of approximately 1.5 inches per second until failure of the anchor system. Figure 181 shows the displacement- and displacement rate-time histories measured at the load point; Figure 182 shows the resulting force-deflection response. The displacement at the end of the rail and the displacement at the groundline of Post 2 are shown in Figure 183. The displacement at Post 1 was not measured due to the displacement transducer malfunctioning. Sequential views of the test from two view-points are shown in Figure 184. The foundation tube at Post 1 began to displace vertically almost immediately at the start of the second loading step. The vertical component of displacement was not measured in the test; however, at 2 inches horizontal deflection of the rail element, the vertical displacement of Post 1 was estimated from the test videos to be 0.5 inches. At approximately 8.1 inches horizontal displacement of the rail element, the foundation tube fully extracted from the ground. Figure 181. Displacement- and displacement rate-time histories measured at the load point on the end of the rail. 0 2 4 6 8 10 12 14 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 D is p la ce m e n t (i n ) D is p la ce m e n t R at e ( in /s ) Time (sec) Displacement Rate Displacement

237 Figure 182. Force-displacement response of the anchor system measured at the load point on the end of the rail. Figure 183. Displacement-time history at the end-of the rail and at the groundline of Post 2. 0 5 10 15 20 25 0 2 4 6 8 10 12 Fo rc e ( ki p s) Displacement (in) FOIL13011B 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 D is p la ce m e n t (i n ) Time (sec) Rail End Post 2

238 Figure 184. Sequential views of Test 13011B. 2 inch disp. 4 inch disp. 6 inch disp. 8 inch disp. 1 inch disp. 2 inch disp. 4 inch disp. 6 inch disp. 8 inch disp. 1 inch disp.

239 As the rail element was pulled during the test, the vertical force on the foundation tube (resulting from the vertical component of force of the anchor cable) was greater than the friction forces between the ground and foundation tube. This behavior, however, was not expected based on the results of previous full-scale crash tests in which vertical displacement of the foundation tube was generally negligible. For example, Figure 185 shows the damage to the upstream anchor system in full-scale crash Test MGSDF-1 conducted by the Midwest Roadside Safety Facility (MwRSF). The horizontal groundline displacement of the foundation tube in the crash test appears to be greater than that of quasi-static Test 13011B, and yet there was no measurable vertical deflection in the dynamic test. Figure 185. Damage to end-terminal anchor in full-scale crash Test MGSDF-1.[Hascal07] Due to the abnormal behavior of the anchor system in Test 13011B, the test results were considered to represent a lower bound for the force-deflection response of the system. Further, if the foundation tube had not extracted suddenly at 8 inches rail deflection, then it was assumed that the force would have continued to increase. Based on these assumptions, the effective static force-deflection response of the system was approximated, as shown in Figure 186. The effect of loading rate on the anchor response was accounted for in the finite element model using a dynamic magnification factor of 1.5 to scale the quasi-static force-deflection curve. As a side note, the anchor strength tests performed in Task 4B included modifications to the test set-up which eliminated many of these issues. The baseline test was again conducted and produced slightly higher, but similar results (see Chapter 13 for more details).

240 Figure 186. Measured and approximated force-deflection response for the end-anchor. Evaluate Effects of Anchor Strength on Guardrail Performance The finite element model of the standard G4(2W) guardrail system, which was developed and validated in an earlier part of this study (See Chapter 7), was used to evaluate the effects of end-anchor strength on the crash performance of the guardrail system. The baseline (undamaged) condition for the guardrail included the DL0 guardrail post model (refer to Chapter 7) and anchor strength corresponding to the results of the end-terminal in Test 13011B with force-deflection response scaled by 1.5 to account for loading rate3, as shown in Figure 186. The various levels of anchor strength were included in the model by simply scaling the baseline anchor force-deflection curve. The crash performance of the guardrail was then evaluated using FEA to determine the effects of anchor strength degradation. The evaluation included anchor strengths ranging from 167 percent of the baseline anchor strength down to 47 percent of the baseline strength. The analysis matrix was shown previously in Table 55. The G4(2W) model used for this study was developed based on the NCHRP Report 350 TL3 compliance test for the guardrail conducted at TTI (i.e., Test 471470-26).[Mak99a] Refer to Chapter 7 for more details on model development. The impact conditions were set to those of full-scale crash Test 471470-26 and involved the 4,568-lb C2500D pickup model impacting the guardrail at 62.6 mph (100.8 km/hr) at an angle of 24.3 degrees.[Mak99a] Due to time and budget constraints, no attempt was made to determine the critical impact point (CIP) for each of the various anchor damage cases. The impact point for all analysis cases was set at 22 inches upstream of Post 14, which corresponded to the CIP for the baseline G4(2W) guardrail in Test 471470-26.[Mak99a] The analyses were conducted for 0.6 seconds of the impact event. 3 The 1.5x scale factor used here was based on comparison of static vs. dynamic post-in-soil tests curves shown in Figure B-3 of MASH. It was later determined through physical tests in this project (not documented here) that the dynamic scale factor for the W6x16 post in soil with density of 144 pcf was 1.4x. 0 5 10 15 20 25 30 35 40 45 0 5 10 Fo rc e ( ki p s) (Displacement (in) FOIL13011B Approximate Static Dynamic (x1.5)

241 Results The analysis model used for the evaluations is shown in Figure 187. Post 4 through Post 21 were modeled using MAT143; while the posts located upstream of the impact zone were modeled using a less computationally demanding material model, MAT13. The material parameters for MAT143 were defined according to deterioration level as described in Chapter 8. MAT13, however, was not calibrated for the various levels of post deterioration. Instead, the material parameters used for MAT13 were adopted from the earlier work by Plaxico, et.al.[Plaxico98] In effect, the line of posts near the downstream anchor should be considered undamaged, or new. Figure 187. Analysis setup for evaluating effects of anchor strength on the performance of the G4(2W). G4(2W) with Undamaged Posts and Various Anchor Strengths The following is an evaluation of the G4(2W) guardrail with various levels of anchor strengths. Sequential views of the FE analysis results for each case are provided in Appendix I. Table 56 provides a summary of barrier damage from the analyses related to rail deflections, anchor movement and splice damage. This information is also presented graphically in Figure 188 and Figure 189. The maximum lateral rail deflection was essentially the same for all cases. This result was not expected, however, and was interpreted to indicate that the tension in the w- beam rail was carried primarily by the line-posts leading up to the end-terminals, resulting in minimal force on the anchor systems. Likewise, the maximum longitudinal deflection of the w- beam rail at the anchor was very similar for each case as well and ranged from 1.3 to 1.8 inches. Regarding the “location of maximum deflection” in Table 56, a negative number indicates that the maximum deflection occurred upstream of the splice at Post 16, while a positive number indicates that maximum deflection occurred downstream of the splice. Since the maximum lateral deflection occurred just upstream of the splice connection at Post 16 for all cases, the impact point used in the analyses was considered to be representative of the critical impact point (CIP) for these damage cases. Deteriorated Posts (MAT143) Undamaged Posts (MAT13) Non-Linear Spring Anchor Stiffness Non-Linear Spring Anchor Stiffness

242 Table 56. Summary of barrier damage evaluation from analyses of undamaged G4(2W) guardrail with various anchor strengths. Figure 188. Summary of barrier damage evaluation from analyses of undamaged G4(2W) guardrail with various anchor strengths. 167% 133% Baseline 67% 47% Maximum Rail Deflection (in) 32.0 32.0 32.0 32.6 32.7 Location of Max Defl. (in) (Relative to Post 16) -31.3 -20.3 -14.8 -6.0 -5.1 Rail Deflection at Post 13 (in) 1.9 1.7 1.8 1.6 1.8 Rail Deflection at Post 14 (in) 12.9 13.0 13.1 12.4 12.6 Rail Deflection at Post 15 (in) 27.7 27.5 27.8 28.2 27.7 Rail Deflection at Post 16 (in) 31.4 31.2 31.1 32.4 32.5 Rail Deflection at Post 17 (in) 17.4 17.8 17.5 18.0 20.7 Rail Deflection at Post 18 (in) 1.2 1.2 1.2 1.3 1.7 Rail Deflection at Post 19 (in) 0.0 0.0 0.0 0.0 0.0 Upstream Anchor Deflection (in) 1.3 1.4 1.4 1.8 1.8 Downstream Anchor Deflection (in) 0.9 0.9 0.9 1.1 1.1 Maximum Strain in splice 1.16 1.15 0.84 1.09 1.25 Event Undamaged Posts (DL0) Varying Anchor Strength -40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0 Undamaged G4(2W) with Various Anchor Strengths Posts DL0 - Anchor 167% Posts DL0 - Anchor 133% Posts DL0 - Anchor Baseline Posts DL0 - Anchor 67% Posts DL0 - Anchor 47%

243 Figure 189. Summary of anchor displacement at rail height from analyses of undamaged G4(2W) guardrail with various anchor strengths. A summary of occupant risk measures computed from the acceleration and angular rate time-histories at the vehicle’s center of gravity is provided in Table 57. This data is also presented graphically in Figures 190 through 192. The results indicate that reduced anchor strength does not significantly affect occupant risk as the magnitude of maximum vehicle decelerations was very similar for all cases. A summary of the maximum effective plastic strains around the splice-bolt holes in the w-beam for each analysis case is shown in Figure 193. The results showed that the potential for splice rupture increased as anchor strength increased or decreased from the baseline anchor strength case. 1.30 1.36 1.38 1.76 1.82 0.86 0.91 0.91 1.08 1.11 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 167% 133% Baseline 67% 47% Varying Anchor Strength A n ch o r D ef le ct io n ( in ) Undamaged G4(2W) with Various Anchor Strengths Upstream Anchor Deflection (in) Downstream Anchor Deflection (in)

244 Table 57. Summary of occupant risk measures from analyses of undamaged G4(2W) guardrail with various anchor strengths. Figure 190. Summary of occupant impact velocity (OIV) from analyses of undamaged G4(2W) guardrail with various anchor strengths. 167% 133% Baseline 67% 47% Occupant Impact Velocity x-direction 5.3 5.4 5.2 5.4 5.4 (m/s) y-direction 5.6 5.6 5.3 5.4 5.5 at time (0.1522 sec) (0.1523 sec) (0.1519 sec) (0.1516 sec) (0.1523 sec) 7.3 7.3 7 7.1 7.2 (0.1473 sec) (0.1477 sec) (0.1471 sec) (0.1466 sec) (0.1476 sec) Ridedown Acceleration 10.3 10.4 10.3 9.6 9.0 (g's) (0.1623 - 0.1723 sec) (0.1525 - 0.1625 sec) (0.1519 - 0.1619 sec) (0.1575 - 0.1675 sec) (0.1650 - 0.1750 sec) 9 9.7 10.7 9.7 9.0 (0.2106 - 0.2206 sec) (0.2196 - 0.2296 sec) (0.2198 - 0.2298 sec) (0.1516 - 0.1616 sec) (0.2108 - 0.2208 sec) 12.3 12.9 13.7 13.1 11.6 (0.1507 - 0.1607 sec) (0.1518 - 0.1618 sec) (0.2003 - 0.2103 sec) (0.1498 - 0.1598 sec) (0.1488 - 0.1588 sec) 0.99 1 0.93 0.97 0.98 (0.1238 - 0.1738 sec) (0.1242 - 0.1742 sec) (0.1219 - 0.1719 sec) (0.1235 - 0.1735 sec) (0.1249 - 0.1749 sec) Max 50-ms moving avg. acc. 7.7 7.6 7.6 7.6 7.6 (g's) (0.1234 - 0.1734 sec) (0.1225 - 0.1725 sec) (0.1216 - 0.1716 sec) (0.1230 - 0.1730 sec) (0.1241 - 0.1741 sec) 6.8 7.0 6.5 6.8 6.8 (0.1254- 0.1754 sec) (0.1257 - 0.1757 sec) (0.1976 - 0.2476 sec) (0.1114 - 0.1614 sec) (0.1258 - 0.1758 sec) 3 2.6 2.4 4.6 3.3 (0.3290 - 0.3790 sec) (0.2631 - 0.3131 sec) (0.3344 - 0.3844 sec) (0.0929 - 0.1429 sec) (0.3312 - 0.3812 sec) Undamaged Posts (DL0) Occupant Risk Factors Varying Anchor Strength (g's) x-direction y-direction z-direction ASI PHD THIV (m/s) x-direction y-direction 0 1 2 3 4 5 6 7 167% 133% Baseline 67% 47% O IV ( m /s ) Anchor Strength (% of Baseline) Occupant Impact Velocity x-dir y-dir

245 Figure 191. Summary of occupant ridedown accelerations (ORA) from analyses of undamaged G4(2W) guardrail with various anchor strengths. Figure 192. Summary of occupant risk measures from analyses of undamaged G4(2W) guardrail with various anchor strengths. 0 2 4 6 8 10 12 167% 133% Baseline 67% 47% O R A ( g) Anchor Strength (% of Baseline) Occupant Ridedown Acceleration x-dir y-dir 0 1 2 3 4 5 6 7 8 167% 133% Baseline 67% 47% 5 0 -m s A ve ra ge A cc . ( g) Anchor Strength (% of Baseline) 50-ms Avg. Acceleration x-dir y-dir

246 Figure 193. Summary of maximum effective plastic strains occurring at the splice-bolt locations at Post 16. G4(2W) with 47 Percent Baseline Anchor Strength and Various Post Deterioration Levels The following is an evaluation of the G4(2W) guardrail for the combination damage mode of reduced anchor stiffness and deteriorated guardrail posts. The anchor stiffness for this analysis was set to 47 percent of the baseline anchor strength and four different post deterioration levels were evaluated. Sequential views of the FEA results for each case are provided in Appendix J. Table 56 provides a summary of barrier damage from the analyses related to rail deflections, anchor movement and splice damage. This information is also presented graphically in Figures 188 and 189. The maximum lateral rail deflection was 32 inches for case DL0 (i.e., undamaged posts) and increased significantly as post deterioration levels increased. This result was similar to that of the G4(2W) with baseline anchor strength and various levels of post-deterioration (see Chapter 8); however, the lateral deflection increased much more significantly for this combination damage mode case. The cause of the increased deflections was attributed to the fact that as deterioration levels for the guardrail posts increased, more and more posts fractured during the impact event. This resulted in fewer posts available to carry the tensile load in the w- beam rail and, thus, higher loads on the anchor systems. Likewise, the maximum longitudinal deflection of the w-beam rail at the anchor also increased significantly for this combination damage mode as post deterioration levels increased - compared to the results from the analyses involving the baseline anchor stiffness. For these analyses cases, the maximum lateral deflection generally occurred downstream of the splice connection at Post 16; thus the impact point used in the analyses was not considered to be representative of the critical impact point (CIP) for these damage cases. 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 167% 133% Baseline 67% 47% Varying Anchor Strength Splice Strains

247 Table 58. Summary of barrier damage evaluation from analyses of G4(2W) with combination of weak anchor and deteriorated posts. Figure 194. Summary of barrier damage evaluation from analyses of G4(2W) with combination of weak anchor and deteriorated posts. DL0 DL1 DL2 DL3 Maximum Rail Deflection (in) 32.7 49.5 73.5 107.0 Location of Max Defl. (in) (Relative to Post 16) -5.1 43.5 75.0 0.0 Rail Deflection at Post 13 (in) 1.8 14.0 44.8 97.4 Rail Deflection at Post 14 (in) 12.6 28.1 53.1 103.2 Rail Deflection at Post 15 (in) 27.7 39.3 60.7 105.3 Rail Deflection at Post 16 (in) 32.5 47.2 69.3 107.0 Rail Deflection at Post 17 (in) 20.7 47.8 73.5 106.3 Rail Deflection at Post 18 (in) 1.7 37.5 71.6 103.5 Rail Deflection at Post 19 (in) 0.0 15.7 64.6 98.2 Upstream Anchor Deflection (in) 1.8 2.3 4.3 20.9 Downstream Anchor Deflection (in) 1.1 1.3 1.6 1.4 Maximum Strain in splice 1.25 1.22 1.15 1.04 Event Varying Post Deterioration Anchor Strength = 47% Baseline -20.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 Anchor Strength = 47% of Baseline w/Varying Post Deterioration DL0 Posts DL1 Posts DL2 Posts DL3 posts

248 Figure 195. Summary of anchor displacement at rail height from analyses of G4(2W) with combination of weak anchor and deteriorated posts. A summary of occupant risk measures computed from the acceleration and angular rate time-histories at the vehicle’s center of gravity is provided in Table 59. This data is also presented graphically in Figures 196 through 198. The results indicated that reduced anchor strength did not significantly affect occupant risk; however there was a slight trend toward decreasing values as post deterioration levels increased. A summary of the maximum effective plastic strains around the splice-bolt holes in the w-beam for each analysis case is shown in Figure 199. The results showed that the potential for splice rupture decreased slightly as post deterioration levels increased; this information may not be reliable, however, since the CIP was not used in the analyses. It is assumed that the results for Cases DL2 and DL3 would likely have been more severe had critical impact conditions been used. The CIP for these cases can be estimated based on the deflection data from Table 58 and assuming that the maximum potential for rail rupture occurs when maximum rail deflection is located at approximately 20 inches upstream of a splice connection. Future work should include analysis cases using the CIP (as estimated here or otherwise determined) for each damage case. 1.82 2.28 4.33 20.91 1.11 1.32 1.64 1.40 0.0 5.0 10.0 15.0 20.0 25.0 DL0 DL1 DL2 DL3 Varying Post Deterioration A n ch o r D ef le ct io n ( in ) Anchor Strength = 47% of Baseline w/Varying Post Deterioration Upstream Anchor Deflection (in) Downstream Anchor Deflection (in)

249 Table 59. Summary of occupant risk measures from analyses of G4(2W) with combination of weak anchor and deteriorated posts. Figure 196. Summary of occupant impact velocity (OIV) from analyses of G4(2W) with combination of weak anchor and deteriorated posts. DL0 DL1 DL2 DL3 Occupant Impact Velocity x-direction 5.4 4.3 3.3 3.3 (m/s) y-direction 5.5 4.7 4.0 3.3 at time (0.1523 sec) (0.1632 sec) (0.1791 sec) (0.1948 sec) 7.2 5.9 5.1 4.5 (0.1476 sec) (0.1573 sec) (0.1727 sec) (0.1853 sec) Ridedown Acceleration 9.0 9.6 5.3 5.9 (g's) (0.1650 - 0.1750 sec) (0.2033 - 0.2133 sec) (0.2487 - 0.2587 sec) (0.4526 - 0.4626 sec) 9.0 9.1 8.0 6.5 (0.2108 - 0.2208 sec) (0.2181 - 0.2281 sec) (0.4893 - 0.4993 sec) (0.3289 - 0.3389 sec) 11.6 10.1 9.0 7.5 (0.1488 - 0.1588 sec) (0.1580 - 0.1680 sec) (0.2413 - 0.2513 sec) (0.3292 - 0.3392 sec) 0.98 0.65 0.57 0.44 (0.1249 - 0.1749 sec) (0.1155 - 0.1655 sec) (0.3053 - 0.3553 sec) (0.4449 - 0.4949 sec) Max 50-ms moving avg. acc. 7.6 5.1 3.4 3.4 (g's) (0.1241 - 0.1741 sec) (0.3408 - 0.3908 sec) (0.2351 - 0.2851 sec) (0.4299 - 0.4799 sec) 6.8 4.8 4.9 3.5 (0.1258 - 0.1758 sec) (0.2179 - 0.2679 sec) (0.3044 - 0.3544 sec) (0.4496 - 0.4996 sec) 3.3 2.0 2.4 1.6 (0.3312 - 0.3812 sec) (0.5022 - 0.5522 sec) (0.5012 - 0.5512 sec) (0.4371- 0.4871 sec) y-direction z-direction Varying Post Deterioration PHD THIV (m/s) x-direction y-direction Anchor Strength = 47% Baseline Occupant Risk Factors (g's) ASI x-direction 0.0 1.0 2.0 3.0 4.0 5.0 6.0 DL0 DL1 DL2 DL3 Varying Post Deterioration O IV ( m /s ) Occupant Impact Velocity x-direction y-direction

250 Figure 197. Summary of occupant ridedown accelerations (ORA) from analyses of G4(2W) with combination of weak anchor and deteriorated posts. Figure 198. Summary of occupant risk measures from analyses of G4(2W) with combination of weak anchor and deteriorated posts. 0.0 2.0 4.0 6.0 8.0 10.0 12.0 DL0 DL1 DL2 DL3 Varying Post Deterioration O R A ( g) Occupant Ridedown Acceleration x-direction y-direction 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 DL0 DL1 DL2 DL3 Varying Post Deterioration 5 0 -m s A ve ra ge A cc . ( g) 50-ms Avg. Acceleration x-direction y-direction

251 Figure 199. Summary of maximum effective plastic strains occurring at the splice-bolt locations at Post 16. Summary and Discussion The purpose of this study was to quantify the effects of various levels of anchor strength on guardrail performance. The focus was strictly on the use of FEA to evaluate the impact response of the G4(2W) guardrail system with various levels of end-anchor strength. The impact conditions for the analyses corresponded to those of NCHRP Report 350 Test 3-11 (i.e., 4400 lb vehicle impacting at 62.2 mph and 25 degrees, nominally). The various causes of reduced anchor strength were evaluated and presented in Chapter 13 where physical testing was used to measure the anchor strength as a function of various damage modes for the end-terminal. The baseline anchor strength used in these analyses was determined through physical testing by measuring the force-deflection response of a standard two-post cable-and-strut end- terminal subjected to tensile loading. An effective static force-deflection response for the baseline anchor system was derived from the results from Test 13011B, which was performed by pulling on the end-of the w-beam rail with a cable and winch system at a displacement rate of 1.0 to 1.5 in/s. For application in the dynamic impact study, the effects of loading rate on the anchor response was accounted for using a dynamic magnification factor of 1.5 to scale the quasi-static force-deflection curve; this response was then used as the baseline for the remainder of the study. Finite element analysis was then used to quantify the effects of various levels of anchor strength degradation on the crash performance of the G4(2W) guardrail system. Two series of analyses were conducted. In the first series, the analyses involved an undamaged G4(2W) with various levels of end-anchor strength. In the second series, the anchor strength was reduced by 47 percent (e.g., end-terminal with single foundation-tube anchor) and the posts were modeled with deteriorated strength properties. The results of the first series of analyses indicated that the performance of the G4(2W) with healthy posts (i.e., DL0 or DL1) was not significantly affected by anchor strength. This was interpreted to indicate that the tension in the w-beam rail was carried primarily by the line-posts leading up to the end-terminals, resulting in minimal force on the anchor systems. 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 DL0 DL1 DL2 DL3 Varying Post Deterioration Splice Strains

252 This result may differ for the G4(1S) guardrail. The wood post (6x8-inch rectangular or 8-inch round cross-section) has a much higher torsional rigidity than the steel W6x9 steel post, and thus the posts of the G4(2W) guardrail do not experience torsional deflections like the posts of the G4(1S). For example, Figure 200 shows a snapshot from full-scale crash test C08C3-2 on the G4(1S) conducted by MGA Research Corporation on August 6, 2008, illustrating the relatively large torsional deflections of the guardrail posts upstream of the impact resulting from the tensile forces in the rail.[Fleck08] This test also resulted in relatively high deflection of the upstream anchor. Figure 201, on the other hand, shows a snapshot from full-scale crash test 471470-26 on the G4(2W) conducted by TTI on May 25, 1994.[Mak99a] In this test, the torsional deflection of the posts upstream of the impact was negligible and there was relatively low deflection of the anchor system. Figure 200. Snapshot from Test C08C3-2 illustrating torsion behavior of posts upstream of impact on the G4(1S).[Fleck08b] Figure 201. Snapshot from 471470-26 illustrating torsion behavior of posts upstream of impact on the G4(2W).[Mak99a] As a further consideration, the impact point on the guardrail in these analyses was at approximately 62 feet downstream of the end-anchor. In this impact scenario, there were enough

253 posts upstream of the impact point carrying the tensile load of the rail such that the loading on the anchor was negligible. However, consideration should be given to the possibility of impact occurring at a point nearer to the anchor which would significantly increase the loading on the anchor. Also, if the w-beam rail detaches from the upstream posts during impact, or if posts upstream of the impact point fracture under the tensile loading of the w-beam, then anchor loads may increase significantly. For example, in full-scale crash test MGSDF-1 conducted by the MwRSF on a reduced-diameter wood post version of the MGS (i.e., 7.2-inch diameter Douglas Fir posts), the rail separated from all the posts upstream of the impact point to the anchor, as illustrated Figure 202, resulting in notable displacement of the anchor. This was, however, a successful test. Figure 202. System damage at upstream anchor resulting from Test MGSDF-1.[Hascall07] This result was demonstrated in this study by simulating Report 350 Test 3-11 impact on the G4(2W) in which the anchor strength was reduced by 47 percent (e.g., end-terminal with single foundation-tube anchor) and four different post deterioration levels. The results of this second analysis series indicated that lateral rail deflections increased significantly as post deterioration levels increased. Compared to the analysis results regarding post deterioration without anchor damage (e.g., see Chapter 8), the lateral deflection of the rail, as well as the load on the anchor system, increased much more significantly for this combination damage mode case. As discussed previously, the increased deflection was attributed to the fact that more posts upstream and downstream of the impact fractured as deterioration levels increased. This resulted in fewer posts available to carry the tensile load in the w-beam rail, thus causing higher loads on the anchor systems. The potential for rail rupture was also inconclusive for this series of analyses due to the fact that the impact point used for the analyses did not correspond to the critical impact point for the individual damage cases.

254 Future work should include analyses of the G4(2W) at impact points nearer to the anchor system. Due to the differences in the torsional rigidity of the W6x9 steel posts of the G4(1S) guardrail, an investigation similar to the one conducted herein should also be conducted to assess effects of anchor strength on the performance of that system; since it is expected that the G4(1S) would have a greater sensitivity to anchor strength. Recommendations As a result of this study, the research team recommends that the repair threshold for the end-terminal for the G4(2W) include damages that result in more than a 30% loss in anchor capacity relative to the baseline anchor strength. When the damage results in more than 50% loss of capacity then the relative priority for repair is high. For end-terminal damage that results in 30% to 50% loss of capacity for the anchor, the relative priority for repair is medium, unless a fixed object is located within 50 inches behind the face of the barrier. In that case, the line posts should also be checked for deterioration damage. If the damage level for the posts is DL1 or greater, then the priority for repair is high, based on lateral deflection limitations of the guardrail. A summary of the recommendations regarding end-terminal damage for the G4(2W) guardrail are presented in Table 60. Table 60. Recommendations for end-terminal damage for the G4(2W). The repair criteria presented here provides assessment and repair criteria for the end- terminal of a G4(2W) guardrail in terms of anchor strength. The evaluation and repair criteria presented herein are later combined with the results of physical tests performed on the anchor system with various types and levels of measureable damage modes (see Chapter 13). Damage Mode Repair Threshold Relative Priority High High - End-Terminal damage that results in more than 30% loss of anchor capacity and - Guardrail line-posts have deterioration levels of DL1 or greater. Medium End-Terminal damage that results in more than 50% reduction in anchor capacity (relative to the baseline anchor strength) should be considered as high priority for repair. Damaged End-Terminal for the G4(2W) If a hazard is located within 50 inches behind the w-beam rail, then the end-terminal should be considered as high priority for repair for the combination damage mode of: Otherwise, end-terminal damage that results in a 30% to 50% loss of anchor capacity (relative to the baseline anchor strength) should be considered as medium priority for repair.

Next: Chapter 10. Combination Damage Mode of Rail Deflection and Rail-Post Connection for the G4(2W) »
Criteria for Restoration of Longitudinal Barriers, Phase II Get This Book
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Guardrails are an important feature of the roadside that are used to shield errant motorists from becoming involved in even more catastrophic crashes by redirecting vehicles away from fixed hazards such as trees and poles and terrain hazards such as steep roadside slopes and fill embankments.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 304: Criteria for Restoration of Longitudinal Barriers, Phase II develops a Field Guide to assist maintenance personnel in making decisions about repairing damaged guardrail installations.

Supplementary material to the document is Appendices A-S.

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