Recommended Guidelines for the Selection of Test Levels 2 Through 5 Bridge Railings (2021)

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184 APPENDIX B: BRIDGE RAIL SELECTION GUIDELINES

185 The following section presents the recommended selection guidelines and the process for the application of the guidelines for the selection of MASH TL2 though TL5 bridge railings. A risk approach applicable to new construction, rehabilitation and retrofitting is recommended. An alternative cost-benefit approach is provided and discussed following the presentation of the process. A discussion of the process and the policy decisions necessary for implementation are also contained later in this section. Appendix B includes only the process, without any discussion. Appendix B is presented in a format that could be inserted directly into the AASHTO LRFD Bridge Design Specification or the Roadside Design Guide. Bridge Rail Risk Assessment Process The selection of the appropriate MASH test level bridge railing for new or rehabilitation construction is dependent on site-specific conditions and results may differ for each side of the bridge. This process, therefore, should be followed for each bridge edge. These selection procedures only apply to the bridge railing itself. Providing appropriate guardrail-bridge rail transitions, adequate guardrail approaches, and appropriate terminals and crash cushions are also important considerations in the complete safety performance of the bridge. Users should refer to the AASHTO Roadside Design Guide for guidance on appropriate transitions, approach guardrails and terminals. The following selection guidelines include six parts: (1) determine the anticipated construction year traffic volume (AADT); (2) estimate the total encroachments expected over the 30-year life of a 1,000-ft section of the bridge; (3) adjust the expected number of encroachments for site-specific conditions; (4) select the test level from the appropriate chart; (5) additional considerations; and (6) if guidelines do not apply. These steps are described in full below. 7. Traffic Conditions – Determine the anticipated construction year traffic volume (AADT) and percent trucks (PT). These selection guidelines assume an annual traffic growth rate of 2% per year and a design life of 30 years. If the anticipated growth rate or design life are significantly different, use the following equation to compute the equivalent construction year traffic volume for use in these selection guidelines: 𝐴𝐴𝐷𝑇 = 0.7430 ∙ 𝐴𝐴𝐷𝑇 ∙ (1 + 𝐺) / where: AADT0 = The anticipated construction year bi-directional traffic volume (use the one-way traffic volume for one-way roads and ramps), AADTEQ = The equivalent construction year bi-directional traffic volume, G = The anticipated annual traffic growth rate where 0≤G≤1. L = The design-life of the bridge railing in years. 8. Encroachments – Estimate the total number of encroachments (NENCR) that will be experienced on a 1,000-ft section of the bridge railing during the life of the bridge railing

186 by entering Table 68 or Figure 30 with the bi-directional construction year AADT from Step 1 and the highway type. a. Do not proportion the value of NENCR based on the length of the bridge. The entire method is based on a per 1,000-ft basis. b. If the AADT of interest falls to the right of the end of the curve for the desired highway type, the level of service for the highway is likely D or worse and these procedures cannot be used; refer to step 6. 9. Site Conditions – Determine the site-specific adjustment factors for the bridge under consideration using the adjustment factors shown in Table 67. Multiply all the adjustments from Table 67 together to obtain fTOT. Find the modified total number of encroachments (NMOD ENCR) either by: a. Drawing a horizontal line in Figure 30 until the curve corresponding to fTOT is obtained (interpolation between lines is acceptable) then reading down to the horizontal axis for the value of the modified total number of encroachments (NMOD ENCR) on 1,000-ft of bridge railing over the 30-year life of the bridge railing or b. Multiplying the estimated encroachments (NENCR) from Table 68 by the total adjustments (fTOT) from Step 2 to obtain the modified total number of encroachments (NMOD ENCR) on 1,000-ft of bridge railing over the 30-year life of the bridge railing. 10. Test Level Selection – Characterize the hazard environment under the bridge as high, medium or low according to the following definitions: HIGH: A high hazard environment below the bridge includes possible interruption to regional transportation facilities (i.e., high-volume highways, transit and commuter rail, etc.) and/or interaction with a densely populated area below the bridge. Penetrating the railing may limit or impose severe limitations on the regional transportation network (i.e., interstates, rail, etc.). Penetrating the railing also has the possibility of causing multiple fatalities and injuries in addition to the injuries associated with the vehicle occupants. A high- hazard environment is also present if penetration or rolling over the bridge railing could lead to the vehicle damaging a critical structural component of the bridge (e.g., a through-truss bridge). MEDIUM: A medium hazard environment below the bridge includes possible interruption to local transportation facilities, large water bodies used for the shipment of goods or transportation of people, and/or damage to an urban area which is not densely populated. Penetrating the railing would limit local transportation routes, however, detours would be possible and

187 reasonable. Penetrating the railing has the possibility of causing at least one non-motor vehicle injury or fatality. LOW: A low hazard environment below the bridge includes water bodies not used for transportation, low-volume transportation facilities, or areas without buildings or houses in the vicinity of the bridge. Penetrating a low hazard railing would have little impact on regional or local transportation facilities. A low hazard railing has no buildings or facilities in the area which present possible non-motor vehicle related victims of a rail penetration. Choose the hazard environment most applicable to the bridge under consideration. Enter the appropriate chart in Figure 32 for the hazard environment selected above, the modified lifetime encroachments per 1,000-ft of bridge edge (NMOD ENCR) from Step 3, and the percent trucks (PT) from Step 1 to select the appropriate MASH test level for the bridge railing. If the point plots above the dashed risk boundary these charts cannot be used and the engineer should refer to step 6. 11. Additional Considerations – The bridge railing selected using this process provides a solution where the risk of observing a severe or fatal injury crash over the design-life of the bridge railing should be less than 0.01 when the specific site conditions evaluated (i.e., traffic volume and mix, geometry, posted speed limit, and access density) are considered. Engineering judgment should be used when unusual or difficult to characterize site conditions are encountered when selecting a bridge railing. Limited numbers of crash tested bridge railings are available at some test levels, therefore, it is possible that the recommended test level barrier for the evaluated site conditions may not be the best choice for some site conditions not explicitly addressed in these selection guidelines. For example, the particular layout of the barrier at the end of a ramp may influence intersection sight distances and require the use of engineering judgment in designing the interchange to determine an appropriate barrier as it approaches the intersection. Another example might be the presence of pedestrians or bicyclists which might benefit from a higher or different type of railing or the use of sidewalks. Some of the factors that should also be considered are: a. TL5 bridge railings may be appropriate for specially designated hazardous material or truck routes. b. Intersection sight distance obstructions created by higher test level bridge railings at the ends of ramps or bridges should be considered and the bridge railings may require transitioning to a lower height approaching the intersection. c. Stopping sight distance on bridges where the radius and design speed plot below the dashed line in Figure 31 may limit the use of higher test level bridge railings. d. The presence of pedestrians, bicyclists, snowmobiles, all-terrain vehicles and other recreational vehicles may affect the choice of bridge railing. e. Crash history especially as it relates to heavy vehicle crashes or bridge rail penetrations may justify higher performance bridge railings.

188 f. Regional concerns about snow removal, hydrological impact of flood waters flowing over the bridge, and maintaining scenic views may also play a role in the selection of bridge railings beyond these selection guidelines. g. The capacity of the bridge deck may limit the choices available for higher test level bridge railings on rehabilitation projects. 12. Guidelines Do Not Apply – There are some situations where these guidelines should not be used, namely: a. The traffic conditions violate the free traffic flow assumption used in developing the guidelines such that the estimate of the number of encroachments is not reliable. Generally, this results from a plot point in Figure 30 that is to the right of the end of the highway-type line. This indicates that the level of service may be D or worse and the basic assumptions of the method are invalid. b. The user may find that the selection plots above the boundary of Figure 32. In such a case the following options should be considered: i. Can the traffic operational conditions (i.e., AADT and percent trucks) be reduced? ii. Are the roadway characteristics (e.g., horizontal curvature, grade, etc.) resulting in large adjustments to the NENCR? Can the geometry be modified to reduce the adjustments? iii. Can the deck and superstructure support a TL6 bridge railing? These situations require a more detailed analysis of the site conditions that examines a broader range of alternatives beyond just the bridge railing test level selection. A solution will probably require the collaboration of traffic operations, geometric design and bridge railing design engineers to either modify the traffic or geometry conditions of the bridge such that these guidelines can be used or perform a crash history investigation to determine the actual performance of the existing bridge railing.

189 Table 67. Encroachment Adjustments. Access Density Lane Width Horizontal Curve Radius N um be r of A cc es s P oi nt s on B ri dg e or w ith in 2 00 ft of ei th er en d U nd iv id ed D iv id ed a nd O ne w ay A vg , L an e W id th (f t) U nd iv id ed D iv id ed a nd O ne w ay H or iz on ta l C ur ve R ad iu s at th e C en te rl in e (ft ) A ll H ig hw ay T yp es 0 1.00 1.00 9≥ 1.50 1.25 950≥R1 4.00 1 1.50 2.00 10 1.30 1.15 1910>R1>950 (2228.2-R1)//318.3 2≤ 2.20 4.00 11 1.05 1.03 1910≤R1 1.00 12≤ 1.00 1.00 950≥R2 2.00 1910>R2>950 (R2-2864.8)/954.9 1910≤R2 1.00 • If road curves toward vehicle on the barrier side use R1. • If road curves away from the vehicle on the barrier side use R2. fACC = fLW = fHC = Lanes in One Direction Posted Speed Grade N o. T hr ou gh L an es in O ne D ir ec tio n U nd iv id ed D iv id ed a nd O ne w ay Po st ed S pe ed L im it (m i/h r) U nd iv id ed D iv id ed a nd O ne w ay Pe rc en t G ra de A ll H ig hw ay T yp es 1 1.00 1.00 <65 1.42 1.18 -6≥G 2.00 2 0.76 1.00 ≥65 1.00 1.00 -6>G>-2 0.5-(G/4) 3≤ 0.76 0.91 For roads with unposted speed limits use the adjustment for <65 mi/hr -2≤G 1.00 fLN = fPSL = fG = fTOT=fACC∙fLN∙fLW∙fG∙fHC∙fPSL =

190 Table 68. AADT – Lifetime Encroachments per 1,000-ft of Bridge Railing. AADT 4 LN DIV 2 LN UNDIV 1 LN ONEWAY AADT 4 LN DIV 2 LN UNDIV 1 LN ONEWAY 500 0.8 1.2 1.7 33,000 11.6 4.3 1,000 1.6 1.9 3.4 34,000 11.8 4.4 2,000 3.1 3.2 6.0 35,000 12.1 4.6 3,000 4.4 3.6 8.1 36,000 12.4 4.7 4,000 5.5 3.6 9.6 37,000 12.6 4.8 5,000 6.5 3.4 10.6 38,000 12.9 5.0 6,000 7.4 3.1 11.4 39,000 13.2 5.1 7,000 8.1 2.7 11.8 40,000 13.5 5.2 8,000 8.8 2.3 12.0 41,000 13.9 5.4 9,000 9.3 2.0 12.0 42,000 14.2 5.5 10,000 9.7 1.9 11.9 43,000 14.5 5.6 11,000 10.1 1.8 11.7 44,000 14.9 5.8 12,000 10.4 1.8 11.6 45,000 15.2 5.9 13,000 10.6 1.8 46,000 15.6 6.0 14,000 10.8 1.9 47,000 15.9 15,000 10.9 2.0 48,000 16.2 16,000 11.0 2.1 49,000 16.6 17,000 11.0 2.2 50,000 16.9 18,000 11.0 2.4 51,000 17.2 19,000 10.9 2.5 52,000 17.6 20,000 10.9 2.6 53,000 17.9 21,000 10.8 2.7 54,000 18.3 22,000 10.7 2.9 55,000 18.6 23,000 10.6 3.0 60,000 20.3 24,000 10.6 3.1 65,000 22.0 25,000 10.5 3.3 70,000 23.7 26,000 10.6 3.4 75,000 25.4 27,000 10.7 3.5 80,000 27.1 28,000 10.8 3.7 85,000 28.7 29,000 10.9 3.8 90,000 30.4 30,000 11.0 3.9 95,000 31,000 11.2 4.1 100,000 32,000 11.4 4.2 105,000 33,000 11.6 4.3 110,000 LOS ≥ D LOS ≥ DLOS ≥ D LOS ≥ D

191 Figure 30. AADT – Lifetime Encroachments/1,000-ft of Bridge Railing Nomograph. 110100 NMO D ENCR 1 10 100 100 1,000 10,000 100,000 N EN C R AADT0 or AADTEQ ) 4 LANE DIVIDED 2 LANW UNDIVIDED 1 LANE ONE WAY

192 Figure 31. Minimum Horizontal Curve Radius Based on Barrier Obstruction to the Stopping Sight Distance Compared to AASHTO Exhibit 3-14. 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 25 30 35 40 45 50 55 60 65 70 75 M in im um C ur ve R ad iu s ( ft) Design Speed (mi/hr) AASHTO Ex. 3-14 with e<=10% Min. Radius Based on Barrier Obstruction to SSD

193 LOW MEDIUM HIGH Figure 32. Test Level Selection Nomograph (Risk<0.01 in 30 years for 1000 ft of bridge railing). TL2 or TL3 TL2 or TL3 TL2 or TL3 TL4 TL4 TL4 TL5 TL5 TL5 Risk Boundary Risk Boundary Risk Boundary