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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
×
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Suggested Citation:"2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS." National Academies of Sciences, Engineering, and Medicine. 2022. Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls. Washington, DC: The National Academies Press. doi: 10.17226/26580.
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11 2 STATE OF PRACTICE FOR BARRIERS AND MSE WALLS This chapter includes background regarding MSE wall design and construction methods, design practice of roadside barriers, roadside barrier crash testing criteria, and design of barriers atop of MSE walls. 2.1 Design of Mechanically Stabilized Earth Wall System MSE walls are composed of three major elements: soil, reinforcing elements, and facing. The individual facing units independently restrained by the soil reinforcements, allow the structure to be nearly as flexible as the soil embankment itself. This inherent flexibility allows the structure to be built on sites where significant total and differential settlement is anticipated (7). Some of the major applications include the solution of problems in location of restricted right-of-way, site with difficult subsurface soil conditions, steepened-slope problems, and other environmental constraints. Another major use of an MSE wall system is in conjunction with bridges. These MSE walls are typically constructed as approach embankments with a roadside barrier system supported on the edge of the wall. This barrier system generally consists of a traffic barrier or bridge rail placed on a continuous footing (e.g., flexible pavement) or structural slab (e.g., rigid pavement). The increase in the use of MSE retaining walls has led the FHWA and state department of transportations to conduct extensive research to improve current understanding of the analysis, design, and construction of MSE walls (1, 8-10). One of the most significant advances of this area is related to the change of MSE walls design procedure from an ASD procedure to the LRFD approach. The LRFD design procedure is now mandated by AASHTO for the design of retaining structures (3). This section provides an explanation of the LRFD method for MSE wall design. 2.1.1 Design and Construction Methods Current methods for designing an MSE wall consist of determining the geometry and the soil reinforcement of the structure to maintain internal and external stability. The analysis can be divided into two main components: external stability and internal stability. The external stability analysis addresses failure modes such as sliding, overturning, bearing capacity, and slope stability failure. Figure 2-1 shows a schematic representation of each failure mode considered for external stability. Each failure mode can be described as follows: - The sliding design ensures that the active force has a very low probability of overcoming the frictional resistance of the system. - The overturning design ensures that the moment generated by the active force only has a very low probability of overcoming the resisting moment due to the weight of the wall mass. - The bearing capacity design ensures that the pressure imposed on the soil due to the self- weight of the structure has a very low probability of exceeding the ultimate bearing capacity of the soil.

12 - The slope stability design ensures that there is only a very low probability of generating a deep seated rotation failure. (a) Sliding (b) Overturning (eccentricity) (c) Bearing capacity d) Deep seated stability (rotational) Figure 2-1 External stability considerations (11) The internal stability design of the MSE wall should address a series of potential internal failure modes such as pull out of the soil reinforcement and yielding of the soil reinforcement. The internal stability design of an MSE wall ensures that the system will behave as a solid block with tensile resistance as shown in Figure 2-2. In this analysis, the geometry and the strength of the reinforcement (strips, bar mats, geogrids, etc.) should be selected to ensure that the system has a very low probability of failing due to pull out of the soil reinforcement or yielding of the soil reinforcement. The maximum tensile load in the reinforcement can be computed by multiplying the vertical earth pressure at the reinforcement level by the lateral earth pressure coefficient, and then by the tributary area of that reinforcement strip (AASHTO LRFD Eq. 11.10.6.2.1-2): max r v v hT K S Sσ= (2-1) where Kr = horizontal pressure coefficient (AASHTO LRFD Figure 11.10.6.2.1-3) σh = horizontal stress due to the soil, rVh Kσσ = σv = vertical earth pressure Sv = vertical spacing of the reinforcement Sh = horizontal spacing of the reinforcement

13 Note: From AASHTO LRFD Bridge Design Specifications, 2014, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used with permission. Figure 2-2 Internal stability considerations (AASHTO LRFD Figure 11.10.7.2.-1) (3). The pullout resistance design of the reinforcement ensures that the system will only have a very low probability of pullout failure due to the maximum static load (Tmax). Only the effective length of the reinforcement located outside of the failure wedge is considered for the computation of the required length for pullout resistance. Then, the total reinforcement length consists of the effective length (Le) and the active length of the reinforcement (La) which is the length within the failure wedge. The equation for computing the pullout resistance is written as (AASHTO LRFD Eq. 11.10.6.3.2-1): * evP F CbLασ= (2-2) where F* = pullout friction factor as shown in Figure 2-3 α = scale effect correction factor (AASHTO LRFD Table 11.10.6.3.2-1) 𝜎 = 𝛾𝑧, z: depth to the reinforcement from the bottom of the moment slab C = overall reinforcement surface area geometry factor based on the gross perimeter of the reinforcement and is equal to 2 for strip, grid and sheet-type reinforcements. b= width of the soil reinforcement Le = length of reinforcement in the resisting zone (effective length).

14 Additional information regarding the external and internal stability analysis of MSE wall is presented in AASHTO LRFD. Note: From AASHTO LRFD Bridge Design Specifications, 2014, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used with permission. Figure 2-3 Default values for the pullout friction factor, F* (AASHTO LRFD Figure 11.10.6.3.2-1) (3). The yield analysis of the soil reinforcement ensures that the reinforcement only has a very low probability of yielding during the service life (e.g., 75 years) of the structure or during an impact event. The analysis is conducted at every level within the wall and at the connection between the reinforcement and the panels. The computations depend on the type of reinforcement being used. Beside the soil reinforcing strips, there are other important components of the MSE walls, such as the concrete leveling pad, precast concrete facing panels, and the backfill material. The concrete leveling pad serve as a flat, level working surface for placement of the concrete panels. The precast concrete facing panels are usually fabricated in nominal dimensions depending on the design and functionality of the wall (e.g., 5 ft (1.52 m) wide by 5 ft (1.52 m) high). Typically, panels are placed with a joint gap in the vertical and in the horizontal direction. The principal objectives of these joints are to ensure proper alignment of the panels, provide adequate permeability, and maximize the flexibility of the wall. Other types of facing exist such as cast-in- place concrete or shotcrete facing and segmental concrete block facing, and are beyond the scope of this study. The best backfill material is a well-graded granular material with no more than 15% fines and a maximum particle size of 4 in. (102 mm). The material should have some important properties such as durability, workability, adequate electromechanical properties, and good permeability. A check on the electromechanical properties is particularly important as they determine the rate at which corrosion of the soil reinforcement may occur.

15 2.1.2 LRFD vs. ASD Design Approach MSE walls are being designed on the basis of the LRFD approach. Prior to the development of the LRFD design procedure, also called limit state design (LSD), MSE walls were designed on the basis of the ASD approach, also called working stress design approach (WSD). The WSD approach consist of applying a global factor of safety to the maximum resistance associated with each of the failure modes considered in the design. Typically, these global factors of safety are based on gathered experience or developed intuition. In LRFD, the external and internal stability of the MSE wall are evaluated at service limit state, strength limit state, and extreme event limit state. At the service limit state, the lateral and vertical wall movements and the overall stability are evaluated. At the strength limit state, the local and global stability under significant load combinations are assessed to ensure the structure has adequate strength to tolerate the imposed loads. The extreme event limit states appraise the survival of a structure after a unique occurrence such as an earthquake, a flood, or a collision. Load combinations for each limit state are specified in AASHTO LRFD (3) The collision forces generated during a vehicle impact is analyzed as an extreme event, under Extreme Event II load combination (AASHTO 3.4.1). The use of LRFD in MSE wall design provides many advantages over the use of ASD. For example, the LRFD separately accounts for the uncertainties in the resistance and in the load. Also, when appropriately calibrated, it can provide more consistent levels of safety in the design of superstructure and substructure components in terms of reliability index. The general formulation of the LRFD design methods can be expressed as:   = = = n i n i iiii RL 1 1 ϕγ (2-3) where γ = load factor L = load φ = resistance factor R = resistance One of the drawbacks inherent in the application of the LRFD design method for MSE wall is that values of γ and φ are difficult to estimate with good precision. This is because large databases are necessary to establish the risk levels. In some cases, those values are calibrated to match the factor of safety use on the ASD design method. Table 2-1 shows some of the resistance and load factors used in the LRFD design approach and the global factor of safety (FS) used in the former ASD design approach. The AASHTO LRFD Bridge Design Specifications (3) provides additional information on the LRFD factors for earth retaining structures including MSE walls.

16 Table 2-1 Comparison between LRFD factors and ASD factors for designing MSE wall. (1)AASHTO LRFD Bridge Design Specification, Sections 3 & 11. (2)Average values used for FS for different failure modes in ASD methodology (11). Typical Application of Load Factors (AASHTO LRFD)(1) Load Factor, γ Minimum Maximum Vertical Earth Pressure, γEV 1.00 1.35 Horizontal Earth pressure, γEH 0.9 1.5 Dead Load of Structural Components, γDC 0.9 1.25 Earth Surcharge, γES 0.75 1.5- Typical Application of Resistance Factors (AASHTO LRFD)(1) Resistance Factor, φ Strength Limit State Extreme Limit State Tensile Resistance of the Strip Reinforcement, φ 0.75 1.0 Pullout Resistance of the Strip Reinforcement, φ 0.9 1.0 Typical FS on ASD Approach(2) Failure Analyses Global FS Sliding ≈1.5 Overturning ≈2.0 Bearing Resistance ≈2.5

17 2.2 Design and Evaluation of Longitudinal Barrier and Bridge Rails This section includes background regarding roadside barrier design and crash testing criteria, analyses of crash test data for TL-4 and TL-5 impacts, and a history of design loads for heavy trucks. 2.2.1 Guidelines for Barrier Evaluation Guidelines for testing and evaluation of roadside barriers systems started in 1962 with Highway Research Circular 482 entitled “Proposed Full-Scale Testing Procedures for Guardrails” (12). This one-page document contained only one test vehicle, six test articles, and three evaluation criteria. NCHRP Report 350, "Recommended Procedures for the Safety Performance Evaluation of Highway Features," was published in 1993 (4). This 132-page document represented a comprehensive update to crash tests and evaluation procedures. It incorporated significant changes and additions to procedures for safety performance evaluation. Also, it included updates reflecting the changing character of the highway network and the fleet characteristics of the vehicles using it (2). NCHRP Report 350 established six test levels for longitudinal barriers. Test levels 1 through 3 (TL-1 to TL-3) relate to passenger vehicles and vary by impact speed and impact angle. Test levels 4 through 6 (TL-4 to TL-6) retain consideration of passenger vehicles, but also incorporate consideration of heavy commercial trucks. TL-4 and TL-5 test matrices include impacts with a SUT and a tractor-van-trailer vehicle, respectively. The AASHTO MASH, which published in October 2009, is an update to NCHRP Report 350. This document was developed under NCHRP Project 22-14(2), “Improvement Procedures for Safety Performance Evaluation of Roadside Feature,” by researchers at the University of Nebraska. Changes include new design test vehicles, revised test matrices, and revised impact conditions. Table 2-2 compares the design test vehicles and impact conditions (i.e., vehicle mass, impact speed, and impact angle) specified by NCHRP Report 350 and MASH. These impact conditions are selected to represent a “practical worst case” scenario. While the impact conditions for passenger vehicles have their foundation in real-world crash data, such data is not as prevalent for large trucks. Therefore, they are based on engineering judgment. Each of the test levels places increasing structural demand on the barrier, which is designed to meet structural adequacy, occupant risk, and vehicle trajectory criteria. Note that the TL-5 impact conditions remained unchanged from NCHRP Report 350 to MASH. However, two of the more important changes incorporated in MASH and of interest to this project affect TL-4. These are: - The impact velocity of the SUT test (Test Designation 4-12) increased from 50 mph (80 km/hr) in NCHRP Report 350 to 56 mph (90 km/hr), and - The test inertial mass of the SUT increased from 17600 lb. in NCHRP Report 350 to 22000 lb. in MASH.

18 These changes increased the IS, or lateral kinetic energy of the test vehicle, by 56%. Table 2-2 Vehicle description incorporated in NCHRP Report 350 and MASH (4-5). Test Level NCHRP Report 350 MASH Test Vehicle Designation and Type Weight (lb.)/ Speed (mph)/ Angle (deg.) Test Vehicle Designation and Type Weight (lb.)/ Speed (mph)/ Angle (deg.) TL-1 700C (Small Car) 1540/31/20 1100C (Passenger Car) 2420/31/25 TL-2 820C (Small Car) 1848/44/20 1500A (Passenger Car) 3300/44/25 TL-3 2000P (Pickup Truck) 4400/62/25 2270P (Pickup Truck) 5000/62/25 TL-4 8000S (Single-Unit Van Truck) 17600/50/15 10000S (SUT) 22000/56/15 TL-5 36000V (Tractor-Van- Trailer) 79300/50/15 36000V (Tractor-Van- Trailer) 79300/50/15 TL-6 36000T (Tractor-Tank Trailer) 79300/50/15 36000T (Tractor-Tank Trailer) 79300/50/15 Note: 1 kg=2.2 lb. 2.2.2 Barrier Design Current design forces for bridge rails are presented in AASHTO LRFD Table A13.2-1 “Design Forces for Traffic Railings” (3). These design forces correspond to test levels defined in NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features (4). For instance, the design loads for TL-4 and TL-5 barriers are 54 kips (240 kN) and 124 kips (552 kN), respectively. These loads were derived using data from an instrumented wall testing program conducted at Texas Transportation Institute (TTI) during the 1980’s (13–14). The principal objective of that research project was to construct an instrumented rigid wall capable of measuring the lateral impact forces associated with light and heavy vehicle impacts. The wall was 40 ft (12.2 m) long, 7.5 ft (2.3 m) tall, and 2 ft (0.6 m) wide. Load cells and accelerometers were mounted on the wall to capture the magnitude and location of the impact load applied to the wall under different impact conditions. The forces were determined by direct measurement using load cells and also computed using the acceleration data. Longitudinal and vertical forces and the load distribution in the wall were not measured. If necessary, the measured forces were adjusted to account for differences in impact conditions and/or rail geometry evaluated using the instrumented wall and those ultimately adopted in test specifications such as NCHRP Report 350. After the instrumented wall testing program, it was observed that the measured dynamic load from full-scale vehicle crash tests were substantially larger than the static loads used in the

19 design of bridge rails following ASD design procedures. This finding does not necessarily mean that railings designed for a static load of 10 kips (44.5 kN) following the AASHTO Standard Specifications for Highway Bridges are inadequate. This is because a railing system will generally have an ultimate strength well above that indicated by ASD procedures. However, the amount of reserve capacity will vary depending on materials and design details, and is not predicted when ASD methods are used. Ultimate strength design procedures provide a more accurate indication of the actual strength of a rail (2). In 1984, Buth et al. (15) recommended that bridge rails be designed based on ultimate strength procedures using yield strength of the material with a FS equal to 1.0. The capacity determined in this manner is compared to the dynamic impact loads determined from data measured or derived from the instrumented wall testing program. Such a design procedure is intended to produce yielding, but not ultimate failure/fracture when a design impact collision occurs. This premise should hold true provided the materials and structural elements have sufficient ductility and ultimate strength substantially greater than yield strength. Ultimate strength design procedures were widely used by roadside safety researchers in the 1980s to develop bridge rails capable of containing buses and trucks. In most cases, the impact performance of the rail was verified through full-scale crash testing. In 1989, these procedures were incorporated into the AASHTO Guide Specifications for Bridge Rails and subsequently into the AASHTO LRFD Bridge Design Specifications published in 1994 (3,16,17). In these procedures, the capacity of a barrier is evaluated using a yield line analysis method as described in Chapter 13 of the AASHTO LRFD Bridge Design Specification (3). The yield line theory considers the plastic strength of all the railing system components. Steel rail systems, concrete rail systems or a combination rail comprised of a steel rail on a concrete parapet can be evaluated using these design procedures. The limiting ultimate capacity of the railing system is calculated based on the appropriate yield line theory. This ultimate capacity is then compared to design forces derived from vehicular loads presented in AASHTO LRFD Table A13.2-1. Typically, the capacity of the railing system is calculated at both midspan of the railing system and at a joint or end of the rail system, as depicted in Figure 2-4.

20 a) Analyses within wall segment b) Analyses near end of wall segment Note: From AASHTO LRFD Bridge Design Specifications, 2014, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used with permission. Figure 2-5 Idealized (a) mid-span and (b) end of rail failure mechanisms (3). 2.2.3 Full-Scale TL-4 Crash Testing Extensive literature exists for designing, analyzing, testing, and evaluating bridge rails and other roadside barrier systems. Most of the research conducted on rail impact has focused on full- scale crash tests. While design impact forces have been defined and design procedures have been developed for bridge rails (AASHTO LRFD), evaluation and assessment of rail impact performance continues to be primarily performance based (i.e., determined through full-scale crash testing) rather than analysis based (i.e., determined through compliance with a design specification).

21 Numerous TL-4 full-scale crash tests were conducted in accordance with NCHRP Report 350 specifications. A rail height of 32 in. (0.812 m) was typically used in most instances to contain and redirect the 8000S truck specified as the NCHRP Report 350 TL-4 test vehicle. A limited number of TL-4 full-scale crash tests have been conducted following the new MASH guidelines. It was determined that a barrier height of 32 in. (0.81 m) is not adequate to contain and redirect the MASH 10000S test vehicle under the new Test 4-12 impact conditions. This was demonstrated in a MASH TL-4 crash test conducted by TTI researchers on a 32-in. (0.81 m) tall New Jersey (NJ) Safety Shape bridge rail as part of NCHRP Project 20-14(03) (18). The length of the test installation was 100 ft (30.5 m). The SUT impacted the barrier at a speed of 57.4 mph (92.3 km/hr.) at an angle of 14.4 degrees. The weight of the test vehicle was 22,090 lb. (10,030 kg) and the ballast center of gravity (CG) height was 63 in. (1.6 m). The calculated IS was 150.4 kip-ft (204 kN-m), 97.5 percent of the target IS. The test failed to meet the structural adequacy criteria specified in MASH due to the SUT rolling over the barrier. This bridge rail had previously met TL-4 impact performance criteria under NCHRP Report 350. TTI researchers conducted another research project funded by the Texas Department of Transportation (TxDOT) with the objective of estimating the minimum barrier height required to contain and redirect the MASH 10000S test vehicle (19). The results of the FE impact simulations indicated that a 36-in. (0.91 m) tall barrier would meet MASH TL-4 requirements. The FE results were then verified through a full-scale crash test. The total weight of the vehicle, the impact velocity and the impact angle were 22,000 lb. (9,982 kg), 57.2 mph (92 km/hr.) and 16.1 degrees, respectively. The 36-in. (0.91-m) tall Single-Slope Traffic Rail (SSTR) successfully contained and redirected the MASH TL-4 SUT. The results of these TL-4 crash tests are summarized in Table 2- 3. Table 2-3 Summary of the TL-4 crash tests Test No. / Agency Ref. No. Guideline Specification Vehicle Weight (lb.) Speed (mph) Angle (deg.) Max. 50- msec. Ave. Lateral Accel. g’s (alat) Barrier Height (in.) Barrier Type Remarks of the Test 476460-1/ TTI (18) MASH 22,090 57.4 14.4 4.1 32 NJ Safety Shape Vehicle rollover/ Fail 420020-9B/ TTI (19) MASH 22,000 57.2 16.1 4.5 36 Single- Slope Barrier Test Pass 2.2.4 Full-Scale TL-5 Crash Testing Some of the early work on bridge rail design was conducted at TTI during the 70s and 80s. During the 70s, the majority of the full-scale tests performed on bridge rails were with passenger cars. Beginning in the 80s, bridge rails began to be designed to have increased structural capacity to contain and redirect large trucks with weights ranging from 50,000 lb. (22,680 kg) to 80,000 lb.

22 (36,288 kg). In 1981, a modification of the Texas traffic rail type C202 concrete parapet was successfully crash tested with a fully loaded tractor-van-trailer (20). The total height of the barrier was 54 in. (1.37 m). The bridge rail did not sustain significant damage but the deck experienced some cracking. Additional full-scale crash tests with fully loaded tractor-trailers were conducted by TTI in 1984 and 1986 (21-22). In the 1984 test, a standard Texas T5 NJ safety shape rail was modified to contain and redirect an 80,000 lb. (36,288 kg) tractor-tank trailer. The height of the barrier was 90 in. (2.3 m). In the 1986 test, an 18-in. (0.46-m) tall metal rail (similar to that used in the Texas type C4 bridge rail system) was mounted on the top a 32 in. (0.81 m) NJ safety shape concrete median barrier (CMB) to provide a total barrier height of 50 in. (1.27 m). The rail successfully contained and redirected an 80,000 lb. (36,288 kg) tractor-van-trailer with only moderate impact damage. Although barrier profile does not seem to have a significant influence on the impact performance of large trucks, it does for passenger vehicles. Therefore, since the higher test levels (TL-4 through TL-6) retain consideration of passenger vehicles, barrier geometry remains an important aspect of the design and evaluation of barriers for large trucks. Table 6-1 of the 2011 AASHTO Roadside Design Guide (23) presents four reinforced CMB designs considered crashworthy for TL-5 impact conditions. The barrier shapes include a vertical wall, NJ profile, single slope, and F-shape. All these barriers have a minimum height requirement of 42 in. (1.07 m). A 42-in. (1.07-m) tall reinforced concrete NJ shape barrier was successfully crash tested with a tractor-van-trailer under TL-5 impact conditions by TTI in 1982 and 1986 (24,25). The research report “Performance Limits of Longitudinal Barrier Systems” (25) indicates that the barriers were capable of containing and redirecting a fully loaded 80,000 lb. (36,288 kg) tractor- van-trailer with minimal damage and no measurable deformation. An unreinforced, 42-in. (1.07 m) tall NJ profile barrier, commonly referred to as the Ontario Tall wall, was successfully tested by TTI in 1990. A vertical reinforced concrete parapet was also successfully crash tested by TTI in 1993 (26). No record could be found of a TL-5 crash test on the single-slope barrier. Two TL-5 full-scale crash tests have been conducted by the Midwest Roadside Safety Facility (MwRSF) at the University of Nebraska. The objective of the first project was to develop an aesthetic, open concrete bridge railing to meet TL-5 safety performance criteria (27). The objective of the second project was to design a new CMB to safely redirect vehicles ranging from small cars to fully loaded tractor-trailers, as specified for NCHRP Report 350 TL-5 performance conditions (28). Both designs addressed issues such as vehicle stability, rollover, and passenger car occupant safety (head ejection). The most recent TL-5 crash tests have been conducted by TTI in 2010 and 2011. The purpose of the tests was to assess the performance of the Schöck ComBAR parapet (29) and the Ryerson/Pultrall parapet (30) according to the safety performance evaluation guidelines specified in MASH. Although no revision was included in MASH for TL-5, these tests represent the first TL-5 crash tests conducted under the MASH specification. The Schöck ComBAR parapet and the Ryerson/Pultrall parapet both contained and redirected the 36000V test vehicle. There was no measurable deformation of the railings during the tests, and the barriers sustained only minor damage.

23 Table 2-4 summarizes the impact conditions, maximum 50-msec. average lateral acceleration and barrier geometry associated with the large truck crash tests reviewed as part of this study. These tests were conducted on a variety of barriers with different heights and geometries. Based on the performance of the different barriers, the AASHTO LRFD Bridge Design Specification (3) has defined 42 in. (1.07 m) as the minimum rail height recommended for TL-5. Table 2-4 also shows that many of the early tests conducted with tractor-van-trailers used sand bags and hay bales for ballast. Because ballast was not rigidly secured to the floor of the trailer, it was able to shift during impact resulting in lower peak forces on the barrier. While these are still acceptable types of ballast, MASH states that “Ballast should be firmly secured to prevent movement during and after the test.” This can result in higher impact loads on the barrier. 2.3 Background on Design Impact Load for Heavy Vehicles Early tests, including the instrumented wall tests, showed that the principal force involved in redirecting articulated trucks was generated by the rear tandem axles of the tractor. A relatively small percentage of the lateral kinetic energy was expended in the redirection of the front axles of the tractor and the rear tandem axles of the trailer. One of the tasks under the project was to estimate the impact loads associated with MASH impact conditions for both TL-4 and TL-5 impacts. Although, as previously discussed, the TL-5 impact conditions in MASH did not change, recent research has suggested that the current TL-5 impact loads specified in the AASHTO LRFD Bridge Design Specification are too low. Therefore, before developing static equivalent design loads for the design of BMS systems on top of MSE walls, the associated dynamic impact loads had to be verified and understood. Consequently, TTI researchers investigated various analytical methods for estimating the design impact load for both MASH TL-4 and TL-5 impact conditions. One of the first methods for predicting barrier impact forces was presented in NCHRP Report 86 (33). This 42-page document contained different service levels for evaluating longitudinal barriers whose test matrices included vehicles ranging from small passenger cars to intercity buses. In addition, it incorporated a series of mathematical equations for predicting the impact loads for different vehicle-barrier impacts. These equations assume that, at the instant of impact, the vehicle motion can be defined by an impact velocity (VI) and an impact angle (θ), as shown in Figure 2-6. The equations can be written as:

24 Table 2-4 Summary of full-scale crash test conducted with tractor-trailer vehicles Test No. / Agency/ Ref. No. Tractor Type Vehicle Weight (lb.)/ Speed (mph)/ Angle (degrees) Max. 50-msec. Ave. Lateral Accel. in g’s (alat) Barrier Height (in.) Barrier/ Ballast 4348-2, TTI (24) Van 80,180/52.8/15 11.4 42 CMB/Sand bags 4798-13, TTI (25) Van 80,180/52.1/16.5 3.1 42 CMB/ Sand bags 416-1, TTI (22) Van 80,080/48.4/15 5.5 50 Modified TX C202 Bridge Rail /Sand bags 230-6, TTI (20) Van 79,770/49.1/15 5.94 54 Modified TX C202 Bridge Rail/Sand bags 911-1, TTI (21) Tank 80,120/51.4/15 5.54 90 Conc. Parapet/ Smooth Red/Water 7046-3/ TTI (13) Van 80,080/55/15.3 N.A. 90 Rigid wall/Sand bags 7046-4, TTI (13) Tank 79,900/54.8/16 N.A. 90 Rigid wall/Water 7046-9, TTI (13) Van 50,000/50.4/14.6 N.A. 90 Rigid wall/Bales of hay 7069-10, TTI (31) Van 50,000/52.2/14.0 4.7 42 F-Shape/Sand bags and bales of hay 7069-13, TTI (32) Van 50,000/50.4/14.6 3.7 42 Concrete Parapet/N.S. 405511-2, TTI (26) Van 79,286/49.8/14.5 5.9 42 Concrete Parapet/N.S. ACBR-1-TL-5, MwRSF (27) Van 78,975/49.4/16.3 N.A. 42 OBR/Steel Panels, concrete barriers and foam blocks TL5-CMB-2, MwRSF (28) Van 79,705/52.7/15.4 N.A. 42 CMB/ Steel panels, concrete barrier and foam 401761-SBG1/ TTI (29) Van 79,220/50.5/15.6 18.6 42 Schöck ComBAR parapet/Sand bags 510605-RYU1/ TTI (30) Van 79,650/49.1/14.6 9.4 42 Ryerson-Pultrall Parapet/concrete N.A. = Not Available; CMB=Concrete Medium Barrier; N.S. = Not Specified; OBR= Open Bridge Rail

25 ( ) ( ) ( )[ ][ ]DBALg V AvgG Ilat +−− = θθ θ cos1sin2 sin22 (2-4) ( ) ( )      −= θθ csccot I E latlong V V GAvgG (2-5) ( )latlat AvgGWAveF = (2-6) ( ) 2max, π latlat AveFF = (2-7) ( ) 2max, πμ latlong AveFF = (2-8) where AvgGlat = average acceleration ratio in the lateral direction (g’s) AvgGlong = average acceleration ratio in the longitudinal direction (g’s) AvgFlat = average impact force in the lateral direction Flatmax = maximum impact force in the lateral direction Flongmax = maximum impact force in the longitudinal direction VI = impact velocity VE = exit velocity θ = impact angle (degrees) g = acceleration of gravity AL = distance from vehicle’s front end to center of mass B = half of vehicle width D = lateral displacement of the barrier W = vehicle weight μ = coefficient of friction between vehicle body and barrier railing Figure 2-6 Mathematical model of vehicle-barrier railing collision (33). Figure 2-7 shows a summary of this work. The mathematical model used to compute dynamic impact forces assumes a sine wave force distribution. The results indicate that the average and the maximum impact forces generated by an 80,000 lb. (36,288 kg) tractor-trailer are

26 approximately 110 kips (489.3 kN) and 168 kips (747.3 kN), respectively. However, these forces were estimated based on an impact speed of 60 mph (96.6 km/hr.) and an impact angle of 15 degrees. If these forces are scaled to an impact speed of 50 mph (80.5 km/hr.) to meet the current MASH criterion for TL-5, the results of the average and maximum impact force would be approximately 75 kips (333.6 kN) and 117 kips (520.4 kN), respectively. The correction is made based on the kinetic energy of the impact, as shown on Figure 2-7. Figure 2-7 Impact force prediction based on NCHRP Report 86 mathematical models (34). It is generally understood that the largest impact load associated with an SUT or articulated tractor-van-trailer is associated with the “backslap” of the SUT or trailer through contact between the rear tandem axles and the barrier. Therefore, another method used to estimate impact force is to simply apply Newton’s second law of motion to the tandem axle impact of the SUT or trailer. This methodology requires measurement or estimation of the lateral acceleration corresponding to the backslap and the portion of the vehicle mass associated with that backslap acceleration. In the late 1980s, researchers at TTI conducted a research project to measure the impact forces generated by collisions of large trucks against barriers (13). The principal objective of this research project was to construct an instrumented rigid wall capable of measuring the impact forces associated with heavy vehicle impacts. The rigid wall was constructed by modifying an existing instrumented wall that was developed to measure the impact forces associated with light vehicles. The load measuring face of the original instrumented wall consisted of four reinforced concrete segments that were each 3.5 ft (1.07 m) tall, 2 ft (0.61 m) thick, and 10 ft (3.05 m) long. Since the original wall was too short to allow a smooth redirection of heavy vehicles, the wall was modified by increasing its height from 3.5 ft (1.07 m) to 7.5 ft (2.29 m). The rest of the dimensions remained unchanged.

27 Each segment of the wall was instrumented with four strain gage load cells and one accelerometer located at its CG. The outputs derived from this instrumentation were used to compute the magnitude and location of the impact force using principals of structural dynamics. Groups of accelerometers were mounted slightly ahead of the CG of both vehicle units to capture the acceleration at the CG of the tractor and the trailer during the test. These accelerometer groups were located behind the anticipated areas of permanent deformation. In addition, accelerometer groups were mounted near the rear of each unit. The information captured by these accelerometers was used to calculate the acceleration associated with the CG of the tractor-trailer. In these analyses, the tractor and the trailer were considered as single rigid bodies undergoing centric impacts. Therefore, the impact force was determined by simply multiplying the mass of each vehicle unit by the component of the acceleration perpendicular to the face of the wall. The total force was found by summing the impact forces for each vehicle unit. The research consisted of three full-scale crash tests with tractor-trailers (13). Two of the tests (Tests 7046-3 and 7046-4) involved a tractor-van-trailer with a weight of approximately 80,000 lb. (36287 kg). The third test (Test 7046-9) involved an impact with a tractor-van-trailer weighing 50,000 lb. (22,680 kg). There were three primary peaks of the measured impact force. The first peak force was associated with the initial impact of the tractor, the second peak force was associated with the impact of the rear tandem axles of the tractor and the front of the trailer, and the third peak force was associated with the backslap of the van-trailer. Table 2-5 summarizes the impact conditions, impact loads, and resultant height of the maximum impact load. The time history of the impact load of the three tests is shown in Figure 2-7 through Figure 2-10. Data collected from the instrumented wall was used to derive barrier design loads for various impact conditions included in the AASHTO Guideline Specification for Bridge Rails (35) and, subsequently, Section 13 of the AASHTO LRFD Bridge Design Specification (3). The AASHTO LRFD specified a design impact force of 54 kips (240 kN) and 124 kips (551.6 kN) for TL-4 and TL-5, respectively. The TL-5 design force of 124 kips (551.6 kN) was scaled for a 42- in. (1.07 m) tall barrier. Vehicular roll during impacts with shorter barrier systems will reduce the maximum impact load associated with a taller barrier with full vehicle contact and less roll.

28 Table 2-5 Summary of the instrumented wall test program with tractor-trailers (13). Test No. Impact Conditions First Peak Load (kips) Second Peak Load (kips) Third Peak Load (kips) Height of Maximum Resultant Force (in) Resultant Height of the Second Peak Force (in) Weight (lb.) Speed (mph) Angle (degrees) 7046-3 80,080 55.0 15.3 66 176 220 70.0 44.0 7046-4 79,900 54.8 16.0 91 212 408 56 40.5 7046-9 50,000 50.4 14.6 39 150 70 70.0 35

29 Figure 2-8 50-msec. average acceleration impact force-Test 7046-3 (13). Figure 2-9 50-msec. average acceleration impact force-Test 7046-4 (13). Figure 2-10 50-msec. average acceleration impact force -Test 7046-9 (13). Researchers at the Midwest Roadside Safety Facility (MwRSF) at the University of Nebraska recently investigated the lateral impact forces associated with TL-5 barrier impacts (36).

30 Linear regression analyses were conducted for a selected number of large trucks crash tests based on the assumption that the lateral impact force is approximately proportional to the kinetic energy (or IS) of a given impact. The analysis was conducted using the total mass of the vehicle and the reaction mass at the central axles of the tractor-trailer vehicle. The results of this analytical investigation yielded two equations: TVX 0.5543 =Y (2-9) RTY= 1.2988X (2-10) where Y = design impact load (kips) XTV = total vehicle IS (kips-ft) XRT = IS of the rear tandem axles of the tractor (kips-ft) Using these relationships, researchers at the MwRSF estimated a TL-5 peak design load ranging from 243 kips (1,081 kN), based on the IS of the total vehicle, to 248 kips (1,103 kN), based on the IS of the tractor’s rear tandem axle. In a second analysis, MwRSF researchers determined the redirective capacity of four existing barrier designs using the yield line analysis procedure. The analysis showed that the standard yield line analytical procedure likely underestimates the redirective capacity of solid, reinforced concrete parapets. They concluded that this may be due to the fact that other factors (e. g., torsional resistance) that likely contribute to the barrier redirective capacity are not accounted for in the analyses. However, since a “modified” yield line analysis is currently unavailable for use in combination with the linear regression analyses, the researchers used a standard yield line- line analysis procedure in combination with a scaled-down design impact load procedure. This procedure was used to estimate the design impact load based on the redirective barrier capacity and the vehicle IS associated with a successfully crash tested 42 in. (1.07 m) tall vertical wall (Test No. 405511-2). The MwRSF researchers determined the redirective capacity of this rigid wall to be 210 kips (934 kN). Since the IS of the crash test was 6.5% below the nominal value (439 kip-ft (596 kJ)), researchers at the MwRSF considered it appropriate to increase the required redirective capacity of the barrier by 6.5%. The results indicated that the impact design load would be 211 kips (939 kN) and 224 kips (996 kN) based on TTI and MwRSF calculations, respectively. Consequently, the revised TL-5 design impact load recommended by the MwRSF researchers was 217 kips (965 kN). 2.4 Roadside Barrier System atop of MSE Walls A roadside barrier system must be designed to contain and safely redirect a vehicle during an impact event. Thus, one important aspect of barrier design is to have sufficient structural capacity to meet or exceed the loads associated with the design impact conditions. When a barrier is placed on top of an MSE wall structure, a moment slab is typically provided to resist the impact load. The moment slab structure is typically designed using a static equilibrium analysis that evaluates both sliding and rotation of the barrier system. Application of the dynamic impact load in this analysis

31 will result in an overly conservative moment slab design. This is due in large measure to the barrier inertial effects that are not accounted for in the static equilibrium analysis. NCHRP Project 22-20 determined a static equivalent barrier load for designing a moment slab to resist a MASH TL-3 impact. The focus of the current project is to define appropriate static equivalent loads and associated design guidelines for MASH TL-4 and TL-5 impacts. The barrier impact also generates forces in the supporting MSE wall reinforcement and wall panels that are superimposed with the static gravity loads that the wall has to resist. The impact load is transferred to the reinforced soil by shear stresses that develop beneath the barrier slab or by direct contact of the barrier with the wall panels (if such contact occurs). To prevent the undesirable transfer of high impact loads to the MSE wall panels below the barrier, a lateral gap (usually ¾ in. (19 mm)) is typically provided between the inside of the coping of the precast barrier unit and the traffic side of the wall panels. 2.4.1 Full-Scale Crash Tests of Barriers on Top of MSE Walls The first full-scale crash test on a precast barrier section atop of an MSE wall was conducted in 1982 by the Terre Armee Internationale in France. This company is closely related to the Reinforced Earth Company (RECO) in the USA. In this test, a 26,500-lb. (12,024-kg) bus impacted the barrier at a speed of 44 mph (70.8 km/hr.) and an angle of 20 degrees. The 5-ft (1.52-m) long precast concrete barrier sections used in the test were 32-in. (0.81 m) tall and had a NJ profile. A 4.1-ft (1.25 m) wide moment slab was cast in place with a joint every 30 ft (9.14 m). The underlying MSE wall was 10 ft (3.05 m) high with two courses of 5 ft (1.52 m) panels attached to 16.4 ft (5 m) long steel reinforcement strips at a density of four strips per 9.84 ft (3 m) of wall. The first and second layers of soil reinforcement were at a depth of 15 in. (0.38 m) and 45 in. (1.14 m) below the bottom of the moment slab. In the test, the MSE wall panels were not damaged and had only minimal movement. All damage was concentrated in the barrier sections. The maximum recorded strip load was 6.5 kips (28.91 kN). In 1995, RECO wrote a report outlining the results of this test, and it was concluded that the minimum density of soil reinforcement was adequate to resist the impact load. In 2004, researchers at Texas A&M University and TTI initiated an extensive research program sponsored by NCHRP to study and evaluate the performance of barriers mounted on top of MSE walls when subjected to light truck impacts. The results of this research effort are summarized in NCHRP Report 663: Design of Roadside Barriers Systems Placed on MSE Retaining Walls (2). The report presents a comprehensive study of the load-transfer mechanism, barrier stability analyses, dynamic pullout resistance tests of steel reinforcing strips and full-scale impact tests of barriers mounted on top of MSE walls. The results of these tests were used to develop a complete design guideline for TL-3 impacts using the LRFD approach. The barrier stability study included a static load test and two dynamic impact tests with a 5,000-lb (2,268-kg) bogie vehicle impacting a concrete parapet (Texas T201). The static load test was conducted prior to the dynamic bogie impact tests. The purpose of this test was to quantify the magnitude of the force required to initiate movement of the BMS system. The barrier and moment slab were 10 ft (3.05 m) long, and the moment slab was 4.5 ft (1.37 m) wide. The measured static load, including soil resistance, was about 9 kips (40 kN). The magnitude of the measured

32 static load was comparable to the recommended static load presented in the AASHTO Guidelines Specification for Bridge Design (35). Upon completion of the static load test, the soil on and around the moment slab was recompacted for the dynamic bogie impact tests. The purpose of these tests was to estimate the forces required to initiate sliding and overturning in the system. In the first dynamic test, the bogie vehicle impacted the barrier at a speed of 13 mph (20.9 km/hr). The estimated impact load, computed from the measured acceleration data at the CG of the bogie, was 42.5 kips (189 kN). In the second dynamic test, the impact speed was increased to 18 mph (28.9 km/hr). The estimated impact load was 54 kips (240 kN). The results of these tests are shown in Figure 2-11. a) Static Test and FEM b) Static Test and Overturning Test Figure 2-11 Comparison of static and dynamic overturning tests (2). The ratio of dynamic load to static load associated with the 13 mph (20.9 km/hr.) and the 18 mph (28.94 km/hr.) bogie tests were 4.2 and 4.9, respectively. These dynamic amplification factors are associated with a tolerable displacement of 1 in. (25.4 mm) measured at the top of the barrier. The difference in this ratio is attributed to the inertial resistance of the system. The results are shown in Figure 2-11. In addition, four full-scale bogie tests were conducted on a 5-ft (1.52-m) high MSE wall with a BMS system. The main objectives of these tests were to quantify the movement of the BMS system as well as the force distribution in the reinforcement strips due to the impact. The tests were conducted using two different reinforcement lengths commonly used in design practice, 8 ft (2.44 m) long (minimum length in construction) and 16 ft (4.88 m) long. The impact speed of the bogie vehicle varied from 20.2 mph (32.5 km/hr.) to 21.8 mph (35.08 km/hr.). The barrier types were a 32-in. (0.81-m) tall NJ shape barrier (Test 1) and a 27-in. (0.69-m) tall vertical wall barrier (Test 2 through Test 4). Figure 2-12 shows the set-up for the four impact tests. The maximum 50- msec. average impact load on the barriers varied from 64.4 kips (286.6 kN) to 73.4 kips (326.5 kN), which are all higher than the 54 kips (240 kN) impact design force associated with AASHTO LRFD for TL-3.

33 Data collected from the results of the barrier stability analyses and the bogie impact tests on the 5-ft (1.52 m) high MSE wall served as a basis to draft a TL-3 design guideline in AASHTO LRFD format. A full-scale crash test on a barrier mounted on top of a 10-ft (3.05-m) tall MSE wall served as the final verification of the guidelines. This test performed acceptably, and the impact test met the evaluation criteria specified for MASH test designation 3-11. Figure 2-13 shows the set-up for the full-scale crash test with pickup truck prior to testing. The summary of the crash test is presented in Figure 2-14. A summary of the results of the stability tests, bogie tests, and full-scale crash test is presented in Table 2-6. Although the wall systems were subjected to loads higher than design conditions in some tests, movement of the wall was considered acceptable in all instances. More details on these tests can be found in NCHRP Report 663. (a) Test 1 b) Test 2 (c) Test 3 (d) Test 4 Figure 2-12 Full-scale test for 5-ft high MSE wall with a bogie (2).

34 Figure 2-13 Barrier on MSE wall prior to testing (2).

35 0.000 s 0.086 s 0.171 s 0.340 s General Information Test Agency ...................... Test No. ........................... Date .................................. Test Article Type .................................. Name ................................ Installation Length ............. Material or Key Elements .. Soil Type and Condition .... Test Vehicle Type/Designation .............. Make and Model ............... Curb .................................. Test Inertial ....................... Dummy .............................. Gross Static ...................... TTI 475350-1 2008-09-25 32 in Vertical Barrier (T-221) MSE Wall 90 ft TxDOT Type B Backfill, Dry 2270P 2004 Dodge Ram 1500 Quad-Cab 4794 lb 4951 lb No. Dummy 4951 lb Impact Conditions Speed .............................. Angle ............................... Location/Orientation ........ Exit Conditions Speed .............................. Angle ............................... Occupant Risk Values Impact Velocity Longitudinal ................. Lateral .......................... Ridedown Accelerations Longitudinal ................. Lateral .......................... THIV ................................ PHD ................................ Max. 0.050-s Average Longitudinal ................. Lateral .......................... Vertical ......................... 63.2 mi/h 25.6 degrees 4.3 ft upstream of 4th joint 54.9 mi/h 7.9 degrees 12.8 ft/s 29.2 ft/s -4.4 Gs 9.2 Gs 34.6 km/hr. 9.3 Gs -6.5 Gs 15.7 Gs -3.7 Gs Post-Impact Trajectory Stopping Distance ................... Vehicle Stability Maximum Yaw Angle .............. Maximum Pitch Angle ............. Maximum Roll Angle ............... Vehicle Snagging .................... Vehicle Pocketing.................... Test Article Deflections Dynamic .................................. Permanent ............................... Working Width ......................... Vehicle Damage VDS ......................................... CDC ........................................ Max. Exterior Deformation OCDI .................................... Max. Occupant Compartment Deformation ...................... OCDI ....................................... 175 ft downstream 6 ft toward traffic 42 degrees @ 1.04 s -10 degrees @ 1.64 s -39 degrees @ 0.58 s No No 0.84 in. (top of barrier) 0.37 in. (bot. of barrier) 0 11LFQ5 11FLEW4 15.75 inches 2.1 inches LF0000100 Figure 2-14 Summary of results for MASH test 3-11 on the MSE wall (2).

36 Table 2-6 Summary of the stability tests, bogie tests, and full-scale crash test conducted under NCHRP Project 22-20 (2). N/A= not applicable Stability Test 1 Stability Test 2 Bogie Test 1 Bogie Test 2 Bogie Test 3 Bogie Test 4 TL-3 Test Barrier Type 27 in. tall 27 in. tall 32 in. tall 27 in. tall 27 in. tall 27 in. tall 32 in. tall Installation Vertical Wall Vertical Wall New Jersey Vertical Wall Vertical Wall Vertical Wall Vertical Wall Reinforcement NA NA 16 ft long Strip 8 ft long 8 ft long Strip 16 ft long Strip 10 ft long Strip (4 per panel) Bar Mat (6 per panel) (4 per panel) (6 per panel) Speed of Bogie 13 mph 18 mph 21.8 mph 20.3 mph 20.19 mph 20.19 mph 63.2 mph Test Results Peak Bogie or Truck -8.5 g -10.9 g -14.45 g -13 g -13.82g -12.69 g -6.5 g (long.) Acceleration 15.67 g (lateral) Barrier 2.8 g 2.5g 7.36 g 10.71 g 10.16 g 13.04 g 1.5 g Moment Slab 2.2 g 3.9 g 1.84 g N/A 1 g N/A 0.52 g Impact Force 42.5 kips 54.1 kips 73.4 kips 66.1 kips 70.17 kips 64.42 kips 83.3 kips Displacement Top of Barrier Dynamic 4.9 in. 7.81 in. 6.14 in. 6.04 in. 5.17 in. 6.02 in. 0.86 in. Permanent 2.4 in. 4.02 in. 3.0 in. 4.0 in. 2.5 in. 3.0 in. 0.37 in. Bottom of Coping Dynamic 0.3 in. 0.32 in. 1.12 in. 0.93 in. 1.16 in. 0.69 in. 0.55 in. Permanent 0 in. 0.1in. 0.55 in. 0.5 in. 0.6 in. 0.22 in. 0.68 in. Panel (Upper Layer) Dynamic N/A N/A 0.63 in. 0.37 in. 0.92 in. 0.3 in. 0.42 in. Permanent N/A N/A 0.24 in. 0.2 in. 0.55 in. 0.07 in. 0.16 in. Panel (Second Layer) Dynamic N/A N/A 0.0 in. 0.1 in. 0.19 in. 0.07 in. 0.26 in. Permanent N/A N/A 0.0 in. 0.02 in. 0.18 in. 0.0 in. 0.04 in. Loads in Strip Upper Layer Max. 50-msec N/A N/A 7.19 kips 1.54 kips 2.13 kips 7.46 kips 1.94 kips Design Load N/A N/A 5.29 kips 1.68 kips 1.64 kips 6.25 kips N/A Design Load (kip/ft) N/A N/A 2.15 kip/ft 1.023 kip/ft 1.01 kip/ft 2.57 kip/ft N/A Second Layer Max. 50-msec N/A N/A -1.2 kips 0.08 kips 1.19 kips 0.15 kips 0.66 kips Design Load N/A N/A -0.88 kips 0.083 kips 0.92 kips 0.13 kips N/A Design Load (kip/ft) N/A N/A -0.36 kip/ft 0.05 kips/ft 0.57 kips/ft 0.05 kips/ft N/A

37 2.4.2 Design of Barriers and MSE Walls for Vehicle Impact Section 11 of the AASHTO LRFD Bridge Design Specification (3) outlines the procedure to design a barrier on top of an MSE wall. The equation presented to calculate the horizontal stress due to the soil weight and the impact load to be resisted by the reinforcement in the wall can be written as follows: ,maxH h hσ σ σ= +Δ (2-11) where hσ = horizontal stress due to the soil weight ( h r vkσ σ= ), kr = horizontal earth pressure coefficient given as 1.7 ka, ,maxhσΔ = horizontal stress due to the impact load Ph1 on the barrier ( ,max 1 12 /h hP lσΔ = ) l1= depth of influence of the impact load down the wall face as shown in Figure 2-15. Note: From AASHTO LRFD Bridge Design Specifications, 2014, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used with permission. Figure 2-15 Distribution of stress from concentrated horizontal loads (AASHTO LRFD Figure 3.11.6.3-2 a) (3). AASHTO LRFD makes use of a pseudo-static impact load (PH1) of 10 kips (44.5 kN) distributed over a barrier length of 5 ft (1.5 m). This load is distributed into the soil reinforcement layer using a simplified vertical distribution described in Figure 2-15. This procedure was inherited from the AASHTO ASD design procedure. NCHRP Report 663 presents a comprehensive set of guidelines for the design of barriers and MSE walls subjected to MASH TL-3 impact. The guidelines address barrier stability as well as pullout and yielding of the soil reinforcement. The barrier stability analysis is conducted using equilibrium equations for overturning and sliding of the BMS system. The applied equivalent static

38 load, that is the static load equivalent to the impact dynamic load that is used in the proportioning of the moment slab, is 10 kips (44.5 kN). A pressure distribution diagram was developed by means of full-scale impact tests for the design of the soil reinforcement against pullout and yielding failure (Figure 2-16). The pullout and yielding resistance of the reinforcing strips are calculated according to AASHTO LRFD. The expected dynamic load for pullout and yielding can be computed as: φ P ≥ γs p s At+ γd pd At (2-12) φ R ≥ γs ps At + γd pd At (2-13) where φ = resistance factor equal to 1.0 (extreme event) φ P= factored static resistance according to AASHTO LRFD Eq. 11.10.6.3.2-1 γs = load factor for static load equal to 1.0 (extreme event) γd = load factor for the impact load equal to1.0 (extreme event) p s = earth pressure At = tributary area of the reinforcing strip p d = dynamic pressure for pullout or yielding analyses as shown in Figure 2-16. φ R= factored resistance to yielding Note that these recommendations will be slightly modified in this report as new data became available. a) Pullout of soil reinforcement b) Yielding of soil reinforcement Figure 2-16 Soil reinforcement pressure distribution (NCHRP Report 663) (2). pd = 230 psf Top Row of Reinforcement Second Row of Reinforcement pd = 315 psf1.8 ft 2.5 ft ps Traffic Barrier Moment Slab Coping ps Traffic Barrier Moment Slab Coping pd = 230 psf Top Row of Reinforcement Second Row of Reinforcement pd = 1200 psf1.8 ft 2.5 ft

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Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls Get This Book
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A major use of Mechanically Stabilized Earth (MSE) walls is as bridge approach embankments, where they are typically constructed with a roadside barrier system supported on the edge of the walls.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 326: Design Guidelines for Test Level 3 through Test Level 5 Roadside Barrier Systems Placed on Mechanically Stabilized Earth Retaining Walls is dedicated to developing guidelines for barrier-moment slab systems placed over MSE walls to resist vehicular impact loads resulting from three test levels.

Supplementary to the document is a presentation. Also, in June 2022, an erratum was posted for this publication: Table 9-4, p. 251, contained incorrect information in the Second Layer column. The table has been corrected in the Web-Only Document.

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