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1 SUMMARY Mechanically Stabilized Earth (MSE) walls with precast concrete facings have been increasingly used in highway applications in recent decades. One major use of MSE walls is as bridge approach embankments, where they are typically constructed with a roadside barrier system supported on the edge of the walls. This barrier system generally consists of a traffic barrier or bridge rail placed on a structural slab (in the case of rigid pavement) or on a continuous footing, also called moment slab (in the case of flexible pavement). When barriers are used with rigid pavement, the barrier is anchored to the structural slab. This anchorage provides stability against sliding and rotation of the barrier to resist the impact of an errant vehicle. In the case of a flexible pavement, a moment slab is used to provide the required inertial resistance against a vehicle impact. This results in the use of a barrier-moment slab system placed over the MSE wall. This report is dedicated to developing guidelines for barrier-moment slab systems placed over MSE walls to resist vehicular impact loads resulting from three test levels. The test levels (Test level 3 (TL-3), Test Level 4 (TL-4) and Test Level 5 (TL-5)) are defined in the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH). National Cooperative Highway Research Program (NCHRP) Research Report 663: Design of Roadside Barrier Systems Placed on MSE Retaining Walls (2), presents guidelines for designing barrier-moment slab systems and MSE walls to withstand TL-3 vehicle impact loads. This research updates and extends the work accomplished under NCHRP Report 663, and it eliminates the need to extrapolate knowledge from a MASH TL-3 impact to a MASH TL-4 or MASH TL-5 impact. The enclosed guidelines include recommendations for designing MASH TL-3, MASH TL-4, and MASH TL-5 barrier systems. The recommendations provide guidance on the dynamic loads applied on the barrier and required for its design, the static loads equivalent to the dynamic loads used in calculation of the moment slab width, and the additional pullout and yielding pressures for the wall reinforcement resulting from vehicle dynamic loading in excess of static. The analyses of the behavior of barrier-moment slab (BMS) systems placed over MSE walls were carried out in two main phases: the analytical study (Phase I) and the experimental study (Phase II). Phase I involved a review of the state of practice for barriers and MSE walls, analytical calculations, static and dynamic finite element simulations using the finite element (FE) software LS-DYNA, and preparation of preliminary guidelines. Phase II included full-scale crash testing of TL-4 and the TL-5 barrier systems, and a full-scale TL-5 quasi-static test. In this phase, the guidelines were revised and finalized in light of data obtained from the full-scale crash testing. The problem statement, project objectives, and research plan to accomplish the project objectives are presented in Chapter 1. Load and Resistance Factor Design (LRFD) design and construction methods of MSE walls are presented in Chapter 2. Background on roadside barrier crash testing criteria, design impact loads for heavy vehicles and design practice of roadside barriers placed atop of MSE retaining wall are also included in Chapter 2. In Chapter 3, a TL-3 simulation was prepared with an updated pickup truck, a Chevrolet Silverado pickup truck model that represents the 4-door, 2270P pickup truck test vehicle specified in MASH. The selected design impact load was 70 kips (311 kN) located at 24 in. (610 mm) above grade. TL-4 and TL-5 impact simulations into rigid barriers of different heights were performed
2 to determine the peak dynamic loads and their distribution along the barrier. The results show that the magnitude, distribution, and resultant height of the impact load are influenced by the height of the barrier. To optimize the barrier design, the recommendations for the TL-4 were divided as follows: a) TL-4-1 associated with a 36-in. (914-mm) tall barrier has a design impact load of 70 kips (311 kN) located at 25 in. (635 mm) above grade and, b) TL-4-2 associated with a 42-in. (1.07-m) tall barrier which has a design impact load of 80 kips (356 kN) located 30 in. (762 mm) above grade. The MASH TL-5 impact simulations were conducted using barrier heights ranging from 42 in. (1.07 m) to a very tall rigid wall. A dramatic increase in the impact load was found between barriers of 42 in. (1.07 m) heights and taller barriers. This load increment was associated with the effect imposed by the trailer and the rigid ballast when it hits the barrier during redirection of the vehicle. To optimize the barrier design, MASH TL-5 analyses were divided as follows: a) MASH TL-5-1 associated with a 42-in. (1.07-m) tall barrier has a design impact load of 160 kips (712 kN) located 34 in. (864 mm) above grade and, b) MASH TL-5-2 associated with barriers taller than 42 in. (1.07-m) has a design impact load of 260 kips (1157 kN) located at 43 in. (1.09 m) above grade. In Chapter 4, a set of full-scale impact simulations on BMS systems were performed to evaluate their dynamic behavior when subjected to MASH TL-4 and MASH TL-5 impacts. The results indicate that the required width of moment slab for TL-4, TL-5-1, and TL-5-2 are 4.5 ft (1.37 m), 7 ft (2.13 m) and 9 ft (2.74 ft), respectively. The analyses were conducted using a 30-ft (9.15 m) long moment slab and a permanent displacement threshold of 1.0 in. (25.4 mm) at the barrier-coping section. Additional analyses (included in Chapter 5 and Chapter 9) were later carried out to further optimize the BMS systems. In Chapter 5, the barrier moment slab systems analyzed in section 4 were placed on top of an MSE wall model to obtain impact forces in the MSE wall reinforcement strips. The analyses were conducted using wall reinforcement of different lengths [10 ft (3.05 m), 16 ft (4.88 m) and 24 ft (7.32 m)]. The results obtained from the shortest strips were used to develop the guideline for pullout resistance, and the results from the longest strips were used to develop guidelines for yielding of the strip reinforcement. Chapter 6 and Chapter 7 document the results of the TL-4-1 and TL-5-1 full-scale crash tests of barrier moment slab systems on an MSE wall, respectively. These full-scale tests were performed to validate the preliminary design guidelines and/or modify them as necessary. The TL- 4-1 roadside barrier successfully contained and redirected the 10000S vehicle, and the TL-5-1 roadside barrier successfully contained and redirected the 36000V vehicle. In each of the tests, the roadside barrier performed acceptably according to the evaluation criteria specified in MASH and the displacement criteria established in this report. Chapter 8 documents a full-scale static test conducted on the same roadside BMS system used for the TL-5 full-scale crash test. The objective of the analysis was to verify the static load required to initiate movement of the system. The ultimate load, determined from the test, was around 100 kips (445 kN) including the shear strength of the soil. The system failed by overturning; however, there was also considerable sliding of the system observed.
3 The guidelines were developed such that the BMS system and the MSE walls have enough strength to resist the impact load and minimize the need for repair works. Design considerations include the rupture or yield of the structural elements, namely the failure of the barrier and coping during impact, and the pullout and yielding of the soil reinforcement during impact. Serviceability considerations include limiting criteria for the maximum dynamic displacement at the top of the barrier and the maximum permanent displacement at the coping level of the barrier. The criteria were developed to minimize the need for repair after the impact based on findings from the full- scale crash tests and the simulations. The maximum dynamic displacement at the top of the barrier during impact was selected as 1.0 in. (25 mm) for TL-3, 1.5 in. (38 mm) for TL-4, and 1.75 (44.5 mm) for TL-5 barrier. This corresponds to a maximum barrier rotation angle of about 2 degrees, and varies due to the different barrier height associated with each test level. The maximum permanent displacement at the bottom of the barrier after impact was selected to be 1 in. (25 mm). This information is included in Chapter 9. This Chapter includes data to back up the guidelines and additional simulations that were carried out in order to finalize the guidelines. The final guidelines are presented in Chapter 10. They address structural and stability design of the barrier-coping system, wall reinforcement analyses for pullout and yielding and structural adequacy of the MSE wall panels. They were developed following AASHTO LRFD design practices and the NCHRP Report 663 procedure (2).
