Design of Roadside Barrier Systems Placed on MSE Retaining Walls (2010)

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163 8.1 Summary of Studies and Tests 8.1.1 Study of Barrier Stability A set of static and dynamic analytical calculations represent- ing increasing levels of complexity were developed. One static load test and two impact tests were performed on a full-scale barrier. Comparison between the analytical results and the results of the three full-scale barrier tests show good agreement. In this study, overturning occurred before sliding. This out- come was shown analytically and confirmed by the full-scale static and dynamic test results. However, both criteria should be checked. There is a significant ratio between the static load and the dynamic load that the barrier can resist. For the 3.05 m (10 ft) barrier tested, the ultimate static load was 40.5 kN (9.1 kips). For the same barrier, the maximum dynamic load in a 20.9 km/h (13 mph) impact test was 189 kN (42.5 kips), which gives a dynamic to static ratio of 4.7. The maximum dynamic load in an 18 mph impact test was 240 kN (54 kips) for a dynamic to static ratio of 5.9. This ratio is due to the inertial resistance of the system. These ratios use a static load and a dynamic load that do not correspond to the same amount of displacement. If a tolerable barrier displacement of 25 mm (1 in.) is targeted, then the static load is still 40.5 kN (9.1 kips) but the dynamic load drops to 170 kN (38.2 kips) for the 13 mph impact test (dynamic to static ratio of 4.2) and to 210 kN (47.2 kips) for the 18 mph impact test (dynamic to static ratio of 5.2). The static load resisted by the dead weight (excluding soil shear strength) of the 3.05 m (10 ft) long barrier is 22.8 kN (5.1 kips). Therefore, for a barrier to resist an equivalent static design load of 44.5 kN (10 kips) with a factor of safety of 1.5, it needs to be at least 9.15 m (30 ft) long. 8.1.2 Pullout Tests on the Reinforcement A series of pullout tests were performed to evaluate the influ- ence of rate effect on the pullout capacity of the reinforcement. Ten tests were conducted: seven on steel reinforcement strips and three on steel bar mats. A load–displacement curve was obtained for each test. The data indicate that there is no particular trend in the effect of the rate of loading. The pullout resistance at the fastest rate is often equal to the resistance at slower rates. Therefore, these tests are an indication that there is no reason to take into account any rate effect on the pullout capacity of the reinforcement during a barrier impact event. The present AASHTO recommendations for calculating the resistance of MSE wall reinforcement to static loading lead to a predicted reinforcement resistance smaller or equal to the actual reinforcement resistance under impact loading (safe condition). On the basis of these few tests, it is suggested that the current AASHTO recommendations be used as-is to calculate the resistance of the reinforcement to impact loads. 8.1.3 Study of 5 ft MSE Wall and Barrier Four reference tests were conducted as summarized in Table 8.1. The impact speeds of the bogie vehicle varied from 32.5 km/h (20.2 mph) to 35.08 km/h (21.8 mph). The bar- rier types used were an 812.8 mm (32 in.) tall N.J. shape bar- rier (Test 1) and a 685.8 mm (27 in.) tall vertical wall barrier (Tests 2 through 4). Wall reinforcement types included 4.88 m (16 ft) steel strips at a density of four per panel (Tests 1 and 4), 2.43 m (8 ft) bar mat (Test 2), and 2.43 m (8 ft) steel strips at a density of six per panel (Test 3). The maximum 50 msec average impact load on the barriers varied from 286.55 kN (64.42 kips) to 326.5 kN (73.4 kips) and are all higher than the 240 kN (54 kips) dynamic force asso- ciated with the design of barriers for AASHTO TL-3 and TL-4 levels. Table 8.1 presents the dynamic and permanent deflection at the top and bottom of the barrier and at the upper and lower layer of reinforcement. Even though the wall systems were subjected to loads higher than design conditions, all move- ments were considered acceptable from a performance point C H A P T E R 8 Summary and Conclusions

164 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 2.5 g 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 0.63 in. 0.37 in. 0.92 in. 0.3 in. 0.42 in. Permanent 0.24 in. 0.2 in. 0.55 in. 0.07 in. 0.16 in. Panel (Second Layer) Dynamic 0.0 in. 0.1 in. 0.19 in. 0.07 in. 0.26 in. Permanent 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 7.19 kips 1.54 kips 2.13 kips 7.46 kips 1.94 kips Design Load N/A 5.29 kips 1.68 kips 1.64 kips 6.25 kips N/A Design Load (kip/ft) 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 -1.2 kips 0.08 kips 1.19 kips 0.15 kips 0.66 kips Design Load N/A N/A N/A N/A N/A N/A N/A N/A N/A -0.88 kips 0.083 kips 0.92 kips 0.13 kips N/A N/ADesign Load (kip/ft) N/A N/A N/A N/A N/A N/A N/A -0.36 kip/ft 0.05 kips/ft 0.57 kips/ft 0.05 kips/ft Table 8.1. Bogie test and TL-3 test results. of view. The wall system with the 2.44 m (8 ft) strip reinforce- ment (Test 3) had the highest panel movements, while the lowest movements were recorded for the configuration that incorporated 4.88 m (16 ft) strips and the vertical wall barrier (Test 4). However, the Test 4 configuration also had the most extensive panel damage. In this test, the top panel exhibited a horizontal flexure crack along a line corresponding to the loca- tion of the top layer of reinforcement. 8.1.4 Study of 10 ft MSE Wall and Barrier The roadside barrier mounted on the edge of the MSE wall performed acceptably according to the evaluation criteria spec- ified for MASH test designation 3-11. The roadside barrier on the MSE wall contained and redirected the 2270P vehicle. The vehicle did not penetrate, underride, or override the instal- lation. No lateral movement of the barrier was noted. No detached elements, fragments, or other debris was present to penetrate or show potential for penetrating the occupant com- partment or to present a hazard to others in the area. Maximum occupant compartment deformation was 53.3 mm (2.1 in.) in the lateral area across the cab. The 2270P vehicle remained upright during and after the collision. Maximum roll was 39 degrees. Occupant risk factors were within the limits speci- fied in MASH. Test results are presented in Table 8.1. 8.2 Conclusions Traffic barriers that can resist vehicle impact without being tied to a structure are needed at the top of MSE walls. These barriers are usually constructed in an L shape so that the impact load on the vertical part of the L can be resisted by the inertia force required to uplift the horizontal part of the L. The design load for such barriers has changed from 44.5 kN (10 kips) to 240 kN (54 kips) over the last decade. This jump has created concern about which load should be used. A design procedure was developed for roadside barrier sys- tems mounted on the edge of a MSE wall. Three components of the structural system are addressed in the design procedure: the barrier–moment slab system, the wall reinforcement, and the wall panel. The stability of the barrier system was investi- gated using static and dynamic analytical solutions, full-scale

static and dynamic impact load tests, and numerical modeling. It was determined that barrier stability can be satisfied using static equilibrium analyses with an equivalent static load of 44.5 kN (10 kips). Using the dynamic barrier load of 240 kN (54 kips) is appropriate for the strength design of the bar- rier but will result in an overly conservative design of the moment slab. Guidelines for MSE wall reinforcement subject to a barrier impact were desired from reinforcement pullout tests, full- scale impacts of barrier systems mounted on an MSE test wall, and numerical modeling. No influence of rate effects was found in the reinforcement pullout tests. Therefore, conven- tional reinforcement design procedures are appropriate for determining the dynamic pullout resistance of the wall rein- forcement. In the dynamic bogie vehicle impact tests, the bar- rier systems were loaded to failure. While the barriers sustained significant damage, the overall behavior of the wall was satis- factory. The displacements of the wall panels were acceptable, and there was no panel damage observed except for a longitu- dinal failure crack in one panel at the upper layer of reinforce- ment in one of the test configurations with 4.88 m (16 ft) long strips. The loads measured in the reinforcement indicate that the reinforcement was brought to its ultimate pullout capac- ity. However, because the impact duration was so short and the displacements were within tolerable limits, this is consid- ered acceptable. The measured maximum dynamic loads in the strips were found to be 3 to 5 times higher than the calcu- lated maximum static loads by AASHTO LRFD guidelines. The measured loads were therefore factored to coincide with current design practice. Pressure diagrams and line loads were developed for the dynamic loads that should be considered in the reinforcement. The full-scale dynamic bogie impact tests and dynamic impact simulations were used to develop design guidelines for the wall panels to resist the moment applied during a barrier impact. The guidelines define recommended design loads due to the increased load in the reinforcement and the contact forces transmitted into the wall panel from direct bearing of the barrier–coping system as appropriate. A full-scale vehicle crash test into a vertical wall barrier mounted on the edge of a 2.74 m (9 ft) tall MSE wall was per- formed to verify the guidelines. The barrier system performed acceptably and met the evaluation criteria of MASH. Damage and displacement of the barrier system and underlying MSE wall were minimal. The resulting guidelines are presented in Chapter 7 of this report. They were developed following AASHTO LRFD design practices and consider two different points of bearing and rotation of the barrier system. One point of rotation is applica- ble if the wall panels are isolated from contact with the coping by presence of a suitable air gap or sufficiently compressible material. The other point of rotation addresses the scenario of direct bearing of the barrier–coping system on top of the wall panels. The design procedures for the barrier system address sliding, overturning, and structural adequacy of the coping and wall panel. The reinforcement design procedure considers pullout and rupture of the reinforcement. The dynamic design loads are specified using both a pressure distribution approach and a line load approach. 165