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100 5 REINFORCEMENT LOADS The BMS systems designed and evaluated in section 4 were placed on top of an approximately 9.8-ft (3 m) tall MSE wall model. The purpose of these numerical simulations included quantifying the movement of the barrier, coping, and moment slab system on top of the MSE wall as well as quantifying the forces in the reinforcement strips in the case of a TL-4 and TL-5 impact. The results were used to help draft a preliminary design guideline for barrier stability, pullout and yielding of the soil reinforcement. In addition, the results were used to design and plan the TL-5 and TL-4 full-scale test installations. 5.1 Full-Scale MSE Wall FE Analyses A total of three MSE wall models with different soil reinforcement lengths were developed for evaluation under TL-4, TL-5-1, and TL-5-2 impacts. The standard cross section was used for the reinforcement strips: 2 in. (50 mm) Ã 0.16 in. (4 mm). The first MSE wall model was built using 10 ft (3.05 m) long strips. This strip length is commonly used in practice as a lower bound for low- height walls and therefore constitutes the critical case for assessing wall displacement during a barrier impact. The second and third wall models were developed using 16 ft (4.88 m) and 24 ft (7.31 m) long strips, respectively. This length of reinforcement is used in practice in many MSE wall installations as the wall height increases up to a typical overpass height. The longer strips can develop more pull out resistance and the wall with the 24 ft (7.31 m) long strips constitutes the critical case for assessing the magnitude and distribution of the impact loads in the reinforcement. In the first two cases (10 ft and 16 ft strips), a density of three strips per panel per layer was considered. The 24 ft (7.32 m) long strips had a density of two strips per panel per layer. Some specific values for the full-scale impact simulations are shown in Table 5-1. Table 5-1 Summary of the full-scale impact simulation for TL-4 and TL-5. Simulation Sequence Barrier Type (Length of Section (ft)) Barrier Capacity(1) (kips) Length of Failure (ft) Moment Slab Width (ft) Strip Reinforcement Length (ft) TL-4 Vertical Wall (10) 89.8 4.2 4.5 10 16 24 TL-5-1 Vertical Wall (15) 161.1 10.3 7.0 10 16 24 TL-5-2 Vertical Wall (15) 323 10.2 9.0 10 16 24 (1) Calculated in accordance with AASHTO A13.3.1
101 5.1.1 MSE Wall Capacity The forces expected in the reinforcement strips due to gravity load were computed according to AASHTO LRFD (3). The MSE wall design guidelines presented in NCHRP Report 663 for TL-3 were used to propose the dynamic load in the reinforcement strips for TL-4 and TL-5. In each case, a scaling factor was used; this factor was equal to the ratio of the impact load for TL-4 and TL-5 over the impact load for TL-3. The pressure distribution to pullout and yielding failure for the TL- 3 case were increased accordingly. For example, the pullout and yielding pressure distributions of the reinforcement strips shown in Figure 2-16 for TL-3 were increased by a factor equal to 1.48 (80 kips/54 kips), 2.96 (160 kips/(54 kips), and 4.81 (260 kips/54 kips) for TL-4, TL-5-1 and TL- 5-2, respectively. These pressure distributions were considered preliminary and would be verified and modified, if necessary, after the full-scale impact simulations. An example of the detailed calculations for designing the MSE wall is presented in Appendix A. The information is also summarized in Table 5-2. In these analyses, the traffic surcharge was not considered. This information ultimately was compared to the forces estimated through numerical simulation. Table 5-2 shows that the expected pullout forces in the first layer of reinforcement for TL-5 impact exceeds the calculated static resistance for pullout. This might indicate that the 10 ft (3.05 m) long reinforcement strips may be temporarily at failure during the short duration of the impact. A BMS system on top of an MSE wall should be designed as a permanent structure (e.g., service life of 75 years). Therefore, the reinforcement strips shall be designed to have an adequate corrosion resistance/durability to ensure a minimum design life of 75 years. Therefore, it is important to provide adequate sacrificial metal thickness, in addition to the required structural thickness of the base metal, to account for corrosion losses and ensure that reinforcement stresses do not exceed the yielding stresses for the full service life of the structure. For example, using the AASHTO LRFD recommended rates and the standard galvanization thickness of 86 µm (3), a sacrificial thickness of 0.06 in. (1.42 mm) is computed for a service life of 75 years. Therefore, for a standard Grade-60 reinforcing strip (2 in. (50 mm) à 0.16 in. (4 mm)), the unfactored tensile capacity at the end of the service life is 12.02 kips (53.5 kN). 5.1.2 Modeling Methodology The BMS models used in the stability analyses for TL-4 and TL-5 impacts were modified and placed on top of a 9.8-ft (2.99 m) tall MSE wall model. The modifications included improvements to the element mesh, changes in material properties and their characterization, incorporation of the MSE wall model, and explicit modeling of steel reinforcement details in the barrier, moment slab, and wall panels. The first step in the simulation process was to initialize the model. The gravity-initialized model was then set up with the SUT or the tractor-trailer vehicle model in order to conduct the impact simulation. Figure 5-1 shows general details of the MSE wall model, and Figure 5-2 shows details for the precast wall panels.
102 Table 5-2 Unfactored resistance and force in the reinforcing strips for TL-5 MSE wall Test Level Strips Length (ft) Layer Depth (ft) Tstatic(1) (kips) Tdynamic(2) (kips) Ttotal=Tstatic +Tdynamic (kips) Resistance to Pullout(1) (kips) TL-4 10 Top 3.0 0.69 1.37 2.06 21.95 (F*=1.63) 10 Second 5.5 1.20 1.36 2.56 3.23 (F*=1.49) 16 Top 3.0 0.69 1.37 2.06 33.12 (F*=1.63) 16 Second 5.5 1.20 1.36 2.56 55.17 (F*=1.49) 24 Top 3.0 1.03 2.05 3.08 44.67 (F*=1.63) 24 Second 5.5 1.80 2.04 3.85 87.75 (F*=1.49) TL-5-1 10 Top 3.6 0.82 2.72 3.54 22.28 (F*=1.60) 10 Second 6.1 1.35 2.72 4.07 33.5 (F*=1.46) 16 Top 3.6 0.82 2.72 3.54 33.66 (F*=1.60) 16 Second 6.1 1.35 2.72 4.07 65.60 (F*=1.46) 24 Top 3.6 1.24 4.07 5.31 55.48 (F*=1.60) 24 Second 6.1 2.03 4.07 6.11 98.40 (F*=1.46) TL-5-2 10 Top 3.7 0.84 4.43 5.27 22.32 (F*=1.60) 10 Second 6.2 1.37 4.42 5.79 33.53 (F*=1.46) 16 Top 3.7 0.84 4.43 5.27 33.71 (F*=1.60) 16 Second 6.2 1.37 4.42 5.82 65.64 (F*=1.46) 24 Top 3.7 1.26 6.65 7.91 55.56 (F*=1.60) 24 Second 6.2 2.06 4.42 8.69 98.46 (F*=1.46) (1) AASHTO LRFD Eq. 11.10.6.3.2-1, assuming Cu=D60/D10=4 as prescribed by AASHTO (2) Modified based on NCHRP Report 663
103 a) Three-dimensional view of the MSE wall model b) Three-dimensional view showing barrier and soil reinforcement Figure 5-1 Components of the MSE wall model.
104 a) Detail of the panels from RECO b) Detail of the panels FE model Figure 5-2 Precast concrete panel details. a) Overview of the MSE Wall Model The FE representation of the BMS system on top of an MSE wall consists of the following components: 4'-138" 9 1 16"9 1 16" 1'-234" 2'-434" 1'-234" 512" 1'-4"1'-4" 212" Typical Location of Vertical Bars 1'-238" 1'-238" 4'-1014" CL Lifting Insert Tie Strip Inside Panel A-A AA Horizontal Bars
105 - Precast, steel reinforced concrete barrier-coping sections - Cast-in-place steel reinforced moment slabs - Backfill and overburden soil material - Precast steel reinforced concrete wall panels - Unreinforced concrete bearing pad and concrete leveling pad - Steel reinforcement shear dowels connecting the moment slab sections - Steel soil reinforcing strips The elements of the barrier, the panels, and the soil surrounding the impact location were re- meshed with an element characteristic size ranging from 0.8 in. (20.3 mm) to 1.5 in. (38.1 mm) to capture the deformation and expected damage to the barrier and the panels with improved accuracy. Figure 5-1 shows details of the components of the MSE wall model. b) Contact Algorithm The methodology followed to model the interface contact between all components of the MSE wall model was similar to that used in the BMS models. The soil reinforcing strips were also modeled using the LS-DYNA feature *CONSTRAINED_LAGRANGE_IN_SOLID. This coupling algorithm permits the reinforcement strip (treated as a slave) to be placed anywhere inside the soil backfill material (treated as a master) without any mesh accommodation. This contact card can be used to model the interaction between the soil and the strip because the relative movement between them is small. It also helps to simulate the passive resistance associated with the penetration of the ribs on the strips during deformation. The connection between the strips and the panels was defined using another LS-DYNA coupling mechanism, *CONTACT_TIED_EDGE_TO_SURFACE (6). c) Material Model and Model Parameters The precast concrete barriers connected to the central 30-ft (9.15 m) long moment slab section and the precast concrete wall panels within that zone were modeled using a nonlinear response concrete material model (LS-DYNA *MAT_159). This material model captures the post peak softening behavior of the barriers and panels due to tensile stresses from the impact load. A damage formulation allows the concrete to lose its ability to carry loads after the failure threshold is reached (42). The reinforcement strips were modeled using a piecewise linear isotropic plasticity model (LS-DYNA *MAT_24) that is representative of an actual stress-strain relationship of a grade 60 steel. The failure criterion was 20%, which means the strips will break at an ultimate strain equal to 20%. The backfill material was modeled with the same material model used in the previous analyses (Modified Cap Soil Model (LS-DYNA *MAT_25) (6)). 5.2 MSE Wall FE Analyses for TL-4 Impact The BMS system model used in the MASH TL-4 barrier stability analysis was placed on top of a 11.5-ft (3.51 m) high, 90-ft (27.43 m) long MSE wall model (Figure 5-3). Three impact simulations were conducted using strips length equal to 10 ft (3.05 m), 16 ft (4.88 m), and 24 ft (7.3 m), as shown in Figure 5-3 (b) through Figure 5-3 (d). Figure 5-4 shows the rebar details of the barrier.
106 The information collected from the FE analyses includes impact force, barrier displacement, and loads and displacements in the reinforcing strips. For record keeping purposes, the barriers and selected strip locations were assigned an alphanumeric designator that describes their horizontal and vertical position as shown in Figure 5-5. The vehicle was aligned to impact at the middle of barrier section 5 (B5) at a speed of 56 mph (90 km/hr.) and an angle of 15 degrees. 5.2.1 Loads and Displacements in the Barrier The maximum 50-msec. average impact load from the FE analyses was 73.8 kips (328.6 kN), as shown in Figure 5-6 and Table 5-3. This peak load was due to the back slap impact of the vehicle and its magnitude was similar in all the analyses regardless of the strip length. The damage profile of the concrete barrier at the time of maximum impact load is shown in Figure 5-7. This barrier damage profile is typically observed in barrier joints or at the end section of a barrier as described by the AASHTO LRFD yield line analyses (3). The damage profile is limited to the surface elements and does not indicate structural failure of the precast concrete barrier section. It might indicate the possibility of some surface damage due to the frictional loads imposed by the tires and the vehicle box. The maximum displacements at the top and bottom of the barrier were, on average, 1.21 in. (30.7 mm) and 0.61 in. (16.7 mm), respectively. The information gathered is summarized in Table 5-3 and Figure 5-8. These displacements are associated with the back slap impact of the vehicle, which causes most of the sliding and rotational displacement in the barrier system. The moment slab joints show little relative displacements indicating an adequate load transfer to the neighboring moment slab sections through the shear dowels. 5.2.2 Loads and Displacement in the Soil Reinforcements The loads in the reinforcement strips were captured at two locations: 7 in. (178 mm) and 36 in. (914 mm) from the wall face (Table 5-4). The first location provides information of the maximum tension load experienced in the strips, while the second location is associated with the maximum tension load due to the gravitational loading according to AASHTO LRFD. Table 5-4 shows the total load for a selected number of reinforcing strips. The results indicate that strip B6-C-1st (ref. Figure 5-5) was subjected to the highest tension load with 4.4 kips (19.6 kN), 5.8 kips (25.8 kN) and 7.0 kips (31.2 kN) for the 10 ft (3.05 m) long, 16 ft (4.9 m) long and 24 ft (7.3 m) long strip models, respectively.
107 a) Three-dimensional view of the TL-4 MSE wall model b) 10 ft long strip model c) 16 ft long strip model d) 24 ft long strip model Figure 5-3 TL-4 MSE wall model with different soil reinforcement lengths.
108 9" 4' 6" 9" 2 5/8" 5" 5 1/2" 5 3/4" 2' 3 4" 1' #6 @ 10"A-A B-B 3'-6" #4 Long. bars (10 in total) #5 @ 8" #4 Long. bars (below grade) #4 Long. bars 3" CLR. 2" CLR. 2" CLR. #9 Shear dowels a) TL-4 BMS system details b) TL-4 BMS system model Figure 5-4 Rebar detail in the barrier and panel for TL-4 impact.
109 Figure 5-5 Elevation view of the MSE wall showing the strip distribution (TL-4 Impact). Figure 5-6 Time history of MASH TL-4 impact load on barriers (50-msec. average). 0 25 50 75 100 0.0 0.2 0.4 0.6 Av e. L at er al F or ce , k ip s Time, sec. 10-ft Strips 16-ft Strips 24-ft Strips
110 Table 5-3 Summary of the impact loads and barrier displacements for the MASH TL-4 impact simulation. Strip Length (ft) Impact Load (kips) Approximate Barrier Displacement (in) Top (dynamic)(1) Bottom (permanent)(2) 10 74.7 1.14 0.54 16 72.8 1.23 0.68 24 74.0 1.27 0.61 Average 73.8 1.21 0.61 (1) Measured at the top of the barrier at the IP location (2) Measured at the coping level of the barrier at the IP location a) Front view of the barrier (back slap impact) b) Back view of the barrier (back slap impact) Figure 5-7 Damage to the 42-in. (1.07 m) tall concrete barrier (TL-4).
111 a) 10 ft long strips b) 16 ft long strips c) 24 ft long strips Figure 5-8 Displacement of the 42 in. (1.07 m) at IP for TL-4 impact. 0.0 0.5 1.0 1.5 0.0 0.2 0.4 0.6 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.3 0.6 0.9 0.0 0.2 0.4 0.6 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational 0.0 0.5 1.0 1.5 0.0 0.2 0.4 0.6 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.3 0.6 0.9 0.0 0.2 0.4 0.6 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational 0.0 0.5 1.0 1.5 0.0 0.2 0.4 0.6 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.3 0.6 0.9 0.0 0.2 0.4 0.6 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational
112 Table 5-4 Summary of the total load for the selected strip location (TL-4 impact) Section 50-msec. Average Strip Load (kips) AASHTO Pullout Resistance(1) (kips) Strip Length 10-ft 16-ft 24-ft 10-ft 16-ft 24-ft At 7 in. from panels At 36 in from panels 10-ft 16-ft 24-ft B4_A_1st 3.8 4.8 4.9 3.0 3.6 4.0 1.95 3.12 4.67 B4_C_1st 2.4 1.9 2.2 2.0 1.7 2.1 1.95 3.12 4.67 B4_D_1st 1.5 1.2 1.3 1.4 1.2 1.3 1.95 3.12 4.67 B4_F_1st 2.0 2.9 1.9 1.7 2.3 1.8 1.95 3.12 4.67 B5_A_1st 3.5 4.6 3.3 2.7 3.5 2.8 1.95 3.12 4.67 B5_C_1st 2.9 3.9 3.1 2.4 3.1 2.8 1.95 3.12 4.67 B5_D_1st 2.6 3.4 3.3 2.2 2.8 2.8 1.95 3.12 4.67 B5_F_1st 3.8 4.5 4.3 3.0 3.5 3.6 1.95 3.12 4.67 B6_A_1st 2.8 3.9 4.6 2.4 3.1 3.8 1.95 3.12 4.67 B6_B_1st 3.4 4.5 5.4 2.8 3.6 4.5 1.95 3.12 4.67 B6_C_1st 4.4 5.8 7.0 3.5 4.2 5.2 1.95 3.12 4.67 B6_D_1st 3.7 5.4 5.6 2.7 4.1 4.4 1.95 3.12 4.67 B5_D_2nd 2.2 1.7 1.6 1.5 1.0 1.5 3.23 5.17 7.75 B5_E_2nd 1.5 1.9 1.5 1.6 1.0 1.4 3.23 5.17 7.75 B3_F_2nd 1.6 1.5 1.7 1.3 1.3 1.7 3.23 5.17 7.75 (1) AASHTO LRFD Eq. 11.10.6.3.2-1, assuming Cu=D60/D10=4 as prescribed by AASHTO The second layer of reinforcement was not significantly stressed as shown in Table 5-4. According to the simulation results, the maximum load in the second layer of reinforcement for the 10 ft (3.05 m) long, 16 ft (4.9 m) long and 24 ft (7.3 m) long strip models was 2.24 kips (10 kN) at strip section B6-D-2nd, 1.9 kips (8.5 kN) at strip section B5-E-2nd, and 1.7 kips (7.6 kN) at
113 strip section B3-F-2nd, respectively. The loadâtime history of the selected strips is presented in Figure 5-9. a) First layer of soil reinforcement b) Second layer of soil reinforcement Figure 5-9 Time history of the total load in the maximum stressed strips (TL-4). The displacements in the strips were minimal. The maximum displacement in the uppermost layer was captured at section B6-C-1st and was 0.17 in. (4.3 mm), 0.15 in. (3.8 mm) and 0.14 in. (3.5 mm) for the 10 ft (3.05 m), 16 ft (4.9 m) and 24 ft (7.3 n) long strip models, respectively. In the second layer of soil reinforcement, the maximum displacements were 0.04 in. (1.0 mm) at section B3-C-2nd, 0.04 in. (1.0 mm) at section B3-C-2nd, and 0.04 in. (1.1 mm) at section B6-C-2nd for the 10 ft (3.05 m), 16 ft (4.9 m) and 24 ft (7.3 m) long strip models, respectively. 0 2 4 6 8 0.0 0.2 0.4 0.6 50 m se c. A ve . L oa d (k ip s) Time, sec. 10-ft Strip (B6-C-1st) 16-ft Strip (B6-C-1st) 24-ft Strip (B6-C-1st) 0 1 2 3 0.0 0.2 0.4 0.6 50 m se c. A ve . L oa d (k ip s) Time, sec. 10-ft Strip (B6-D-2nd) 16-ft Strip (B5-E-2nd) 24-ft Strip (B3-F-2nd)
114 The distribution of the total load in the highest stressed soil reinforcing strip (B6-C-1st) was also studied. Based on the results shown in Figure 5-10, the load distribution appears to be linear for shorter strips with a slightly nonlinear behavior as the strip length increases. Note that the highest load in the strip is at the panel connection unlike the case of the gravity load. It is also clear that the maximum strip loads are higher than the failure loads calculated by AASHTO at the time of maximum load. However, due to the short duration of the superimposed impact load, this possible failure condition is acceptable from the displacement point of view. The strips movement is very small and pullout failure does not have time to occur. Assuming a linear distribution along the strip length and zero load at the end of the strip, the average skin friction developed at the interface between the soil and the strip is 1.02 kip/ft2 (48.8 kPa), 0.9 kip/ft2 (43.1 kPa), and 0.71 kip/ft2 (34 kPa) for the 10 ft (3.05 m), 16 ft (4.88 m) and 24 ft (7.32 m) long strip models, respectively. This corresponds to f* value of 1.63. Figure 5-10 Distribution of the total load at section B6-C-1st (TL-4). 5.2.3 Wall Panel Analyses The results of the numerical analyses show that the wall panels do not experience any damage during the TL-4 impact. The compressive strains were minimal and they do not represent any risk of structural failure of the wall panels due to the impact load. The dynamic and permanent displacements of the wall panels in the impact region for the three models are shown in Figure 5-11. It is observed that the wall displacement decreases as the strip length increases. The maximum dynamic and permanent displacement at the top of the top wall panel were 0.37 in. (9.4 mm) and 0.13 in. (3.3 mm), respectively. These displacements are associated with the soil reinforcement length of 10 ft (3.05 m). 0.0 2.0 4.0 6.0 8.0 0 5 10 15 20 25 To ta l L oa d (k ip s) Strip Length from the Panel Face (ft) 10-ft Model (t=0.32 sec.) 16-ft Model (t=0.32 sec.) 24-ft Model (t=0.17 sec.) Static Load Total Load
115 a) Dynamic displacement (impact region) b) Permanent displacement (impact region) Figure 5-11 Displacements of the wall panels (TL-4).
116 The permanent displacement of the 24 ft (7.3 m) long strip is almost zero. The increase in wall stiffness due to a long strip length (24 ft (7.3 m)) decreases the wall displacement considerably, but also increases the load in the strips. 5.3 MSE Wall FE Analyses for TL-5 Impact on a 42-in. (1.07 m) Tall Barrier (TL-5-1) The 42-in. (1.07 m) tall vertical wall barrier and the 7-ft (2.13 m) wide moment slab evaluated in the TL-5-1 stability analyses were placed on top of the same MSE wall model described previously for the TL-4 impact. The objectives of the analyses included quantification of the impact force, barrier displacement, loads and displacements in the reinforcing strips, and understanding of the load-transfer mechanism. The alphanumeric designators that describe the horizontal and vertical position of the reinforcing strips in the wall are shown in Figure 5-12. The vehicle was aligned to impact the middle of barrier section 3 (B3) at a speed of 56 mph (90 km/hr.) and an angle of 15 degrees. Figure 5-13 shows details of the vertical wall barrier and Figure 5-14 displays the MSE wall including the IP and the downstream section. To prevent the transfer of a high impact load into the top MSE wall panel, it is common construction practice to provide a gap between the throat face of the coping of the precast barrier and the back traffic face of the wall panel. A 1.5 in. (38.1 mm) gap was incorporated into the model. Adjacent moment slabs were connected to one another using three No.11 steel dowel bars embedded 18 in. (457 mm) into each moment slab. The vertical wall barrier was extended 45 ft (13.7 m) beyond the MSE wall to help fully redirect the vehicle downstream. Figure 5-12 Elevation view of the MSE wall showing the strip distribution (TL-5 Impact).
117 5 3/4" 3'-6" 3' 1' #7 @ 8" 512" 1'-4" 8" 1' 7' 112" 1' A-A B-B #7 Long. bars (10 in total) #4 Long. bars #4 Long. bars #5 @ 8" #11 Shear dowels 3" CLR. 3" CLR. 2" CLR. #4 @ 8" (below grade) a) TL-5-1 BMS system details b) TL-5-1 BMS system model Figure 5-13 Rebar detail in the barrier and panel for TL-5-1 impact.
118 a) Three-dimensional view of the TL-5-1 MSE wall model b) Downstream view Figure 5-14 TL-5 MSE wall model showing the profile of the 42-in. (1.07 m) tall barrier and embedded soil strip. 5.3.1 Loads and Displacements in the Barrier The magnitude of the lateral impact load for the three models is shown in Figure 5-15. The time history of the impact load indicates that, on average, the first peak load is 48.6 kips (216.3 kN), the second peak load is 118.6 kips (527.8 kN) and the third peak load is 167.3 kips (744.5 kN). Note that these loads also include the component of the frictional load on top of the barrier, which is significant for this barrier height.
119 Figure 5-15 Time history of MASH TL-5-1 impact load on barriers (50-msec. average). Damage to the 42-in. (1.07 m) tall concrete barriers and the moment slab is shown in Figure 5-16. The damage exhibited by the barrier is typical of an end section failure mechanism and it is due to the impact load imposed by the rear tandem axles of the trailer. The length of the end section damage profile is approximately 9.4 ft (2.9 m), which is slightly smaller than the theoretical failure length computed using the yield line analyses procedure (10.4 ft (3.2 m)). The damage profiles shown in Figure 5-16(a) and Figure 5-16(b) are limited to the surface elements, and they do not indicate failure of the barrier. The maximum displacement at the top and bottom of the 42-in. (1.07 m) tall barrier was, on average, 1.48 in. (37.6 mm) and 0.81 in. (20.6 mm), respectively. The information gathered from the three models is summarized in Table 5-5 and the displacementâtime history is shown in Figure 5-17. The permanent displacement of the barrier at the coping section meets the criterion specified for these analyses (<= 1 in. (25.4 mm)). Figure 5-17 shows that most of the rotational displacement at the top of the barrier is recoverable after impact reducing the risk of snagging in subsequent impacts with small passenger cars. 0 50 100 150 200 0.0 0.3 0.6 0.9 1.2 Av e. L at er al F or ce , k ip s Time, sec. 10-ft Strips 16-ft Strips 24-ft Strips 1 2 3
120 a) Front view of the barrier (impact of the rear axle of the trailer) b) Back view of the barrier (impact of the rear axle of the trailer) Figure 5-16 Damage to the 42-in. (1.07 m) tall concrete barrier (TL-5-1). Table 5-5 Summary of the impact loads and barrier displacements for the MASH TL-5-1 impact simulation. Strip Length (ft) Impact Load (kips) Approximate Barrier Displacement (in.) Top (dynamic)(1) Bottom (permanent)(2) 10 163.6 1.60 0.87 16 165.4 1.53 0.95 24 172.8 1.31 0.61 Average 167.3 1.48 0.81 (1) Measured at the top of the barrier at the IP location (2) Measured at the coping level of the barrier at the IP location
121 a) 10 ft model b) 16 ft model c) 24 ft model Figure 5-17 Displacement of the 42 in. (1.07 m) at IP for TL-5-1. 5.3.2 Loads and Displacement in the Soil Reinforcement Table 5-6 summarizes the tension loads in the first and second layers of reinforcement strips. The maximum strip loads were 5.5 kips (24.5 kN), 8.5 kips (37.8 kN) and 9.46 kips (42.1 kN) for the 0.0 0.5 1.0 1.5 2.0 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.3 0.5 0.8 1.0 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational 0.0 0.5 1.0 1.5 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.5 1.0 1.5 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational 0.0 0.5 1.0 1.5 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.3 0.5 0.8 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational
122 10 ft (3.05 m) long, 16 ft (4.9 m) long and 24 ft (7.3 m) long strips, respectively. The peak load in the strips is generated by the impact of the trailer and occurs at 0.84 sec, which is shortly after the maximum impact load in the barrier (t=0.80 sec.) (see Figure 5-18). Contributions to the dynamic load in the reinforcing strips include two primary sources: a) the horizontal and vertical loads transferred to the top wall panel through the barrier-coping section, and b) shearing force on the soil surface due to sliding of the system. Table 5-6 Summary of the total load for the selected strip location (TL-5-1impact). Section 50-msec. Average Strip Load (kips) AASHTO Pullout Resistance(1) (kips) Strip Length 10-ft 16-ft 24-ft 10-ft 16-ft 24-ft At 7 in. from panels At 36 in from panels 10-ft 16-ft 24-ft B3_A_1st 3.55 2.29 2.89 3.38 2.22 2.56 2.45 3.90 5.86 B3_C_1st 3.82 4.28 3.54 3.63 4.10 3.11 2.45 3.90 5.86 B3_D_1st 5.36 8.41 9.46 4.66 8.25 6.15 2.45 3.90 5.86 B3_E_1st 5.52 8.52 - 5.16 8.51 6.23 2.45 3.90 5.86 B3_F_1st 5.33 8.45 8.30 4.89 8.10 6.53 2.45 3.90 5.86 B3_G_1st 4.63 4.42 5.60 4.27 3.96 4.89 2.45 3.90 5.86 B3_I_1st 5.48 5.74 6.82 4.47 4.38 5.09 2.45 3.90 5.86 B4_A_1st 4.01 3.13 4.34 3.41 2.84 3.72 2.45 3.90 5.86 B4_C_1st 3.72 3.06 2.78 3.27 2.92 2.86 2.45 3.90 5.86 B4_D_1st 4.15 3.83 2.73 3.64 3.52 2.78 2.45 3.90 5.86 B4_F_1st 4.92 4.85 3.38 4.62 4.70 6.20 2.45 3.90 5.86 B4_I_2nd 2.95 2.76 2.49 2.32 2.27 2.01 3.80 6.07 9.10 B4_G_2nd 2.50 3.21 3.47 2.07 2.62 2.09 3.80 6.07 9.10 B3_D_2nd 2.35 2.78 2.78 1.83 2.18 1.68 3.80 6.07 9.10 1 AASHTO LRFD Eq. 11.10.6.3.2-1
123 a) First layer of soil reinforcement b) Second layer of soil reinforcement Figure 5-18 Time history of the total load in the maximum stressed strips (TL-5-1). The total loads in the second layer of soil reinforcement were 2.95 kips (13.1 kN) at section B4-I-2nd, 3.21 kips (14.28 kN) at section B4-G-2nd and 3.47 kips (14.6 kN) at section B4-G-2nd for the 10 ft (3.05 m) long, 16 ft (4.9 m) long and 24 ft (7.3 m) long strip, respectively. These loads are significantly smaller than the total loads in the uppermost layer of strips. This behavior is typical of these systems as has been observed previously in full-scale crash tests (1,45). The force- time histories for selected strips are presented in Figure 5-18. The maximum displacement of the uppermost layer of strips occurred at section B3-D-1st and was 0.30 in. (7.6 mm), 0.26 in. (6.6 mm) and 0.20 in. (5.1 mm) for the 10 ft (3.05 m), 16 ft (4.9 m), and 24 ft (7.3 m) long strips, respectively. In the second layer of soil reinforcement, the maximum displacements were 0.12 in. (3.1 mm) at section B4-I-2nd, 0.09 in. (2.3 mm) at section 0 3 6 9 12 0.0 0.4 0.8 1.2 50 m se c. A ve . L oa d (k ip s) Time, sec. 10-ft Strip (B3-E-1st) 16-ft Strip (B3-E-1st) 24-ft Strip (B3-D-1st) 0 1 2 3 4 0.0 0.4 0.8 1.2 50 m se c. A ve . L oa d (k ip s) Time, sec. 10-ft Strip (B4-I-2nd) 16-ft Strip (B4-G-2nd) 24-ft Strip (B4-G-2nd)
124 B3-F-2nd, and 0.08 in. (2 mm) at section B4-G-2nd for the 10 ft (3.05 m), 16 ft (4.9 m) and 24 ft (7.3 m) long strips, respectively. The distribution of the total load in strip section B3-E-1st (10 ft (3.05 m) and 16 ft (4.88 m) long strip) and strip section B3-D-1st (24 ft (7.32 m) long strip) are shown in Figure 5-19. The load distribution in the strip can be approximated by a triangular distribution. The slope of the line is about the same for all three cases indicating that the strip-soil friction is the same. The very high values of the maximum load compared to the calculated values by AASHTO tend to indicate that the strips are at pull out failure. However, the strip movements are not large and pullout failure does not occur because the time of load application is very short. Assuming a linear distribution along the strip length, the average skin friction developed at the soil-strip interface is 1.35 kip/ft2 (64.6 kPa), 1.40 kip/ft2 (67 kPa), and 1.05 kip/ft2 (20.3 kPa) for the 10 ft (3.05 m), 16 ft (4.88 m) and 24 ft (7.32 m) long strips, respectively. Figure 5-19 Distribution of the total load at section B3-E-1st and B3-D-1st (TL-5-1). 5.3.3 Wall Panel Analyses The results of the numerical analyses show that the wall panels located underneath the IP were significantly stressed at the level of the first layer of soil reinforcement, as shown in Figure 5-20. Since the wall panels do not have sufficient steel reinforcement to prevent tension cracks due to excessive bending moment, they might experience some cracking when subjected to this level of impact load. This observation will be investigated during the TL-5-1 full-scale crash test. Several options are available if a need arises to minimize damage of the top panels: (1) increase the moment slab width, (2) provide a larger gap between the face of the coping and the face of the panel, and/or (3) increase the strength of the top panel. 0 4 8 0 5 10 15 20 25 To ta l L oa d (k ip s) Strip Length from the Panel Face (ft) 10-ft Model (t=0.225sec.) (B3_E_1st) 16-ft Model (t=0.855 sec.) (B3_E_1st) 24-ft Model (t=0.833 sec.) (B3_D_1st) Static Load Total Load
125 Figure 5-20 Damage profile of the panel at B3 (below IP) for TL-5-1 impact. The dynamic and permanent displacements of the wall panels in the impact region of the three models are shown in Figure 5-21. The maximum dynamic and permanent displacements at the top of the wall panels for the 10 ft (3.05 m) long strips were 0.50 in. (12.7 mm) and 0.24 in. (5.8 mm), respectively. The permanent displacement of the wall panels for the 24 ft (7.3 m) long strip decreases to almost zero. (a) Dynamic displacement (impact region) Figure 5-21 Displacements at the wall panels (TL-5-1).
126 (b) Permanent displacement (impact region) Figure 5-21 Displacements at the wall panels (TL-5-1) (Continued). 5.4 MSE Wall FE Analyses for TL-5 Impact on a 48-in. (1.22 m) Tall Barrier (TL-5-2) The 48-in. (1.22 m) tall vertical wall barrier and the 9-ft (2.74 m) wide moment slab were placed on top of an MSE wall model with reinforcing strips lengths equal to 10 ft (3.05 m), 16 ft (4.88 m) and 24 ft (7.32 m) to evaluate impact performance under TL-5 conditions. The objectives of the analyses included quantification of the impact force, barrier displacement, and loads and displacements of the reinforcing strips. The barrier and wall displacements were significantly greater that those accepted for the TL-3, TL-4 and TL-5-1 systems. Therefore, an additional simulation was performed on a 48-in. (1.22 m) tall vertical wall barrier with a 12-ft (3.66 m) moment slab width and 16-ft (4.88 m) reinforcement length. The alphanumeric designators used in this analysis were similar to those used for the TL- 5-1 impact (see Figure 5-12). The vehicle was positioned to impact the middle of barrier section 3 (B3) at a speed of 50 mph (80 km/hr) and an angle of 15 degrees. Figure 5-22 and Figure 5-23 show details of the 48-in. (1.22 m) tall vertical wall barrier with the 9-ft (2.74 m) moment slab width and the underlying MSE wall, including the IP. Except for the larger moment slab width, the details for the system with 12 ft (3.66 m) moment slab are similar to those of the system with 9 ft (2.74 m) moment slab width. A horizontal gap of 1.5 in. (38.1 mm) was modeled between the inside face of the barrier coping and the back face of the panel. The moment slabs were connected using three No.11 steel bars embedded 18 in. (457 mm) the sides of adjacent moment slabs. The vertical wall barriers were also extended 45 ft (13.7 m) beyond the end of the MSE wall to help ensure full redirection of the tractor-van-trailer.
127 4' 3'-4" 1'-2" #7 @ 8" 512" 1'-812" 1' 9' 112" 10" A-A B-B 712" #7 Long. bars (12 in total) #6 @ 8" #11 Shear dowels 2" CLR. #4 @ 8" #4 Long. bars (below grade) #4 Long. bars 3" CLR. 3" CLR. (a) TL-5-2 BMS system details (b) Typical TL-5-2 BMS system model Figure 5-22 Rebar detail in the barrier and panel for TL-5-2 impact.
128 a) Three-dimensional view of the TL-5-2 MSE wall model b) Downstream view Figure 5-23 TL-5 MSE wall model showing the profile of the 48-in. (1.22 m) tall barrier and embedded soil strips. 5.4.1 Loads and Displacements in the Barrier The loads and displacements in the barrier were obtained for four models: three models with different lengths of soil reinforcement strips (10 ft (3.05 m), 16 ft (4.88 m) and 24 ft (7.32 m)) with a 9 ft (2.74 m) moment slab width, and a system with 16 ft (4.88) long reinforcement and 12 ft (3.66 m) moment slab width. The average of the peak loads for the three strip lengths for the first,
129 second and third impact of the tractor-van-trailer for the 9-ft (2.74 m) wide moment slab were 64 kips (285 kN), 232 kips (1032 kN), and 260 kips (1157 kN), respectively. For the model with the 12-ft (3.66 m) wide moment slab, the peak loads for the first, second and third impact of the tractor- van-trailer were 65 kips (289 kN), 234 kips (1041 kN) and 269 kips (1197 kN), respectively. The magnitude of the lateral impact load versus time for the four models is shown in Figure 5-24. As shown in the figure, the resulting plots for the models are almost overlapping. Figure 5-24 Time history of MASH TL-5-2 impact load on barriers (50-msec. average). Damage to the 48-in. (1.22 m) tall concrete barrier and moment slab is shown in Figure 5-25. As in the case of the previous analyses, the damage exhibited by the barrier is typical of an end section failure mechanism and is due to the impact load imposed by the trailer. The length of the end section damage profile was approximately 11.5 ft (3.5 m). The ultimate capacity of the end of the barrier was 323 kips (1437.4 kN) with a theoretical failure length of 10.2 ft (3.1 m) computed using the yield line analysis procedure. The damage profiles shown in Figure 5-25(a) and Figure 5-25(b) are limited to the surface elements, and do not indicate failure of the barrier. The barrier displacements for the different systems are summarized in Table 5.7. The maximum displacement at the top and bottom of the 48-in. (1.22 m) tall barrier was 2.61 in. (66.3 mm) and 1.60 in. (40.6 mm), respectively, for the 9 ft (2.74 m) moment slab and 10 ft (3.05 m) reinforcement strips. The displacementâtime history for the 9 ft (2.74 m) moment slab systems is shown in Figure 5-26. The permanent displacements of the barriers at the coping section for the three models with 9 ft (2.74 m) moment slab width exceeded the threshold criterion of 1 in. (25.4 mm) specified in Chapter 4. 0 100 200 300 400 0.0 0.3 0.6 0.9 1.2 Av e. La te ra l Fo rc e, ki ps Time, sec. 10-ft Strips/9-ft MS 16-ft Strips/9-ft MS 24-ft Strips/9-ft MS 16-ft Strips/12-ft MS 1 2 3
130 (a) Front view of the barrier (impact of the trailer) (b) Back view of the barrier (impact of the trailer) Figure 5-25 Damage to the 48-in. (1.22 m) tall concrete barrier (TL-5-2). Table 5-7 Summary of the impact loads and barrier displacements for the MASH TL-5-2 impact simulations. Strip Length (ft)/Moment Slab Width (ft) Impact Load (kips) Approximate Barrier Displacement (in) Top (dynamic)(1) Bottom (permanent)(2) 10/9 249 2.61 1.60 16/9 254 2.50 1.32 24/9 268 2.36 1.35 16/12 270 1.85 0.68 Average(3) 257 2.49 1.42 (1) Measured at the top of the barrier at the IP location (2) Measured at the coping level of the barrier at the IP location (3) Average of the values from the model moment slab width of 9 ft (2.74 m)
131 a) 10-ft reinforcement length and 9-ft moment slab width b) 16-ft reinforcement length and 9-ft moment slab width c) 24-ft reinforcement length and 9-ft moment slab width Figure 5-26 Displacement of the 48 in. (1.22 m) barrier for TL-5-2 impact. This result unexpectedly differs from the TL-5-2 simulation results of the 48-in. (1.22-m) tall barrier with 9-ft (2.74 m) wide moment slab presented in Chapter 4. This may be attributed to the different displacement mechanisms exhibited by the systems for the different points of rotation. The BMS system simulated in Chapter 4 was modeled to rotate around point A. The system 0.0 1.0 2.0 3.0 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.5 1.0 1.5 2.0 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational 0.0 1.0 2.0 3.0 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.5 1.0 1.5 2.0 0.0 0.3 0.6 0.9 1. D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational 0.0 1.0 2.0 3.0 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Barrier Bottom Barrier Top 0.0 0.5 1.0 1.5 2.0 0.0 0.3 0.6 0.9 1.2 D isp la ce m en t a t I P, in . Time, sec. Sliding Rotational
132 simulated in this chapter was modeled on top of the MSE wall panels and, therefore, was designed to rotate about point B. The overturning capacity around point A is generally less than that around point B. Therefore, the moment slab width determined for point of rotation A (Chapter 4) was expected to perform acceptably when the point of rotation was moved to point B. However, it appears that the mode of displacement changed between the two points of rotation. The system rotating around point A showed more dynamic rotation than sliding (Figure 4.14 a shows a dynamic rotation of 1.75 in. (44 mm) versus dynamic sliding less than 0.5 in. (13 mm)), whereas the system presented in this chapter, which rotated around point B, generally shows more dynamic sliding than rotation (see Figure 5.26). Consequently, an additional simulation TL-5-2 simulation of the 48-in. (1.22-m) tall barrier with a 12-ft (3.66 m) wide moment slab was performed to satisfy the 1 in. (25.4 mm) permanent displacement threshold. The results of the analysis with the 12-ft (3.66 m) wide moment slab section and 16 ft (4.88 m) long reinforcing strips are presented in Figure 5-27. The barrier system had a permanent displacement at the coping section of 0.68 in. (17.3 mm). This permanent displacement is 51% of the displacement for the system with the 9-ft (2.74 m) wide moment slab section and 16 ft (4.88 m) long reinforcing strips and satisfies the 1-in.ch permanent displacement criterion. Figure 5-27 Displacement for TL-5-2 impact with 12 ft (3.66 m) moment slab and 16 ft (4.88 m) long reinforcing strips. 5.4.2 Loads and Displacements in the Soil Reinforcement Table 5-8 summarizes the maximum 50-msec. average loads in the first and second layer of soil reinforcement for the TL-5-2 impact simulations. For the models with 9 ft (2.74 m) moment slab width, the maximum load in the reinforcing strips was measured in the 24 ft (7.31 m) long strip model at section B4-F-1st and was 10.55 kips (46.95 kN). The maximum load in the 10 ft (3.05 m) and 16 ft (4.88 m) long strip models was 7.16 kips (31.9 kN) at strip section B3-I-1st and 9.02 kips (41.14 kN) at strip section B3-A-1st, respectively. For the model with 12 ft (3.66 m) moment slab width, the maximum load in the 16 ft (4.88 m) reinforcing strips was 11.7 kips (52 kN) at strip section B3-A-1st. The maximum tension load in the 10 ft (3.05 m) long strip model occurred when the corner of the trailer and the rear tandem axles of the tractor hit the barrier at t=0.225 sec (second peak load in the barrier). For the longer strips, the maximum tension load was due to the impact of the trailer with the barrier (third peak load). Figure 5-28 shows the time history of the load for the selected reinforcement strips.
133 Table 5-8 Summary of the total load for the selected strip location (TL-5-2 impact) Section 50-msec. Average Strip Load (kips) AASHTO Pullout Resistance(1) (kips) Strip Length 10-ft 16-ft 24-ft 16-ft 10-ft 16-ft 24-ft 16 ft (2) (2) (2) (3) (2) (2) (2) (3) At 7 in. from panels At 36 in from panels 10-ft 16-ft 24-ft B3_A_1st 6.12 9.02 6.88 11.67 4.77 8.24 5.96 7.49 2.47 3.95 5.93 B3_C_1st 5.39 4.42 8.09 5.41 4.37 3.75 7.25 3.72 2.47 3.95 5.93 B3_D_1st 6.93 3.37 10.46 2.98 7.67 3.15 10.18 2.28 2.47 3.95 5.93 B3_F_1st 6.23 2.81 10.55 2.84 7.89 2.76 9.46 2.28 2.47 3.95 5.93 B3_G_1st 6.76 6.97 10.31 6.95 6.48 5.66 9.40 4.39 2.47 3.95 5.93 B3_I_1st 7.16 7.19 10.37 7.57 5.79 5.75 8.65 5.29 2.47 3.95 5.93 B4_A_1st 5.59 4.40 6.01 8.34 3.95 4.25 5.61 5.03 2.47 3.95 5.93 B4_C_1st 4.74 1.84 5.80 5.50 3.76 1.56 5.40 3.40 2.47 3.95 5.93 B4_D_1st 4.27 1.96 5.19 3.70 4.02 1.79 4.89 2.96 2.47 3.95 5.93 B4_F_1st 4.87 3.46 6.73 4.24 4.62 3.21 6.30 3.90 2.47 3.95 5.93 B4_G_1st 6.38 8.25 8.60 9.98 7.88 7.55 10.05 6.80 2.47 3.95 5.93 B4_I_1st 6.09 7.82 8.33 8.54 6.28 9.00 8.54 6.44 2.47 3.95 5.93 B4_I_2nd 3.99 2.88 2.34 3.98 2.47 2.46 2.21 2.54 3.81 6.09 9.14 B3_A_2nd 2.89 2.90 2.40 3.76 1.99 2.80 2.23 2.58 3.81 6.09 9.14 B3_D_2nd 3.50 1.92 2.70 1.98 2.16 1.79 2.36 1.80 3.81 6.09 9.14 (1) AASHTO LRFD Eq. 11.10.6.3.2-1(2) (2) Model with 9 ft (2.75 m) moment slab width (3) Model with 12 ft (3.66 m) moment slab width
134 a) First layer of soil reinforcement b) Second layer of soil reinforcement Figure 5-28 Time history of the total load in the maximum stressed strips (TL-5-2). The maximum 50-msec. average loads in the second layer of soil reinforcement for the 9 ft (2.75 m) moment slab were 3.99 kips (17.8 kN) at section B4-I-2nd, 2.90 kips (12.9 kN) at section B3-A-2nd, and 2.70 kips (15.71 kN) at section B3-D-2nd for the 10 ft (3.05 m) long, 16 ft (4.9 m) long, and 24 ft (7.3 m) long strip models, respectively. The maximum 50-msec. average load in 0 3 6 9 12 0.0 0.4 0.8 1.2 50 m se c. A ve . L oa d (k ip s) Time, sec. 10-ft Strip (B3-I-1st)/9ft MS 16-ft Strip (B3-A-1st)/9ft MS 24-ft Strip (B4-F-1st)/9ft MS 16-ft Strip (B3-A-1st)/12ft MS 0 1 2 3 4 0.0 0.4 0.8 1.2 50 m se c. A ve . L oa d (k ip s) Time, sec. 10-ft Strip (B4-I-2nd)/9ft MS 16-ft Strip (B3-A-2nd)/9ft MS 24-ft Strip (B3-D-2nd)/9ft MS 16-ft Strip (B3-A-1st)/12ft MS
135 the second layer for the 12 ft (3.66m) moment slab model with 16 ft (4.9 m) long reinforcement strips was 3.98 (17.7 kN) at section B4-I-2nd. The time history of the selected strips is presented in Figure 5-28. The maximum dynamic and permanent displacement of the uppermost layer of soil reinforcement for the 9 ft (2.75 m) moment slab models were obtained at section B3-D-1st for the 10 ft (3.05 m) long strip model and were 0.63 in. (16 mm) and 0.55 in. (14 mm), respectively. The maximum permanent displacement of the strips in the 16 ft (4.88 m) and 24 ft (7.32 m) long strip models were 0.51 in. (13 mm) at section B3-A-1st and 0.08 in. (2.03 mm) at section B3-G-1st, respectively. In the second layer of strips, the maximum dynamic displacements were 0.18 in. (4.5 mm) at section B3-E-2nd (10 ft (3.05 m) strips), 0.12 in. (3 mm) at section B4-I-2nd (16 ft (4.9 m) strips) and 0.08 in. (2 mm) at section B4-G-2nd (24 ft (7.3 m) strips). The permanent displacements of the second layer were minimal. The maximum load in the reinforcing strips did not occur at the same location along the barrier for the four analyses as shown in Table 5-8. This is attributed to the fact that different strip lengths create different overall kinematic behavior of the wall components when subjected to the impact load. As the strip length increases, the system becomes stiffer and less movement takes place during impact. Therefore, the strip length also affects the soil-structure interaction of the system. However, the difference in load magnitude from the strips located at the impacted region is not significant. The distribution of the total load in the most highly stressed reinforcing strips is shown in Figure 5-29 for the models with 9 ft (2.75 m) moment slab width. These distributions are similar to the ones observed in the previous analyses for MASH TL-5-1. However, in this case, the slope of the load distribution curve (friction) is larger than for MASH TL-5-1. This indicates that the apparent coefficient of friction (F*) developed during the impact loading has increased. The average skin friction developed at the soil-strip interface is 1.89 kip/ft2 (90.5 kPa), 1.47 kip/ft2 (70.4 kPa), and 0.84 kip/ft2 (40.2 kPa) for the 10 ft (3.05 m), 16 ft (4.88 m) and 24 ft (7.32 m) long strip models, respectively. In addition, the strip elements closest to the wall panels experienced the highest load and some bending near the connection with the wall panels. A short distance from the wall panels, the load drops and then increases again This behavior is thought to be due to bending of the strips because of the rotational movement of the panels during impact. Despite the high load in the reinforcing strips, the overall behavior of the wall is acceptable and the permanent wall movements are within tolerable limits. 5.4.3 Wall Panel Analyses As in the case of the analyses conducted for MASH TL-5-1, the wall panels located underneath the IP (below B3) were significantly stressed at the level of the first layer of soil reinforcement. Therefore, they might experience light tension cracks due to high bending moment during impact.
136 Figure 5-29 Distribution of the total load (TL-5-2). The dynamic and permanent displacement of the wall panels for the four models (9 ft (2.75 m) moment slab width with 10 ft (3.05 m), 16 ft (4.88 m) and 24 ft (7.32 m) long reinforcement strips, and 12 ft (3.66m) moment slab width with 16 ft (4.88 m) long reinforcement strips) is shown in Figure 5-30. The maximum dynamic and permanent displacement at the top of the wall panels was 1.10 in. (27.9 mm) and 0.91 in. (23.1 mm), respectively. These displacements were attained in the 9 ft (2.75m) moment slab model with 10 ft (3.05m) reinforcement strips. For the longest strip model (24 ft (7.32 m)), the permanent displacement was minimal. 5.5 Conclusions The following conclusions are based on and limited to the content of this chapter: 1. The impact loads in the barriers computed from the MASH TL-4, MASH TL-5-1 and MASH TL-5-2 impact simulations were similar in magnitude to the loads observed in the rigid barrier analyses conducted in Chapter 3. This is because the levels of displacements associated with the barrier-coping sections are small. Therefore, the load levels are similar to those associated with rigid barriers. 0 4 8 12 0 5 10 15 20 25 To ta l L oa d (k ip s) Strip Length from the Panel Face (ft) 10-ft Model (t=0.225 sec.) (B3-I-1st) 16-ft Model (t=0.780 sec.) (B3-A-1st) 24-ft Model (t=0.760 sec.) (B3-F-1st) Static Load Total Load
137 (a) Dynamic displacement (b) Permanent displacement Figure 5-30 Displacements at the wall panels (TL-5-2). 2. The permanent displacement at the coping section for MASH TL-4 and MASH TL-5-1 impacts were within tolerable limits for the moment slab widths and reinforcement strip lengths analyzed. The permanent displacement at the coping section of the barrier with 9- ft (2.74 m) wide moment slab section for MASH TL-5-2 was on average 1.4 in. (35.6 mm), which is larger than the threshold limit of 1.0 in. (25.4 mm). For this reason, a model with 12 ft (3.66m) moment slab width and 16 ft (4.88) reinforcement strips was included in the study. The permanent displacement for this system was 0.68 in. (17.3 mm).
138 3. The loads in the reinforcing strips were found to be larger than the static resistance for pullout. This means that the strips are at failure during the impact event, but only for a very short duration. During this short period of time, the strips and wall panels do not displace significantly and, therefore, the displacements are within acceptable limits. 4. The permanent displacement at the top of the wall panels for TL-5-1 impact would cause light tension cracks. The MASH TL-5-2 impact simulation indicated that the wall panels could be subjected to excessive movement when using 10 ft (3.05 m) long strips, greater than those expected in TL-5-1. Therefore, recommendations for pullout pressure for MASH TL-5-2 impact will be based on 16 ft (4.88 m) long strips. This will help to prevent excessive permanent movement of the wall components during a TL-5-2 impact. 5. The damage profile of the wall panels during the TL-5-1 and TL-5-2 impact simulations indicates that some of the wall panels might experience tension cracks near the first layer of wall reinforcement due to impact load. However, thin hairline cracks in the wall panels are considered acceptable and typically no restoration of the wall panels should be required. 6. Since the performance of the reinforcing strips was adequate for the different analyses, this indicates that the average design strip load in excess of static for each impact simulation can be used to develop the design guideline for pullout of the reinforcement. For example, for TL-5-1, the resistance (P) for a 10 ft (3.05 m) long strip was calculated to be 2.28 kips (10.14 kN) for the upper most layer and 3.5 kips (15.6 kN) for the second layer using Eq. (2-2) in Chapter 2 of AASHTO LRFD (AASHTO 11.10.6.3.2-1). The static load due to the earth pressure was 0.82 kips (3.64 kN). Therefore, the controlling design load in excess of the static load due to static earth pressures was calculated to be 1.46 kips (6.5 kN). Then, the pullout pressure design load for the uppermost layer for a density of three strips per panel per layer with a tributary area of 2.57 ft2 (0.24 m2) is approximately 568 psf (27.2 kPa) (1460 lb./2.57 ft2=568 psf). 7. The pullout analyses at the second layer for TL-5-1 followed the procedure used at the first layer. The total load was 4.3 kips (dynamic load of 2.95 kips (13.12 kN) and static load of 1.35 kips (6 kN) which is greater than the calculated pullout resistance from AASHTO (3.5 kips (15.6 kN)). But the displacements are tolerable. Therefore, the controlling dynamic load in excess of the static load (2.15 kips (9.56 kN)) was used as the controlling dynamic load for pullout design. Then, the pullout pressure design load for the second layer for a density of three strips per panel per layer with a tributary area of 3.94 ft2 is approximately 545 psf (26 kPa) (2150 lb. /3.94 ft2=545 psf). 8. The maximum total load experienced by the first layer of reinforcing strips was computed using the 24 ft (7.32 m) long strip model and it was 9.46 kips (42.1 kN). Therefore, it seems appropriate to use the actual load experienced by the strips to develop the guidelines for yielding analyses. Based on that, the controlling dynamic load for yielding design is 8.22 kips (36.6 kN) (total load minus static load). Then, the yielding pressure for a density of two strips per panel per layer with a tributary area of 4.60 ft2 is 1786 psf (89.34 kPa) (8220 lb. /4.6 ft2=1786 psf). 9. The yielding analyses at the second layer followed the procedure used at the first layer. The maximum tension load in the reinforcing strips was 3.21 kips (14.28 kN) (maximum load at the second layer of 16 ft (4.88 m) long strip). The excess dynamic load is 1.86 kips (8.3 kN). Then, the yielding pressure for a density of three strips per panel per layer with a tributary area 3.94 ft2 (0.37 m2) is 472 psf (22.6 kPa) (1860 lb./3.94 ft2 = 472 psf).
139 10. The recommended dynamic pressure distribution for pullout and yielding analyses of the soil reinforcing strip will be revised after the TL-5-1 full-scale crash test. In addition, detailed calculations of pullout and yielding pressure are presented in section 9.3.
