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

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169 A P P E N D I X I AASHTO LRFD Format Design Guideline SECTION 1 DESIGN GUIDELINES 1.1 SCOPE This section provides guidelines to design three components: the barrier–moment slab, the MSE wall reinforcement, and the wall panel. The guidelines are applicable for TL-3 and TL-4 criteria as defined in Section 13 of AASHTO LRFD Bridge Design Specifications, and for inextensible MSE wall reinforcement (e.g., strips, bar mats) Depending on the design, two points of rotation are possible as shown in Figure 1.1. The point of rotation should be determined based on the interaction between the barrier coping and top of the wall panel. With reference to Figure 1.1, the point of rotation should be taken as Point A if the top of the wall panel is isolated from contact with the coping by the presence of an air gap or a sufficiently compressible material. The point of rotation should be taken as Point B if there is direct bearing between the bottom of the coping and the top of the wall panel or level-up concrete. Rotation Point, A C.G. Rotation Point, B Panel Leveling pad Traffic Barrier Overburden Soil Moment Slab Coping Figure 1.1. Barrier–moment slab system for design guideline.

170 1.3 NOTATION horizontal shear load on top of the panel (kips) h a = mo ment arm taken as the vertical distance between th e point of impact of the dyna mi c force and the point of rotation A (ft) h b = mo ment arm taken as the vertical distance between th e point of impact of the dyna mi c force and the point of rotation B (ft) h c = mo L d = dynam ic load (kips) L s = static load equivalent to the dynam ic impact force (kips) l vertical distance from the to p of the panel to the upperm ost reinforcem ent layer (ft) l A = horizontal distance from the center of gravity of the weight to the point of rotation A (ft). l B = horizontal distance from the center of gravity of the weight to the point of rotation B (ft). M = static mo me nt resistance to overturning of the barrier–m om ent slab system (ki ps-ft) M d = Ultimate mo ment resistance of the coping of the barrier–m om ent slab system in be nding (kips-ft M i = moment applied to the panel during impact (kips-ft) M u = ultimate moment resistance of the wall panel (kips-ft) P = static resistance to sliding of the barrier–moment slab system (kips) P r = static resistance to pullout of the reinforcement (kips) p d = dynam ic pressure diagram for pullout or rupture of the reinforcem ent (psf) Q d = dynam ic line load diagram for pullout or rupture of the reinforcem ent (lb/ft) R = resistance for rupture of the reinforcem ent (kips) t thickness of the panel (ft) V vertical load transferred from the barrier to the panel during the im pact (kips) W = weight of the m onolithic section of barrier and mo ment slab between joints plus any ma terial laying on top of the mo me nt slab (kips) = load factors = resistance factors r = friction angle of the soil–moment slab interface (°) s = friction angle of the soil (°) v = vertical soil stress (ksf) 1.2 DEFINITIONS Rotation Point A—The rotation point of a barrier– mo ment slab system if the top of the wall panel is isolated from c ontact with th e coping by the presence of an air gap or a sufficiently compressible material as shown in Figure 1.1. Rotation Point B—The rotation point of a barrier – mo ment slab syst em if there is direct bearing between the bottom of the coping and the top of the wall pane l or level-up concrete as shown in Figure 1.1. ment arm taken as the vertical distance between the point of impact of the dynamic force and the middle of the weakest section of the coping (ft)

171 1.4 GUIDELINES FOR THE BARRIER 1.4.1 General The barrier, the coping, and moment slab should be safe against structural failure. Any section along the coping and moment slab should not fail in bending when the barrier is subjected to the design impact load. Two modes of stability failure are possible in addition to structural failure of the barrier system. They are sliding and overturning of the barrier–moment slab system. The equivalent static load defined in this section should be used for sizing the moment slab. The design for structural capacity of the barrier, coping, and moment slab should follow the design recommended in Section 13 of AASHTO LRFD Bridge Design Specifications including the loads. Width of moment slabs should range between 4.5 ft to 10 ft. Length of moment slabs should range between 20 ft to 60 ft. Dimensions outside these ranges can be used provided it is shown that sufficiently rigid body behavior is achieved. C1.4.1 Much of the knowledge and experience with MSE structures and traffic barriers have been with design as specified in Sections 11 and 13 of AASHTO LRFD Bridge Design Specifications. In these recommendations it is assumed that a barrier–moment slab design would generate 1 in. movement or less at the top of the barrier during impact. This 1 in. dynamic movement is considered acceptable as it would likely require little or no repair and should not affect the impact performance of the barrier system. 1.4.2 Sliding of the Barrier The factored static resistance ( P) to sliding of the barrier–moment slab system along its base should satisfy the following condition (Figure 1.4.1): P Ls (1.4.2-1) Ls = equivalent static load (10 kips) = resistance factor (0.8) (AASHTO LRFD Bridge Design Specifications Table 10.5.5-1) C1.4.2 The equivalent static load should be applied to the length of the moment slab between joints. Any coupling between adjacent moment slabs or friction that may exist between free edges of the moment slab and the surrounding soil should be neglected. = load factor (1.0) [extreme event] P = static resistance (kips) The static force (P) should satisfy the following condition: P = W tan r (1.4.2-2) where: W = weight of the monolithic section of barrier and moment slab between joints (with an upper limit of 60 ft) plus any material laying on top of the moment slab r = friction angle of the soil–moment slab interface (°) If the soil–moment slab interface is rough (e.g., cast in place), r is equal to the friction angle of the soil s. If the soil–moment slab interface is smooth (e.g., precast), r should be reduced accordingly 2 tan 3 s .

172 1.4.3 Overturning of the Barrier The factored static moment resistance ( M) of the barrier–moment slab system to overturning should satisfy the following condition (Figure 1.4.1): M Ls (hA or hB) (1.4.3-3) where: Ls = equivalent static load (10 kips) = resistance factor (0.9) = load factor (1.0) [extreme event] ha = moment arm taken as the vertical distance from the point of impact due to the dynamic force to the point of rotation A hb = moment arm taken as the vertical distance from the point of impact due to the dynamic force to the point of rotation B M = static moment resistance (kips-ft) M should be calculated as: M = W (lA or lB) (1.4.3-4) where: W = weight of the monolithic section of barrier and moment slab plus any material laying on top of the moment slab lA = horizontal distance from the center of gravity of the weight W to the point of rotation A lB = horizontal distance from the center of gravity of the weight W to the point of rotation B The moment contribution due to any coupling between adjacent moment slabs, shear strength of the overburden soil, or friction which may exist between the backside of the moment slab and the surrounding soil should be neglected. 1.4.4 Design of the Coping The critical section of the coping must be designed to resist the applicable impact load conditions for the appropriate test level as defined in Section 13 of AASHTO LRFD Bridge Design Specifications (Figure 1.4.2).

173 Rotation Point, A C.G. hA lA Fs Ls W Rotation Point, B lBhB He Panel Leveling pad Traffic Barrier Overburden Soil Moment Slab Coping He = effective height of the impact force (AASHTO LRFD Bridge Design Specifications Figure A13.2-1). Figure 1.4.1. Barrier–moment slab system for barrier design guideline (sliding and overturning). Rotation Point, A C.G. Rotation Point, B Panel Leveling pad Traffic Barrier Overburden Soil Moment Slab Coping Critical section Figure 1.4.2. Coping and possible critical section.

174 1.5 GUIDELINES FOR THE SOIL REINFORCEMENT 1.5.1 General The reinforcement guidelines should ensure that the reinforcement does not pullout or break during the impact of the chosen vehicle. C1.5.1 In this section, the recommendations for the load in the reinforcement due to the impact are based on a pressure diagram and line load diagram back calculated by using the design loads in excess of static earth pressure loads recorded in the tests. The design load for pullout is different from the design load for rupture. The reason is that the design load for pullout is an equivalent static load while the design load for rupture is a measured dynamic load. 1.5.2 Pullout of the Soil Reinforcement 1.5.2.1 Pressure distribution approach The factored ultimate static resistance ( Pr) to pullout of the reinforcement should satisfy the following condition: Pr s p s At+ d pd At (1.5.2-1) where, resistance factor (1.0) s load factor for static load (1.0) ps = static earth pressure At = the tributary area of the reinforcement unit pd = dynamic pressure distribution to pullout of the reinforcement (Figure 1.5.1) d = load factor for dynamic load (1.0) C1.5.2.1 The reinforcement resistance (Pr) should be calculated by the equation shown in AASHTO LRFD Section 11.10.6.3.2. The traffic surcharge should not be added as it is already included in the measured load during the experiments. 1.5.2.2 Line load approach The factored static resistance ( Pr) to pullout of the reinforcement should satisfy the following condition: Pr s p s At + d Qd SL (1.5.2-2) where, resistance factor (1.0) C1.5.2.2 The reinforcement resistance (Pr) should be calculated by the equation shown in AASHTO LRFD Section 11.10.6.3.2. s load factor for static load (1.0) ps = static earth pressure At = the tributary area of the reinforcement unit

175 d = load factor for dynam ic load (1.0) Q d = dyna mic line load to pull out of the reinforcement (Figure 1.5.2) p d = 230 psf Top Row of Reinforcemen t Second Row of Reinforcemen t p d = 315 psf 1.8 ft 2.5 ft p s Traffic Barrier Moment Slab Copi ng Figure 1.5.1. Pressure distribution (p d ) for reinforcement pullout. Top Row of Reinforcement Second Row of Reinforcement Q d =575 lb/ft Q d =575 lb/ft < 2.7 ft < 1 ft Traffi c Ba rrie r Moment Slab Copin g p s Figure 1.5.2. Line load (p d ) for reinforcement pullout.

176 1.5.3 Rupture of the Soil Reinforcement 1.5.3.1 Pressure distribution approach The factored resistance ( R) to rupture of the reinforcement should satisfy the following condition: R s ps At + d pd At + LL p LL At (1.5.3-1) where, resistance factor (1.0) s load factor for static load (1.0) ps = static earth pressure At = the tributary area of the reinforcement unit pd = dynamic pressure distribution to rupture of the reinforcement (Figure 1.5.3) d = load factor for dynamic load (1.0) C1.5.3 In this section, the recommendations for the load in the reinforcement due to the impact are based on a pressure diagram and line load diagram back calculated by using the design loads in excess of static earth pressure loads recorded in the tests. C1.5.3.1 The factored resistance ( R) to rupture of the reinforcement is specified in Article 11.10.6.4. The cross section of the reinforcement can be subject to corrosion in the long term, depending on the expected time of burial and the composition of the soil, sand, or aggregate. (AASHTO LRFD Section 11.10.6.4.2). 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 Figure 1.5.3. Pressure diagram (pd) for reinforcement rupture. 1.5.3.1 Line load approach The factored resistance ( R) to rupture of the reinforcement should satisfy the following condition: R s p s At + d Qd SL + LL p LL At (1.5.3-2) C1.5.3.2 The resistance ( R) to rupture of the reinforcement should be calculated by the equation shown in AASHTO LRFD Section 11.10.6.4. The cross section of the reinforcement can be subject to corrosion in the long term, depending on the

177 where, resistance factor (1.0) s load factor for static load (1.0) ps = static earth pressure At = the tributary area of the reinforcement unit d = load factor for dynamic load (1.0) expected time of burial and the composition of the soil, sand, or aggregate. (AASHTO LRFD Section 11.10.6.4.2). Qd = dynamic line load to rupture of the reinforcement (Figure 1.5.4) SL = longitudinal spacing of the reinforcement unit Top Row of Reinforcement Second Row of Reinforcement Qd=2160 lb/ft Qd=575 lb/ft < 2.7 ft < 1 ft Traffic Barrier Moment Slab Coping ps Figure 1.5.4. Line load (Qd) for reinforcement rupture. 1.6 GUIDELINES FOR THE WALL PANEL The wall panels must be designed to resist the dynamic pressure distributions defined in Figure 1.5.3, Section 1.5.3.1. The wall panel should have sufficient structural capacity to resist the maximum design rupture load for the wall reinforcement. The static load is not included because it is not located at the panel connection.