The guidelines provided herein are based on NCHRP Report 663 with the exception that a resistance factor of 0.5 shall be used to determine rotational resistance. This guideline is applicable for TL3, TL4 and TL5 barrier systems as defined in Section 13 of AASHTO LRFD Bridge Design Specifications.
Figure 1 Global Stability of Barrier–Moment Slab System
Structural Capacity
Global Stability
The Equivalent Static Load (ESL) is assumed to distribute over the length of the moment slab through rigid body behavior. 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. Minimum and Maximum Dimensions
Rigid body behavior may be increased from 60 feet to a maximum of 120 feet if the torsional rigidity constant of the moment slab is proportionately increased and the reinforcing steel is designed to resist combined shear, moment, and torsion from TL impulse loads.
For example: Rigid Body Length = (J’/J_{60})x(60 ft.) < 120 feet. The torsional rigidity constant for moment slabs shall be based on a solid rectangle using the following formula:
where: 2a = total width of moment slab 2b = average depth of moment slab For example: Minimum Moment Slab Width = 48 inches: a=24 inches Minimum Moment Slab Average Depth = 10 inches: b=5 inches J= J_{60} = 13,900 in^{4}
Sliding of the Barrier The factored static resistance to sliding (fP) of the barrier–moment slab system along its base shall satisfy the following condition fP ≥ gL_{s} (1) where: Ls = equivalent static load (10 kips for TL3 and TL4) (23 kips for TL5) f= resistance factor (0.8) Supersedes AASHTO 10.5.5.3.3—Other Extreme Limit States g=
load factor (1.0) for TL3 and TL4 [crash tested extreme event] P = static resistance (kips) P shall be calculated as: P = W tan f_{r} (2) where: W = weight of the monolithic section of barrier and moment slab between joints or assumed length of rigid body behavior whichever is less, plus any material laying on top of the moment slab f_{r} = friction angle of the soil on the moment slab interface (°) If the soil–moment slab interface is rough (e.g., cast in place), f_{r} is equal to the friction angle of the soil f_{s}. If the soil–moment slab interface is smooth (e.g., precast), tan f_{r} shall be reduced accordingly (0.8 tan f_{s}
Overturning of the Barrier The factored static moment resistance (fM) of the barriermoment slab system to overturning shall satisfy the following condition (Figure 1 fM ≥ gL_{s} h_{a} (3) where: A = point of rotation, where the toe of the moment slab makes contact with compacted backfill adjacent to the fascia wall L_{w} = width of moment slab L_{s} = equivalent static load (10 kips for TL3 and TL4) (23 kips for TL5) f= resistance factor (0.5) Supersedes AASHTO 10.5.5.3.3—Other Extreme Limit States and NCHRP Report 663 g=
load factor (1.0) for TL3 and TL4 [crash tested extreme event] h_{a} = moment arm taken as the vertical distance from the point of impact due to the dynamic force (top of the barrier) to the point of rotation A M = static moment resistance (kipsft) M shall be calculated as: M = W (L_{a}) (4) W = weight of the monolithic section of barrier and moment slab between joints or assumed length of rigid body behavior whichever is less, plus any material laying on top of the moment slab L_{a} = horizontal distance from the center of gravity of the weight W to point of rotation A 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. GUIDELINES FOR THE SOIL REINFORCEMENT
The wall panel shall have sufficient structural capacity to resist the maximum design rupture load for the wall reinforcement designed in accordance with the WSDOT Geotechnical Manual, Chapter 15. The static load is not included because it is not located at the panel connection.
Much of the design experience with barrier–moment slab systems for Structural Earth Walls and Geosynthetic Walls is based on traffic barrier loads specified in Sections 11 and 13 of the AASHTO LRFD Bridge Design Specifications. The TL loads listed in Section 13 are dynamic impulse loads that are inconsistent with static equilibrium calculation used to evaluate sliding and overturning of the moment slab. NCHRP 663 provides an equivalent static load methodology for TL3 and TL4 barriers and recommends a maximum distribution length of 60 feet for a moment slab system that is 4.5 feet wide with an average depth of 0.75 feet. The WSDOT minimum dimensions of 4.0 feet wide and 0.83 feet average depth were developed as a reasonable equivalent to the moment slab that was used for NCHRP 663. NCHRP 663 assumed that a barrier–moment slab design would generate 1 in. movement or less at the top of the barrier during impact. This 1 inch 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. WSDOT isolates the top of the wall panel from contact with the moment slab and requires the point of rotation to be where the toe of the moment slab makes contact with the compacted backfill. Resistance factors of 0.8 for extreme event sliding and 0.5 for extreme event overturning are specified to provide design tolerance for unexpected variation in wall backfill and deviations in wall alignment.
Signs, Barriers, Approach Slabs, and Utilities Chapter 10 Delete Article 10.3.3 and replace with following: 10.3.3 Traffic Barrier Moment Slab A. General The guidelines provided herein are based on NCHRP Report 663 with the exception that a resistance factor of 0.5 shall be used to determine rotational resistance. This guideline is applicable for TL3, TL4 and TL5 barrier systems as defined in Section 13 of AASHTO LRFD Bridge Design Specifications.
Figure 1 Global Stability of Barrier–Moment Slab System
The structural capacity of the barrier and concrete moment slab shall be designed using impulse Traffic Loads (TL3, TL4, TL5) in accordance with Sections 5 and 13 of AASHTO LRFD Bridge Design Specifications. Any section along the moment slab should not fail in shear, bending, or torsion when the barrier is subjected to the design impact load. The moment slab reinforcement shall be designed to resist forces developed at the base of the barrier. The torsion capacity of the moment slab must be equal to or greater than the traffic barrier moment generated by the specified TL impulse load (TL3, TL4, TL5) applied to the top of the barrier. 2. Global Stability Sliding and overturning stability of the moment slab shall be based on an Equivalent Static Load (ESL) applied to the top of the traffic barrier. For TL3 and TL4 barrier systems, the ESL shall be 10 kips. For TL5 barrier systems, the ESL shall be 23 kips. The Equivalent Static Load (ESL) is assumed to distribute over the length of the moment slab through rigid body behavior. 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. 3. Minimum and Maximum Dimensions Moment slabs shall have a minimum width of 4.0 feet measured from the point of rotation to the heel of the slab and a minimum average depth of 0.83 feet. Moment slabs meeting these minimum requirements are assumed to provide rigid body behavior up to a length of 60 feet for end barrier and interior barrier impacts. Rigid body behavior may be increased from 60 feet to a maximum of 120 feet if the torsional rigidity constant of the moment slab is proportionately increased and the reinforcing steel is designed to resist combined shear, moment, and torsion from TL impulse loads.
For example: Rigid Body Length = (J’/J_{60})x(60 ft.) < 120 feet. The torsional rigidity constant for moment slabs shall be based on a solid rectangle using the following formula:
where: 2a = total width of moment slab 2b = average depth of moment slab For example: Minimum Moment Slab Width = 48 inches: a=24 inches Minimum Moment Slab Average Depth = 10 inches: b=5 inches J= J_{60} = 13,900 in^{4}
4. Sliding of the Barrier The factored static resistance to sliding (fP) of the barrier–moment slab system along its base shall satisfy the following condition (Figure 2) fP ≥ gL_{s} (1) where: Ls = equivalent static load (10 kips for TL3 and TL4) (23 kips for TL5) f= resistance factor (0.8) Supersedes AASHTO 10.5.5.3.3—Other Extreme Limit States g=
load factor (1.0) for TL3 and TL4 [crash tested extreme event] P = static resistance (kips) P shall be calculated as: P = W tan f_{r} (2) where: W = weight of the monolithic section of barrier and moment slab between joints or assumed length of rigid body behavior whichever is less, plus any material laying on top of the moment slab f_{r} = friction angle of the soil on the moment slab interface (°) If the soil–moment slab interface is rough (e.g., cast in place), f_{r} is equal to the friction angle of the soil f_{s}. If the soil–moment slab interface is smooth (e.g., precast), tan f_{r} shall be reduced accordingly (0.8 tan f_{s}
5. Overturning of the Barrier The factored static moment resistance (fM) of the barriermoment slab system to overturning shall satisfy the following condition (Figure 1 fM ≥ gL_{s} h_{a} (3) where: A = point of rotation, where the toe of the moment slab makes contact with compacted backfill adjacent to the fascia wall L_{w} = width of moment slab L_{s} = equivalent static load (10 kips for TL3 and TL4) (23 kips for TL5) f= resistance factor (0.5) Supersedes AASHTO 10.5.5.3.3—Other Extreme Limit States and NCHRP Report 663 g=
load factor (1.0) for TL3 and TL4 [crash tested extreme event] h_{a} = moment arm taken as the vertical distance from the point of impact due to the dynamic force (top of the barrier) to the point of rotation A M = static moment resistance (kipsft) M shall be calculated as: M = W (L_{a}) (4) W = weight of the monolithic section of barrier and moment slab between joints or assumed length of rigid body behavior whichever is less, plus any material laying on top of the moment slab L_{a} = horizontal distance from the center of gravity of the weight W to point of rotation A 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.
Design of the soil reinforcement shall be in accordance with the WSDOT Geotechnical Design Manual, Chapter 15.
The wall panels shall be designed to resist the dynamic pressure distributions as defined in the WSDOT Geotechnical Manual, Chapter 15. The wall panel shall have sufficient structural capacity to resist the maximum design rupture load for the wall reinforcement designed in accordance with the WSDOT Geotechnical Manual, Chapter 15. The static load is not included because it is not located at the panel connection.
