Load Rating of Precast Box Culverts Brian Barngrover, P.E. January 18,2018

What is a Load Rating? Rigid Rugged Resilient

What is a Load Rating? “Units of live load that can be supported by a bridge before the structure reaches a defined limit state” Rigid Rugged Resilient

What is a Limit State? Rigid Rugged Resilient

What is a Limit State? (from LRFD) – “A condition beyond which the bridge or component ceases to satisfy the provisions for which is was designed.” Rigid Rugged Resilient

Simple example: Simple example of a bridge. Rigid Rugged Resilient

Simple example: Rigid Rugged Resilient You have determined that your limit state is a crack at the centerline. Rigid Rugged Resilient

Simple example: Rigid Rugged Resilient Your unit of live load is an elephant Rigid Rugged Resilient

Simple example: RF = 2.0 Rigid Rugged Resilient Based on your calculations, placing two elephants on this bridge is just enough to cause that centerline crack. Therefore, for this bridge, the RF = 2.0 Rigid Rugged Resilient

Where are Load Ratings defined? TODO: Need picture of cover of load rating manual here In my opinion load ratings should be a chapter of LRFD Bridge Design Specification, as all elements of the rating equation are defined therein. Rigid Rugged Resilient

Rating Equation (from MBE): Rigid Rugged Resilient

Rating Equation: RF = (C – DL) / LL Rigid Rugged Resilient

Rating Equation: RF = (C – DL) / LL C = Capacity of the bridge element (limit state) Rigid Rugged Resilient

Rating Equation: RF = (C – DL) / LL C = Capacity of the bridge element (limit state) DL = Dead load effects (essentially all non-live loads) Rigid Rugged Resilient

Rating Equation: RF = (C – DL) / LL C = Capacity of the bridge element (limit state) DL = Dead load effects (essentially all non-live loads) LL = Live load effects (based on one ‘unit’ of live load) Rigid Rugged Resilient

Which culverts need to be rated? Only bridges are load rated Bridges were defined as having clear spans > 20’ The definition of a bridge has been changing Rigid Rugged Resilient

Required Load Ratings Rigid Rugged Resilient

Required Load Ratings Inventory Rating Rigid Rugged Resilient

Required Load Ratings Inventory Rating Operating Rating Rigid Rugged Resilient

Trucks used in Rating Factors Rigid Rugged Resilient

Vehicles used in Rating Factors Design Vehicle Typically HL-93, “notional” truck, not a real one, used to design the bridge, millions of cycles Rigid Rugged Resilient

Vehicles used in Rating Factors Design Vehicle Permit Vehicle Represents real trucks, typically defined by the state, far fewer number of cycles, allows for lower load factors Rigid Rugged Resilient

Vehicles used in Rating Factors Design Vehicle Permit Vehicle Legal Vehicle Also represents real trucks, not typically required for design, usually reserved for existing bridges that do not rate using Design Vehicles, used for posting of load limits, can be used for design if required by a state Rigid Rugged Resilient

Load Rating Culverts Which limit states are checked? Rigid Rugged Resilient

Load Rating Culverts Which limit states are checked? Flexure Rigid Rugged Resilient

Load Rating Culverts Which limit states are checked? Flexure Shear Rigid Rugged Resilient

Load Rating Culverts Which limit states are checked? Flexure Shear Where are these limit states checked? Rigid Rugged Resilient

Load Rating Culverts Which limit states are checked? Flexure Shear Where are these limit states checked? At the critical sections Rigid Rugged Resilient

Where are the critical sections? Rigid Rugged Resilient

Where are the critical sections? For flexure, there are three possible locations (+ mid-span) Rigid Rugged Resilient

Where are the critical sections? For flexure, there are three possible locations Rigid Rugged Resilient

Where are the critical sections? For flexure, there are three possible locations Rigid Rugged Resilient

Where are the critical sections? For flexure, there are three possible locations Rigid Rugged Resilient

Where are the critical sections? For shear, there are two possible locations (+ mid- span) Rigid Rugged Resilient

Where are the critical sections? For shear, there are two possible locations Rigid Rugged Resilient

Where are the critical sections? For shear, there are two possible locations Rigid Rugged Resilient

Getting to 1.0 Maximum moment/maximum shear Axial loads Steel yields Critical section locations – can you move them? Haunches in the structural model Always distribute wheel loads Always check iterative beta Critical section depths Rigid Rugged Resilient

Problems with Rating Culverts Rigid Rugged Resilient

Problems with Rating Culverts Older culverts design using LFD or ASD Rigid Rugged Resilient

Problems with Rating Culverts Older culverts design using LFD or ASD Culverts at depths where LL can be neglected Rigid Rugged Resilient

Live ETCulvert Demo Rigid Rugged Resilient

Questions?