A Paper from the Transportation Research Record Influence of Cover Soil Depth on the Load Rating of Reinforced Concrete Box Culverts A Paper from the Transportation Research Record Timothy A Wood, PhD April 22, 2015

INTRODUCTION, SCOPE and RESEARCH PROBLEM This dissertation evaluates the influence of cover soil depth on the load rating of cast-in-place (CIP), reinforced-concrete (RC) box culverts. Load Rating The analytical determination of the live load carrying capacity of a structure

INTRODUCTION, SCOPE and RESEARCH PROBLEM This dissertation evaluates the influence of cover soil depth on the load rating of cast-in-place (CIP), reinforced-concrete (RC) box culverts. Load Rating The analytical determination of the live load carrying capacity of a structure Cast-in-place, reinforced-concrete box culverts. Cover soil depth The distance from the ground surface to the top slab of the culvert

LOAD RATING POLICY Culvert load rating is one component required by the National Bridge Inspection Standards (NBIS) The NBIS is referenced by Code of Federal Regulations (23 CFR 650 Subpart C) Concerned with “the proper safety inspection and evaluation of all highway bridges…” including reinforced concrete box culverts with cumulative spans greater than 20ft. The CFR references the AASHTO Manual for Bridge Evaluation (MBE) as the document of technical authority for all components of the NBIS, including load rating.

Load Rating Equation LOAD RATING CONCEPT RF = the rating factor C = the structural capacity of the member D = the dead load effect on the member L = the live load effect on the member I = the impact factor, IM A1 = 1.3 = factor for dead loads A2 = 2.17 for Inventory Level factor for live loads 1.3 for Operating Level factor for live loads Three major components: Capacity, Dead Load, Live Load

Demand Types Critical Sections For every Bending Moment Corners LOAD RATING CONCEPT Demand Types Critical Sections For every Bending Moment Corners Top Slab Shear Midspans Wall Axial Thrust Bottom Slab

PRODUCTION-SIMPLIFIED DEMAND MODELS Structural Frame Analysis 2D, linear-elastic, direct-stiffness, structural-frame model Loads applied to structure SSHB defined loads Load Cases Total Load Case Reduced Lateral Load Case CULVLR uses CULV5 – TxDOT’s culvert analysis software Called “Level 1” in the Culvert Rating Guide Sketch Live Load distribution on white board

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESEARCH PROBLEM January 13, 2015 Cover Soil Depth D = vertical distance from top of pavement to top of the culvert top slab, ft D It is appropriate to define variables. First, we are talking about culverts. In particular, bridge-class, reinforced concrete box culverts such as the one showing in this image.. Cover soil depth, as used in this presentation, means the vertical distance from the top of pavement to the top of the top culvert slab. TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESEARCH PROBLEM January 13, 2015 Load Rating Levels Inventory Rating (IR) …the “live load which can safely utilize the [culvert] for an indefinite period of time” Operating Rating (OR) …a larger load intended to identify “the maximum permissible live load to which the structure [culvert] may be subjected“ -AASHTO Manual for Bridge Evaluation A culvert’s load rating is defined by AASHTO policy, as published in the Manual for Bridge Evaluation. Conceptually, the load rating is a type of live load safety factor for the structure, expressed in terms of an HS truck tonnage. Two ratings are defined: INVENTORY. OPERATING.. TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 2 3 4 6 7 8 9 January 13, 2015 Illustration When I say the cover soil depth vs. load rating relationship is non-linear, here is what I mean. This is a typical plot of cover soil depth vs. load rating. Note the axes. Cover soil depth is shown on the vertical (Y) axis, increasing from top to bottom. Load rating is shown on the horizontal (X) axis, increasing from left to right. Note the two “curves”. The solid line is the IR. The dashed line is the OR. Note the highlighted rating designation at HS20. This is the load threshold which culvert designs must achieve to avoid load posting. And finally, note the blue shaded region. This region is the design cover soil depth for a particular culvert standard. So in this case, the OR is above HS20 in the shaded (blue) cover soil range, which is good. TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESEARCH PROBLEM January 13, 2015 Culvert structures… presume a constant (nominal) cover soil depth [typical] Culvert design standards… address a range of cover soil thicknesses [typical] The influence of cover soil depth on load rating is highly non-linear [typical] Standard designs may or may not work over the full range of cover soil depths The argument to be developed in this paper – which is another way of saying why this paper is important – is showing here… TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

METHOD – TxDOT archives: 1,081 pre-1980 standard designs Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 January 13, 2015 METHOD – TxDOT archives: 1,081 pre-1980 standard designs 25 +/- DESIGNS ON ONE SHEET! TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

METHOD – Three Design Eras Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 METHOD – Three Design Eras January 13, 2015 PRE-WWII INTERSTATE HWY RE-RELEASE ERA ERA UPDATE DOES NOT INCLUDE POST-2000 DESIGNS TRB 94th Meeting SESSION 614: Modeling of the Pipe-Soil System

METHOD – Typ. Section, Pre WWII Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 METHOD – Typ. Section, Pre WWII January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

METHOD – Typ. Section, Interstate Hwy Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 METHOD – Typ. Section, Interstate Hwy January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

METHOD – Perform load rating calculations at 0.5ft increments Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 METHOD – Perform load rating calculations at 0.5ft increments January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 METHOD – Analysis using Culvert Rating Guide, direct-stiffness model, and CULVLR January 13, 2015 TRB 94th Meeting SESSION 614: Modeling of the Pipe-Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 METHOD January 13, 2015 For this project Digitized 1,081 design standards Calculated ratings at 0.5ft increments for each standard 24,015 unique culvert load ratings >15 million load rating factors TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

RESULTS – Three types (or forms) of rating vs. depth relationships Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESULTS – Three types (or forms) of rating vs. depth relationships January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

RESULTS – Load Distributions Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESULTS – Load Distributions January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESULTS – Increasing January 13, 2015 Rating increased by at least 5 HS tons from 2ft to 6ft of cover soil 1. Non-linear relationship 2. Direct Traffic means 0 to 2 ft of cover soil Live load completely unattenuated by cover soil Dead load is increasing Impact factor decreases with increasing cover soil thickness Net result… load rating tends to be constant in this depth range 3. In rating vs. depth relationship Typically occurs at slightly greater depth than maximum load rating After this, the load rating precipitously decreases to zero (or less) Net result… dead load overwhelms capacity at the controlling critical section TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESULTS – Decreasing January 13, 2015 Rating maximized under 1ft of cover soil Rating decreased between 2ft to 6ft of soil Rating failed under dead load above 6ft soil TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESULTS – Constant January 13, 2015 Rating did not categorize as “increasing” or “decreasing” TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

RESULTS – Three types (or forms) of rating vs. depth relationships Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 RESULTS – Three types (or forms) of rating vs. depth relationships January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Characteristics of Culvert Population by Culvert Era Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 Characteristics of Culvert Population by Culvert Era January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

Characteristics of Culvert Population by Design Cover Depth Range Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-4795 Characteristics of Culvert Population by Design Cover Depth Range January 13, 2015 TRB 94th Annual Meeting SESSION 614: Modeling of the Pipe Soil System

CONCLUSION Cover Soil Depth Interaction of live load and dead load with depth cause a non-linear interaction between cover soil depth and load rating for a structure There is no a priori worst case cover soil depth Trends with design era, geometry and design cover soil depth