1. 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

2. 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

3. 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

4. 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.

5. 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

6. 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

7. 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

8. 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

9. 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

10. Culvert Cover Soil Depth vs. Load Rating TRB Paper 15-47952 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

11. 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

12. METHOD – TxDOT archives: 1,081 pre-1980 standard designs
Culvert Cover Soil Depth vs. Load Rating
TRB Paper
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

13. METHOD – Three Design Eras
Culvert Cover Soil Depth vs. Load Rating
TRB Paper
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

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

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

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

17. 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

18. 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

19. RESULTS – Three types (or forms) of rating vs. depth relationships
Culvert Cover Soil Depth vs. Load Rating
TRB Paper
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

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

21. 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

22. 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

23. 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

24. RESULTS – Three types (or forms) of rating vs. depth relationships
Culvert Cover Soil Depth vs. Load Rating
TRB Paper
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

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

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

27. 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