1. ACPA 2014 Pipe School – Houston, TX
Direct Design
Hugh Martin, P.E.
Hanson Pipe & Precast

2. Direct Design Approach2
Direct Design Approach
Direct Design vs. Indirect Design
Concrete Design vs. D-Load Design

3. Indirect Design Method3
Indirect Design Method
Analyze Pipe Installation and Support Conditions
Calculate Total Load on Pipe
Calculate and Apply Bedding Factors
Determine Pipe Class / D-Load

4. Direct Design Method Define Pipe Material Properties4
Direct Design Method
Define Pipe Material Properties
Specify Installation Conditions
Calculate Total Load on Pipe
Calculate Design Forces
Design Pipe to Resist Design Forces

5. References
AASHTO LRFD Bridge Design Specifications, 6th Ed., American Association of State Highway and Transportation Officials, 2012.
Standard Practice for Direct Design of Buried Precast Concrete Pipe Using Standard Installations (SIDD), ASCE 15-98, American Society of Civil Engineers, 2000.
Concrete Pipe Technology Handbook, American Concrete Pipe Association, 2001.

6. Installation Methods
Trench
Embankment
Tunnel

7. AASHTO LRFD
“Standard installations for both embankments and trenches shall be designed for positive projection, embankment loading conditions where Fe shall be taken as the vertical arching factor, VAF, specified in Table for each type of standard installation.”

8. Loads Earth Load Internal Fluid Load Pipe Weight Surcharge Load
Live Load

9. Earth Load
Final Grade
WE = PL x Fe
Prism Load (PL) H
Where: ( ) c B 8 π 4 H w PL ú û ù ê ë é ÷ ø ö ç è æ - + = Bc
(O.D.)
WE = w x Bc x H x Fe (AASHTO LRFD )

10. Fluid Load Effects are of little significance in small diameter pipe.
Fluid weight is calculated as:

11. Pipe Weight
Always included in direct design.
Pipe weight is given by:

12. Live Loads
Highway Live Load
Railroad Live Load
Aircraft Live Load

13. AASHTO HL-93 Live Load
H20 vs. HS20

14. Tire Contact Area LRFD (3.6.1.2.5) 10”x20” Footprint
LFD 6.4.2: “When the depth of fill is less than 2 feet, the wheel load shall be distributed as in slabs with concentrated loads.”

15. Live Load Distribution
“Live load shall be distributed to…concrete pipe with less than 2.0 ft. of fill as specified in Article ” – LRFD a
E = S (for Axle Load)
E = in.
S = Span (ft.)
‘l’ = 0” or 1” per LFD’s reference to “concentrated loads”. ‘l’ = 10” H per LRFD
For skewed culverts???

16. Live Load Distribution Factor (LLDF) 2 ft. of Cover or Greater
Structure Type
Transverse to Span
Parallel to Span
Concrete Pipe with Depth 2 ft. or Greater
1.15 for Diameter 2 ft. or Less
1.75 for Diameter 8 ft. or Greater
Linearly Interpolate for LLDF Between 2 ft. and 8 ft.
Same as Transverse
All Other Culverts and Buried Structures
1.15

17. Live Load Distribution – LRFD 3.6.1.2.6b
Live Load Applied From “Interacting Wheels” As Opposed to Axles.
Axle Load Interaction Depth

18. Live Load Distribution
Previously: Le = L (¾ Bc)
For 36” RCP, Le = L ’
Currently: Le = L Di
For 36” RCP, Le = L ’

19. Live Load Distribution
For single-span culverts, the effects of live load may be neglected where the depth of fill is more than 8.0 ft and exceeds the span length;” – LRFD a

20. AASHTO LRFD – Lane Load
“Where the slab spans primarily in the transverse direction…[and] where the slab spans primarily in the longitudinal direction…only the axle loads of the design truck or design tandem of Articles and , respectively, shall be applied.” – LRFD Article

21. Calculate Design Forces
Uniform Load System
Radial Load System
SIDD Pressure Distribution

22. Uniform Load System
Rational Approximation Also Allowed per LRFD
Uniformly distributed vertical and horizontal components of pressure.
First proposed by J.M. Paris in the early 1920’s.

23. Radial Load System
Pressures act normal to the pipe surface and vary as a trigonometric function.
First presented by H.C. Olander in 1950.

24. SIDD Pressure Distribution
Resulted from a long range research program initiated by the ACPA in the 1970’s.
Commonly referred to as the Heger Pressure Distribution.

25. Vertical Arching Factors (SIDD)
INSTALLATION Fe
TYPE
TYPE
TYPE
TYPE

26. SIDD Pressure Distribution
The following equations are used to calculate the internal pipe moments, thrusts, and shears:

27. SIDD Pressure Distribution
Type 1 Installation

28. SIDD Pressure Distribution
Type 2 Installation
-.437

29. SIDD Pressure Distribution
Type 3 Installation

30. SIDD Pressure Distribution
Type 4 Installation

31. SPIDA Finite Element Analysis
Soil
Pipe
Interaction
Design
Analysis

32. Pipe Wall Design
“The procedures for analysis and design are similar to those used for other reinforced concrete structures.” – LRFD C
Design procedure is based on “limit states” criteria to ensure adequate pipe strength and serviceability.

33. Direct Design – Articles 12.5.3 & 12.10.4.1
Flexure & Thrust
Steel
Concrete
Crack Control
Diagonal Tension (Shear)
7 ksi Max. Conc. Comp. Strength
Radial Tension

34. Radial Tension

35. Pipe Wall Design Steps:
Determine required reinforcement areas based on the tensile yield strength limit.
Check radial tension.
Check compressive strains.
Check shear.
Modify design if any strength limit is exceeded.
Design stirrup reinforcement (if req’d.).
Check crack width.

36. Example:

56. Same Procedure for Outer Cage

57. Same Procedure for Crown