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Live Load Design of Concrete Pipe
Live Load Design of Concrete Pipe
Outline
• Critical Items• Comparison of AASHTO Methods• Testing and Research• Latest Information
• AASHTO Standard Spec• 1.75
• AASHTO LRFD Initially• 1.15 or 1.0
• AASHTO LRFD Currently
• 1.15 up to 24 inch ID• 1.75 for 96 inch ID and
above• Linear Interpolation in
between these sizes
Live Load Distribution Factor – A Critical Item
• Industry• 1.3125 x OD
• AASHTO • Previously
• No Comment• Currently
• 0.06 x ID
Distribution Through the Pipe – A Critical Item
Current Designs Based On:
• NCHRP Report 647, “Recommended Design Specifications for Live Load Distribution to Buried Structures”
• “The results of the comparison of predictions from computer models with data from actual field test was often poor; extenuating circumstances are discussed in Section 2.1.3.”
Constant Change (D-Loads – lbs/ft/ft)
Type 2 Installation
Depth (ft) Code Pipe Inside Diameter (in)
12 36 60
1 Standard Spec 1161 761 671
LRFD 1998 1700 850 800
LRFD 2014 1492 1244 948
2 Standard Spec 667 557 530
LRFD 1998 1050 700 675
LRFD 2014 1322 1137 875
Industry Testing• Participants
• Forterra (formerly Hanson Precast Products)• Rinker Materials• Oldcastle Precast• Forterra (formerly Cretex Companies Inc.)• County Materials Corp. (formerly Independent Concrete
Pipe Company)• Scurlock Industries
Rigid Rugged Resilient10
Distribution Through Pipe
Coeffm = 14500 Do-1.58+0.755
CoeffI = 242 Do-1.97+0.855
Moment from Live Load –Bedding Factor
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Old/Proposed
Moment from Live Load –Bedding Factor
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CURRENT
Comparison TableType 2 Installation
Depth (ft) Code Pipe Inside Diameter (in)
12 36 60
1 LRFD 1998 1700 850 800
LRFD 2014 1492 1244 948
Proposed 1659 920 998
2 LRFD 1998 1050 700 675
LRFD 2014 1322 1137 875
Proposed 993 749 717
NCHRP 20-7, Task 316
Rigid Rugged Resilient15
NCHRP 20-07, Task 316Moments at the Crown From a 24.7 kip wheel load
Diameter (in) Depth (ft) LRFD 2007(in-kips/ft)
LRFD 2013(in-kips/ft)
Test(in-kips/ft)
24 4 5.8 5.8 2.3
24 2 17.0 17.0 6.0
24 1 29.4 29.4 10.1
48 4 18.9 16.8 5.3
48 2 36.9 33.5 11.6
48 1 56.9 53.1 18.5
Rigid Rugged Resilient16
Moments at the Crown From a 24.7 kip wheel loadDiameter (in) Depth (ft) LRFD 2013
(in-kips/ft)Test
(in-kips/ft)Proposed(in-kips/ft)
24 4 5.7 2.3 5.1
24 2 9.9 6.0 7.3
24 1 17.5 10.1 9.1
48 4 20.3 5.3 17.2
48 2 26.0 11.6 18.3
48 1 46.5 18.5 22.6
Rigid Rugged Resilient17
Rigid Rugged Resilient18
𝑙𝑙𝑤𝑤 =𝑙𝑙𝑡𝑡12 + 1.75𝐻𝐻 𝑙𝑙𝑤𝑤 =
𝑙𝑙𝑡𝑡12 + 1.15𝐻𝐻
H = 1 ft
lw = 2.58 ft lw = 1.98 ft
Standard LRFD
Live Load
Load Angle = 180Bf = 2.2
Load Angle = 98Bf = 1.9
24 inch RCP at 1 foot – LLDF = 1.75 and 1.15
Revised Live Load Bedding FactorsHeight of Fill
Pipe Diameter (inches)
feet 12 24 36 48 60 72 84
0.5 2.12.2
1.6 1.7
1.4 1.3 1.21.3
1.1 1.1
1.0 2.2 1.92.2
1.61.7
1.5 1.4 1.3 1.3
1.5 2.2 2.2 1.82.1
1.71.8
1.5 1.4 1.4
2.0 2.2 2.2 2.02.2
1.82.0
1.71.8
1.5 1.5
2.5 2.2 2.2 2.2 2.02.2
1.82.0
1.71.8
1.7
3.0 2.2 2.2 2.2 2.2 2.02.2
1.82.2
1.8
3.5 2.2 2.2 2.2 2.2 2.2 2.02.2
1.9
4.0 2.2 2.2 2.2 2.2 2.2 2.12.2
2.02.1
Rigid Rugged Resilient20
Using Revised Bedding Factors
Moments at the Crown From a 24.7 kip wheel loadDiameter (in) Depth (ft) LRFD 2013
(in-kips/ft)Test
(in-kips/ft)Proposed(in-kips/ft)
24 4 5.7 2.3 5.1
24 2 9.9 6.0 7.3
24 1 17.5 10.1 10.4
48 4 20.3 5.3 17.2
48 2 26.0 11.6 18.3
48 1 46.5 18.5 22.6
Rigid Rugged Resilient21
CL value
Rigid Rugged Resilient22
CL = 1
Lw
Lw
CL < 1
CL Value
• Plastic Pipe • Metal Pipe
Rigid Rugged Resilient23
Box Culvert Shear Design
For Fills Equal to or Greater Than 2 feet
Rigid Rugged Resilient24
Rigid Rugged Resilient25
Equation for Beta
Crack Spacing Parameter
Moment at the Location Being Checked for Shear
Crack Spacing Parameter
Add:
sxe = the incorporation of the effect of maximum aggregate size into thecrack spacing parameter. The requirement that sxe equal or exceed 12 inches need not apply to slabs.
Limit sxe to 12 inches?
Rigid Rugged Resilient31
Moment at the Location Being Checked for Shear• Add:
• In continuous slabs not containing prestressing, the value of |Mu| for the location being analyzed may be utilized without consideration of the
|Mu|min = |Vu – Vp|dv limitation.
Why the Vu x dv requirement?
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If Mv = 0, and Nv = 0
Tension Face
Compression Face
Tension in the Entire Element
β versus Span
Rigid Rugged Resilient34
β versus ρ (depth limit removed)
Rigid Rugged Resilient35
β versus ρ (no limit)
Rigid Rugged Resilient36
Behavior and Strength in Shear of Beams and Frames Without Web
Reinforcement
Rigid Rugged Resilient37
Roger Diaz De Cossio and Chester P. Seiss
“A study of the test results in Table 3 and 4 reveals also that shear capacity isroughly a linear function of tensile steel percentage.”
De Cossio Equation for Shear
Rigid Rugged Resilient38
5.14.5.3 Method
Rigid Rugged Resilient39
5.8.3 With No Restrictions
Rigid Rugged Resilient40
Minimum Mu Required?
Rigid Rugged Resilient41
Why the Minimum Mu?
Rigid Rugged Resilient42
What Has Been Implied?
• The minimum value of sx should be removed, or at least reduced to 5 inches.
• Relative Agreement
• The requirement that Mu be not less than Vu*dvshould be removed, at least for frames/continuous members
• Additional discussion required
Rigid Rugged Resilient43
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