Direct Design and Indirect Design of Concrete Pipe Part 2 Josh Beakley March 2011
Latest in Design Methods? AASHTO LRFD Bridge Design Specifications 2010 Direct Design Method for Concrete Pipe 1993?
LRFD5732FlexuralResistance 5.7.3.2 Flexural Resistance
Assumptions for Strength and Extreme Event Limit States Section 5.7.2 1 Factored resistance of concrete components shall be based on conditions of equilibrium and strain compatibility 2 Strain is directly proportional to distance from neutral axis 3 Maximum compression strain at the extreme concrete compression fiber is 0.003
Assumptions for Strength and Extreme Event Limit States Section 5.7.2 4 Stress in the reinforcement is based on a stress-strain strain curve representative of the steel or on an approved mathematical representation 5 The tensile strength of the concrete is neglected 6 The concrete compressive stress-strain strain distribution is assumed to be rectangular, parabolic
Assumptions for Strength and Extreme Event Limit States Section 5.7.2 7 The net tensile strain in the extreme tension steel is determined from a linear strain distribution 8 The use of compression reinforcement in conjunction with additional tension reinforcement is permitted to increase the strength of flexural members.
What do we have in Section 12 for Pipe?
c = 0.003 s =? = y Strain compatibility -1 Strain is directly proportional to distance from neutral axis 2 Maximum compression strain = 0.003003 3 Tensile strain determined from a linear strain distribution - 7
085*f` 0.85 c *b*a ba a = 1 *C Nu Mu A s f y Equilibrium -1 Tensile strength of concrete is neglected 5 Concrete stress-strain strain distribution is rectangular 6
Was Anything Missed? 4 Stress in the reinforcement is based on a stress-strain strain curve representative of the steel or on an approved mathematical representation 8 The use of compression reinforcement in conjunction with additional tension reinforcement is permitted to increase the strength of flexural members.
Review Some Factors for Difference Reinforcement Proportions Size Factor Steel Reinforcement Properties Double Reinforcement
B - Wall Pipe Class III in Type 2 16 ft. Advanced Steel Properties and Combined Moments 27 inch Pipe Advanced Steel Properties H f (ft) 18 15 12 H r (ft) H cr (ft) H s (ft) Section 12
B - Wall Pipe Class III in Type 2 16 ft. Advanced DR and Steel Mod 42 inch Pipe Double Reinforced H f (ft) 19 15 13 H r (ft) H cr (ft) 18 H s (ft) 17 Section 12
B - Wall Pipe Class III in Type 2 16 ft. Advanced DR and Steel Mod 54 inch Pipe Double Reinforced H f (ft) 19 14 14 H r (ft) H cr (ft) H s (ft) 15 Min Section 12
B - Wall Pipe Class III in Type 2 16 ft. 66 inch Pipe H f (ft) Advanced DR and Steel Mod Double Reinforced Section 12 H r (ft) H cr (ft) H s (ft) 15 15 15
Where are the Benefits? Size Factor - < 36 inches Steel Reinf. Properties - < 54 inches Double Reinforcement - < 60 inches
PIPE DESIGN PROGRAMS
18 inch Pipe Class III
Class III 18 Inch Pipe Per ASTM C 76
PIPECAR Direct Design 18 inch B-Wall Pipe at 16 Feet
PIPECAR 3.E.B. Design 18 inch B-Wall Pipe Class III
How Does PIPECAR 3EB Calculate Flexural Steel?
M = Pr X
How Does PIPECAR 3EB Calculate Flexural Steel? A s f y (d-a/2) = PrX
How Does PIPECAR 3EB Calculate Flexural Steel? A s f y (d-a/2) = PrX
How Does PIPECAR 3EB Calculate Flexural Steel? PrX α α = lbs/ft/ft P = lbs/ft P = S i x α where S i is in feet r = (S i + h)/2 where S i is in inches
How Does PIPECAR 3EB Calculate Flexural Steel? X = X = (2) (12) 85 x C m x C mp 0.282 C m x C mp C m = 1.0 for circular pipes (shape factor) C mp = coefficient for increased bending strength based on plastic behavior
X = 0.282/Cmp
The C mp Factor C s = coefficient for flexural reinforcement at springlines based on ASTM Specifications C mp = 1.108108 for a 18 inch B-wall pipe with C s = 06 0.6
X = 0.282/C mp X = 0.255
We calculate the moment at a specific location without any consideration of moment distribution as a result of the pipe size or reinforcement proportions
How Does PIPECAR 3EB Calculate Flexural Steel? A si = 0.098 A si = 0.078078 if C s = 10 1.0 were used
42 inch Pipe Class III
PipePac 42 Inch Pipe
ASTM C 76 42 inch Class III, B- Wall A si = 0.21
42 inch Pipe at 16 feet of Fill
42 inch Pipe at 16 feet of Fill
42 Inch Class III Pipe in the Three-Edge Bearing Module
How Does PIPECAR 3EB Calculate Flexural Steel?
How Does PIPECAR 3EB Calculate Flexural Steel? A si = 0.241 Not accounting for: Plastic behavior, steel stress passed yield, and double reinforcement. A si = 0.224 Not accounting for: steel stress passed yield and double reinforcement - Not accounting for double reinforcement
HAVE WE KNOWN ABOUT THIS FOR A LONG TIME AND JUST NEVER ADDRESSED IT?
Shear
Calculating Shear in Concrete Research presented in (15) has shown that the critical internal shear force in a flexural component subject to a distributed loading, such as buried pipe, pe, is not necessarily the maximum shear force, because shear strength is reduced by the presence and severity of flexural cracking. Because of this, the nondimensional ratio of moment to shear, M/Vd is a significant consideration in the evaluation of the adequacy of shear strength of a buried pipe. ACPA Concrete Pipe Technology Handbook Pipe
Section 12 Shear (Pipe)
Strain & Shear
Section 5 Shear (Concrete Design)
Proposed Change for Shear Design of V c 2 v b d Concrete Pipe (ASTM C 361 and ASCE) f' c F d F x F c F x 2.2 1 2.75 0. 25 x u Based on the Shear Design Method in the CHBDC and LRFD Codes. x u M u v 0.9d.5V u v cot v E s A si 0.4 N u ve 0.5 N u p
Summary The Direct Design Method is more conservative than the Indirect Design Method for pipes governed by flexure. The Direct Design is more conservative than the Indirect Design Method for small diameter pipes evaluated for shear. The Direct Design Method is a valuable tool for the simplified design of large diameter pipe, which h was its intended d use.
Summary Some of the conservatisms result from: Reinforcement Proportions Size Factor Steel Reinforcement Properties Double Reinforcement A concrete pipe could be more accurately designed following the prescribed assumptions in Section 5 on Concrete Design than it can following the required equations in Section 12 of the LRFD Code.
Summary The concrete pipe pp industry should not be punished in smaller size pipe simply because it made an effort to provide a simplified design process for large diameter pipe pp to make the design engineers life easier.
HDPE Indirect Design Method Stub Compression Test
AASHTO 2011 Section 12.12.3.10 10 As an alternate to determining the effective area by the calculation procedure presented above, the results of the stub compression test, AASHTO T 341-10, 10, may be used
The End