How can I model a pipe that is supported over all its length on an open trench or gutter
The response below is based on open trench or gutter, meaning the pipe is only supported at its base, or is only semi-embedded. At this time there is no direct support selection that represents a continuously supported pipe. The requested has been logged as an enhancement under the following: TFS-E113001: Add option for above ground continuous support with friction. For an alternate workflow, the following 2 options may be considered:
Note Definitions for the soil parameters:
k1 – soil stiffness value at yield displacement of the soil (lb/in/ft) – notice that as the value decreases, the Yield Displacement will increase – Yield Disp. = p1 / k1
p1 – maximum soil resistance per unit length of pipe
k2 – stiffness of soil after yield has occurred (conservative to assume a small value)
The Buried pipe option can be used to model semi-embedded (debris builds up on the sides of the pipe over time) or non-embedded piping.
The difficulty is calculating a transverse horizontal (lateral) and longitudinal soil stiffness.
a) The vertical up soil stiffness K1, P1 and K2 can be taken as a negligibly small values but cannot use 0.00. b) The transverse horizontal K1 and P1 is usually taken as low values. Higher values are used if semi-embedded pipe is anticipated. c) Transverse vertical down K1 and p1 can be calculated as non-zero with H=0
d) The longitudinal K1 and P1 would again be modeled as negligibly small values with Z=0 but there may be an alternative equation in some textbook which would calculate a non-zero longitudinal K1 and P1 although it is suspect it would be a low value since only line contact of soil with the pipe is assumed.
Note: the reason for using non-zero soil property values is to avoid analysis errors. You may wish to enter multiple non-zero K1 and P1 values to evaluate the longitudinal frictional stiffness effect back on the pipe system. It is recommend to set all final stiffness, K2 = 0.1 lb/in/ft (0.006Kg/m/mm) to avoid convergence problems.
For Transverse Horizontal Soil Properties:
Initially assume a friction factor and calculate stiffness based on that. That is assume friction factor v=0.4 to 1.0, then P1 for soil will be v*Wp. (Wp is pipe weight per unit length, including coating and contents). K1 can be calculated assuming a small yield disp (e.g. 1 cm)
For Longitudinal Soil Properties:
Same as for transverse properties except use a lower friction factor (suggest ½ the value).
Vertical Upward Soil Properties:
The vertical up soil stiffness K1, P1 and K2 can be taken as negligibly small value.
For Vertical Downward Soil Properties:
Use equation D-16 from AutoPIPE help. Obtain the initial value of Nc by assuming an H/B of ½ (ref. fig. D-12). Note that this is an iterative process. Observe the vertical displacement and adjust the amount of friction for the transverse and longitudinal properties accordingly as the contact area increases. If the downward displacement reaches the centerline of the pipe, assume values for fully buried pipe for Vertical Downward Soil Properties. Use half of the values for fully buried pipe for the Transverse Horizontal and Longitudinal Soil Properties. Note that it is recommend all final stiffness be set to a negligibly small sucah as K2 = 0.1 lb/in/ft (0.006Kg/m/mm).
The procedure for loose sand is basically the same as for that of clay. However for vertical downward properties the first 2 terms of equation D-15 should be used as opposed to D-16.
Question: Where can I get values for K1.,P1, K2?
Answer: Suggested resources:
a. Geotechnical engineer
b. Perform calculations based on Codes or Reference documents seen in AutoPIPE help:
i. American Lifeline Alliance
ii. Pipeline Research Council Int.
iii. AutoPIPE
c. Project documentation
d. Hire a consulting firm to perform site test and provide a report
An alternative approach would be to model multiple closely spaced V-stop supports with a friction coefficient defined. The spacing between V-stops should be sufficient to prevent any pipe settling under gravity loads.
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