Question:
Why does it appear that soil is not being applied to the model?
Answer:
Most common cause of the issue is that the model is being analyzed as a Linear Analysis when it should be analyzed as a Non-Linear Analysis.
See AutoPIPE help for details on how to perform a Non-Linear Analysis.
Question #1:
What is an adequate spacing required for piping points (nodes) and soil points?
Answer:
There is no one answer to this question. See AutoPIPE's help for suggested soil spring spacing based on Zone 1, Zone 2, or Zone 3 piping. These are only suggestions may need it increase/decrease soil points as required for convergence issues.
Question #2:
The more the nodes, some of the load cases (temperature, pressure and user-defined) do not converge
Answer:
If a model does not converge, please see the following AutoPIPE help section:
Help > Contents> Search Tab> enter "Failure to Converge" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information.
Review the *.LOG file to understand where the failure is occurring and address the location. See AutoPIPE help for details about the *.LOG file.
Question #3:
What is the most soil deformation that AutoPIPE can handle?
Answer:
Soil Deformations
This can be a difficult problem, since the settlement is such a large soil deformation of 1600mm All beam theory piping analysis programs such as AutoPIPE, Caesar, pipeplus are based on small deformation theory and large displacements can give an ill-conditioned solution and erroneous results.
As an example, large deformation across a large span of 200m. The maximum slope of the pipe may be of the order of 0.025 radians and the solution should be ok
As a check that AutoPIPE can handle these large deformations use the following rule of thumb. Check that the maximum slope angle in radians of the deformed pipe = approx. the sin (slope angle) then the solution should be ok.
One method to simulate subsidence of known amount e.g 25mm see below. Set the yield P1 = 25 as shown below and K1 (initial) value = 0.001 and calculated soil stiffness = K2 (final)
SOIL PROPERTIES
Soil Initial K yield P Final K Yield disp
ID Dirn (N/mm/mm ) (N/m ) (N/mm/mm ) (mm )
------ ------- ---------- --------- ---------- ----------
SOIL1 Horiz. 6.540 699861.00 0.000 07.0118
Long 17.540 43859.00 0.000 2.5005
Vert Up 2.790 139303.00 0.000 49.9291
Vert Dn 0.001 25.00 23.030 24.9999
One method:
If the maximum settlement is unknown, then model the soft waterlogged soil over the known span and known firmer soil at ends. Apply the over-burden sand weight as UDL load or vertical wind load (to simulate the pressure of the 4m sand) on the pipe and observe the deformation and check the resulting maximum slope.
Note: ensure the final soil stiffness values K2 is not = 0 but a finite value e.g 0.001. The vertical down K2 value should be known (ie final soil conditions after settlement) and the P1 (soil yield ) value can be 'tweeked' to achieve the distance between the soft and final soil condition. Check that the actual pipe movement reaches the final soil condition otherwise the solution may become ill-conditioned.
Maybe the sand has a influence on calculating the upward soil stiffness but this direction should not be important..
Question #4:
Is there an error in the program. Why is the high stiffness values being used for the low Stiffness calculations and vise-versa Low values being used for the high stiffness calculations:
Answer:
This is not an error. The Yield Displacement is calculated via formula YD = P1/K1. However while calculating the K1 & P1 stiffness values we use low yield displacement values for high stiffness value and vice versa. This behavior is also described in the note Note: High yield disp. is used for low stiffness. Just below the Pipe direction selection on the dialog (shown below).
For the parameters, the following rule is applied:
1. If the user updates a value in the low column, which is greater than high column value, the low column value is assigned to the high column value.
2. If the user updates a value in the high column, which is lower than the value in low column, there is no update to the low column.
This behavior is missing from the online help and will be added in a future version of the program.
Question #5:
What is the best method to model soil settlement in AutoPIPE when 25.4mm (1 inch) of pipe settlement is required in the model when there is a probability of over -excavation and greater than 25.4mm (1 inch)?
a.Impose an upward displacement on the support
or
b. Impose a downward displacement on the soil
Do soil springs act in both tension and compression?
What is the best way to model pipe settlement where over-excavation has occurred?
Answer:
In our opinion, approach (b) is better as the displacement is applied in a more discretized fashion than in (a). In AutoPIPE, imposed displacements can be applied only at points which are supported or buried. If the imposed displacements are applied as in (b), there will be compression in the soil springs below the pipe and tension in the soil springs above the pipe.
Yes, soil springs act in tension as well as compression as per the assumption of bilinear behavior. Please refer to AutoPIPE help for more information.
We think that it is more reasonable to retain the soil springs below and impose displacements on the soil points to model the soil settlement in areas of over-excavation. You may also consider using incline supports with a gap of 1" below the pipe so that the resistance of soil below the pipe is engaged after the pipe displaces 1" to close the gap (similar to scenario #2 in the figure below). However, the yielding of the soil cannot be modeled using this approach of using incline supports to model soil stiffness.
Closer to the support, the pipe displacement is relatively less compared to mid-span. If large imposed displacements are modeled close to the support you might see spurious high stresses. ( WYMIWYG - what you model is what you get.)
Yes, not modeling top soil might be a reasonable approach. As there is lesser restraint after removing these soil springs, the pipe support reactions are likely to be lesser as you have rightly observed.
Question #6
What is the correct metric unit for the variable "f" at the bottom of the Edit Soil Properties window near Soil Overburden Loads? I am confused because the displayed imperial unit is psi, whereas the metric unit is N/mm.
Answer: Yes, this is an oversight in the program (AutoPIPE V8i 09.06.00.xx and lower). If you convert the PSI value over to metric units the value displayed would have the units of N/mm^2.
The development team is aware of this issue and will fix in a future version of the program.
Question #7:
Open the soil dialog screen and press Edit Soil Parameters button, set Calculation method = AutoPIPE, notice that the soil parameter "horizontal stiffness parameter ki" is always has the units of lb/in3 when the input units are set to SI. The lb/in3 units are the same in the output listing. The numerical value of the ki parameter is the same in the output listing regardless of units selected. is this an error?
Answer:
No it is not an error.
Open the Edit Soil Parameters dialog screen, press the help button, under "AutoPIPE only parameters" select the hyperlink for "Horizontal Stiffness Parameter, ki" and read the online help.
The reason for this is that there is no conversion in the unit files for lbs/in^3. We will require adding an additional unit in all our unit files to represent this lb/in3. Otherwise, the same value is used regardless of units selected in the model.
Item #8:
What type of discretized soil foundation is implemented in AutoPIPE (Example Winkler Soil Model, Filanenko Borodich Model, continuous model etc...
Answer:
To be added in the future.
Item #9:
How does imposing a support displacement effect the soil spring at the imposed support displacement and how does it effect adjacent springs ?
Answer:
Look at it this way, each support can be considered a spring with a base. The base is attached to the imaginary ground in the program. When you apply "Impose Support Displacement" to a support, the base will physically moved when a specific load case is applied during the analysis. These values are specified when inserting an "Impose Support Displacement". As mentioned in the online help, soil properties are essentially spring supports at intervals with the correct stiffness values.
Therefore, "Impose Support Displacement" can be assigned and will affect Soil point locations. Furthermore, only those soil points that actually have "Imposed Support Displacement" assigned will move when the specified load case is applied during the analysis. This action will affect surrounding piping because the pipe is not made of bubble gum.
Example:
If a set of soils points between two node points have a 5 inch Impose Support Displacement assigned. When the indicated load case is analyzed, these soil point bases will try to moved the 5 inches, while the rest of the adjacent soil springs will resist this movement but ultimately will be displaced by a fraction of that 5 inches until the impose support displacement no longer affects the adjacent soil spring. Typically the pipe will have a bell shape curve to it.
Item #10:
Is there anyway to use K1, P1, K2 soil parameters to hand calculate the displacement seen in AutoPIPE's output report for verification the program is correctly using the soil values?
Answer:
It is important to note that AutoPIPE uses a non-linear iterative process of displacements to determine the actual spring stiffness for soil. P1 is the limiting force that determines which soil stiffness is used. This process is very complex.
The numerical verification of the soil spring analysis in AutoPIPE is done by comparing results to the following reference:
"Buried piping - An analysis procedure update", ASME Publication PVP- Vol 777, ASME Pressure Vessels and Piping Conference, Portland, June 1983 (402258).
Item #11:
We have a case in which the steel buried piping system has following layers,
Layer 1: Soil depth of 1m
Layer 2: Concrete slab of 0.15m
Layer 3: Compressed Stabilized Material 0.50m
Layer 4: Vehicle of 80 Ton
How to consider these concrete slab level?
Answer:
There is no direct method to consider Concrete layer in between two soil layers. Workaround in this condition is to add soil properties of layer 1 at the required pipe sections.
Then calculate equivalent pressure on pipe due to weight of concrete (Layer 2), Compressed Stabilized Material (Layer 3) and weight of vehicle (Layer 4). Add this pressure as a Soil Overburden Loads.
Open the soil properties and click on the Soil Overburden Loads,
In Soil Overburden Loads Tab choose User Defined against Surface Live Loads type, Enter the calculated Pressure on pipe due to surface loads.