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 online 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 occuring and address the location. See omline 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. 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 and upward displacement on the support or b. A downwward displacement ont he soil Do soil springs act in both tension and compression? what is the besst 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.
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