| Applies To | |||
| Product(s): | AutoPIPE, | ||
| Version(s): | 2004, XM, & V8i | ||
| Area: | Modeling | ||
| Original Author: | Bentley Technical Support Group | ||
Problem:
what procedure and information is available for non-linear Wind load analysis?
Solution:
Let us consider this simple example: Let us assume a vertical pipe with a horizontal support that has gaps of 1” on each direction, imaged below.
Let us also assume that under operating conditions (T1) the pipe moves 0.8” in the +X direction. That means the pipe can only move 0.2 inches for wind in +X and 1.8 inches for wind in –X direction. That is assuming the wind load acts under operating load. If however, the wind is acting under shutdown or ambient temperature, the displacements due to wind can be a full 1” in each direction. Let us assume W1 is in the +X direction.
Analyze using the nonlinear option:
Initial case for Occ. loads: GR
Notice that depending on the wind case W1 amplitude, pipe displacements up to 1.8 inches could be obtained in +X direction when adding up load cases GW1 and T1 since GW1 is limited by 1 inch gap. So 1+0.8 for T1 will give 1.8 inches which is not true in the +X direction.
To correctly analyze the nonlinear effect of wind and other occasional loads in AutoPIPE we recommend the following.
(Option 1) Analyze the system by applying occasional load under NON-OPERATING condition
This can be done easily using the nonlinear analysis option:
Initial case for Occ. loads: GR
This is the default in most cases in AutoPIPE. AutoPIPE will automatically create code combination Sus+W1 for stresses. It also creates a consistent non-code combination GW1 for support forces and displacements. In this case displacements GW1 are limited to +1 inches for our wind example. If you manually create a user case GW1+T1 then this is not consistent with the selected initial state and would have displacements up to 1.8 inches. So DO NOT manually create GW1+T1. Instead do the following:
(Option 2) Rerun the analysis with occasional load under OPERATING condition:
Initial case for Occ. loads: OP1
AutoPIPE will automatically create code combination Sus+W1 for stresses. This W1 is different from the one obtained under the previous sequence or initial state since it is evaluated using GT1W1-GT1. The previous one was evaluated from GW1-GR. AutoPIPE also creates a consistent non-code combination GT1W1 for support forces and displacements. GT1W1 will correctly be limited to 0.2 inches in this case. In this case the displacements would not exceed specified gap limits for supports.
Notice AutoPIPE will by default create for you non-code combinations that are consistent with the load sequence (i.e initial state) selected. In general if you manually create a user non-code case, it is very likely that it will have inconsistent forces or displacements. Please refer to the PDF files posted on April 17 2007 (understanding load sequencing) for more information.
In order to capture nonlinear effects for wind under operating conditions in all directions we recommend the following:
Define 4 wind load cases as follows:
W1 for Wind in +X
W2 for Wind in –X
W3 for Wind in +Z
W4 for Wind in -Z
Analyze using the option
Initial case for Occ. loads: OP1
This will automatically create 4 occasional code cases (for stresses):
Sus+W1
Sus+W2
Sus+W3
Sus+W4
And 4 consistent non-code combinations (for support forces and displacements):
GT1W1
GT1W2
GT1W3
GT1W4
These load cases will be consistent with support gaps and friction limits.
If wind loads are to be under non-operating conditions, use a separate analysis set with initial state option changed:
Initial case for Occ. loads: GR
The nice thing is that AutoPIPE does create for you the non-code combinations that are consistent with the initial state selected.