Applies To
	Product(s):	AutoPIPE
	Version(s):	2004, XM, V8i
	Environment:	N/A
	Area:	warning message
	Original Author:	Bentley Technical Support Group
	Date Logged & Current Version	Mar. 2015 09.06.01.10

Problem:

The following warning message is located in the model consistency check report:

* * * W A R N I N G - MODEL * * *
W726-14: No anchors in current system

What does it mean and how to avoid?

Solution:

As mentioned in warning message, the current model does not have an anchor component. If model does not include any fixity, you are experiencing very high stresses and displacements. To stabilize the system and obtain accurate results, you must define at least one fixity or anchor point in the model, which will help achieve the objectives listed below: 

• • AutoPIPE models need at least one anchor or fixity to define a stable reference point; otherwise, the entire system can float in space.
  • Without an anchor, the solver cannot distinguish between actual pipe deformation and overall system translation, causing rigid body motion.
  • The stiffness matrix becomes singular (non-invertible) if no point is fixed, making it impossible to solve equilibrium equations.
  • Anchors provide boundary conditions that stabilize the model and prevent calculation instability.
  • Real-world piping systems are always connected to something rigid, so anchors simulate these physical constraints.
  • At least one anchor is required to eliminate rigid body motion and provide a reference for thermal expansion and load distribution.
  • If exact stiffness is unknown, start with a rigid anchor and refine later with flexible anchors as needed.

Typically, if a model does not have an anchor, it would allow the piping system to rotate about one or more of the 3 global axis (i.e. Global X, Y, or Z) thus resulting in the following Error message being displayed when running an analysis:

---------------------------
Error
---------------------------
E801-1: FATAL ERROR : Unstable system. Need support at node xxx Dir X
---------------------------
OK
---------------------------

However, if a model can be sufficiently restrained on all 3 global axis then no anchor is necessary. The easiest way to tell if a model is sufficiently restrained, run an analysis  and see if the E801-1 Fatal Error message appears. If the Error message appears, there are 2 options:

1. Add a support as mentioned in the error message

or

2. Insert an anchor (i.e. Rigid or Flexible).

Note: if your actual as-built system does not include a rigid anchor, consider adding a flexible anchor with very low values for all 6 Translational and Rotational stiffness values. 

See Also

Warnings Messages displayed in AutoPIPE

Bentley AutoPIPE