01. AutoPIPE's Flexible Joint Dialog Explained

Applies To
Area: Modeling
Original Author:Bentley Technical Support Group

Nov 2014, AutoPIPE V8i


AutoPIPE's Flexible Joint Dialog Explained

Also, which direction is Y-shear on a vertical pipe?


 The following will provide additional information about the Flexible joint dialog. After a model is opened, select a run node point, and press Insert> Flexible joint> the following screen will appear.


Name of Point:

Start point of flexible joint


Length of bellows.

Typical units: Ft (mm)


AutoPIPE will display the default joint length components in each of the global axis coordinate directions in these fields, based on the direction of the previous component. Another words, the distance between Flexible joints end point connections is that distance when the joint is being installed or when the joint is not under operating (temperature, pressure, etc..) loads.

See AutoPIPE help axis directions. At this time, user can not  specify the horizontal axis directions, these are fixed based on the direction of the segment and vertical Axis designation.

Axial Stiffness:

Amount of resistance for change in axial length of the bellows from its free length in a direction parallel to its longitudinal axis.

Typical units: lbs/in (N/mm)

Shear stiffness (Y-axis & Z-axis):

the relative stiffness of one end of the bellows to the other end in a direction perpendicular to the longitudinal axis. Local Y & Z axis values are typically the same stiffness.

Typical units: lbs/in (N/mm)

Torsional :

Amount of resistance in the rotation about the axis through the center of the bellows (twisting) 

Typical units: Ft-lbs / deg (N-m/deg)

Bending stiffness (Local Y-axis & Z-axis):

resist angular rotation along the local longitudinal axis of the bellows towards a point of rotation

Typical units: ft-lbs / deg  (N-m/deg)


weight of the bellows.

Typical units: lbs (kg) (N-m/deg)

Note: For static loads, the weight of contents is automatically calculated based on the pipe properties pipe size and specific gravity. For Dynamic loads the static weight is multiplied by the Fluid density factor on the modal dialog. 

Pressure Area:

The effective cross sectional area; usually based on the mean diameter of the flexible joint convolutions. This area is then multiplied by the internal pressure to obtain the axial thrust due to internal pressure. This thrust force is applied to adjacent equipment or piping if a rigorous pressure extension analysis is specified.

Typical units: sq in (cm^2)


Which direction is Y-shear on a vertical pipe?


The answer is simple, if you know the pipe's local direction. the Flexible joint dialog asks the user to enter the Axial Stiffness and Torsional stiffness. These settings are easy to understand when referring to the information above. However the Y & Z Shear / Bending stiffness can be a little confusing for some new users.  Press the "Help" button on the Flexible Joint dialog to understand these setting. In the AutoPIPE help will inform the reader that the dialog is asking for the Local shear stiffness and Local bending  stiffness. Now the question is, how to find the local coordinate system where the Flexible Joint has been modeled?

Note: regardless of pipe orientation, be mindful of the pipe's segment direction, modeled going down or was it modeled going up? Node numbering is a good indication of segment direction.


In order to find the local coordinate system, use 1 of 2 methods: 1. AutoPIPE help or 2. By creating an example model and testing it..

1. AutoPIPE Help:

The easy method would be to refer to the following AutoPIPE help section: 

Help > Contents> Contents Tab> Reference Information> Results Interpretation> 

->Straight Pipe Forces and Moments: Local Option

-> Support Forces for Guides and Line Stops without Gaps

-> Curved Pipe Forces and Moments: Local Option

This page will document in detail on how to find the local coordinate direction.

2. By creating an example model and testing it:

Create a new model that has multiple vertical pipe runs with a flexible joint and a set of anchors on each end. See image below.

For this example, Pipe size, pipe material, Piping Code, etc.. is of no great concern, pick some default values from the drop-down listings. In this example model note that Segments A and B have been modeled going Down, while Segments C and D have been modeled going Up. 

The objective of this model is to determine which direction is Local Y and Local Z for values entered into the Flexible joint dialog. Consider a Flexible Joint where all stiffness values are set to rigid except one that is set to a low stiffness (see settings below)

Now apply Impose Support Displacement at the bottom of each segment in different directions, as shown below:

If the stiffness setting is set to a low value and moves in that direction the top part of the piping will remain stationary while the Flexible Joint and lower piping flexes. If the stiffness value is entered in the wrong axis direction the entire length of the pipe run will try to bend. 

Run the analysis and animate the displacement results:

Clearly one can see that segments A and C are bending over the entire length of the segment, while segment B and D are clearly flexing from B01 & D02 down. Therefore  in this example the Local Y direction is on the global Z axis for both segment directions Up and Down.

Next Repeat the test but change the Flexible Joint stiffness accordingly:

Run the analysis and animate the displacement results:


Now, one can see that things have changed a bit, segments A and C are now flexing from A01 and C02 down but the top part of the pipe segments remains rigidly straight, while segment B and D are clearly flexing over the entire length. Therefore  in this example the Local Z direction is on the global X axis for both segment directions Up and Down.

Fair to say that the Y-shear and Y-bending stiffness are in the same local axis direction, so no need to test for that, however you could run that test yourself  as needed . 

Note: the Flexible Joint dialog is not concerned with +ve or -ve local axis direction. The flexible joint stiffness settings restrict movement on the respective axis irrelevant of  + or - axis direction. Example: one does not enter a -ve stiffness value for the -ve local axis direction. All Stiffness settings are entered using +ve values.


These values required by this dialog are specific to each flexible joint manufactured, please contact the company manufacturing your assembly to request the above information required by AutoPIPE. By using Rigid in any field, it is essentially meaning that the expansion joint will be infinitely Rigid for that plane of motion. Entering Rigid may be required by some modeling approaches but the user is cautioned to understand the implications of doing so.

If you are modeling a flexible joint with multiple convoluted sections (i.e. universal expansion, pressure balanced, etc..), do not use the overall assembly values. You must model each individual bellows and piping with their specific values.

See Also

Flexible Joint Piping Components

Bentley AutoPIPE