Transient Force Calculations in HAMMER

Product(s): Hammer
Version(s): 08.11.XX.XX, 10.XX.XX.XX
Area: Output and Reporting

Transient Force - Background

This article provides background information on transient force in Bentley HAMMER

Note: for background information on Transient Force calculations, see the section further below.

By default, HAMMER does not calculate force results, but you can set up the model to run these calculations. To do this, go to Analysis > Calculation Options and double-click the active Transient Calculation Option to view the properties. In the properties, find the attribute "Calculation Transient Force?" and set this to True.

Once this is done, compute the model.

There a couple of different ways to view the results. First, under the Time Histories tab of the Transient Results Viewer, you can choose "Force X", "Force Y", "Force Z" "Force Magnitude", or "Force X, Y, Z and Magnitude" as the "Graph type". Select the end point where you would like to see the force results, then click "Plot".

*For more information on how the forces are calculated please see this wiki technote.

In addition, you can see the magnitude of the force, as well as the force in the x, y, and z directions at each node in the model by viewing the Transient Output Log (Report > Transient Analysis Reports > Transient Analysis Output Log). Search for the section "Maximum Forces at Every Node" for the data. This will list the maximum force for each node and the time that the maximum force occurred. 

Transient Force Calculations

To understand the meaning of the X/Y/Z force values (fx, fy, fz), think of the model plan view, or the top-down view. Think of the node in question as the 0,0 origin of a standard graph, with the X axis extending to the right and the Y axis extending up. The "X" force would be the force exerted from left to right. So, a negative value would mean a force exerted towards the left of the node. The "Y" force would be the force from down to up. So, a negative value for Y would mean a force exerted down. The Z force would be the force in the Z direction - perpendicular to the plane of the graph. So, a negative Z force would mean a force exerted toward the ground.

These X/Y/Z values are useful when determining required bracings (thrust blocks, restraints, etc) since it gives you the force for the important 3 directions, when the total (resultant) was at it's maximum.

Note:

  • There are two components of force at a pipe end section (pipe end point at a node): Longitudinal force and Transverse force. These two forces affect the total component forces in X, Y and Z direction.
  • Longitudinal force at a pipe end section is affected by the pipe cross-section area, velocity, and pressure.
  • Transverse force is affected by the direction of the pipe and the weight of the fluid in the pipe.
  • Pipe direction is affected by the pipe start and stop node coordinates (X, Y, and the elevation Z). For this reason, force calculations will work best with a scaled model. If the model is not scaled, you should be mindful of the coordinates, as a shift in the coordinates can impact the force calculations. For instance, if you manually lay out a system that is meant to have no gradient in the Y direction (i.e., a horizontal line), if there the line is not exactly horizontal, it will make the results different compared to a horizontal layout.
  • The weight of fluid is affected by the pipe length and pipe cross-section area.
  • Pipe length can be the user-defined length or scaled length. You will want to make sure the data is accurate, as length can be important in force calculations. For a vertical pipe, the start and stop node can have the same X-Y coordinates (or close to overlapping). The user only needs to get correct pipe length and node elevations to see accurate results, though the plan view appearance of the model may not be ideal. 

Note: HAMMER models can be exported to an i-model (File > Export > Publish i-model). This file can then be opened in MicroStation to see a 3-D representation of the system, along with results that are available in the Properties dialog. Starting with HAMMER V8i SELECTseries 5, the user can also export to a Map Mobile i-model to view the system on handheld devices or tablets.

Pipe Bends (vertices)

In determining the pipe direction, only pipe start and stop node coordinates (X, Y and elevation Z) are used. The bending points (vertices) in the drawing of a pipe are ignored. You will need to place junctions (or another node element) at the vertex.

Balancing effects

You may wonder if the transient force calculations in HAMMER account for balancing effects of opposing pipe bends.

HAMMER doesn't show net forces on a pipeline; it just shows the forces at individual nodes. If you add the net forces on individual nodes, you might find they balance out at certain times during a simulation, but HAMMER doesn't attempt to show that. In order to do that properly, HAMMER would need to be augmented with numerous other components (like pipe supports, hangers, anchor blocks, etc.) and it would end up looking more like AutoPIPE (see link below).

Note: An Enhancement (#958426) has been filed to export transient force results to AutoPIPE acceptable format (.TIH and .THL) files

See Also

Additional explanation of force calculations in HAMMER is found in the Help documentation under Contents > Modeling Capabilities > Hydraulic Transient Pressure Analysis > Analysis of Transient Forces and also under Contents > Bentley HAMMER V8i Theory and Practice > Transient Forces.

The z force for my model appears unusually large. Why?

"Fluid Transient" - AutoPIPE Load Case

How to create / import a Time History file into AutoPIPE?

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