Force Sets Tab - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

The Force Sets tab is available when Relief Loads (spectrum), Water Hammer/Slug Flow (spectrum), and Time History are selected for Analysis Type in the Dynamic Analysis window.

Spectrum or time history analysis can have multiple force sets.

Force spectrum analyses, such as a relief valve loading, differ from earthquake analyses because there is no implicit definition of the load distribution. For example, the loading for earthquakes is uniform over the entire structure and proportional to the pipe mass. For relief valves and other point loadings, the load is not uniformly distributed and is not proportional to the mass. A water hammer load is proportional to the speed of sound and the initial velocity of the fluid. Its point of application is at subsequent elbow-elbow pairs. Force spectrum analyses require more information than the more common earthquake simulations: the load magnitude, direction, and location. Forces that occur together are grouped into like-numbered force sets and are manipulated in the analysis together. For example, the following shows two different loading levels of the same type of load:

Force

Direction

Node

Force Set #

-3400

Y

35

1

-1250

Y

35

2

For a skewed load, force components belong to the same force set, because the components always occur together:

Force

Direction

Node

Force Set #

-2134

Y

104

1

-2134

X

104

1

Force Spectrum Workflow

The general procedure for applying a force spectrum load is as follows:

  1. Determine the pulse time history acting at a single node or over a group of nodes. The pulse waveform must be the same for all nodes in a group, but the maximum pulse amplitude may vary.

  2. To convert a time history to a response spectrum, use the DLF/Spectrum Generator to build a DLF versus frequency file for the time-pulse waveform. This is a standard shock table file. This step is not needed for a time history analysis. The data is automatically added to the dynamic input and can be saved to a separate file.

  3. On the Spectrum Definitions tab or Time History Definitions tab, define the DLF versus frequency file just created as a force spectrum data file with linear interpolation along the frequency axis and linear interpolation along the ordinate axis. Begin the shock name with a #. The software then reads the shock table from the data file.

  4. Determine the maximum force magnitude that acts on each node subject to the pulse load.

  5. On the Force Sets tab, specify the maximum amplitude of the dynamic load, the direction, and the nodes.

    If the force-time profiles are normalized to 1.0, the maximum magnitudes of the loads are entered here. If the profiles are entered using their actual values, the force set values are entered as 1.0.

  6. On the Spectrum Load Cases tab or Time History Load Cases tab, enter the force spectrum name (defined in the Spectrum Definitions tab), the table multiplication factor (usually 1.0), a direction, and the Force Set # (defined on the Force Sets tab). This step defines the link between the force spectrum and the force loading pattern.

  7. Set up any other parameters needed to run the spectrum analysis. Perform error checking, and after there are no fatal errors, run the analysis.

  • You can include any number of user comment lines by clicking Cmt. There can be any number of line entries in the Force spectrum data.

  • If there are multiple force spectrum components in a single dynamic load case, carefully select the combination method. The same rules that cover earthquake shocks and components apply to force spectrum shocks and components