Spectrum/Time History Load Cases Tab - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

The Spectrum Load Cases tab is available when Earthquake (spectrum), Relief Loads (spectrum) and Water Hammer/Slug Flow (spectrum) are selected for Analysis Type in the Dynamic Analysis window.

The Time History Load Cases tab is available when Time History is selected for Analysis Type in the Dynamic Analysis window. A time history analysis has only one load case.

Load cases consist of simultaneously applied spectra. Each spectrum in the case is assigned a direction and factor.

Additional Spectrum Options

The following options are only available for the Earthquake (spectrum), Relief Loads (spectrum) and Water Hammer/Slug Flow (spectrum) analysis types.

Editing Load Case

Specifies a load case to edit.

Stress Types

Specifies the stress type for the load case:

  • OPE - Stress from operating loads.

  • OCC - Stress from occasional short-term loads.

  • SUS - Stress from primary sustained loads.

  • EXP - Stress from secondary thermal expansion loads.

  • FAT - Stress from fatigue loads.

Fatigue Cycles

Specifies the number of fatigue cycles. This option is only available when FAT is selected for Stress Types.

Directives

Displays the Directive Builder dialog box.

Add New Load Case

Adds a new load case.

Delete Current Load Case

Deletes the current load case.

Load Cases for Force Spectrum

Spectrum load cases for force spectrum analyses are set up differently than spectrum load cases for earthquake analyses. Force spectrum analyses must link a force multiplier spectrum to a force set.

A load case definition consists of one or more lines, as shown below. The direction specified on this line does not need to be the direction of the load (which is specified in the force set). This direction is used for labeling and designation of independent versus dependent loadings.

Spectrum

Factor

Dir.

Force Set #

TESTFILE

1.0

Y

1

Complexity increases as the number of components in the load case goes beyond one, and as the time history phenomena being modeled deviates from true impulse type loading. For more information, see Examples.

Load Cases for Earthquakes

For earthquakes, the direction defines the orientation of the uniform inertial loading. Earthquakes typically have X, Y, and Z components. The factor is used to modify the magnitude of the shock. For example, the seismic evaluation of a piping system includes two load cases:

  • 1.0 times (100% of) the El Centro spectrum in the X direction and 0.67 times (67% of) the El Centro spectrum in the Y direction.

  • 1.0 in Z and 0.67 in Y.

CAESAR II also supports options for independent support motion earthquakes, where parts of the system are exposed to different shocks. For example, a piping system is supported from both ground and building supports. Because the building filters the earthquake, supports attached to the building are not exposed to the same shock as the supports attached to the ground. Two different shock inputs are required: one for the ground supports and one for the building supports. To specify an independent support motion shock, the software requires thenode range that defines a group of supports. The maximum displacement (seismic anchor movements) of the support attachment point must also be specified.

The example below shows a typical uniform support earthquake specification and a typical independent support motion earthquake:

* UNIFORM SUPPORT MOTION EARTHQUAKE INPUT

ELCENTRO 1 X

ELCENTRO 1 Z

ELCENTRO .667 Y

* INDEPENDENT SUPPORT MOTION EARTHQUAKE INPUT

HGROUND 1 X 1 100 1 0.25

HGROUND 1 Z 1 100 1 0.25

VGROUND 1 Y 1 100 1 0.167

HBUILDING 1 X 101 300 1 0.36

HBUILDING 1 Z 101 300 1 0.36

VBUILDING 1 Y 101 300 1 0.24

The uniform support motion earthquake contains only components of the El Centro earthquake acting uniformly through all the supports. There is a 33% reduction in the earthquake’s magnitude in the Y direction.

The independent support motion earthquake above has two different support groups: 1-100 and 101-300. The 1-100 group is exposed to a ground spectrum. The 101-300 group is exposed to a building spectrum. Different horizontal and vertical components are used for the ground and the building spectra. The last values specified are the seismic support movements (that is the Anchor Movement).

Stress Types can be assigned to the spectrum load cases. If FAT is selected, you must also enter a value for Fatigue Cycles, the number of anticipated load cycles.

Load Case for Time History

Only a single load case is defined for time history analysis. The direction entry (Dir.) is used only for labeling, not as an analytic input value.