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h1h2. Translational acceleration loads

*Acceleration loads* are used to simulate the ground motion of a [time-history|Time-history analysis] acceleration record. CSI Software assumes complete fixity for all supports, then automatically computes acceleration loads at each node and element. Translational-acceleration loads are taken as the negative of the product of assembled joint mass and input acceleration.

At any point in a structure, translational acceleration is given by the cross product of the position vector (relative to the origin of rotation) and the acceleration vector. Resultant force is then the negative of the product of this translational-acceleration value and the translational mass. For example, RY acceleration would generate MY, FX, and FZ values.

Please see the [CSI Analysis Reference Manual|doc:Analysis Reference Manual] (chapter: Load Cases, section: Acceleration Loads) for further discussion on acceleration loads.

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It is not possible to display the calculated acceleration loads in a tabular format.
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h1h2. Rotational acceleration loads

Rotational acceleration is calculated independently from rotational inertia. This is done by applying, at the global origin, a unit rotation about the axis considered for rotational-acceleration computation.

While applying a rotational-acceleration load during time-history analysis, users may specify a coordinate system and an angle from the vertical Z-axis. Rotational acceleration is then applied at the origin of that coordinate system, about the designated axis.

Rotational acceleration is constant through all points in a structure.

Rotational inertia may induce negative moment values.

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h1. Translational Acceleration Loads

To apply the acceleration loads, the program assumes that the restrained joints do not move. The applied force at a joint is simply the negative of the assembled mass at a joint times the input acceleration.

h1. Rotational Acceleration Loads

Rotational inertia is NOT needed for rotational acceleration. A unit rotation is applied about the given axis at the global origin. In time-history cases, you can specify a coordinate system and angle (about Z), and the rotational acceleration will be applied about the corresponding axis in that system at that origin.

At any point in the structure, the rotational acceleration is equal to that at the origin, and negative moments will be generated for rotational inertia, if any. At any point in the structure, the translational acceleration is given by the cross product of the position vector from the origin of rotation and the acceleration vector. The negative of this translational acceleration times the translational mass will be the force. The RY acceleration generates MY, FX, and FZ.

This is basic mechanics, the conjugate to summing moments at a point from moments and forces distributed throughout the structure.
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