TR.32.12.2 Generation of Seismic Loads
This command is used to generate seismic loads using previously specified load definitions.
Built-in algorithms will automatically distribute the base shear among appropriate levels and the roof per the relevant code specifications. The following general format should be used to generate loads in a particular direction.
General Format
LOAD i
code LOAD { X | Y | Z } (f1) (DECCENTRICITY f2) (ACCIDENTAL f3)
Where:
code = { 1893 | AIJ | COL | CFE | GB | IBC | NRC | NTC | RPA | TUR | UBC}
| Parameter | Default Value | Description |
|---|---|---|
| LOAD i | - | Load case number. |
| X | Y | Z f1 | 1.0 | Optional factor to be used to multiply the seismic Load. May be negative. |
| DEC f2 | 0.0 | Multiplying factor for natural torsion –arising due to static eccentricity which is the difference between center of mass and center of rigidity of a rigid floor diaphragm– to be used to multiply the seismic horizontal torsion load. Must be a positive value (greater than 1.0) or exactly 0.0. |
| ACC f3 | 1.0 | Multiplying factor for accidental torsion, to be used to multiply the seismic accidental torsion load. May be negative (otherwise, the default sign for MY is used based on the direction of the generated lateral forces). |
Use only horizontal directions. This means that seismic loads should only be applied in the X and Z directions with Y up (or X and Y directions with Z up).
- The floor must be modeled as a rigid diaphragm.
- A positive value (greater than 1.0) for DEC must be provided. Seismic load is applied at center of mass instead of center of rigidity which incorporates the effect that a value less than or equal to 1.0 will yield. Placing seismic load at center of mass of a rigid diaphragm automatically includes inherent torsion in analysis corresponding to static eccentricity (the difference between center of mass and center of rigidity). Providing DEC parameter as 0.0 for a model having rigid diaphragm to ignore inherent torsion is not possible.
- The ACC command must not be present in seismic definition (i.e., in the DEFINE code LOAD command). If present, the natural torsion factor will be ignored and only the accidental torsion for all seismic loads will be considered.
The design eccentricity for calculating horizontal torsion is the DEC + ACC values. When ACC is negative, it becomes DEC - ACC (i.e., the torsion magnitudes are always additive).
Dynamic Eccentricity
- the rotational component of ground motion about the vertical axis,
- the difference between computed and actual values of the mass, stiffness, or strength, and
- uneven live mass distribution.
| edi = DEC×esi + ECC×bi |
| = | ||
| = | ||
| = |
Only TOR ECC 0.05 or TOR ECC -0.05 can also be defined without specifying DEC 1.0 since it is the default that is included in the analysis.
Notes
-
The static seismic load cases should be provided as the first set of load cases. Other (non-seismic) primary load case specified before a seismic load case is not acceptable. Additional loads such as MEMBER LOADS and JOINT LOADS may be specified along with the seismic load under the same load case.
Example of Incorrect Usage: The error here is that the UBC cases appear as the 3rd and 4th cases, when they should be the 1st and 2nd cases.
⋮ LOAD 1 SELFWEIGHT Y -1 LOAD 2 JOINT LOAD 3 FX 45 LOAD 3 UBC LOAD X 1.2 JOINT LOAD 3 FY -4.5 LOAD 4 UBC LOAD Z 1.2 MEMEBER LOAD 3 UNI GY -4.5 PERFORM ANALYSIS
Example of Correct Usage
⋮ SET NL 10 ⋮ LOAD 1 UBC LOAD X 1.2 JOINT LOAD 3 FY -4.5 PERFORM ANALYSIS CHANGE LOAD 2 UBC LOAD Z 1.2 MEMBER LOAD 3 UNI GY -4.5 PERFORM ANALYSIS CHANGE LOAD 3 SELFWEIGHT Y -1 LOAD 4 JOINT LOAD 3 FX 45 PERFORM ANALYSIS LOAD LIST ALL
-
If the static seismic cases
- are to be factored later in a REPEAT LOAD command;
- or if the static seismic case is to be used in a tension/compression analysis;
- or if re-analysis (i.e., two analysis commands without a CHANGE or new load case in between);
Example of Incorrect Usage: The error here is that the CHANGE command is missing before Load Case 2.
⋮ LOAD 1 UBC LOAD X 1.2 SELFWEIGHT Y -1 JOINT LOAD 3 FY -4.5 PDELTA ANALYSIS LOAD 2 UBC LOAD Z 1.2 SELFWEIGHT Y -1 JOINT LOAD 3 FY -4.5 PDELTA ANALYSIS
Example of Correct Usage
⋮ LOAD 1 UBC LOAD X 1.2 SELFWEIGHT Y -1 JOINT LOAD 3 FY -4.5 PDELTA ANALYSIS Change LOAD 2 UBC LOAD Z 1.2 SELFWEIGHT Y -1 JOINT LOAD 3 FY -4.5 PDELTA ANALYSIS CHANGE
- Up to 8 sesimic load cases may be entered.
-
The REPEAT LOAD specification cannot be used for load cases involving seismic load generation unless each seismic case is followed by an analysis command then CHANGE.
Example of repeat load using a seismic load case:
⋮ LOAD 1 UBC LOAD X 1.0 PDELTA ANALYSIS CHANGE LOAD 2 SELFWEIGHT Y -1 PDELTA ANALYSIS CHANGE LOAD 3 REPEAT LOAD 1 1.4 2 1.2 PDELTA ANALYSIS
- If seismic load generation is performed for the X and the Z (or Y if Z up) directions, the command for the X direction must precede the command for the Z (or Y if Z up) direction.
UBC Example
In the following example, notice that the first three load cases are UBC load cases. They are specified before any other load cases.
DEFINE UBC LOAD ZONE 0.2 K 1.0 I 1.5 TS 0.5 SELFWEIGHT JOINT WEIGHT 1 TO 100 WEIGHT 5.0 101 TO 200 WEIGHT 7.5 LOAD 1 UBC in X-Direction UBC LOAD X DEC 1.0 ACC 0.05 JOINT LOAD 5 25 30 FY -17.5 PERFORM ANALYSIS CHANGE LOAD 2 UBC in X-Direction UBC LOAD X DEC 1.0 ACC -0.05 JOINT LOAD 5 25 30 FY -17.5 PERFORM ANALYSIS CHANGE LOAD 3 UBC in Z-Direction UBC LOAD Z DEC 0.0 ACC 0.05 PERFORM ANALYSIS CHANGE LOAD 4 Dead load SELFWEIGHT LOAD COMBINATION 4 1 0.75 2 0.75 3 1.0
IS 1893 Example
In the following example, the first two load cases are the 1893 load cases. They are specified before any other load case.
DEFINE 1893 Load ZONE 0.05 RF 1.0 I 1.5 SS 1.0 SELFWEIGHT JOINT WEIGHT 7 TO 12 WEIGHT 17.5 13 TO 20 WEIGHT 18.0 MEMEBER WEIGHT 1 TO 20 UNI 2.0 LOAD 1 1893 Load in X-Direction 1893 LOAD X JOINT LOAD 5 25 30 FY -17.5 LOAD 2 1893 Load in Z-Direction 1893 LOAD Z LOAD 3 Dead Load SELFWEIGHT LOAD COMBINATION 4 1 0.75 2 0.75 3 10