# TR.31.2.12 IBC 2000/2003 Load Definition

The specifications of the IBC 2000, and 2003 codes for seismic analysis of a building using a static equivalent approach have been implemented as described in this section. Depending on this definition, equivalent lateral loads will be generated in horizontal direction(s).

## General Format

There are 2 stages of command specification for generating lateral loads. This is the first stage and is activated through the DEFINE IBC 2000 or 2003 LOAD command.

DEFINE IBC ( { 2000 | 2003 } ) (ACCIDENTAL) LOAD

` ibc-spec`

`weight-data`

Refer to Common Weight Data for information on how to specify structure weight for seismic loads.

Where:

ibc-spec = { SDS f1 SD1 f2 S1 f3 I f4 RX f5 RZ f6 SCLASS f7 (CT f8) (PX f9) (PZ f10) }

The seismic load generator can be used to generate lateral loads in the X & Z directions for Y up or X & Y for Z up. Y up or Z up is the vertical axis and the direction of gravity loads (See the
`SET Z UP` command in
TR.5
Set Command Specification). All vertical coordinates of the floors above the base must be positive and the vertical axis must be perpendicular to the floors.

The implementation details of the respective codes are as follows:

## IBC 2000

On a broad basis, the rules described in section 1617.4 of the IBC 2000 code document have been implemented. These are described in pages 359 thru 362 of that document. The specific section numbers, those which are implemented, and those which are not implemented, are as follows:

Implemented sections of IBC 2000 | Omitted sections of IBC 2000 |
---|---|

1617.4.1 1617.4.1.1 1617.4.2 1617.4.2.1 1617.4.3 1617.4.4 1617.4.4.4 |
1617.4.4.1 1617.4.4.2 1617.4.4.3 1617.4.4.5 1617.4.5 1617.4.6 |

## IBC 2003

On a broad basis, the rules described in section 1617.4 of the IBC 2003 code document have been implemented. This section directs the engineer to Section 9.5.5 of the ASCE 7 code. The specific section numbers of ASCE 7-2002, those which are implemented, and those which are not implemented, are shown in the table below. The associated pages of the ASCE 7-2002 code are 146 thru 149.

Implemented sections of IBC 2003 (ASCE 7-02) | Omitted sections of IBC 2003 (ASCE 7-02) |
---|---|

9.5.5.2 9.5.5.2.1 9.5.5.3 9.5.5.3.1 9.5.5.3.2 9.5.5.4 9.5.5.5 Portions of 9.5.5.5.2 |
9.5.5.5.1 9.5.5.5.2 9.5.5.6 9.5.5.7 |

## Methodology

The design base shear is computed in accordance with Eqn. 16-34 of IBC 2000 and Eqn. 9.5.5.2-1 of ASCE 7-02:

V = C_{s}W

The seismic response coefficient, C_{s}, is determined in accordance with Eqn. 16-35 of IBC 2000 / Eqn. 9.5.5.2.1-1 of ASCE 7-02:

C_{s} need not exceed the limit given in Eqn. 16-36 of IBC 2000 / Eqn. 9.5.5.2.1-2 of ASCE 7-02:

C_{s} shall not be taken less than the lower limit given in Eqn. 16-37 of IBC 2000 / Eqn. 9.5.5.2.1-3 of ASCE 7-0:

C_{s} = 0.044 S_{DS}I_{E}

In addition, for structures for which the 1-second spectral response, S_{1}, is equal to or greater than 0.6g, the value of the seismic response coefficient, C_{s}, shall not be taken less than the limit given in Eqn. 16-38 of IBC 2000 / Eqn. 9.5.5.2.1-4 of ASCE 7-02:

For an explanation of the terms used in the above equations, please refer to the relevant IBC and ASCE 7-02 codes.

## Procedure Used by the Program

Steps used to calculate and distribute the base shear are as follows:

- The Time Period of the structure is calculated based on section 1617.4.2 of IBC
2000, and section 9.5.5.3 of ASCE 7-02 (IBC 2003) This is reported in the output
as T
_{a}. - The period is also calculated in accordance with the Rayleigh method. This is reported in the output as T.
- you may override the Rayleigh based period by specifying a value for
`PX`or`PZ`depending on the direction of the IBC load. - The governing Time Period of the structure is then chosen between the above two periods, and the additional guidance provided in clause 1617.4.2 of IBC 2000, section 9.5.5.3 of ASCE 7-02 (IBC 2003) or section 12.8.2.1 of ASCE 7-05 (IBC 2006). The resulting value is reported as "Time Period used."
- The Design Base Shear is calculated based on equation 16-34 of IBC 2000, equation 9.5.5.2-1 of ASCE 7-02 (IBC 2003) or equation 12.8-1 of ASCE 7-05 (IBC 2006). It is then distributed at each floor using the rules of clause 1617.4.3, equations 16-41 and 16-42 of IBC 2000. For IBC 2003, using clause 9.5.5.4, equations 9.5.5.4-1 & 9.5.5.4-2 of ASCE 7-02.
- If the
`ACCIDENTAL`option is specified, the program calculates the additional torsional moment. The lever arm for calculating the torsional moment is obtained as 5% of the building dimension at each floor level perpendicular to the direction of the IBC load (clause 1617.4.4.4 of IBC 2000, and section 9.5.5.5.2 of ASCE 7-02 for IBC 2003 ). At each joint where a weight is located, the lateral seismic force acting at that joint is multiplied by this lever arm to obtain the torsional moment at that joint.

## Example 1

DEFINE IBC 2003 LOAD SDS 0.6 SD1 0.36 S1 0.31 I 1.0 RX 3 RZ 4 SCL 4 CT 0.032 SELFWEIGHT JOINT WEIGHT 51 56 93 100 WEIGHT 1440 101 106 143 150 WEIGHT 1000 151 156 193 200 WEIGHT 720

## Example 2

The following example shows the commands required to enable the program to generate the lateral loads. Refer to TR.32.12 Generation of Loads for details.

LOAD 1 ( Seismic Load in X Direction ) IBC LOAD X 0.75 LOAD 2 ( Seismic Load in Z Direction ) IBC LOAD Z 0.75

The Examples manual contains examples illustrating load generation involving IBC and UBC load types.