# TR.31.2.13 IBC 2006/2009 Seismic Load Definition

The specifications of the seismic loading chapters of the International Code Council 2006 & 2009 code and the ASCE 7-05 (including Supplement #2) code for seismic analysis of a building using a static equivalent approach are available in the program. Depending on the definition, equivalent lateral loads will be generated in the horizontal direction(s).

## General Format

There are two stages of command specification for
generating lateral loads. This is the first stage and is activated through the
`DEFINE IBC 2006 LOAD` command.

DEFINE IBC 2006 (ACCIDENTAL) LOAD

map-spec ibc06-spec

`weight-data`

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

Where:

map-spec = { ZIP f_{1}| LAT f_{2}LONG f_{3}| SS f_{4}S1 f_{5}}

Where:

ibc06-spec = { RX f_{6}RZ f_{7}I f_{8}TL f_{9}SCLASS f_{10}(CT f_{11}) (PX f_{12}) (PZ f_{13}) (K f_{14}) (FA f_{15}) (FV f_{16}) }

Where:

## Implementation in STAAD.Pro

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 rules described in section 1613 of the ICC IBC-2006 code (except 1613.5.5) have been implemented. This section directs the engineer to the ASCE 7-2005 code. The specific section numbers of ASCE 7— those which are implemented, and those which are not implemented—are shown in the table below.

Implemented sections of IBC 2006/2009 (ASCE 7-2005) |
Omitted sections of IBC 2006/2009 (ASCE 7-2005) |
---|---|

11.4 | 12.8.4.1 |

11.5 | 12.8.4.3 and onwards |

12.8 |

Additionally, Supplement #2 of ASCE 7-05—as referenced by IBC 2009—specifies a different equation to be used for the lower bound of the seismic response coefficient, which has also been implemented.

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

- The Time Period of the structure is
calculated based on section 12.8.2.1 of ASCE 7-05 (IBC 2006/2009). 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 (Items f7 and f8) 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 section 12.8.2 of ASCE 7-05 (IBC 2006). The resulting value is reported as "Time Period used" in the output file.
- The Design Base Shear is calculated based on equation 12.8-1 of ASCE 7-05 (IBC 2006). It is then distributed at each floor using the rules of clause 12.83, equations 12.8-11, 12.8-12 and 12.8-13 of ASCE 7-05.
- 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 (section 12.8.4.2 of ASCE 7-05 for IBC 2006). 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. - The amplification of accidental torsional moment, as described in Section 12.8.4.3 of the ASCE 7-05 code, is not implemented.
- The story drift determination as explained in Section 12.8.6 of the ASCE 7-05 code is not implemented in STAAD.Pro.

## Methodology

The design base shear is computed in accordance with the following equation (equation 12.8-1 of ASCE 7-05):

V = C_{s}W

The seismic response coefficient, C_{s}, is
determined in accordance with the following equation (equation 12.8-2 of ASCE
7-05):

C_{s} =
S_{DS}/[R/I_{E}]

_{s}need not exceed the following limits defined in ASCE 7-05 (equations 12.8-3 and 12.8-4):

- C
_{s}= S_{D1}/[T⋅(R/I)] for T ≤ T_{L} - C
_{s}= S_{D1}· T_{L}/[T^{2}(R/I)] for T > T_{L}

However, C_{s} shall not be less than (equation
12.8-5 of ASCE 7-05, supplement #2):

C_{s} = 0.044 ·
S_{DS} · I ≥ 0.01

In addition, per equation 12.8-6 of ASCE 7-05, for
structures located where S_{1} is equal to or greater than 0.6g,
C_{s} shall not be less than

C_{s} = 0.5 ·
S_{1}/(R/I)

For an explanation of the terms used in the above equations, please refer to the IBC 2006/2009 and ASCE 7-05 codes.

## Example 1

DEFINE IBC 2006 LAT 38.0165 LONG -122.105 I 1.25 RX 2.5 RZ 2.5 SCLASS 4 - TL 12 FA 1 FV 1.5 SELFWEIGHT JOINT WEIGHT 51 56 93 100 WEIGHT 650 MEMBER WEIGHT 151 TO 156 158 159 222 TO 225 324 TO 331 UNI 45

## 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 this information.

LOAD 1 (SEISMIC LOAD IN X DIRECTION) IBC LOAD X 0.75 LOAD 2 (SEISMIC LOAD IN Z DIRECTION) IBC LOAD Z 0.75