The purpose of this command is to define and generate static equivalent seismic loads as per Japanese specifications using a static equivalent approach similar to those outlined by UBC. Depending on this definition, equivalent lateral loads will be generated in horizontal direction(s). The implementation is as per Article 88 in the "Building Codes Enforcement Ordinance 2006".

## General Format

DEFINE  AIJ (ACCIDENTAL) LOAD
AIJ-spec
weight-data

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

Where:

AIJ-spec
= { ZONE f1 CO f2 TC f3 ALPHA f4 }

where:

ParameterDescription
ZONE f1 Zone factor (0.7, 0.8, 0.9 or 1.0 )
CO f2 Normal coefficient of shear force ( 0.2 or 1.0 )
TC f3 Period defined by ground support specification ( 0.4, 0.6 or 0.8 sec)
ALPHA f4 Ratio of steel height to overall building height, which are used in the calculation of Rt.

## Generation of AIJ Seismic Load

General format to provide Japanese Seismic load in a primary load case:

LOAD i
AIJ LOAD {X | Y | Z} (f5) (ACC f6)

Where:

ParameterDescription
AIJ LOAD { X | Y | Z } f5 optional factor to multiply horizontal seismic load.
ACC f6 multiplying factor for Accidental Torsion, to be used to multiply the AIJ accidental torsion load (default = 1.0). May be negative (otherwise, the default sign for MY is used based on the direction of the generated lateral forces).
Note: Choose horizontal directions only.

If the ACCIDENTAL option is specified, the accidental torsion will be calculated per the AIJ specifications. The value of the accidental torsion is based on the "center of mass" for each level. The "center of mass" is calculated from the SELFWEIGHT, JOINT WEIGHTs and MEMBER WEIGHTs you have specified

Note: For additional details on the application of a seismic load definition used to generate loads, refer to TR.32.12.2 Generation of Seismic Loads.

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.

## Methodology

Seismic zone coefficient and parameter values are supplied by the user through the DEFINE AIJ LOAD command.

Program calculates the natural period of building Tutilizing the following equation:

 T = h(0.02 + 0.01α)

where
 h = height of building (m) α = Alpha, ratio of steel height for overall height

Design spectral coefficient, Rt, is calculated utilizing T and Tc as follows:

where
 Tc = Period defined by ground support specification for Rt.

αi is calculated from the weight values provided in the Define AIJ Load command.

 αi = Wi / W

where
 Wi = sum of weight from a top to floor i. W = all weight

The seismic coefficient of floor, Ci, is calculated using:

Ci = Z Rt Ai Co

where
 Z = zone factor Co = normal coefficient of shear force
$A i = 1 + 2 T ( 1 / a i − a i ) 1 + 3 T$

Seismic load shear force, Qi, of each floor is calculated by Ci and Wi:

Qi = CiWi

where
 Wi = sum of weight from a top to i floor

Load value of each floor Pi is calculated by seismic load shear force Qi.

Pi = Qi– Qi+1

The total lateral seismic load is distributed by the program among different levels.

## Example

DEFINE AIJ LOAD
ZONE 0.8 CO 0.2 TC 0.6 ALPHA 1.0
JOINT WEIGHT
51 56 93 100 WEIGHT 1440
101 106 143 150 WEIGHT 1000
151 156 193 200 WEIGHT 720