# TR.31.2.1 RPA (Algerian) Seismic Load

The purpose of this command is to define and generate static equivalent seismic loads as per RPA specifications using a static equivalent approach similar to those outlined by RPA. Depending on this definition, equivalent lateral loads will be generated in horizontal direction(s).

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.

## General Format

DEFINE RPA (ACCIDENTAL) LOAD

` rpa-spec`

`weight-data`

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

Where:

rpa-spec = { A f1 Q f2 RX f3 RZ f4 STYPE f5 CT f6 CRDAMP f7 (PX f8) (PZ f9) }

## Generation of RPA Seismic Load

General format to provide RPA Seismic load in any load case:

`LOAD i`

RPA LOAD {X | Y | Z} (f10) (ACC f11)

Where:

Parameter | Description |
---|---|

LOAD i | the load case number |

RPA LOAD {X | Y | Z} f10 | factor to multiply horizontal seismic load. |

ACC f11 | multiplying factor for Accidental Torsion, to be used to multiply the RPA 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). |

`ACCIDENTAL`option is specified, the accidental torsion will be calculated per the RPA 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 WEIGHT`s and

`MEMBER WEIGHT`s you have specified.

## Methodology

The design base shear is computed in accordance with section 4.2.3 of the RPA 99 code. The primary equation, namely 4-1, as shown below, is checked.

V = (A D Q)W / R

Where:

- W = total weight on the structure
- A = zone coefficient
- D = average dynamic amplification factor
- R = lateral R factor
- Q = structural quality factor

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

Program calculates the natural period of building T utilizing clause 4.2.4 of RPA 99.

Design spectral coefficient (D) is calculated utilizing T as,

D = 2.5η when 0 ≤ T ≤
T_{2} |

=
2.5η(T_{2}/T)^{2/3} when T_{2} ≤ T ≤ 0.3 sec. |

=
2.5η(T_{2}/3)^{2/3}(3/T)^{5/3} when T > 0.3
sec. |

Where:

- η = factor of damping adjustment (Eq. 4.3)
- T
_{2}= specific period (Table 4.7)

Total lateral seismic load, V is distributed by the program among different levels.

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