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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


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


rpa-spec = { A f1 Q f2 RX f3 RZ f4 STYPE f5 CT f6 CRDAMP f7 (PX f8) (PZ f9) }
A f1 Seismic zone coefficient. Use a fractional value such as 0.08, 0.15, 0.2, 0.3, 0.05, etc., instead of an integer value.
Q f2 Importance factor
RX f3 Coefficient R for lateral load in X direction – table 4.3
RZ f4 Coefficient R for lateral load in Z direction – table 4.3
STYPE f5 Soil Profile Type
CT f6 Coefficient from table 4.6 of RPA 99
CRDAMP f7 Critical damping factor
PX f8 Optional Period of structure (in sec) in X direction
PZ f9 Optional Period of structure (in sec) in Z direction to be used as fundamental period of the structure instead of the value calculated by the program using Rayleigh method.

Used for Y if the SET Z UP command is used.

Generation of RPA Seismic Load

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

RPA LOAD {X | Y | Z} (f10) (ACC f11)


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).
Note: If the 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 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 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


  • 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 ≤ T2

= 2.5η(T2/T)2/3 when T2 ≤ T ≤ 0.3 sec.

= 2.5η(T2/3)2/3(3/T)5/3 when T > 0.3 sec.


  • η = factor of damping adjustment (Eq. 4.3)
  • T2 = 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.


A 0.15 Q 1.36 STYP 2 RX 3 RZ 4 CT 0.0032 –
CRDAMP 30 PX .027 PZ 0.025
51 56 93 100 WEIGHT 1440
101 106 143 150 WEIGHT 1000
151 156 193 200 WEIGHT 720