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TR.31.2.21 UBC 1994 or 1985 Load Definition

This set of commands may be used to define the parameters for generation of UBC-type equivalent static lateral loads for seismic analysis. Depending on this definition, equivalent lateral loads will be generated in horizontal direction(s).

General Format

{ ubc-1994-spec | ubc-1985-spec }

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


ubc-1994-spec  = { ZONE f1 I f2 RWX f3 RWZ f4 S f5 (CT f8) (PX f9) (PZ f10) }
ubc-1985-spec = { ZONE f1 K f6 I f2 (TS f7) }


ZONE f1 The seismic zone coefficient (0.2, 0.3 etc.). Instead of using an integer value like 1, 2, 3 or 4, use the fractional value like 0.075, 0.15, 0.2, 0.3, 0.4, etc.
I f2 The importance factor. Used for both 1985 and 1994 specifications.
RWX f3 (UBC 1994 spec only) Numerical co-efficient Rw for lateral load in Z-directions
RWZ f4 (UBC 1994 spec only) Numerical co-efficient Rw for lateral load in Z-directions
S f5 (UBC 1994 spec only) Site co-efficient for soil characteristics
K f6 (UBC 1985 spec only) Horizontal force factor
TS f7 (UBC 1985 spec only) Period of structure (in seconds) in the X- direction.
CT f8 (UBC 1994 spec only) Value of the term Ct which appears in the equation of the period of the structure per Method A. See note f for details.
PX f9 (UBC 1994 spec only) Period of structure (in seconds) in the X direction.    
PZ f10 (UBC 1994 spec only) Period of structure (in seconds) in the Z direction.

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

Note: For additional details on the application of a seismic load definition used to generate loads, refer to GUID-6D9B5C48-9FFF-4548-BFB4-ACAE1E973743.


  1. If the option ACCIDENTAL is used, the accidental torsion will be calculated per UBC 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, MEMBER WEIGHT, ELEMENT WEIGHT, FLOOR WEIGHT, and ONEWAY WEIGHT commands you have specified.

  2. In ubc-spec for 1985 code, specification of TS is optional. If TS is specified, resonance co-efficient S is determined from the building period T and user provided TS using UBC equations. If TS is not specified, the default value of 0.5 is assumed.

  3. By providing either PX or PZ or both, you may override the period calculated by STAAD for Method B of the UBC Code. The user defined value will then be used instead of the one recommended by UBC per equation 28.5 of UBC 94. If you do not define PX or PZ, the period for Method B will be calculated by the program per equation 28.5.

  4. Some of the items in the output for the UBC analysis are explained below.


    The CALC PERIOD is the period calculated using the Rayleigh method (Method B as per UBC code).  For UBC in the x-direction, the USED PERIOD is PX.  For the UBC in the z-direction, the USED PERIOD is PZ.  If PX and PZ are not provided, then the used period is the same as the calculated period for that direction.  The used period is the one substituted into the critical equation of the UBC code to calculate the value of C.

  5. In the analysis for UBC loads, all the supports of the structure have to be at the same level and have to be at the lowest elevation level of the structure.

  6. If the value of Ct is not specified, the program scans the Modulus of Elasticity (E) values of all members and plates to determine if the structure is made of steel, concrete or any other material. If the average E is smaller than 2000 ksi, Ct is set to 0.02. If the average E is between 2000 & 10000 ksi, Ct is set to 0.03. If the average E is greater than 10,000 ksi, Ct is set to 0.035. If the building material cannot be determined, Ct is set to 0.035. Ct is in units of seconds/feet¾ or in units of seconds/meter¾. Ct < 0.42 if the units are in feet, and Ct > 0.42 if the units are in meter.


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. 

Total lateral seismic force or base shear is automatically calculated by STAAD using the appropriate UBC equations (All symbols and notations are per UBC).

UBC 1994: Equation 1



UBC 1984: Equation 2

V = ZIK · CS · W

Base shear V may be calculated by STAAD using either the 1994 procedure (equation 1) or the 1985 procedure (equation 2). The user should use the appropriate "ubc-spec" (see General Format) to instruct the program accordingly.

Procedure Used by the Program

STAAD utilizes the following procedure to generate the lateral seismic loads.

  1. You must specify seismic zone co-efficient and desired ubc-spec (1985 or 1994) following the DEFINE UBC LOAD command.

  2. Program calculates the structure period T.

  3. Program calculates C from appropriate UBC equation(s) utilizing T.

  4. Program calculates V from appropriate equation(s). W is obtained from SELFWEIGHT, JOINT WEIGHT, MEMBER WEIGHT, ELEMENT WEIGHT, FLOOR WEIGHT, and ONEWAY WEIGHT commands specified following the DEFINE UBC LOAD command. The weight data must be in the order shown.

    Note: If both mass table data (SELFWEIGHT, JOINT WEIGHT, MEMBER WEIGHT, etc. options) and a REFERENCE LOAD are specified, these will be added algebraically for a combined mass.
  5. The total lateral seismic load (base shear) is then distributed by the program among different levels of the structure per UBC procedures.


ZONE 0.2 I 1.0 RWX 9 RWZ 9 S 1.5 CT 0.032
17 TO 48 WEIGHT 2.5
49 TO 64 WEIGHT 1.25
17 TO 48 FY -2.5
_SLAB1 FLOAD 0.045
_ROOF ONE 0.035 GY