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TR.31.2.23 UBC 1997 Load Definition

This feature enables one to generate horizontal seismic loads per the UBC 97 specifications using a static equivalent approach. 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



ubc-1997-spec = { ZONE f1 I f2 RWX f3 RWZ f4 STYPE f5 NA f6 NV f7 (CT f8) (PX f9) (PZ f10)}


ZONE f1 Seismic zone coefficient. 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 Importance factor
RWX f3 Numerical coefficient R for lateral load in X direction
RWZ f4 Numerical coefficient R for lateral load in Z direction
STYPE f5 Soil Profile type. Valid range of values are integers 1 through 5. These are related to the values shown in Table 16-J of the UBC 1997 code in the following manner:
  1. SA
  2. SB
  3. SC
  4. SD
  5. SE
Note: The soil profile type SF is not supported.
NA f6 Near source factor Na
NV f7 Near source factor Nv
CT f8 Optional CT value to calculate time period based on Method A (see Note 7)
PX f9 Optional Period of structure (in sec) in X-direction to be used in Method B
PZ f10 Optional Period of structure (in sec) in Z-direction to be used in Method B

The seismic zone factor (ZONE) in conjunction with the soil profile type (STYPE), Near source factor (NA), and the Near source factor (NV), is used to determine the values of seismic coefficients Ca and Cv from Tables 16-Q and 16-R of the UBC 1997 code.

If the ACCIDENTAL option is specified, the accidental torsion will be calculated per the 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 WEIGHTs and MEMBER WEIGHTs you have specified. 

seismic-weights =


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

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 1630.2.1 of the UBC 1997 code. The primary equation, namely, 30-4 of UBC 1997, as shown below, is checked.

V = CvI/(RT)⋅W

In addition, the following equations are checked :

Equation 30-5 – The total design base shear shall not exceed

V = 2.5⋅CaI/R⋅W

Equation 30-6 - The total design base shear shall not be less than

V = 0.11⋅CaIW

Equation 30-7 – In addition, for Seismic Zone 4, the total base shear shall also not be less than

V = 0.8⋅ZNvI/R⋅W

For an explanation of the terms used in the above equations, please refer to the UBC 1997 code.

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

Procedure Used by the Program

Steps to calculate base shear are as follows:

  1. Time Period of the structure is calculated based on clause 1630.2.2.1 (Method A) and 1630.2.2.2 (Method B).
  2. You may override the period that the program calculates using Method B by specifying a value for PX or PZ (Items f9 and f10) depending on the direction of the UBC load. The specified value will be used in place of the one calculated using Method B.
  3. The governing Time Period of the structure is then chosen between the above-mentioned two periods on the basis of the guidance provided in clause 1630.2.2.2.
  4. From Table 16-Q and 16-R, Ca and Cv coefficients are calculated.
  5. The Design Base Shear is calculated based on clause 1630.2.1 and distributed at each floor using the rules of clause 1630.5.
  6. 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 UBC load (clause 1630.6). 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.
  7. 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.
  8. Due to the abstractness of the expression "Height above foundation," in STAAD, height, h, is measured above supports. If supports are staggered all over the vertical elevations of the structure, it is not possible to calculate "h" if one doesn't have a clear elevation level from where to measure "h". Also, the code deals with distributing the forces only on regions above the foundation. If there are lumped weights below the foundation, it is not clear as to how one should determine the lateral forces for those regions.

Example 1

ZONE 0.38 I 1.0 STYP 2 RWX 5.6 RWZ 5.6 NA 1.3 NV 1.6 CT 0.037
51 56 93 100 WEIGHT 1440
101 106 143 150 WEIGHT 1000
151 156 193 200 WEIGHT 720
12 17 24 UNI 25.7
YRA 9 11 FLOAD 200 XRA -1 21 ZR -1 41
234 TO 432 PR 150

Example 2

The following example shows the commands required to enable the program to generate the lateral loads. See TR.32.12 Generation of Loads for this information.

UBC  LOAD  X 0.75
UBC LOAD  Z 0.75

The UBC / IBC input can be provided in two or more lines using the continuation mark (hyphen) as shown in the following example :

ZONE 3.000 -
I 1.00 RWX 1.100 -
RWZ 1.200 STYP 5.000 NA 1.40 NV 1.50 CT -
1.300 PX 2.100 PZ 2.200