Notes

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.

   CALC / USED PERIOD

   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/feet^¾ or in units of seconds/meter^¾. Ct < 0.42 if the units are in feet, and Ct > 0.42 if the units are in meter.

Philosophy

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

V = ZIC W

Rw

UBC 1984: Equation 2

V = ZIK · C_S · 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.