UBC Seismic Loads 

In ICAB/PS, the ratio V/W shall be determined using section UBC 1630.2
The following sections are extracts of the UBC 1997.
The International Conference of Building Officials (ICBO, http://www.icbo.org/) publishes a family of codes, each correlated with the Uniform Building Code (UBC) to provide jurisdictions with a complete set of buildingrelated regulations for adoption.
Chapter 16, division IV of the UBC 1997
The purpose of the earthquake provisions herein is primarily to safeguard against major structural failures and loss of life, not to limit damage or maintain function.
1628 – Symbols and notations
T= elastic fundamental period of vibration, in seconds, of the structure in the direction under consideration
V= the total design lateral force or shear at the base
W= the total seismic dead load
1629.8.2 Simplified static
The simplified static lateralforce procedure set forth in section 1630.2.3 may be used for the following structures of Occupancy Category 4 or 5:
1630.2 Static Force Procedure
1630.2.1 Design base shear
The total design base shear in a give direction shall be determined from the following formula:
_{}
The total design base shear need not exceed the following:
_{}
The total design base shear not be less than the following:
_{}
In addition, for seismic Zone 4, the total base shear shall also not be less than the following:
_{}
1630.2.3 Simplified design base shear
Structures conforming to the requirements of section 1629.8.2 may be designed using this procedure.
1630.2.3.2 Base shear
The total design base shear in a given direction shall be determined from the following formula:
_{}
where the value C_{a} shall be based on Table 16Q for the soil profile type. When the soil properties are not known in sufficient detail to determine the soil profile type, Type S_{D }shall be used in Seismic Zones 3 and 4, and Type S_{E} shall be used in Seismic Zones 1, 2A and 2B. In Seismic Zone 4, the NearSource Factor, N_{a}, need not be greater than 1.3 if none of the following structural irregularities are present: Type 1, 4 or 5 of Table 16L, or Type 1 or 4 of Table 16M.
1630.3 Determination of Seismic Factors
Structures conforming to the requirements of section 1629.8.2 may be designed using this procedure.
1630.3.1 Determination of W_{0}
For specific elements of the structure, as specifically identified in this code, the minimum design strength shall be the product of the seismic force over strength factor W_{0} and the design seismic forces set forth in Section 1630. For both Allowable Stress Design and Strength Design, the Seismic Force Over strength Factor W_{0}, shall be taken from Table 16N.
1630.3.2 Determination of R
The notation R shall be taken from Table 16N.
1630.5 Vertical Distribution of Force
The total force shall be distributed over the height of the structure in conformance with the following formulas in the absence of a more rigorous procedure.
_{}
The concentrated force Ft at the top, which is in addition to Fn, shall be determined from the formula:
_{}
The value of T used for the purpose of calculating Ft shall be the period that corresponds with the design base shear. Ft need not exceed 0.25V and may be considered as zero where T is 0.7 second or less. The remaining portion of the base shear shall be distributed over the height of the structure, including Level n, according to the following formula:
_{}
At each level designed as x, the force Fx shall be applied over the area of the building in accordance with the mass distribution at that level. Structural displacements and design seismic forces shall be calculated as the effect of Forces Fx and Ft applied at the appropriate levels above the base.