Load Generation - Seismic Loads

Building Seismic Loads can be automatically generated according to the equivalent static methods of the following codes:

Seismic load can only be applied at diaphragm/floor levels. The program will automatically calculate the center of mass and the 5% accidental eccentricity for the various seismic load cases.


Seismic Weight

The Seismic Weight of each diaphragm is the total tributary weight associated with it depending on the Load Combination chosen for calculating these forces on the Seismic Loads dialog box.

While computing the seismic weight at a particular diaphragm, the self weight of the members/columns and plates between any two diaphragms is equally distributed amongst these diaphragms. Any weight, or load included in the specified load combination, supported between diaphragms is distributed to the diaphragm above and below in inverse proportion to its distance from each diaphragm.

The total seismic weight of the whole structure is the sum of the seismic weights associated with all diaphragms and the weight associated with the base level. The base shear is always computed using the total seismic weight. The total seismic weight can be viewed using the Scaling Factor Dialog. To get here click the button in the Load Combinations spreadsheet.

For additional advice on this topic, please see the RISA Tips & Tricks webpage at risa.com/post/support. Type in Search keywords: Diaphragms.

Seismic Design Parameters

The parameters used in the seismic calculations may be viewed or changed by selecting Seismic from the Load Generators section of the Advanced tab.

In RISA-3D, the weight used for the calculation of seismic loads is based solely upon the Load Combination specified as the Seismic Weight LC entered in the Seismic Loads Dialog shown below:

For additional advice on this topic, please see the RISA Tips & Tricks webpage at risa.com/post/support. Type in Search keywords: Generate Seismic.

Seismic Load Parameters

Code currently allows you to choose the code which will be used for seismic load generation. For reference, sections of the 2016 edition of ASCE 7 will be cited to explain the various entries.

T represents the input natural periods in each lateral direction. These would typically be determined from an eigensolution analysis. If these values are not entered, then the program will calculate this using the Approximate Fundamental Period as defined in section of ASCE 7-16. This value is entered for each of the Global horizontal directions.

Ct is the building period coefficient as defined in of ASCE 7-16. It is used in conjunction with the Ct exponent "x" to determine the Approximate Fundamental Period. These are defined for each of the Global horizontal directions.


R is the Response Modification Factor as defined in table 12.14-1 of ASCE 7-16. It provides a reduction for the design seismic force based on the ductility of the system. This is defined for each of the Global horizontal directions.


Base Elevation determines the height at which the structure is assumed to be connected to the ground. This is important for hillside structures or structures with sub-grade floor levels. A certain amount of structure self weight may be associated with base level (or sub-grade levels) of the structure. The Add Base Weight check-box determines if that self weight will be added into the base shear to be distributed as lateral force through the height of the structure per section 12.8.3 of ASCE 7-16. If no elevation is chosen for base elevation, then the lowest joint in the structure will be assumed to be the base elevation.

Risk Categoryis used to determine the importance factor assigned to the structure per table 1.5-2 of ASCE 7-16.

SD1 represents the 5% damped spectral response Design acceleration for a 1.0 second period.

SDS represents the 5% damped spectral response Design acceleration for short period response.

S1 represents the 5% damped spectral response Mapped acceleration for a 1.0 second period.

TL represents the point at which the structural response is assumed to transition from a velocity controlled response to a displacement controlled response. These values are shown on Figures 22-14 through 22-17 in the ASCE 7-16.

Seismic Weight LC is used to dictate which load combination should be used to define the weight of the structure when the seismic event is assumed to occur. In ASCE 7-16 this would be based on the criteria in section 12.7.2.

Seismic Load Results

When you activate RISA-3D via the Director Menu, the program will calculate the appropriate seismic loads and present the calculations in a printable report. You may open the seismic load generator at any time to view, print, or recalculate the seismic loads.

Seismic Generation Input  

This section displays the user all the relevant design data entered so that it can be included on print outs with the Seismic Load results.

Seismic Generation Detail Results

This section reports the values used to obtain the Base Shear in each of the two global directions.

TX and TZ are the periods which was actually used to determine an upper limit for Cs in equations 12.8-3 and 12.8-4 of ASCE 7-16 (if applicable). A user defined period can exceed the upper limit period.

Ta is the approximate period calculated per equation 12.8-7 of ASCE 7-16.

TLimit is the maximum allowable period to be used in equations 12.8-3 and 12.8-4, calculated per Table 12.8-1 of ASCE 7-16.

Importance Factor is determined from Table 1.5-2 of ASCE 7-16, based on the specified Risk Category.

Design Category is determined in Section 11.6 of the ASCE 7-16 and reported here.

V (Base Shear) is calculated using the Governing Equation listed next to it.

Governing Equation is the equation which was used to calculate the base shear. This is typically from 12.8 of ASCE 7-16.

CS is the seismic response coefficient used to calculate the seismic base shear, V.

Seismic Generation Force Results 

This section displays information used in distributing the seismic force to each diaphragm or story level. This includes the calculated Height and Weight of each diaphragm, the calculated Force in each horizontal direction and the calculated location of the Center of Gravity of the diaphragm (CG).


Seismic Generation Diaphragm Results 

This section displays information used in calculating the accidental torsion values. This includes the Width and Length of each diaphragm and the distance used for the accidental eccentricity.


Semi-Rigid Seismic Loads

When running a combined RISAFloor/RISA-3D model the program has the ability to create Semi-Rigid Seismic loads and apply them to the diaphragm.  The seismic load is calculated by taking the Total Seismic Weight and converting it into a horizontal direction by multiplying by the seismic response coefficient Cs.

The program will apply a Diaphragm Surface load which represents the seismic contribution of the Slab weight and any additional Dyn Load.  There will be horizontal point loads and line loads at the top of the columns and walls which represent their respective contribution of the seismic weight. Any point, line or area loads that are "Dyn Mass" will also be converted as horizontal seismic load applied directly to the diaphragm. Below shows an example of the Earthquake loads applied into a simple L-shaped building.