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

- ASCE 7-2016 / IBC 2015 / IBC 2018
- ASCE 7-2010 / IBC 2012
- ASCE 7-2005 / IBC 2006 / IBC 2009
- ASCE 7-2002 / IBC 2003
- IBC 2000
- 1997 UBC
- 2001 CBC (California Amended UBC)
- Mexican NTC-04
- Indian IS 1893:02
- Canadian NBC-2015
- Canadian NBC-2010
- Canadian NBC-2005

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.

Note

- The Seismic Loads generated by the program are calculated for Building Structures ONLY and may not apply to non-building structures.
- The Seismic Loads generated by the program consider accidental eccentric loading as well. If you plan on using the Load Combination Generator in the Load Combinations spreadsheet, you must use the "X and Z w/Eccentric" Seismic Load option. Otherwise the eccentric BLCs that have been generated will never actually be applied.

The **Seismic Weight** of each

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

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

The parameters used in the seismic calculations may be viewed or changed **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**.

**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 12.8.2.1 of ASCE 7-16. This value is entered for each of the Global horizontal directions.

**C**_{t}

**Note:**

- You can either input the period manually in the Global horizontal directions, or you can input Ct and the Ct exponent "x" and the program will use Eq. 12.8-7 of the ASCE 7-16 for calculation of the period.

**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.

Note:

- The program offers a single R value input in each direction. There are situations where the lower portion of the structure may have a different R value than the upper portion. This can be typical with a concrete pedestal supporting a wood structure. In this case, two sets of load combinations would be created, one set in which only the Wood design check-box would be checked in the Load Combinations spreadsheet and one set in which only the Concrete design check-box is checked. In this case, the R value for the concrete pedestal would be input in the Seismic Load generator. Then, in the Load Combinations spreadsheet, the wood load combinations would have their seismic load factors factored by the ratio of the wood R value over the concrete R value. In this way, the two R values can be taken into account in the same direction.

**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 Category**is used to determine the importance factor assigned to the structure per table 1.5-2 of ASCE 7-16.

**SD**_{1}

**SD**_{S}

**S**_{1}

**T**_{L}

**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.

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.

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.

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

**T**_{X}
**T**_{Z}

**T**_{a}

**T**_{Limit}

**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.

**C**_{S}

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**).

Note:

- In ASCE 7-16 there is no required seismic loading required for structures which fall under Seismic Design Category A. Instead, notional loads should be applied.

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.

Note

- The magnitudes of the seismic loads are based on the settings under the Seismic tab of the Model Settings dialog.
- The Joints / Nodes used to apply seismic load may appear as a "diamond" shape pattern of unattached nodes at that diaphragm level. While these nodes may not be attached to any beams or framing, they are attached to the diaphragm at that floor level. To get rid of these nodes, it is necessary to re-run the Seismic force generation with the design code set to NONE.
- Any equations based on NON-building structures are not currently taken into account for the seismic load calculations.

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.

Note

- For a RISA-3D only model you will need to manually apply your loads and build your diaphragm with plates. See the Semi-Rigid Diaphragms section for more information on this.
- Any Openings in the slab (Concrete Floor Slabs) will have the self weight correctly accounted for.
- The program does not currently account for accidental torsion for Semi-Rigid diaphragms. The load categories ELX+Z, ELX-Z are exactly the same as ELX and likewise for ELZ and ELZ+X, ELZ-X.