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Understanding Seismic Design Categories (SDC)

We hear it from our customers everyday, trying to wrap your head around all the different aspects involved in determing what is required on any project can be tedious. Whether you are trying to gain better understanding of how seismic requirements are determined, or just trying to complete what is required on a given project ISAT can help.

If you are unsure of what Seismic Design Category (SDC) your project may fall under it can typically be found in the structural notes section of your construction drawings, or they may also be listed in the project specification. If you are still having trouble determining what category it falls under, ISAT has staff available that can walk you through your project details and help you figure it out. Once you have figured out the SDC of your project, you can then determine the seismic requirements by using ISAT's online Seismic Requirement Calculator.

What is a Seismic Design Category

If A Seismic Design Category is a classification assigned to a structure based on it's occupancy category, and the severity of the design earthquake ground motion. The category assignment can range from A-F, and can be defined as the following:

11.6 SEISMIC DESIGN CATEGORY

Structures shall be assigned a Seismic Design Category in accordance with Section

11.6.1.1. Occupancy Category I, II, or III structures located where the mapped spectral response acceleration parameter at 1-s period, S1, is greater than or equal to 0.75 shall be assigned to Seismic Design Category E. Occupancy Category IV structures located where the mapped spectral response acceleration parameter at 1-s period, S1, is greater than or equal to 0.75 shall be assigned to Seismic Design Category F. All other structures shall be assigned to a Seismic Design Category based on their Occupancy Category and the design spectral response acceleration parameters,
SDS and SD1, determined in accordance with Section 11.4.4. Each building and structure shall be assigned to the more severe Seismic Design Category in accordance with Table 11.6-1 or

11.6-2, irrespective of the fundamental period of vibration of the
structure, T .

Where S1 is less than 0.75, the Seismic Design Category is permitted to be determined from Table 11.6-1 alone where all of the following apply:

1. In each of the two orthogonal directions, the approximate fundamental period of the structure, Ta, determined in accordance with Section 12.8.2.1 is less than 0.8Ts , where Ts is determined in accordance with Section 11.4.5.

2. In each of two orthogonal directions, the fundamental period of the structure used to calculate the story drift is less than Ts.

3. Eq. 12.8-2 is used to determine the seismic response coefficient Cs .

4. The diaphragms are rigid as defined in Section 12.3.1 or for diaphragms that are flexible, the distance between vertical elements of the seismic force-resisting system does not exceed 40 ft.

Where the alternate simplified design procedure of Section 12.14 is used, the Seismic Design Category is permitted to be determined from Table 11.6-1 alone, using the value of SDS determined in Section 12.14.8.1.

11.7 DESIGN REQUIREMENTS FOR SEISMIC DESIGN CATEGORY A

11.7.1 Applicability of Seismic Requirements for Seismic Design Category A Structures. Structures assigned to Seismic Design Category A need only comply with the requirements of.

Section 11.7. The effects on the structure and its components due to the forces prescribed in this section shall be taken as E and combined with the effects of other loads in accordance with the load combinations of Section 2.3 or 2.4. For structures with damping systems, see Section 18.2.1.

11.7.2 Lateral Forces. Each structure shall be analyzed for the effects of static lateral forces applied independently in each of two orthogonal directions. In each direction, the static lateral forces at all levels shall be applied simultaneously. For purposes of analysis, the force at each level shall be determined using Eq. 11.7-1 as follows:

Fx = 0.01wx (11.7-1)

where

Fx = the design lateral force applied at story x, and

wx = the portion of the total dead load of the structure, D, located or assigned to Level x

11.7.3 Load Path Connections. All parts of the structure between separation joints shall be interconnected to form a continuous path to the lateral force-resisting system, and the connections shall be capable of transmitting the lateral forces induced by the
parts being connected. Any smaller portion of the structure shall be tied to the remainder of the structure with elements having design strength of not less than 5 percent of the portion’s weight. This connection force does not apply to the overall design of the lateral force-resisting system. Connection design forces need not exceed the maximum forces that the structural system can deliver to the connection.

11.7.4 Connection to Supports. A positive connection for resisting a horizontal force acting parallel to the member shall be provided for each beam, girder, or truss either directly to its supporting elements, or to slabs designed to act as diaphragms. Where
the connection is through a diaphragm, then the member’s supporting element must also be connected to the diaphragm. The connection shall have a minimum design strength of 5 percent of the dead plus live load reaction.

11.7.5 Anchorage of Concrete or Masonry Walls. Concrete and masonry walls shall be anchored to the roof and all floors and members that provide lateral support for the wall or that are supported by the wall. The anchorage shall provide a direct
connection between the walls and the roof or floor construction. The connections shall be capable of resisting the horizontal forces specified in Section 11.7.3, but not less than a minimum strength level horizontal force of 280 lb/ linear ft (4.09 kN/m)
of wall substituted for E in the load combinations of Section 2.3 or 2.4.