Section 1613.5.1 of the International Code Council’s 2006 International Building Code (IBC) defines Seismic Design Category A based on the mapped spectral response acceleration parameters, SS and S1. The section states that where the mapped spectral response acceleration parameters meet the criteria: SS ≤ 0.15g and S1 ≤ 0.04g, the building can be assigned to Seismic Design Category A. The other option is to use Tables 1613.5.6(1) and 1613.5.6(2), which are based on the design spectral response acceleration parameters, SDS and SD1, and the occupancy category of the building. Based on this option, a building is assigned to Seismic Design Category A if SDS < 0.167g and SD1 < 0.067g. Note that in either case, the “and” requires that both conditions be satisfied. Following is a summary of the criteria for Seismic Design Category A:
Case 1: Seismic Design Category A if both SS ≤ 0.15g and S1 ≤ 0.04g.
Case 2: Seismic Design Category A if both SDS < 0.167g and SD1 < 0.067g.
Note that in either case, the building is assigned to Seismic Design Category A irrespective of the occupancy category of the building. In case 1 this is because the classification is based on mapped values only and is independent of the occupancy category; in case 2 this is because all occupancy categories (I, II, III, and IV) are in Seismic Design Category A in Tables 1613.5.6(1) and 1613.5.6(2), below.
Answers to FAQ’s:
Q: The base shear for the equivalent lateral force procedure is a function of the effective seismic weight, W. The seismic weight not only includes the dead load of the structure, but also four additional items described in Section 12.7.2 of ASCE 7-05, which are 25 percent of the floor live load in a storage building, a partition load, the weight of permanent equipment, and 20 percent of the design snow load where Pf exceeds 30 pounds per square foot (psf). I recently noticed that in Section 11.7.2 of ASCE 7-05 the lateral force is not the seismic weight, but simply 1 percent of the dead load for buildings in Seismic Design Category A. Is this correct?
A: Yes. The equation for the determination of the design lateral force at any level (Fx = 0.01wx) has been simplified in Seismic Design Category A so that wx is only the tributary dead load at that level. The 1 percent of dead load force level is not intended to represent a true seismic lateral force, but rather to provide a minimum level of strength relative to the mass of the structure to ensure a minimum level of structural integrity and performance. It should be noted that the term “dead load” is defined in Section 3.1 of ASCE 7-05 as the weight of materials of construction incorporated into the building as well as fixed service equipment, including the weight of cranes. Section 3.1.2 further describes items that typically constitute the weight of fixed service equipment.
The Fx = 0.01wx force at each level requires a complete load path from the point of origin to resisting element, which includes all members and associated connections as described in ASCE 7-05 Section 11.7.3. For the majority of cases in Seismic Design Category A, this design requirement will likely have no impact on the design because these lateral forces will generally be less than the lateral forces from wind design.
Note that there is a separate 5 percent dead plus live load reaction requirement in ASCE 7-05 Section 11.7.4 that is in addition to the seismic force requirement. It applies to connections resisting horizontal forces acting parallel to beams, girders, and trusses. This requirement is intended to provide minimum ties to resist the effects of horizontal movement of beams, girders, and trusses at their supports.
Q: I also noticed that the lateral force equation for Seismic Design Category A does not include the importance factor, I, as do the base shear equations for the equivalent lateral force procedure. Is it not necessary to multiply the minimum lateral force, Fx = 0.01wx, for SDC A structures by the importance factor, I?
A: That is correct. The lateral force is not related to the structural system used, site soil type, period of the structure, or importance factor. All of the seismic-related design requirements for buildings in Seismic Design Category A are included in ASCE 7-05 Section 11.7. The lateral force prescribed in Section 11.7.2 is to be taken as E and combined with the other load effects in the load combinations specified in IBC Section 1605.2 for strength design/load and resistance factor design or IBC Section 1605.3 for allowable stress design.
Disclaimer: The building official has the final authority in rendering interpretations of the code as prescribed in IBC Section 104.1. Opinions by ICC staff are based solely on the published model code and do not consider federal, state, or local amendments, ordinances, or policies.
John R. Henry, P.E., is the principal staff engineer at the International Code Council’s Los Angeles office. He can be reached at email@example.com.