IBC/ASCE 7 Seismic Provisions for Seismic Design Category A Structures
Did you know that if a structure is assigned to Seismic Design Category (SDC) A, you need not comply with the seismic provisions subsequent to ASCE 7-10 Section 11.7? In fact, you will not find mention of SDC A in any section after Section 11.7 of Chapter 11 nor in ASCE 7-10 Chapters 12-23.
The Seismic Subcommittee of ASCE 7 deliberately organized the seismic provisions in this manner for the convenience of the user, beginning with the ASCE 7-05 edition. This could be, by far, the most user-friendly feature of the ASCE 7 seismic provisions, and we’re sure those designing structures assigned to SDC A would agree. This saves the designer from having to sift through the seismic analysis methods and irregularity tables as well as spares them from calculations such as determining ρ, base shear, and drift, just to name a few.
Also, did you know that Section 1613.3.1 of the 2012 IBC and 2015 IBC (and ASCE 7-10 Section 11.4.1) allows you to automatically assign SDC A to structures located where S1 is less than or equal to 0.04g and Ss is less than or equal to 0.15g? This SDC designation is irrespective of risk category and soil classification.
Here is an illustration of areas which automatically qualify as SDC A locations. As long as a structure is located in the green zones, it only needs to comply with ASCE 7-10 Section 11.7.
ASCE 7-10 Section 11.7 requires SDC A structures to only comply with the general structural integrity requirements of ASCE 7-10 Section 1.4. Nonstructural components in SDC A structures are specifically exempt from seismic design requirements. The reason for this is that SDC A structures are located where the seismic ground motions are below those associated with damage of any significance.
ASCE 7-10 Section 1.4 requires every structure to be provided with a continuous load path in accordance with Section 1.4.2. Every structure is also required to have a complete lateral force-resisting system with adequate strength to resist the forces indicated in Section 1.4.3. All members of the structural system are required to be connected to their supporting members in accordance with Section 1.4.4. Structural walls (bearing walls or shear walls; both terms are defined in Section 11.2) are required to be anchored to diaphragms and other members providing lateral support in accordance with Section 1.4.5.
We would like to discuss the requirements of Section 1.4.3 briefly, because they probably come the closest to seismic requirements. The section requires each structure to be loaded in each orthogonal direction with lateral loads as shown in the following figure. The structure is to be analyzed elastically under these notional or theoretical loads. The bending moments, shear forces, axial forces at various critical sections of various structural members, as obtained from these analyses, are to be combined with those caused by the gravity (dead, live, snow) loads using the load combinations in Section 220.127.116.11 or 18.104.22.168.
Usually we are relaying messages in our blogs about the many requirements you need to meet. With this blog, we’re pleased to share how few requirements need to be met for SDC A structures.