Sample Question:
A new 4-story outpatient clinic (Occupancy Category III) is being designed in Seattle, WA. The structure is a Special Reinforced Concrete Moment Frame building founded on Site Class D soil. The design team is determining the seismic design parameters.
Which primary codes and standards must be referenced for the seismic design of this structure? (List at least 3)
How is the Seismic Design Category (SDC) determined for this building? Outline the key steps and required parameters.
What is the Seismic Importance Factor (Ie) for this structure, and why?
Where would the design team find the mapped spectral acceleration parameters (Sₛ and S₁) for the site?
Why is the selected structural system (Special Reinforced Concrete Moment Frame) significant in the context of the determined SDC?
Answer Guide & Key Concepts:
Applicable Codes/Standards:
International Building Code (IBC): The overarching building code adopted by most US jurisdictions. It references ASCE 7 for seismic loads and ACI 318 for concrete design.
ASCE/SEI 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures): Provides the methodology for calculating seismic design forces, defining SDC, and specifying detailing requirements based on SDC.
ACI 318-19 (Building Code Requirements for Structural Concrete): Provides specific design, detailing, and construction requirements for concrete structural systems, including seismic provisions tailored to the SDC and the specific seismic-force-resisting system (e.g., "Special" moment frames).
(Other potential: AISC 341 for steel if applicable, local amendments to IBC).
Determining Seismic Design Category (SDC): The steps involve:
Identify Occupancy Category (IBC Table 1604.5): Given as III (Essential Facilities - includes outpatient clinics).
Determine Mapped Spectral Accelerations (ASCE 7 Ch. 22): Obtain Sₛ (Short Period) and S₁ (1-Second Period) for the site location (Seattle, WA) using the USGS Hazard Tool or maps/equations in ASCE 7. (Seattle will have high values).
Determine Site Class (ASCE 7 Ch. 20): Given as D (Stiff Soil).
Calculate Site Coefficients & Design Spectral Accelerations (ASCE 7 Sec. 11.4):
Fₐ (Table 11.4-1) adjusts Sₛ for site class → SMS = Fₐ * Sₛ
Fᵥ (Table 11.4-2) adjusts S₁ for site class → SM1 = Fᵥ * S₁
SDS = (2/3) * SMS
SD1 = (2/3) * SM1
Determine SDC (ASCE 7 Tbl 11.6-1 & Tbl 11.6-2):
Use SDS value and Occupancy Category in Table 11.6-1.
Use SD1 value and Occupancy Category in Table 11.6-2.
The most severe category (highest letter) from the two tables governs. For Occupancy III in a high seismic region like Seattle, the SDC will likely be D or E.
Seismic Importance Factor (Ie):
Value: Ie = 1.25
Why: ASCE 7 Table 1.5-2 assigns Ie based on Occupancy Category. Category III structures (Essential Facilities) require an Ie = 1.25. This factor increases the design seismic forces to provide a higher margin of safety and reduce the risk of damage for structures deemed essential post-earthquake.
Source for Mapped Spectral Accelerations (Sₛ and S₁):
The design team would use the USGS Seismic Design Web Application or the maps, equations, and coefficients provided in ASCE 7, Chapter 22 (specifically Section 22.3 for the Conterminous United States). The USGS tool is the most common and up-to-date method.
Significance of Structural System Choice:
ASCE 7 Table 12.2-1 defines permissible seismic-force-resisting systems and assigns them a Seismic Force-Resisting System (SFRS) designation (e.g., C1 for Special Reinforced Concrete Moment Frames) and corresponding Response Modification Coefficient (R), Overstrength Factor (Ω₀), and Deflection Amplification Factor (Cd).
The SDC determined in Step 2 imposes limitations on the types of SFRS permitted (ASCE 7 Tbl 12.2-1 has footnotes listing SDC restrictions). Special Moment Frames (SMF) are among the most ductile systems (high R factor) and are permitted for all SDCs, including the highest (E and F). Choosing an SMF system is appropriate and necessary for a high SDC building like this clinic. Using a less ductile system (e.g., an "Ordinary" moment frame) might not be permitted for the anticipated high SDC.