What Is Ordinary vs. Special Seismic Force-Resisting System?
Ordinary, intermediate, and special seismic systems are the ductility hierarchy: the ordinary (the minimal detailing — the low-seismic; the R=3), the intermediate (the moderate — the R=4-5), and the special (the ductile — the high-seismic; the R=6-8). The design: the higher the R (the lower the forces), the stricter the detailing (the capacity design, the confinement). The trade — the forces vs. the ductility.
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The hierarchy
Ordinary (the prescriptive — the minimal; the SDC A-C; the R's low), intermediate (the moderate — the detailing; the SDC D's some), and special (the ductile — the tested; the SDC D-F; the R's high). The moment frames, the shear walls, the braced frames — each's three levels.
The trade-off
The R-factor (the higher — the lower the design forces; the special's 8 vs. the ordinary's 3), the detailing (the stricter — the confinement, the capacity, the connections; the labor and the inspection), and the cost (the total — the forces' savings vs. the detailing's premium; the local's economy).
Selection
The SDC (the code's — the permitted systems; the height limits), the performance (the special's — the ductile; the damage's), and the peer (the high-seismic's — the review). The hierarchy is the seismic's — the ductility designed, the forces reduced.
The bottom line
Ordinary (R=3, minimal detailing) vs. special (R=6-8, ductile capacity design) seismic systems trade lower design forces for stricter detailing — select by SDC, height limits, and local construction economics.
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