Choosing a Structural System: Why Table 4.1 Is Not Enough on Its Own

1. The table's three columns

TBDY 2018 Table 4.1 lists 46 structural systems in six groups: cast-in-place RC, precast RC, steel, light-gauge steel, masonry and timber. For each it gives three values:

• R, the behaviour factor: how much the elastic seismic force may be reduced (see TBDY-01).
• D, the overstrength factor: used to amplify internal forces that must be protected against brittle failure (section 5 below).
• Permitted BYS: the building height classes in which the system may be used (see TBDY-04). "BYS ≥ 3" means the system may not be used in BYS 1 and 2, the tallest buildings.

2. Three ductility levels

Each group is divided into three levels: high ductility, mixed (a limited-ductility frame together with high-ductility walls) and limited ductility. High ductility means a larger R and a wider height range, in exchange for the strict detailing rules of Chapter 7 (confinement, strong column–weak beam, shear safety). Limited ductility means simpler details, in exchange for a small R and use only in low buildings.

For a cast-in-place RC frame, for example: high ductility (A11) has R = 8 and BYS ≥ 3; limited ductility (A31) has R = 4 and BYS ≥ 7. In the long-period range the limited system's seismic force is twice that of the high-ductility one.

3. Two buildings, two answers

Consider two buildings in the same province (DTS 1): a 30 m residential building and the site's three-storey school block (H_N = 10.2 m). The residential building is DTS 1 and BYS 4; the school, being BKS 1, is DTS 1a and BYS 7. Which cast-in-place RC systems may be used? (Values from the site's Table 4.1 code pack, BYS from the site's engine.)

For the residential building the table suffices: the limited systems and two mixed ones are excluded by the BYS column. But look at the school column: the limited-ductility frame A31 appears permitted by Table 4.1 (BYS ≥ 7, and the school is BYS 7). It still may not be used, because the rule lies outside the table, in Clause 4.3.4.1(a).

4. Rules outside the table

• Clause 4.3.4.1(a): in buildings of DTS 1a, 2a, 3a and 4a — BKS 1 buildings such as hospitals and schools — limited-ductility systems may not be used.
• Clause 4.3.4.1(b): in DTS 1a and 2a buildings with BYS ≤ 6, mixed systems may not be used either. The school example is BYS 7, so mixed systems are permitted for it; one storey taller (BYS 6) and they would not be.
• Clause 4.3.4.2: the ductility level must be the same in both orthogonal directions.
• Clause 3.6.2.3: with a weak-storey (B1) irregularity, R is reduced by 1.25·η_ci,min (see TBDY-03).
• Clauses 13.1.4 and 13.1.5: tall buildings (BYS 1) may use only high-ductility RC systems A12–A15, B12, B13 or high-ductility steel systems C12, C14, C15; only in DTS 4 are mixed systems A21, A22, C21, C22 also allowed.
• Clause 4.3.4.10: rigid perimeter walls around basements are not counted as part of wall or wall–frame systems.

5. Where is D used?

The use of D applies the capacity-design principle:

• Ductile actions, D = 1 (Clause 4.3.5.2): internal forces where yielding is allowed, such as bending moment and tension, are used in reduced form.
• Brittle actions, D > 1 (Clause 4.3.5.3): internal forces where yielding is not allowed, such as shear in RC members and connection forces in steel, are amplified by D. In high-ductility systems, however, the amplified force need not exceed the force consistent with yielding of the permitted sections.

The reasoning: when a building yields in an earthquake, the yielding members carry more force than the design calculation assumed (overstrength). Brittle members not designed for that larger force would fail before the ductile ones, and the whole benefit of ductility would be lost.

6. Checklist for selection

1. Find DTS from the building use class and S_DS; is there an "a"?
2. Find BYS from H_N and DTS.
3. Check the system's permitted BYS in Table 4.1.
4. Check Clause 4.3.4.1: if DTS has "a", no limited system; if BYS ≤ 6 and DTS 1a/2a, no mixed system either.
5. If BYS 1, go to Chapter 13.
6. If there is a weak storey, reduce R.